WorldWideScience

Sample records for accretion disc radii

  1. In situ accretion of gaseous envelopes on to planetary cores embedded in evolving protoplanetary discs

    Science.gov (United States)

    Coleman, Gavin A. L.; Papaloizou, John C. B.; Nelson, Richard P.

    2017-09-01

    The core accretion hypothesis posits that planets with significant gaseous envelopes accreted them from their protoplanetary discs after the formation of rocky/icy cores. Observations indicate that such exoplanets exist at a broad range of orbital radii, but it is not known whether they accreted their envelopes in situ, or originated elsewhere and migrated to their current locations. We consider the evolution of solid cores embedded in evolving viscous discs that undergo gaseous envelope accretion in situ with orbital radii in the range 0.1-10 au. Additionally, we determine the long-term evolution of the planets that had no runaway gas accretion phase after disc dispersal. We find the following. (I) Planets with 5 M⊕ cores never undergo runaway accretion. The most massive envelope contained 2.8 M⊕ with the planet orbiting at 10 au. (II) Accretion is more efficient on to 10 M⊕ and 15 M⊕ cores. For orbital radii ap ≥ 0.5 au, 15 M⊕ cores always experienced runaway gas accretion. For ap ≥ 5 au, all but one of the 10 M⊕ cores experienced runaway gas accretion. No planets experienced runaway growth at ap = 0.1 au. (III) We find that, after disc dispersal, planets with significant gaseous envelopes cool and contract on Gyr time-scales, the contraction time being sensitive to the opacity assumed. Our results indicate that Hot Jupiters with core masses ≲15 M⊕ at ≲0.1 au likely accreted their gaseous envelopes at larger distances and migrated inwards. Consistently with the known exoplanet population, super-Earths and mini-Neptunes at small radii during the disc lifetime, accrete only modest gaseous envelopes.

  2. On the illumination of neutron star accretion discs

    Science.gov (United States)

    Wilkins, D. R.

    2018-03-01

    The illumination of the accretion disc in a neutron star X-ray binary by X-rays emitted from (or close to) the neutron star surface is explored through general relativistic ray tracing simulations. The applicability of the canonical suite of relativistically broadened emission line models (developed for black holes) to discs around neutron stars is evaluated. These models were found to describe well emission lines from neutron star accretion discs unless the neutron star radius is larger than the innermost stable orbit of the accretion disc at 6 rg or the disc is viewed at high inclination, above 60° where shadowing of the back side of the disc becomes important. Theoretical emissivity profiles were computed for accretion discs illuminated by hotspots on the neutron star surfaces, bands of emission and emission by the entirety of the hot, spherical star surface and in all cases, the emissivity profile of the accretion disc was found to be well represented by a single power law falling off slightly steeper than r-3. Steepening of the emissivity index was found where the emission is close to the disc plane and the disc can appear truncated when illuminated by a hotspot at high latitude. The emissivity profile of the accretion disc in Serpens X-1 was measured and found to be consistent with a single unbroken power law with index q=3.5_{-0.4}^{+0.3}, suggestive of illumination by the boundary layer between the disc and neutron star surface.

  3. Superluminous accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Sikora, M [Cambridge Univ. (UK). Inst. of Astronomy; Polska Akademia Nauk, Warsaw. Centrum Astronomiczne)

    1981-07-01

    Upper limits are computed for the total luminosities and collimation of radiation from thick, radiation supported accretion discs around black holes. Numerical results are obtained for the 'extreme' discs with rsub(out) = 10/sup 3/ GMsub(BH)/c/sup 2/, the angular momentum of the black hole being Jsub(BH) = 0.998 GMsub(BH)/c. The high luminosity (L approximately 8.5 Lsub(Edd)) and substantial collimation of radiation found for these discs indicate that such discs can explain both the high luminosities of quasars and similar objects and may produce some of the observed beams and jets.

  4. Vortex survival in 3D self-gravitating accretion discs

    Science.gov (United States)

    Lin, Min-Kai; Pierens, Arnaud

    2018-04-01

    Large-scale, dust-trapping vortices may account for observations of asymmetric protoplanetary discs. Disc vortices are also potential sites for accelerated planetesimal formation by concentrating dust grains. However, in 3D discs vortices are subject to destructive `elliptic instabilities', which reduces their viability as dust traps. The survival of vortices in 3D accretion discs is thus an important issue to address. In this work, we perform shearing box simulations to show that disc self-gravity enhances the survival of 3D vortices, even when self-gravity is weak in the classic sense (e.g. with a Toomre Q ≃ 5). We find a 3D, self-gravitating vortex can grow on secular timescales in spite of the elliptic instability. The vortex aspect-ratio decreases as it strengthens, which feeds the elliptic instability. The result is a 3D vortex with a turbulent core that persists for ˜103 orbits. We find when gravitational and hydrodynamic stresses become comparable, the vortex may undergo episodic bursts, which we interpret as interaction between elliptic and gravitational instabilities. We estimate the distribution of dust particles in self-gravitating, turbulent vortices. Our results suggest large-scale vortices in protoplanetary discs are more easily observed at large radii.

  5. Modeling the response of a standard accretion disc to stochastic viscous fluctuations

    Science.gov (United States)

    Ahmad, Naveel; Misra, Ranjeev; Iqbal, Naseer; Maqbool, Bari; Hamid, Mubashir

    2018-01-01

    The observed variability of X-ray binaries over a wide range of time-scales can be understood in the framework of a stochastic propagation model, where viscous fluctuations at different radii induce accretion rate variability that propagate inwards to the X-ray producing region. The scenario successfully explains the power spectra, the linear rms-flux relation as well as the time-lag between different energy photons. The predictions of this model have been obtained using approximate analytical solutions or empirically motivated models which take into account the effect of these propagating variability on the radiative process of complex accretion flows. Here, we study the variation of the accretion rate due to such viscous fluctuations using a hydro-dynamical code for the standard geometrically thin, gas pressure dominated α-disc with a zero torque boundary condition. Our results confirm earlier findings that the time-lag between a perturbation and the resultant inner accretion rate variation depends on the frequency (or time-period) of the perturbation. Here we have quantified that the time-lag tlag ∝f-0.54 , for time-periods less than the viscous time-scale of the perturbation radius and is nearly constant otherwise. This, coupled with radiative process would produce the observed frequency dependent time-lag between different energy bands. We also confirm that if there are random Gaussian fluctuations of the α-parameter at different radii, the resultant inner accretion rate has a power spectrum which is a power-law.

  6. Accretion disc boundary layers - geometrically and optically thin case

    International Nuclear Information System (INIS)

    Regev, Oded; Hougerat, A.A.

    1988-01-01

    The method of matched asymptotic expansions is applied to an optically and geometrically thin boundary layer between an accretion disc and the accreting star. Analytical solutions are presented for a particular viscosity prescription in the boundary layer. For a typical example we find that the disc closely resembles standard steady-disc theory. It is identical to it everywhere save a narrow boundary layer, where the temperature increases rapidly inward (by an order of magnitude), the angular velocity achieves maximum and decreases to its surface value and other variables also undergo rapid changes. This and previous work can now be used to calculate the emission from accretion discs including the boundary layers for a wide range of parameters. (author)

  7. Black hole accretion discs and screened scalar hair

    Energy Technology Data Exchange (ETDEWEB)

    Davis, Anne-Christine; Jha, Rahul [Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge, CB3 0WA (United Kingdom); Gregory, Ruth, E-mail: acd@damtp.cam.ac.uk, E-mail: r.a.w.gregory@durham.ac.uk, E-mail: r.jha@damtp.cam.ac.uk [Centre for Particle Theory, Durham University, South Road, Durham, DH1 3LE (United Kingdom)

    2016-10-01

    We present a novel way to investigate scalar field profiles around black holes with an accretion disc for a range of models where the Compton wavelength of the scalar is large compared to other length scales. By analysing the problem in ''Weyl' coordinates, we are able to calculate the scalar profiles for accretion discs in the static Schwarzschild, as well as rotating Kerr, black holes. We comment on observational effects.

  8. Collimation of particle beams from thick accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Sikora, M [N. Copernicus Astronomical Center, Warszawa (Poland); Wilson, D B [Cambridge Univ. (UK). Inst. of Astronomy

    1981-11-01

    The acceleration and collimation of particle beams in the funnel of thick accretion discs is studied in the approximation that the flow is optically thin. Such flows can be collimated to within approximately 0.1 radians by sufficiently thick discs. The flow cannot convert more than a small fraction of the disc's (super-Eddington) luminosity into the energy flow of a narrow beam without being optically thick.

  9. Accretion disc origin of the Earth's water.

    Science.gov (United States)

    Vattuone, Luca; Smerieri, Marco; Savio, Letizia; Asaduzzaman, Abu Md; Muralidharan, Krishna; Drake, Michael J; Rocca, Mario

    2013-07-13

    Earth's water is conventionally believed to be delivered by comets or wet asteroids after the Earth formed. However, their elemental and isotopic properties are inconsistent with those of the Earth. It was thus proposed that water was introduced by adsorption onto grains in the accretion disc prior to planetary growth, with bonding energies so high as to be stable under high-temperature conditions. Here, we show both by laboratory experiments and numerical simulations that water adsorbs dissociatively on the olivine {100} surface at the temperature (approx. 500-1500 K) and water pressure (approx. 10⁻⁸ bar) expected for the accretion disc, leaving an OH adlayer that is stable at least up to 900 K. This may result in the formation of many Earth oceans, provided that a viable mechanism to produce water from hydroxyl exists. This adsorption process must occur in all disc environments around young stars. The inevitable conclusion is that water should be prevalent on terrestrial planets in the habitable zone around other stars.

  10. Evidence for accreted component in the Galactic discs

    Science.gov (United States)

    Xing, Q. F.; Zhao, G.

    2018-06-01

    We analyse the distribution of [Mg/Fe] abundance in the Galactic discs with F- and G-type dwarf stars selected from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) archive. The sample stars are assigned into different stellar populations by using kinematic criteria. Our analysis reveals the chemical inhomogeneities in the Galactic thick disc. A few of metal-poor stars in the thick disc exhibit relatively low [Mg/Fe] abundance in respect to the standard thick-disc sample. The orbital eccentricities and maximum Galactocentric radii of low-α metal-poor stars are apparently greater than that of high-α thick-disc stars. The orbital parameters and chemical components of low-α stars in the thick disc suggest that they may have been formed in regions with low star formation rate that were located at large distances from the Galactic centre, such as infalling dwarf spheroidal galaxies.

  11. Radio emission from Sgr A*: pulsar transits through the accretion disc

    Science.gov (United States)

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2017-06-01

    Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in the wind. During the pulsar's transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity Lsd ˜ 3 × 1035 erg s-1 and a wind Lorentz factor of γw ˜ 104 a quasi-steady synchrotron emission is established with luminosities in the 1-10 GHz range comparable to the observed one.

  12. The gravitational microlens influence on X-ray spectral line generated by an AGN accretion disc

    Directory of Open Access Journals (Sweden)

    Popović L.Č.

    2001-01-01

    Full Text Available The influence of gravitational microlensing on the X-ray spectral line profiles originated from a relativistic accretion disc has been studied. Using a disc model, we show that microlensing can induce noticeable changes in the line shapes when the Einstein ring radius associated with the microlens is of a size comparable to that of the accretion disc. Taking into account the relatively small size of the X-ray accretion disc, we found that compact objects (of about a Solar mass which belong to the bulge of the host galaxy can produce significant changes in the X-ray line profile of AGN.

  13. Hydrodynamic Simulations of the Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds

    Science.gov (United States)

    Ressler, S. M.; Quataert, E.; Stone, J. M.

    2018-05-01

    We present Athena++ grid-based, hydrodynamic simulations of accretion onto Sagittarius A* via the stellar winds of the ˜30 Wolf-Rayet stars within the central parsec of the galactic center. These simulations span ˜ 4 orders of magnitude in radius, reaching all the way down to 300 gravitational radii of the black hole, ˜32 times further in than in previous work. We reproduce reasonably well the diffuse thermal X-ray emission observed by Chandra in the central parsec. The resulting accretion flow at small radii is a superposition of two components: 1) a moderately unbound, sub-Keplerian, thick, pressure-supported disc that is at most (but not all) times aligned with the clockwise stellar disc, and 2) a bound, low-angular momentum inflow that proceeds primarily along the southern pole of the disc. We interpret this structure as a natural consequence of a few of the innermost stellar winds dominating accretion, which produces a flow with a broad distribution of angular momentum. Including the star S2 in the simulation has a negligible effect on the flow structure. Extrapolating our results from simulations with different inner radii, we find an accretion rate of ˜ a few × 10-8M⊙/yr at the horizon scale, consistent with constraints based on modeling the observed emission of Sgr A*. The flow structure found here can be used as more realistic initial conditions for horizon scale simulations of Sgr A*.

  14. Beyond the standard model of the disc-line spectral profiles from black hole accretion discs

    Czech Academy of Sciences Publication Activity Database

    Sochora, Vjačeslav; Karas, Vladimír; Svoboda, Jiří; Dovčiak, Michal

    2014-01-01

    Roč. 54, č. 4 (2014), s. 301-304 ISSN 1210-2709 R&D Projects: GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : accretion discs * black hole physics * galactic nuclei Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  15. Magnetized advective accretion flows: formation of magnetic barriers in magnetically arrested discs

    Science.gov (United States)

    Mondal, Tushar; Mukhopadhyay, Banibrata

    2018-05-01

    We discuss the importance of large-scale strong magnetic field in the removal of angular momentum outward, as well as the possible origin of different kinds of magnetic barrier in advective, geometrically thick, sub-Keplerian accretion flows around black holes. The origin of this large-scale strong magnetic field near the event horizon is due to the advection of the magnetic flux by the accreting gas from the environment, say, the interstellar medium or a companion star, because of flux freezing. In this simplest vertically averaged, 1.5-dimensional disc model, we choose the maximum upper limit of the magnetic field, which the disc around a black hole can sustain. In this so called magnetically arrested disc model, the accreting gas either decelerates or faces the magnetic barrier near the event horizon by the accumulated magnetic field depending on the geometry. The magnetic barrier may knock the matter to infinity. We suggest that these types of flow are the building block to produce jets and outflows in the accreting system. We also find that in some cases, when matter is trying to go back to infinity after knocking the barrier, matter is prevented being escaped by the cumulative action of strong gravity and the magnetic tension, hence by another barrier. In this way, magnetic field can lock the matter in between these two barriers and it might be a possible explanation for the formation of episodic jet.

  16. Estimation of bipolar jets from accretion discs around Kerr black holes

    Science.gov (United States)

    Kumar, Rajiv; Chattopadhyay, Indranil

    2017-08-01

    We analyse flows around a rotating black hole and obtain self-consistent accretion-ejection solutions in full general relativistic prescription. Entire energy-angular momentum parameter space is investigated in the advective regime to obtain shocked and shock-free accretion solutions. Jet equations of motion are solved along the von Zeipel surfaces computed from the post-shock disc, simultaneously with the equations of accretion disc along the equatorial plane. For a given spin parameter, the mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock location. Interestingly, we obtain all types of possible jet solutions, for example, steady shock solution with multiple critical points, bound solution with two critical points and smooth solution with single critical point. Multiple critical points may exist in jet solution for spin parameter as ≥ 0.5. The jet terminal speed generally increases if the accretion shock forms closer to the horizon and is higher for corotating black hole than the counter-rotating and the non-rotating one. Quantitatively speaking, shocks in jet may form for spin parameter as > 0.6 and jet shocks range between 6rg and 130rg above the equatorial plane, while the jet terminal speed vj∞ > 0.35 c if Bernoulli parameter E≥1.01 for as > 0.99.

  17. A lower limit to the accretion disc radius in the low-luminosity AGN NGC 1052 derived from high-angular resolution data

    Science.gov (United States)

    Reb, Lennart; Fernández-Ontiveros, Juan A.; Prieto, M. Almudena; Dolag, Klaus

    2018-05-01

    We investigate the central sub-arcsec region of the low-luminosity active galactic nucleus NGC 1052, using a high-angular resolution dataset that covers 10 orders of magnitude in frequency. This allows us to infer the continuum emission within the innermost ˜17 pc around the black hole to be of non-thermal, synchrotron origin and to set a limit to the maximum contribution of a standard accretion disc. Assuming the canonical 10 per cent mass-light conversion efficiency for the standard accretion disc, its inferred accretion power would be too low by one order of magnitude to account for the observed continuum luminosity. We thus introduce a truncated accretion disc and derive a truncation radius to mass-light conversion efficiency relation, which we use to reconcile the inferred accretion power with the continuum luminosity. As a result we find that a truncated disc providing the necessary accretion power must be truncated at rtr ≳ 26 rg, consistent with the inner radius derived from the observations of the Fe Kα line in the X-ray spectrum of this nucleus. This is the first time to derive a limit on the truncation radius of the accretion disc from high-angular resolution data only.

  18. A laboratory plasma experiment for studying magnetic dynamics of accretion discs and jets

    OpenAIRE

    Hsu, S. C.; Bellan, P. M.

    2002-01-01

    This work describes a laboratory plasma experiment and initial results which should give insight into the magnetic dynamics of accretion discs and jets. A high-speed multiple-frame CCD camera reveals images of the formation and helical instability of a collimated plasma, similar to MHD models of disc jets, and also plasma detachment associated with spheromak formation, which may have relevance to disc winds and flares. The plasmas are produced by a planar magnetized coaxial gun. The resulting...

  19. Quasi-periodic oscillations and the global modes of relativistic, MHD accretion discs

    Science.gov (United States)

    Dewberry, Janosz W.; Latter, Henrik N.; Ogilvie, Gordon I.

    2018-05-01

    The high-frequency quasi-periodic oscillations that punctuate the light curves of X-ray binary systems present a window on to the intrinsic properties of stellar-mass black holes and hence a testbed for general relativity. One explanation for these features is that relativistic distortion of the accretion disc's differential rotation creates a trapping region in which inertial waves (r-modes) might grow to observable amplitudes. Local analyses, however, predict that large-scale magnetic fields push this trapping region to the inner disc edge, where conditions may be unfavourable for r-mode growth. We revisit this problem from a pseudo-Newtonian but fully global perspective, deriving linearized equations describing a relativistic, magnetized accretion flow, and calculating normal modes with and without vertical density stratification. In an unstratified model we confirm that vertical magnetic fields drive r-modes towards the inner edge, though the effect depends on the choice of vertical wavenumber. In a global model we better quantify this susceptibility, and its dependence on the disc's vertical structure and thickness. Our calculations suggest that in thin discs, r-modes may remain independent of the inner disc edge for vertical magnetic fields with plasma betas as low as β ≈ 100-300. We posit that the appearance of r-modes in observations may be more determined by a competition between excitation and damping mechanisms near the ISCO than by the modification of the trapping region by magnetic fields.

  20. On the diversity and statistical properties of protostellar discs

    Science.gov (United States)

    Bate, Matthew R.

    2018-04-01

    We present results from the first population synthesis study of protostellar discs. We analyse the evolution and properties of a large sample of protostellar discs formed in a radiation hydrodynamical simulation of star cluster formation. Due to the chaotic nature of the star formation process, we find an enormous diversity of young protostellar discs, including misaligned discs, and discs whose orientations vary with time. Star-disc interactions truncate discs and produce multiple systems. Discs may be destroyed in dynamical encounters and/or through ram-pressure stripping, but reform by later gas accretion. We quantify the distributions of disc mass and radii for protostellar ages up to ≈105 yr. For low-mass protostars, disc masses tend to increase with both age and protostellar mass. Disc radii range from of order 10 to a few hundred au, grow in size on time-scales ≲ 104 yr, and are smaller around lower mass protostars. The radial surface density profiles of isolated protostellar discs are flatter than the minimum mass solar nebula model, typically scaling as Σ ∝ r-1. Disc to protostar mass ratios rarely exceed two, with a typical range of Md/M* = 0.1-1 to ages ≲ 104 yr and decreasing thereafter. We quantify the relative orientation angles of circumstellar discs and the orbit of bound pairs of protostars, finding a preference for alignment that strengths with decreasing separation. We also investigate how the orientations of the outer parts of discs differ from the protostellar and inner disc spins for isolated protostars and pairs.

  1. Hot-spot model for accretion disc variability as random process

    Czech Academy of Sciences Publication Activity Database

    Pecháček, Tomáš; Karas, Vladimír; Czerny, B.

    2008-01-01

    Roč. 487, č. 3 (2008), s. 815-830 ISSN 0004-6361 R&D Projects: GA AV ČR IAA300030510; GA MŠk(CZ) LC06014 Grant - others:ESA(XE) ESA-PECS project No. 98040 Institutional research plan: CEZ:AV0Z10030501 Keywords : black holes * accretion discs Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.153, year: 2008

  2. Dissipative advective accretion disc solutions with variable adiabatic index around black holes

    Science.gov (United States)

    Kumar, Rajiv; Chattopadhyay, Indranil

    2014-10-01

    We investigated accretion on to black holes in presence of viscosity and cooling, by employing an equation of state with variable adiabatic index and multispecies fluid. We obtained the expression of generalized Bernoulli parameter which is a constant of motion for an accretion flow in presence of viscosity and cooling. We obtained all possible transonic solutions for a variety of boundary conditions, viscosity parameters and accretion rates. We identified the solutions with their positions in the parameter space of generalized Bernoulli parameter and the angular momentum on the horizon. We showed that a shocked solution is more luminous than a shock-free one. For particular energies and viscosity parameters, we obtained accretion disc luminosities in the range of 10- 4 - 1.2 times Eddington luminosity, and the radiative efficiency seemed to increase with the mass accretion rate too. We found steady state shock solutions even for high-viscosity parameters, high accretion rates and for wide range of composition of the flow, starting from purely electron-proton to lepton-dominated accretion flow. However, similar to earlier studies of inviscid flow, accretion shock was not obtained for electron-positron pair plasma.

  3. HEROIC: 3D general relativistic radiative post-processor with comptonization for black hole accretion discs

    Science.gov (United States)

    Narayan, Ramesh; Zhu, Yucong; Psaltis, Dimitrios; Saḑowski, Aleksander

    2016-03-01

    We describe Hybrid Evaluator for Radiative Objects Including Comptonization (HEROIC), an upgraded version of the relativistic radiative post-processor code HERO described in a previous paper, but which now Includes Comptonization. HEROIC models Comptonization via the Kompaneets equation, using a quadratic approximation for the source function in a short characteristics radiation solver. It employs a simple form of accelerated lambda iteration to handle regions of high scattering opacity. In addition to solving for the radiation field, HEROIC also solves for the gas temperature by applying the condition of radiative equilibrium. We present benchmarks and tests of the Comptonization module in HEROIC with simple 1D and 3D scattering problems. We also test the ability of the code to handle various relativistic effects using model atmospheres and accretion flows in a black hole space-time. We present two applications of HEROIC to general relativistic magnetohydrodynamics simulations of accretion discs. One application is to a thin accretion disc around a black hole. We find that the gas below the photosphere in the multidimensional HEROIC solution is nearly isothermal, quite different from previous solutions based on 1D plane parallel atmospheres. The second application is to a geometrically thick radiation-dominated accretion disc accreting at 11 times the Eddington rate. Here, the multidimensional HEROIC solution shows that, for observers who are on axis and look down the polar funnel, the isotropic equivalent luminosity could be more than 10 times the Eddington limit, even though the spectrum might still look thermal and show no signs of relativistic beaming.

  4. Proto-planetary disc evolution and dispersal

    Science.gov (United States)

    Rosotti, Giovanni Pietro

    2015-05-01

    rates and is therefore the ideal candidate for driving disc evolution. Another process that can influence discs is a close encounter with another star. In this thesis we develop a model to study the effect of stellar dynamics in the natal stellar cluster on the discs, following for the first time at the same time the stellar dynamics together with the evolution of the discs. We find that, although close encounters with stars are unlikely to change significantly the mass of a disc, they can change substantially its size, hence imposing an upper limit on the observed disc radii. Finally, we investigated in this thesis whether discs can be reformed after their dispersal. If a star happens to be in a region that is currently forming stars, it can accrete material from the interstellar medium. This mechanism may result in the production of "second generation" discs such that in a given star forming region a few percent of stars may still possess a disc, in tentative agreement with observations of so called "old accretors", which are difficult to explain within the current paradigm of disc evolution and dispersal.

  5. The ephemeris and variations of the accretion disc radius in IP Pegasi

    International Nuclear Information System (INIS)

    Wood, J.H.; Robinson, E.L.

    1989-01-01

    We present timings of 37 previously unpublished eclipses of the dwarf nova IP Pegasi, obtained in 1986 through to 1988, and combine these with earlier published timings in order to update the orbital ephemeris and to examine the behaviour of the accretion disc radius through the outburst cycle. (author)

  6. Optically thin core accretion: how planets get their gas in nearly gas-free discs

    Science.gov (United States)

    Lee, Eve J.; Chiang, Eugene; Ferguson, Jason W.

    2018-05-01

    Models of core accretion assume that in the radiative zones of accreting gas envelopes, radiation diffuses. But super-Earths/sub-Neptunes (1-4 R⊕, 2-20 M⊕) point to formation conditions that are optically thin: their modest gas masses are accreted from short-lived and gas-poor nebulae reminiscent of the transparent cavities of transitional discs. Planetary atmospheres born in such environments can be optically thin to both incident starlight and internally generated thermal radiation. We construct time-dependent models of such atmospheres, showing that super-Earths/sub-Neptunes can accrete their ˜1 per cent-by-mass gas envelopes, and super-puffs/sub-Saturns their ˜20 per cent-by-mass envelopes, over a wide range of nebular depletion histories requiring no fine tuning. Although nascent atmospheres can exhibit stratospheric temperature inversions affected by atomic Fe and various oxides that absorb strongly at visible wavelengths, the rate of gas accretion remains controlled by the radiative-convective boundary (rcb) at much greater pressures. For dusty envelopes, the temperature at the rcb Trcb ≃ 2500 K is still set by H2 dissociation; for dust-depleted envelopes, Trcb tracks the temperature of the visible or thermal photosphere, whichever is deeper, out to at least ˜5 au. The rate of envelope growth remains largely unchanged between the old radiative diffusion models and the new optically thin models, reinforcing how robustly super-Earths form as part of the endgame chapter in disc evolution.

  7. Blinded by the light: on the relationship between CO first overtone emission and mass accretion rate in massive young stellar objects

    Science.gov (United States)

    Ilee, J. D.; Oudmaijer, R. D.; Wheelwright, H. E.; Pomohaci, R.

    2018-04-01

    To date, there is no explanation as to why disc-tracing CO first overtone (or `bandhead') emission is not a ubiquitous feature in low- to medium-resolution spectra of massive young stellar objects, but instead is only detected toward approximately 25 per cent of their spectra. In this paper, we investigate the hypothesis that only certain mass accretion rates result in detectable bandhead emission in the near infrared spectra of MYSOs. Using an analytic disc model combined with an LTE model of the CO emission, we find that high accretion rates (≳ 10-4 M⊙yr-1) result in large dust sublimation radii, a larger contribution to the K-band continuum from hot dust at the dust sublimation radius, and therefore correspondingly lower CO emission with respect to the continuum. On the other hand, low accretion rates (≲ 10-6 M⊙yr-1) result in smaller dust sublimation radii, a correspondingly smaller emitting area of CO, and thus also lower CO emission with respect to the continuum. In general, moderate accretion rates produce the most prominent, and therefore detectable, CO first overtone emission. We compare our findings to a recent near-infrared spectroscopic survey of MYSOs, finding results consistent with our hypothesis. We conclude that the detection rate of CO bandhead emission in the spectra of MYSOs could be the result of MYSOs exhibiting a range of mass accretion rates, perhaps due to the variable accretion suggested by recent multi-epoch observations of these objects.

  8. Viscous driving of global oscillations in accretion discs around black holes

    Czech Academy of Sciences Publication Activity Database

    Miranda, R.; Horák, Jiří; Lai, D.

    2015-01-01

    Roč. 446, č. 1 (2015), s. 240-253 ISSN 0035-8711 R&D Projects: GA MŠk(CZ) LH14049 Grant - others:Rada Programu interní podpory projektů mezinárodní spolupráce AV ČR(CZ) M100031202 Institutional support: RVO:67985815 Keywords : accretion discs * hydrodynamics * waves Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.952, year: 2015

  9. Dust inflated accretion disc as the origin of the broad line region in active galactic nuclei

    Science.gov (United States)

    Baskin, Alexei; Laor, Ari

    2018-02-01

    The broad line region (BLR) in active galactic nuclei (AGNs) is composed of dense gas (˜1011 cm-3) on sub-pc scale, which absorbs about 30 per cent of the ionizing continuum. The outer size of the BLR is likely set by dust sublimation, and its density by the incident radiation pressure compression (RPC). But, what is the origin of this gas, and what sets its covering factor (CF)? Czerny & Hryniewicz (2011) suggested that the BLR is a failed dusty wind from the outer accretion disc. We explore the expected dust properties, and the implied BLR structure. We find that graphite grains sublimate only at T ≃ 2000 K at the predicted density of ˜1011 cm-3, and therefore large graphite grains (≥0.3 μm) survive down to the observed size of the BLR, RBLR. The dust opacity in the accretion disc atmosphere is ˜50 times larger than previously assumed, and leads to an inflated torus-like structure, with a predicted peak height at RBLR. The illuminated surface of this torus-like structure is a natural place for the BLR. The BLR CF is mostly set by the gas metallicity, the radiative accretion efficiency, a dynamic configuration and ablation by the incident optical-UV continuum. This model predicts that the BLR should extend inwards of RBLR to the disc radius where the surface temperature is ≃2000 K, which occurs at Rin ≃ 0.18RBLR. The value of Rin can be tested by reverberation mapping of the higher ionization lines, predicted by RPC to peak well inside RBLR. The dust inflated disc scenario can also be tested based on the predicted response of RBLR and the CF to changes in the AGN luminosity and accretion rate.

  10. Evolution of viscous discs. 3. Giant discs in symbiotic stars

    Energy Technology Data Exchange (ETDEWEB)

    Bath, G T [Oxford Univ. (UK). Dept. of Astrophysics; Pringle, J E [Cambridge Univ. (UK). Inst. of Astronomy

    1982-10-01

    The structure of time-dependent accretion discs in giant binaries with separation of the order of 10/sup 13/ cm is examined. Radiative ..cap alpha..-viscosity discs with ..cap alpha.. of order unity accreting on to main-sequence stars at accretion rates which generate luminosities greater than a giant companion decay on time-scales of the same order as the binary period, unlike those in dwarf nova binaries which decay on time-scales 100 times longer than the binary period. This results from the lower gravitational potential and consequent larger disc thickness (relative to the radius) of luminous 'giant' discs accreting at high accretion rates. The eruptions of the symbiotic binary C I Cygni are modelled by an ..cap alpha.. = 1 disc with outer radius 8.5 x 10/sup 12/ cm and a sequence of five mass-transfer bursts at rates between 1.5 x 10/sup 21/ and 4 x 10/sup 22/g s/sup -1/.

  11. On the timing behaviour of PSR B1259-63 under the propeller torque from a transient accretion disc

    Science.gov (United States)

    Yi, Shu-Xu; Cheng, K. S.

    2018-05-01

    The γ-ray pulsar binary system PSR B1259-63 flares in GeV after each periastron. The origin of these flares is still under debate. Recently, in 2017, we proposed a mechanism that might explain the GeV flares. In that model, a transient accretion disc is expected to be formed from the matter that was gravity-captured by the neutron star from the main-sequence companion's circumstellar disc. The transient accretion disc exerts a spin-down torque on the neutron star (i.e. the propeller effect), which might be traceable via pulsar timing observations of PSR B1259-63. In this paper, we consider the propeller effect phenomenologically using a parameter χ, which describes the coupling between the disc matter and the neutron star. Comparing the expected timing residuals with recent observations by Shannon et al., we conclude that the angular momentum transfer is very weak (with the coupling parameter χ ≤ 10-4).

  12. Accretion disc dynamo activity in local simulations spanning weak-to-strong net vertical magnetic flux regimes

    Science.gov (United States)

    Salvesen, Greg; Simon, Jacob B.; Armitage, Philip J.; Begelman, Mitchell C.

    2016-03-01

    Strongly magnetized accretion discs around black holes have attractive features that may explain enigmatic aspects of X-ray binary behaviour. The structure and evolution of these discs are governed by a dynamo-like mechanism, which channels part of the accretion power liberated by the magnetorotational instability (MRI) into an ordered toroidal magnetic field. To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations. The strength of the self-sustained toroidal magnetic field depends on the net vertical magnetic flux, which we vary across almost the entire range over which the MRI is linearly unstable. We quantify disc structure and dynamo properties as a function of the initial ratio of mid-plane gas pressure to vertical magnetic field pressure, β _0^mid = p_gas / p_B. For 10^5 ≥ β _0^mid ≥ 10 the effective α-viscosity parameter scales as a power law. Dynamo activity persists up to and including β _0^mid = 10^2, at which point the entire vertical column of the disc is magnetic pressure dominated. Still stronger fields result in a highly inhomogeneous disc structure, with large density fluctuations. We show that the turbulent steady state βmid in our simulations is well matched by the analytic model of Begelman et al. describing the creation and buoyant escape of toroidal field, while the vertical structure of the disc can be broadly reproduced using this model. Finally, we discuss the implications of our results for observed properties of X-ray binaries.

  13. Erratum: Spiral structure in the accretion disc of the binary IP Pegasi

    Science.gov (United States)

    Steeghs, D.; Harlaftis, E. T.; Horne, Keith

    1998-05-01

    The paper `Spiral structure in the accretion disc of the binary IP Pegasi' was published in Mon. Not. R. Astron. Soc. 290, L28-L32 (1997). Figs 1 and 2 of the paper (grey-scale images) did not reproduce well, and are reprinted here (Fig. 1 overleaf). Colour versions of the images are available on the World Wide Web:http://www-star.st-and.ac.uk/^~ds10/spirals.html

  14. Accretion discs around neutron stars

    International Nuclear Information System (INIS)

    Pringle, J.E.

    1982-01-01

    If the central object in the disc is a neutron star, then we do not need the disc itself to produce the X-rays. In other words, the disc structure itself is not important as long as it plays the role of depositing matter on the neutron star at a sufficient rate to produce the X-ray flux. Similarly, in the outer disc regions, the main disc luminosity comes from absorption and reradiation of X-ray photons and not from the intrinsic, viscously-produced, local energy production rate. These two points indicate why in the compact binary X-ray sources confrontation between disc theory and observations is not generally practicable. For this reason I will divide my talk into two parts: one on observational discs in which I discuss what observational evidence there is for discs in the compact X-ray sources and what the evidence can tell the theorist about disc behaviour, and the other on theoretical discs where I consider in what ways theoretical arguments can put limits or cast doubt on some of the empirical models put forward to explain the observations. (orig.)

  15. Fate of an accretion disc around a black hole when both the viscosity and dark energy is in effect

    Energy Technology Data Exchange (ETDEWEB)

    Dutta, Sandip; Biswas, Ritabrata [The University of Burdwan, Department of Mathematics, Burdwan, West Bengal (India)

    2017-10-15

    This paper deals with the viscous accretion flow of a modified Chaplygin gas towards a black hole as the central gravitating object. A modified Chaplygin gas is a particular type of dark energy model which mimics of radiation era to phantom era depending on the different values of its parameters. We compare the dark energy accretion with the flow of adiabatic gas. An accretion disc flowing around a black hole is an example of a transonic flow. To construct the model, we consider three components of the Navier-Stokes equation, the equation of continuity and the modified Chaplygin gas equation of state. As a transonic flow passes through the sonic point, the velocity gradient being apparently singular there, it gives rise to two flow branches: one in-falling, the accretion and the other outgoing, the wind. We show that the wind curve is stronger and the wind speed reaches that of light at a finite distance from the black hole when dark energy is considered. Besides, if we increase the viscosity, the accretion disc is shortened in radius. These two processes acting together make the system deviate much from the adiabatic accretion case. It shows a weakening process for the accretion procedure by the work of the viscous system influencing both the angular momentum transport and the repulsive force of the modified Chaplygin gas. (orig.)

  16. Dynamic processes during accretion into a black hole

    Directory of Open Access Journals (Sweden)

    G. S. Bisonvatyi-kogan

    2001-01-01

    Full Text Available Accretion disc theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disc. One of the most important new invention of this theory was a phenomenological treatment of the turbulent viscosity, known as “alpha” prescription, when the (rϕ component of the stress tensor was approximated by (αP with a unknown constant α This prescription played the role in the accretion disc theory as well important as the mixing-length theory of convection for stellar evolution. Sources of turbulence in the accretion disc are discussed, including nonlinear hydrodynamic turbulence, convection and magnetic filed role. In parallel to the optically thick geometrically thin accretion disc models, a new branch of the optically thin accretion disc models was discovered, with a larger thickness for the same total luminosity. The choice between these solutions should be done of the base of stability analysis. The ideas underlying the necessity to include advection into the accretion disc theory are presented and first models with advection are reviewed. The present status of the solution for a low-luminous optically thin accretion disc model with advection is discussed and the limits for an advection dominated accretion flows (ADAF imposed by the presence of magnetic field are analyzed.

  17. Modelling hard and soft states of Cygnus X-1 with propagating mass accretion rate fluctuations

    Science.gov (United States)

    Rapisarda, S.; Ingram, A.; van der Klis, M.

    2017-12-01

    We present a timing analysis of three Rossi X-ray Timing Explorer observations of the black hole binary Cygnus X-1 with the propagating mass accretion rate fluctuations model PROPFLUC. The model simultaneously predicts power spectra, time lags and coherence of the variability as a function of energy. The observations cover the soft and hard states of the source, and the transition between the two. We find good agreement between model predictions and data in the hard and soft states. Our analysis suggests that in the soft state the fluctuations propagate in an optically thin hot flow extending up to large radii above and below a stable optically thick disc. In the hard state, our results are consistent with a truncated disc geometry, where the hot flow extends radially inside the inner radius of the disc. In the transition from soft to hard state, the characteristics of the rapid variability are too complex to be successfully described with PROPFLUC. The surface density profile of the hot flow predicted by our model and the lack of quasi-periodic oscillations in the soft and hard states suggest that the spin of the black hole is aligned with the inner accretion disc and therefore probably with the rotational axis of the binary system.

  18. Star-disc interaction in galactic nuclei: formation of a central stellar disc

    Science.gov (United States)

    Panamarev, Taras; Shukirgaliyev, Bekdaulet; Meiron, Yohai; Berczik, Peter; Just, Andreas; Spurzem, Rainer; Omarov, Chingis; Vilkoviskij, Emmanuil

    2018-05-01

    We perform high-resolution direct N-body simulations to study the effect of an accretion disc on stellar dynamics in an active galactic nucleus (AGN). We show that the interaction of the nuclear stellar cluster (NSC) with the gaseous accretion disc (AD) leads to formation of a stellar disc in the central part of the NSC. The accretion of stars from the stellar disc on to the super-massive black hole is balanced by the capture of stars from the NSC into the stellar disc, yielding a stationary density profile. We derive the migration time through the AD to be 3 per cent of the half-mass relaxation time of the NSC. The mass and size of the stellar disc are 0.7 per cent of the mass and 5 per cent of the influence radius of the super-massive black hole. An AD lifetime shorter than the migration time would result in a less massive nuclear stellar disc. The detection of such a stellar disc could point to past activity of the hosting galactic nucleus.

  19. Magnetic fields in giant planet formation and protoplanetary discs

    Science.gov (United States)

    Keith, Sarah Louise

    2015-12-01

    Protoplanetary discs channel accretion onto their host star. How this is achieved is critical to the growth of giant planets which capture their massive gaseous atmosphere from the surrounding flow. Theoretical studies find that an embedded magnetic field could power accretion by hydromagnetic turbulence or torques from a large-scale field. This thesis presents a study of the inuence of magnetic fields in three key aspects of this process: circumplanetary disc accretion, gas flow across gaps in protoplanetary discs, and magnetic-braking in accretion discs. The first study examines the conditions needed for self-consistent accretion driven by magnetic fields or gravitational instability. Models of these discs typically rely on hydromagnetic turbulence as the source of effective viscosity. However, magnetically coupled,accreting regions may be so limited that the disc may not support sufficient inflow. An improved Shakura-Sunyaev ? disc is used to calculate the ionisation fraction and strength of non-ideal effects. Steady magnetically-driven accretion is limited to the thermally ionised, inner disc so that accretion in the remainder of the disc is time-dependent. The second study addresses magnetic flux transport in an accretion gap evacuated by a giant planet. Assuming the field is passively drawn along with the gas, the hydrodynamical simulation of Tanigawa, Ohtsuki & Machida (2012) is used for an a posteriori analysis of the gap field structure. This is used to post-calculate magnetohydrodynamical quantities. This assumption is self-consistent as magnetic forces are found to be weak, and good magnetic coupling ensures the field is frozen into the gas. Hall drift dominates across much of the gap, with the potential to facilitate turbulence and modify the toroidal field according to the global field orientation. The third study considers the structure and stability of magnetically-braked accretion discs. Strong evidence for MRI dead-zones has renewed interest in

  20. Migration of accreting giant planets

    Science.gov (United States)

    Crida, A.; Bitsch, B.; Raibaldi, A.

    2016-12-01

    We present the results of 2D hydro simulations of giant planets in proto-planetary discs, which accrete gas at a more or less high rate. First, starting from a solid core of 20 Earth masses, we show that as soon as the runaway accretion of gas turns on, the planet is saved from type I migration : the gap opening mass is reached before the planet is lost into its host star. Furthermore, gas accretion helps opening the gap in low mass discs. Consequently, if the accretion rate is limited to the disc supply, then the planet is already inside a gap and in type II migration. We further show that the type II migration of a Jupiter mass planet actually depends on its accretion rate. Only when the accretion is high do we retrieve the classical picture where no gas crosses the gap and the planet follows the disc spreading. These results impact our understanding of planet migration and planet population synthesis models. The e-poster presenting these results in French can be found here: L'e-poster présentant ces résultats en français est disponible à cette adresse: http://sf2a.eu/semaine-sf2a/2016/posterpdfs/156_179_49.pdf.

  1. Corotation resonance and overstable oscillations in black hole accretion discs: general relativistic calculations

    Czech Academy of Sciences Publication Activity Database

    Horák, Jiří; Lai, D.

    2013-01-01

    Roč. 434, č. 4 (2013), s. 2761-2771 ISSN 0035-8711 R&D Projects: GA MŠk ME09036; GA ČR(CZ) GAP209/11/2004 Grant - others: NASA (US) NNX12AF85G; NSF(US) AST-1008245; NSF(US) AST-1211061; Rada Programu interní podpory projektů mezinárodní spolupráce AV ČR(CZ) M100031202 Institutional support: RVO:67985815 Keywords : accretion discs * hydrodynamics * X-ray binaries Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.226, year: 2013

  2. The response of the Fe K\\alpha line to changes in the X-ray illumination of accretion discs

    OpenAIRE

    Ballantyne, D. R.; Ross, R. R.

    2002-01-01

    X-ray reflection spectra from photoionized accretion discs in active galaxies are presented for a wide range of illumination conditions. The energy, equivalent width (EW) and flux of the Fe K line are shown to depend strongly on the ratio of illuminating flux to disc flux, Fx/Fdisc, the photon index of the irradiating power-law, \\Gamma, and the incidence angle of the radiation, i. When Fx/Fdisc \\leq 2 a neutral Fe K line is prominent for all but the largest values of \\Gamma. At higher illumin...

  3. Magnetorotational instability and dynamo action in gravito-turbulent astrophysical discs

    Science.gov (United States)

    Riols, A.; Latter, H.

    2018-02-01

    Though usually treated in isolation, the magnetorotational and gravitational instabilities (MRI and GI) may coincide at certain radii and evolutionary stages of protoplanetary discs and active galactic nuclei. Their mutual interactions could profoundly influence several important processes, such as accretion variability and outbursts, fragmentation and disc truncation, or large-scale magnetic field production. Direct numerical simulations of both instabilities are computationally challenging and remain relatively unexplored. In this paper, we aim to redress this neglect via a set of 3D vertically stratified shearing-box simulations, combining self-gravity and magnetic fields. We show that gravito-turbulence greatly weakens the zero-net-flux MRI. In the limit of efficient cooling (and thus enhanced GI), the MRI is completely suppressed, and yet strong magnetic fields are sustained by the gravito-turbulence. This turbulent `spiral wave' dynamo may have widespread application, especially in galactic discs. Finally, we present preliminary work showing that a strong net-vertical-flux revives the MRI and supports a magnetically dominated state in which the GI is secondary.

  4. Accreting Millisecond Pulsars: Neutron Star Masses and Radii

    Science.gov (United States)

    Strohmayer, Tod

    2004-01-01

    High amplitude X-ray brightness oscillations during thermonuclear X-ray bursts were discovered with the Rossi X-ray Timing Explorer (RXTE) in early 1996. Spectral and timing evidence strongly supports the conclusion that these oscillations are caused by rotational modulation of the burst emission and that they reveal the spin frequency of neutron stars in low mass X-ray binaries. The recent discovery of X-ray burst oscillations from two accreting millisecond pulsars has confirmed this basic picture and provided a new route to measuring neutron star properties and constraining the dense matter equation of state. I will briefly summarize the current observational understanding of accreting millisecond pulsars, and describe recent attempts to determine the mass and radius of the neutron star in XTE J1814-338.

  5. Effects of local thermodynamics and of stellar mass ratio on accretion disc stability in close binaries

    Science.gov (United States)

    Lanzafame, G.

    2009-08-01

    Inflow kinematics at the inner Lagrangian point L1, gas compressibility, and physical turbulent viscosity play a fundamental role on accretion disc dynamics and structure in a close binary (CB). Physical viscosity supports the accretion disc development inside the primary gravitational potential well, developing the gas radial transport, converting mechanical energy into heat. The Stellar-Mass-Ratio (SMR) between the compact primary and the secondary star (M1/M2) is also effective, not only in the location of the inner Lagrangian point, but also in the angular kinematics of the mass transfer and in the geometry of the gravitational potential wells. In this work we pay attention in particular to the role of the SMR, evaluating boundaries, separating theoretical domains in compressibility-viscosity graphs where physical conditions allow a well-bound disc development, as a function of mass transfer kinematic conditions. In such domains, the lower is the gas compressibility (the higher the polytropic index γ), the higher is the physical viscosity (α) requested. In this work, we show how the boundaries of such domains vary as a function of M1/M2. Conclusions as far as dwarf novae outbursts are concerned, induced by mass transfer rate variations, are also reported. The smaller M1/M2, the shorter the duration of the active-to-quiet and vice-versa transitional phases. Time-scales are of the order of outburst duration of SU Uma, OY Car, Z Cha and SS Cyg-like objects. Moreover, conclusions as far as active-quiet-active phenomena in a CB, according to viscous-thermal instabilities, in accordance to such domains, are also reported.

  6. Apparent quasar disc sizes in the "bird's nest" paradigm

    Science.gov (United States)

    Abolmasov, P.

    2017-04-01

    Context. Quasar microlensing effects make it possible to measure the accretion disc sizes around distant supermassive black holes that are still well beyond the spatial resolution of contemporary instrumentation. The sizes measured with this technique appear inconsistent with the standard accretion disc model. Not only are the measured accretion disc sizes larger, but their dependence on wavelength is in most cases completely different from the predictions of the standard model. Aims: We suggest that these discrepancies may arise not from non-standard accretion disc structure or systematic errors, as it was proposed before, but rather from scattering and reprocession of the radiation of the disc. In particular, the matter falling from the gaseous torus and presumably feeding the accretion disc may at certain distances become ionized and produce an extended halo that is free from colour gradients. Methods: A simple analytical model is proposed assuming that a geometrically thick translucent inflow acts as a scattering mirror changing the apparent spatial properties of the disc. This inflow may be also identified with the broad line region or its inner parts. Results: Such a model is able to explain the basic properties of the apparent disc sizes, primarily their large values and their shallow dependence on wavelength. The only condition required is to scatter a significant portion of the luminosity of the disc. This can easily be fulfilled if the scattering inflow has a large geometrical thickness and clumpy structure.

  7. Mismatch and misalignment: dark haloes and satellites of disc galaxies

    Science.gov (United States)

    Deason, A. J.; McCarthy, I. G.; Font, A. S.; Evans, N. W.; Frenk, C. S.; Belokurov, V.; Libeskind, N. I.; Crain, R. A.; Theuns, T.

    2011-08-01

    We study the phase-space distribution of satellite galaxies associated with late-type galaxies in the GIMIC suite of simulations. GIMIC consists of resimulations of five cosmologically representative regions from the Millennium Simulation, which have higher resolution and incorporate baryonic physics. Whilst the disc of the galaxy is well aligned with the inner regions (r˜ 0.1r200) of the dark matter halo, both in shape and angular momentum, there can be substantial misalignments at larger radii (r˜r200). Misalignments of >45° are seen in ˜30 per cent of our sample. We find that the satellite population aligns with the shape (and angular momentum) of the outer dark matter halo. However, the alignment with the galaxy is weak owing to the mismatch between the disc and dark matter halo. Roughly 20 per cent of the satellite systems with 10 bright galaxies within r200 exhibit a polar spatial alignment with respect to the galaxy - an orientation reminiscent of the classical satellites of the Milky Way. We find that a small fraction (˜10 per cent) of satellite systems show evidence for rotational support which we attribute to group infall. There is a bias towards satellites on prograde orbits relative to the spin of the dark matter halo (and to a lesser extent with the angular momentum of the disc). This preference towards co-rotation is stronger in the inner regions of the halo where the most massive satellites accreted at relatively early times are located. We attribute the anisotropic spatial distribution and angular momentum bias of the satellites at z= 0 to their directional accretion along the major axes of the dark matter halo. The satellite galaxies have been accreted relatively recently compared to the dark matter mass and have experienced less phase-mixing and relaxation - the memory of their accretion history can remain intact to z= 0. Understanding the phase-space distribution of the z= 0 satellite population is key for studies that estimate the host halo

  8. The Accretion-Ejection Mechanisms in X-ray Binaries: an Unified View

    International Nuclear Information System (INIS)

    Petrucci, P. O.; Foellmi, C.; Ferreira, J.; Henri, G.; Cabanac, C.; Belmont, R.; Malzac, J.

    2009-01-01

    We present a new keplerian accretion disc solution, the so-called Jet Emitting Disc (JED hereafter), which is part of global self-consistent disc-jet MHD structure. In our framework, a large scale, organized vertical magnetic field is threading the JED giving birth, when conditions are met, to stationnary self-collimated non relativistic jets. The main condition is that the magnetic pressure P mag must be of the order of the total pressure P tot in the JED and a direct consequence is a jet torque largely dominating the viscuous torque. This in turn implies an accretion velocity of the order of the sound speed and then a density much lower than a standard accretion disc. Moreover, most of the accretion power P acc being extracted by the jet, only part of it (<50%) is liberated in the JED as heating power.

  9. Planet population synthesis driven by pebble accretion in cluster environments

    Science.gov (United States)

    Ndugu, N.; Bitsch, B.; Jurua, E.

    2018-02-01

    The evolution of protoplanetary discs embedded in stellar clusters depends on the age and the stellar density in which they are embedded. Stellar clusters of young age and high stellar surface density destroy protoplanetary discs by external photoevaporation and stellar encounters. Here, we consider the effect of background heating from newly formed stellar clusters on the structure of protoplanetary discs and how it affects the formation of planets in these discs. Our planet formation model is built on the core accretion scenario, where we take the reduction of the core growth time-scale due to pebble accretion into account. We synthesize planet populations that we compare to observations obtained by radial velocity measurements. The giant planets in our simulations migrate over large distances due to the fast type-II migration regime induced by a high disc viscosity (α = 5.4 × 10-3). Cold Jupiters (rp > 1 au) originate preferably from the outer disc, due to the large-scale planetary migration, while hot Jupiters (rp meaning that more gas giants are formed at larger metallicity. However, our synthetic population of isolated stars host a significant amount of giant planets even at low metallicity, in contradiction to observations where giant planets are preferably found around high metallicity stars, indicating that pebble accretion is very efficient in the standard pebble accretion framework. On the other hand, discs around stars embedded in cluster environments hardly form any giant planets at low metallicity in agreement with observations, where these changes originate from the increased temperature in the outer parts of the disc, which prolongs the core accretion time-scale of the planet. We therefore conclude that the outer disc structure and the planet's formation location determines the giant planet occurrence rate and the formation efficiency of cold and hot Jupiters.

  10. Production of the entire range of r-process nuclides by black hole accretion disc outflows from neutron star mergers

    Science.gov (United States)

    Wu, Meng-Ru; Fernández, Rodrigo; Martínez-Pinedo, Gabriel; Metzger, Brian D.

    2016-12-01

    We consider r-process nucleosynthesis in outflows from black hole accretion discs formed in double neutron star and neutron star-black hole mergers. These outflows, powered by angular momentum transport processes and nuclear recombination, represent an important - and in some cases dominant - contribution to the total mass ejected by the merger. Here we calculate the nucleosynthesis yields from disc outflows using thermodynamic trajectories from hydrodynamic simulations, coupled to a nuclear reaction network. We find that outflows produce a robust abundance pattern around the second r-process peak (mass number A ˜ 130), independent of model parameters, with significant production of A spike at A = 132 that is absent in the Solar system r-process distribution. The spike arises from convection in the disc and depends on the treatment of nuclear heating in the simulations. We conclude that disc outflows provide an important - and perhaps dominant - contribution to the r-process yields of compact binary mergers, and hence must be included when assessing the contribution of these systems to the inventory of r-process elements in the Galaxy.

  11. MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

    Directory of Open Access Journals (Sweden)

    Lii Patrick

    2014-01-01

    Full Text Available Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

  12. Simulations of small solid accretion on to planetesimals in the presence of gas

    Science.gov (United States)

    Hughes, A. G.; Boley, A. C.

    2017-12-01

    The growth and migration of planetesimals in a young protoplanetary disc are fundamental to planet formation. In all models of early growth, there are several processes that can inhibit grains from reaching larger sizes. Nevertheless, observations suggest that growth of planetesimals must be rapid. If a small number of 100 km sized planetesimals do manage to form in the disc, then gas drag effects could enable them to efficiently accrete small solids from beyond their gravitationally focused cross-section. This gas-drag-enhanced accretion can allow planetesimals to grow at rapid rates, in principle. We present self-consistent hydrodynamics simulations with direct particle integration and gas-drag coupling to estimate the rate of planetesimal growth due to pebble accretion. Wind tunnel simulations are used to explore a range of particle sizes and disc conditions. We also explore analytic estimates of planetesimal growth and numerically integrate planetesimal drift due to the accretion of small solids. Our results show that, for almost every case that we consider, there is a clearly preferred particle size for accretion that depends on the properties of the accreting planetesimal and the local disc conditions. For solids much smaller than the preferred particle size, accretion rates are significantly reduced as the particles are entrained in the gas and flow around the planetesimal. Solids much larger than the preferred size accrete at rates consistent with gravitational focusing. Our analytic estimates for pebble accretion highlight the time-scales that are needed for the growth of large objects under different disc conditions and initial planetesimal sizes.

  13. Stellar photospheric abundances as a probe of discs and planets

    Science.gov (United States)

    Jermyn, Adam S.; Kama, Mihkel

    2018-06-01

    Protoplanetary discs, debris discs, and disrupted or evaporating planets can all feed accretion on to stars. The photospheric abundances of such stars may then reveal the composition of the accreted material. This is especially likely in B to mid-F type stars, which have radiative envelopes and hence less bulk-photosphere mixing. We present a theoretical framework (CAM), considering diffusion, rotation, and other stellar mixing mechanisms to describe how the accreted material interacts with the bulk of the star. This allows the abundance pattern of the circumstellar material to be calculated from measured stellar abundances and parameters (vrot, Teff). We discuss the λ Boötis phenomenon and the application of CAM on stars hosting protoplanetary discs (HD 100546, HD 163296), debris discs (HD 141569, HD 21997), and evaporating planets (HD 195689/KELT-9).

  14. White dwarf radii and boundary-layer constraints in three dwarf novae

    International Nuclear Information System (INIS)

    Wood, J.H.

    1990-01-01

    The structure of the boundary layer between the accretion disc and white dwarf in three quiescent dwarf novae is explored with high signal-to-noise eclipse light curves obtained by phase folding 12-20 eclipses. Models of the eclipse shapes of various white dwarf/boundary layer configurations that might be at the centres of the accretion discs are calculated and compared with observations of the eclipses in Z Cha, OY Car and HT Cas. Possible models for the central objects are found to be a white dwarf with or without its lower hemisphere occulted by the disc, or a white dwarf with an optically thick boundary layer significantly extended in latitude up and down its sides. The most likely of these models for each system is an unocculted white dwarf with no boundary layer contributing significantly to the optical flux, or a white dwarf totally covered by an optically thick boundary layer. (author)

  15. Compact binary merger and kilonova: outflows from remnant disc

    Science.gov (United States)

    Yi, Tuan; Gu, Wei-Min; Liu, Tong; Kumar, Rajiv; Mu, Hui-Jun; Song, Cui-Ying

    2018-05-01

    Outflows launched from a remnant disc of compact binary merger may have essential contribution to the kilonova emission. Numerical calculations are conducted in this work to study the structure of accretion flows and outflows. By the incorporation of limited-energy advection in the hyper-accretion discs, outflows occur naturally from accretion flows due to imbalance between the viscous heating and the sum of the advective and radiative cooling. Following this spirit, we revisit the properties of the merger outflow ejecta. Our results show that around 10-3 ˜ 10-1 M⊙ of the disc mass can be launched as powerful outflows. The amount of unbound mass varies with the disc mass and the viscosity. The outflow-contributed peak luminosity is around 1040 ˜ 1041 erg s-1. Such a scenario can account for the observed kilonovae associated with short gamma-ray bursts, including the recent event AT2017gfo (GW170817).

  16. Hyper-Eddington accretion in GRB

    International Nuclear Information System (INIS)

    Janiuk, A.; Czerny, B.; Perna, R.; Di Matteo, T.

    2005-01-01

    Popular models of the GRB origin associate this event with a cosmic explosion, birth of a stellar mass black ho le and jet ejection. Due to the shock collisions that happen in the jet, the gamma rays are produced and we detect a burst of duration up to several tens of seconds. This burst duration is determined by the lifetime of the central engine, which may be different in various scenarios. Characteristically, the observed bursts have a bimodal distribution and constitute the two classes: short (t < 2 s) and long bursts. Theoretical models invoke the mergers of two neutron stars or a neutron star with a black hole, or, on the other hand, a massive star explosion (collapsar). In any of these models we have a phase of disc accretion onto a newly born black hole: the di se is formed from the disrupted neutron star or fed by the material fallback from the ejected collapsar envelope. The disc is extremely hot and dense, and the accretion rate is orders of magnitude higher than the Eddington rate. In such physical conditions the main cooling mechanism is neutrino emission, and one of possible ways of energy extraction from the accretion disc is the neutrino-antineutrino annihilation

  17. Turbulence in Accretion Discs. The Global Baroclinic Instability

    Science.gov (United States)

    Klahr, Hubert; Bodenheimer, Peter

    The transport of angular momentum away from the central object is a sufficient condition for a protoplanetary disk to accrete matter onto the star and spin it down. Magnetic fields cannot be of importance for this process in a large part of the cold and dusty disk where the planets supposedly form. Our new hypothesis on the angular momentum transport based on radiation hydro simulations is as follows: We present the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian accretion disks. We show by analytical considerations and three-dimensional radiation hydro simulations that, in particular, protoplanetary disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that this baroclinic flow is unstable and produces turbulence. These findings are currently tested for numerical effects by performing barotropic simulations which show that imposed turbulence rapidly decays. The turbulence in baroclinic disks draws energy from the background shear, transports angular momentum outward and creates a radially inward bound accretion of matter, thus forming a self consistent process. Gravitational energy is transformed into turbulent kinetic energy, which is then dissipated, as in the classical accretion paradigm. We measure accretion rates in 2D and 3D simulations of dot M= - 10-9 to -10-7 Msolar yr-1 and viscosity parameters of α = 10-4 - 10-2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the (r-φ) plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.

  18. Physics of accretion and ejection processes: a multi-wavelengths study of galactic X-ray binaries

    International Nuclear Information System (INIS)

    Prat, Lionel

    2010-01-01

    This manuscript is dedicated to the study of the accretion and ejection processes in X-ray Binaries, using radio and X-ray observations as well as numerical simulations. The links and interplay between the accretion disc, the corona and the compact jet. In an introductory part, I first describe the main observational and theoretical properties of the X-ray binaries. I especially emphasize the aspects required to understand the work reported in this manuscript. I also describe the main X-ray and radio observatories used during this work. Then, the first part of this manuscript is dedicated to the accretion processes in X-ray Binaries. I use high energy observations to study one High Mass X-ray Binary (IGR J19140+0951) and two Low-Mass X-ray Binaries (XTE J1818-245 and H1743-322). In the case of IGR Jl9140+0951, observations show that the luminosity generated by the accretion processes can deeply alter the stellar wind. In the case of the two Low Mass X-ray Binaries, I estimate several important parameters of the Systems using the behavior of their accretion discs. The second part is dedicated to the interplay between the accretion disc and the other components of the Systems, namely the corona and the compact jet. Using simultaneous X-ray and radio observations, I show that the corona undergo a strong evolution prior to a discrete ejection of matter, in the case of several binary Systems. In the case of GRS 1915+105, evolution of the corona and detection of a discrete ejection appear within a few seconds, while for other sources it takes a few hours. I study also the link between the accretion disc and the compact jet using a correlation between radio and X-ray flux: depending on the System, the link between the accretion energy brought by the accretion disc and the luminosity of the jet is different, indicating that different physical processes are at work. Finally, the third part is dedicated to numerical simulations of the accretion disc, in the case where an

  19. Symbiotic stars - a binary model with super-critical accretion

    Energy Technology Data Exchange (ETDEWEB)

    Bath, G T [National Radio Astronomy Observatory, Charlottesville, Va. (USA)

    1977-01-01

    The structure of symbiotic variables is discussed in terms of a binary model. Disc accretion by a main sequence star or white dwarf at rates close to the Eddington limit produces an ultraviolet continuum source near the accreting star surface. This generates a variable, radiatively-driven, out-flowing wind. The wind is optically thick and the disc luminosity is absorbed and scattered and thus degraded into the optical region. Variations in the rate of mass loss in the wind lead to optical eruptions through shifts in the position of, and conditions in, the last scattering surface. The behaviour of Z And determined by Boyarchuk is shown to be in agreement with such a model. The conditions in the out-flowing wind are discussed. Limits on the mass loss rate are derived from conditions at the surface of the accreting star. It is suggested that variable out-flow in the wind is generated by fluctuations in disc luminosity produced by changes in the giant companions rate of mass transfer. The relation between symbiotic variables and classical and dwarf novae is discussed.

  20. Changes in the metallicity of gas giant planets due to pebble accretion

    Science.gov (United States)

    Humphries, R. J.; Nayakshin, S.

    2018-06-01

    We run numerical simulations to study the accretion of gas and dust grains on to gas giant planets embedded into massive protoplanetary discs. The outcome is found to depend on the disc cooling rate, planet mass, grain size, and irradiative feedback from the planet. If radiative cooling is efficient, planets accrete both gas and pebbles rapidly, open a gap, and usually become massive brown dwarfs. In the inefficient cooling case, gas is too hot to accrete on to the planet but pebble accretion continues and the planets migrate inward rapidly. Radiative feedback from the planet tends to suppress gas accretion. Our simulations predict that metal enrichment of planets by dust grain accretion inversely correlates with the final planet mass, in accordance with the observed trend in the inferred bulk composition of Solar system and exosolar giant planets. To account for observations, however, as many as ˜30-50 per cent of the dust mass should be in the form of large grains.

  1. Inner disc obscuration in GRS 1915+105 based on relativistic slim disc model

    Czech Academy of Sciences Publication Activity Database

    Vierdayanti, K.; Sądowski, A.; Mineshige, L.S.; Bursa, Michal

    2013-01-01

    Roč. 436, č. 1 (2013), s. 71-81 ISSN 0035-8711 Institutional support: RVO:67985815 Keywords : accretion discs * black hole physics * GRS 1915+105 Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.226, year: 2013

  2. The profiles of Fe K α line from the inhomogeneous accretion flow

    Science.gov (United States)

    Yu, Xiao-Di; Ma, Ren-Yi; Li, Ya-Ping; Zhang, Hui; Fang, Tao-Tao

    2018-05-01

    The clumpy disc, or inhomogeneous accretion flow, has been proposed to explain the properties of accreting black hole systems. However, the observational evidence remains to be explored. In this work, we calculate the profiles of Fe K α lines emitted from the inhomogeneous accretion flow through the ray-tracing technique, in order to find possible observable signals of the clumps. Compared with the skewed double-peaked profile of the continuous standard accretion disc, the lines show a multipeak structure when the emissivity index is not very steep. The peaks and wings are affected by the position and size of the cold clumps. When the clump is small and is located in the innermost region, due to the significant gravitational redshift, the blue wing can overlap with the red wing of the outer cold disc/clump, forming a fake peak or greatly enhancing the red peak. Given high enough resolution, it is easier to constrain the clumps around the supermassive black holes than the clumps in stellar mass black holes due to the thermal Doppler effect.

  3. The Effects of High Density on the X-ray Spectrum Reflected from Accretion Discs Around Black Holes

    Science.gov (United States)

    Garcia, Javier A.; Fabian, Andrew C.; Kallman, Timothy R.; Dauser, Thomas; Parker, Micahel L.; McClintock, Jeffrey E.; Steiner, James F.; Wilms, Jorn

    2016-01-01

    Current models of the spectrum of X-rays reflected from accretion discs around black holes and other compact objects are commonly calculated assuming that the density of the disc atmosphere is constant within several Thomson depths from the irradiated surface. An important simplifying assumption of these models is that the ionization structure of the gas is completely specified by a single, fixed value of the ionization parameter (xi), which is the ratio of the incident flux to the gas density. The density is typically fixed at n(sub e) = 10(exp 15) per cu cm. Motivated by observations, we consider higher densities in the calculation of the reflected spectrum. We show by computing model spectra for n(sub e) approximately greater than 10(exp 17) per cu cm that high-density effects significantly modify reflection spectra. The main effect is to boost the thermal continuum at energies 2 approximately less than keV. We discuss the implications of these results for interpreting observations of both active galactic nuclei and black hole binaries. We also discuss the limitations of our models imposed by the quality of the atomic data currently available.

  4. Gamma-burst emission from neutron-star accretion

    Science.gov (United States)

    Colgate, S. A.; Petschek, A. G.; Sarracino, R.

    1983-01-01

    A model for emission of the hard photons of gamma bursts is presented. The model assumes accretion at nearly the Eddington limited rate onto a neutron star without a magnetic field. Initially soft photons are heated as they are compressed between the accreting matter and the star. A large electric field due to relatively small charge separation is required to drag electrons into the star with the nuclei against the flux of photons leaking out through the accreting matter. The photon number is not increased substantially by Bremsstrahlung or any other process. It is suggested that instability in an accretion disc might provide the infalling matter required.

  5. Super-Eddington accretion on to the neutron star NGC 7793 P13: Broad-band X-ray spectroscopy and ultraluminous X-ray sources

    Science.gov (United States)

    Walton, D. J.; Fürst, F.; Harrison, F. A.; Stern, D.; Bachetti, M.; Barret, D.; Brightman, M.; Fabian, A. C.; Middleton, M. J.; Ptak, A.; Tao, L.

    2018-02-01

    We present a detailed, broad-band X-ray spectral analysis of the ultraluminous X-ray source (ULX) pulsar NGC 7793 P13, a known super-Eddington source, utilizing data from the XMM-Newton, NuSTAR and Chandra observatories. The broad-band XMM-Newton+NuSTAR spectrum of P13 is qualitatively similar to the rest of the ULX sample with broad-band coverage, suggesting that additional ULXs in the known population may host neutron star accretors. Through time-averaged, phase-resolved and multi-epoch studies, we find that two non-pulsed thermal blackbody components with temperatures ∼0.5 and 1.5 keV are required to fit the data below 10 keV, in addition to a third continuum component which extends to higher energies and is associated with the pulsed emission from the accretion column. The characteristic radii of the thermal components appear to be comparable, and are too large to be associated with the neutron star itself, so the need for two components likely indicates the accretion flow outside the magnetosphere is complex. We suggest a scenario in which the thick inner disc expected for super-Eddington accretion begins to form, but is terminated by the neutron star's magnetic field soon after its onset, implying a limit of B ≲ 6 × 1012 G for the dipolar component of the central neutron star's magnetic field. Evidence of similar termination of the disc in other sources may offer a further means of identifying additional neutron star ULXs. Finally, we examine the spectrum exhibited by P13 during one of its unusual 'off' states. These data require both a hard power-law component, suggesting residual accretion on to the neutron star, and emission from a thermal plasma, which we argue is likely associated with the P13 system.

  6. Discovery of a strong soft X-ray excess in Mkn 335 -evidence for an accretion disc

    International Nuclear Information System (INIS)

    Pounds, K.A.; Stanger, V.J.; Turner, T.J.; King, A.R.; Czerny, B.

    1987-01-01

    EXOSAT observations of Mkn 335 reveal a hard power-law spectrum above approx. 1 keV, typical for Seyfert 1 galaxies, but with unusually strong variability on time-scales of approx. 1-2 hr. In addition an intense soft X-ray component is found to dominate the overall spectrum of Mkn 335 below approx. 0.6 keV. Both soft and hard components increased in strength by a factor approx. 6 between 1983 November and 1984 December. These observations are discussed in terms of models in which an accretion disc feeds a central massive black hole. It is found that radiation pressure only allows a thermal origin for the soft X-radiation if the black hole mass is 7 solar masses, as suggested also by the observed variability time-scale. (author)

  7. Demonstration of a magnetic Prandtl number disc instability from first principles

    OpenAIRE

    Potter, William J.; Balbus, Steven A.

    2017-01-01

    Understanding what determines the strength of MHD turbulence in accretion discs is a question of fundamental theoretical and observational importance. In this work we investigate whether the dependence of the turbulent accretion disc stress ($\\alpha$) on the magnetic Prandtl number (Pm) is sufficiently sensitive to induce thermal-viscous instability using 3D MHD simulations. We first investigate whether the $\\alpha$-Pm dependence, found by many previous authors, has a physical or numerical or...

  8. Magnetic fields and accretion discs around static black holes

    International Nuclear Information System (INIS)

    Dadhich, N.

    1982-01-01

    Some aspects of accretion onto static black holes immersed in a uniform magnetic field are investigated. The Ernst metric is employed to find the 'Keplerian' angular momentum distribution and the efficiency of mass-to-energy conversion for a plasma and for test particles. Under almost all physically reasonable conditions for hydrodynamic accretion the effect of the magnetic field is small. However, for test particles the effect can be very important and the efficiency can approach unity. (author)

  9. Accretion onto stellar mass black holes

    Science.gov (United States)

    Deegan, Patrick

    2009-12-01

    I present work on the accretion onto stellar mass black holes in several scenarios. Due to dynamical friction stellar mass black holes are expected to form high density cusps in the inner parsec of our Galaxy. These compact remnants may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. I build a simple but detailed time-dependent model of such emission. Future observations of the distribution and orbits of the gas in the inner parsec of Sgr A* will put tighter constraints on the cusp of compact remnants. GRS 1915+105 is an LMXB, whose large orbital period implies a very large accretion disc and explains the extraordinary duration of its current outburst. I present smoothed particle hydrodynamic simulations of the accretion disc. The models includes the thermo-viscous instability, irradiation from the central object and wind loss. I find that the outburst of GRS 1915+105 should last a minimum of 20 years and up to ˜ 100 years if the irradiation is playing a significant role in this system. The predicted recurrence times are of the order of 104 years, making the duty cycle of GRS 1915+105 to be a few 0.1%. I present a simple analytical method to describe the observable behaviour of long period black hole LMXBs, similar to GRS 1915+105. Constructing two simple models for the surface density in the disc, outburst and quiescence times are calculated as a function of orbital period. LMXBs are an important constituent of the X-ray light function (XLF) of giant elliptical galaxies. I find that the duty cycle can vary considerably with orbital period, with implications for modelling the XLF.

  10. Criteria for retrograde rotation of accreting black holes

    Science.gov (United States)

    Mikhailov, A. G.; Piotrovich, M. Yu; Gnedin, Yu N.; Natsvlishvili, T. M.; Buliga, S. D.

    2018-06-01

    Rotating supermassive black holes produce jets and their origin is connected to the magnetic field that is generated by accreting matter flow. There is a point of view that electromagnetic fields around rotating black holes are brought to the hole by accretion. In this situation the prograde accreting discs produce weaker large-scale black hole threading magnetic fields, implying weaker jets than in retrograde regimes. The basic goal of this paper is to find the best candidates for retrograde accreting systems in observed active galactic nuclei. We show that active galactic nuclei with low Eddington ratio are really the best candidates for retrograde systems. This conclusion is obtained for kinetically dominated Fanaroff-Riley class II radio galaxies, flat-spectrum radio-loud narrow-line Seyfert I galaxies and a number of nearby galaxies. Our conclusion is that the best candidates for retrograde systems are the noticeable population of active galactic nuclei in the Universe. This result corresponds to the conclusion that in the merging process the interaction of merging black holes with a retrograde circumbinary disc is considerably more effective for shrinking the binary system.

  11. Observational constraints on neutron star masses and radii

    Energy Technology Data Exchange (ETDEWEB)

    Coleman Miller, M. [University of Maryland, Department of Astronomy and Joint Space-Science Institute, College Park, MD (United States); Lamb, Frederick K. [University of Illinois at Urbana-Champaign, Center for Theoretical Astrophysics and Department of Physics, Urbana, IL (United States); University of Illinois at Urbana-Champaign, Department of Astronomy, Urbana, IL (United States)

    2016-03-15

    Precise and reliable measurements of the masses and radii of neutron stars with a variety of masses would provide valuable guidance for improving models of the properties of cold matter with densities above the saturation density of nuclear matter. Several different approaches for measuring the masses and radii of neutron stars have been tried or proposed, including analyzing the X-ray fluxes and spectra of the emission from neutron stars in quiescent low-mass X-ray binary systems and thermonuclear burst sources; fitting the energy-dependent X-ray waveforms of rotation-powered millisecond pulsars, burst oscillations with millisecond periods, and accretion-powered millisecond pulsars; and modeling the gravitational radiation waveforms of coalescing double neutron star and neutron star - black hole binary systems. We describe the strengths and weaknesses of these approaches, most of which currently have substantial systematic errors, and discuss the prospects for decreasing the systematic errors in each method. (orig.)

  12. Accretion on to Magnetic White Dwarfs

    Directory of Open Access Journals (Sweden)

    Wickramasinghe Dayal

    2014-01-01

    The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is “blobby”. This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence for blobby accretion in the generally lower field and disced IPs.

  13. Eclipsing the innermost accretion disc regions in AGN

    Czech Academy of Sciences Publication Activity Database

    Sanfrutos, M.; Miniutti, G.; Dovčiak, Michal; Agis-Gonzalez, B.

    2016-01-01

    Roč. 337, 4-5 (2016), s. 546-551 ISSN 0004-6337 Institutional support: RVO:67985815 Keywords : accretion disks * black hole physics * relativistic effects Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 0.916, year: 2016

  14. The origin of diverse α-element abundances in galaxy discs

    Science.gov (United States)

    Mackereth, J. Ted; Crain, Robert A.; Schiavon, Ricardo P.; Schaye, Joop; Theuns, Tom; Schaller, Matthieu

    2018-04-01

    Spectroscopic surveys of the Galaxy reveal that its disc stars exhibit a spread in [α/Fe] at fixed [Fe/H], manifest at some locations as a bimodality. The origin of these diverse, and possibly distinct, stellar populations in the Galactic disc is not well understood. We examine the Fe and α-element evolution of 133 Milky Way-like galaxies from the EAGLE simulation, to investigate the origin and diversity of their [α/Fe]-[Fe/H] distributions. We find that bimodal [α/Fe] distributions arise in galaxies whose gas accretion histories exhibit episodes of significant infall at both early and late times, with the former fostering more intense star formation than the latter. The shorter characteristic consumption timescale of gas accreted in the earlier episode suppresses its enrichment with iron synthesised by Type Ia SNe, resulting in the formation of a high-[α/Fe] sequence. We find that bimodality in [α/Fe] similar to that seen in the Galaxy is rare, appearing in approximately 5 percent of galaxies in our sample. We posit that this is a consequence of an early gas accretion episode requiring the mass accretion history of a galaxy's dark matter halo to exhibit a phase of atypically-rapid growth at early epochs. The scarcity of EAGLE galaxies exhibiting distinct sequences in the [α/Fe]-[Fe/H] plane may therefore indicate that the Milky Way's elemental abundance patterns, and its accretion history, are not representative of the broader population of ˜L⋆ disc galaxies.

  15. The origin of blueshifted absorption features in the X-ray spectrum of PG 1211+143: outflow or disc

    Science.gov (United States)

    Gallo, L. C.; Fabian, A. C.

    2013-07-01

    In some radio-quiet active galactic nuclei (AGN), high-energy absorption features in the X-ray spectra have been interpreted as ultrafast outflows (UFOs) - highly ionized material (e.g. Fe XXV and Fe XXVI) ejected at mildly relativistic velocities. In some cases, these outflows can carry energy in excess of the binding energy of the host galaxy. Needless to say, these features demand our attention as they are strong signatures of AGN feedback and will influence galaxy evolution. For the same reason, alternative models need to be discussed and refuted or confirmed. Gallo and Fabian proposed that some of these features could arise from resonance absorption of the reflected spectrum in a layer of ionized material located above and corotating with the accretion disc. Therefore, the absorbing medium would be subjected to similar blurring effects as seen in the disc. A priori, the existence of such plasma above the disc is as plausible as a fast wind. In this work, we highlight the ambiguity by demonstrating that the absorption model can describe the ˜7.6 keV absorption feature (and possibly other features) in the quasar PG 1211+143, an AGN that is often described as a classic example of a UFO. In this model, the 2-10 keV spectrum would be largely reflection dominated (as opposed to power law dominated in the wind models) and the resonance absorption would be originating in a layer between about 6 and 60 gravitational radii. The studies of such features constitute a cornerstone for future X-ray observatories like Astro-H and Athena+. Should our model prove correct, or at least important in some cases, then absorption will provide another diagnostic tool with which to probe the inner accretion flow with future missions.

  16. A New Paradigm for Gamma Ray Bursts: Long Term Accretion Rate Modulation by an External Accretion Disk

    Science.gov (United States)

    Cannizzo, John; Gehrels, Neil

    2009-01-01

    We present a new way of looking at the very long term evolution of GRBs in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence the efficacy of the process that extracts rotational energy from the black hole and inner accretion disk. The pre-Swift paradigm of achromatic, shallow-to-steep "breaks" in the long term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an "external disk" whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power law decay to the GRB light curves. In this model, the different canonical power law decay segments in the GRB identified by Zhang et al. and Nousek et al. represent different physical states of the accretion disk. We identify a physical disk state with each power law segment.

  17. Likelihood inference for unions of interacting discs

    DEFF Research Database (Denmark)

    Møller, Jesper; Helisova, K.

    2010-01-01

    This is probably the first paper which discusses likelihood inference for a random set using a germ-grain model, where the individual grains are unobservable, edge effects occur and other complications appear. We consider the case where the grains form a disc process modelled by a marked point...... process, where the germs are the centres and the marks are the associated radii of the discs. We propose to use a recent parametric class of interacting disc process models, where the minimal sufficient statistic depends on various geometric properties of the random set, and the density is specified......-based maximum likelihood inference and the effect of specifying different reference Poisson models....

  18. Papaloizou-Pringle instability suppression by the magnetorotational instability in relativistic accretion discs

    Science.gov (United States)

    Bugli, M.; Guilet, J.; Müller, E.; Del Zanna, L.; Bucciantini, N.; Montero, P. J.

    2018-03-01

    Geometrically thick tori with constant specific angular momentum have been widely used in the last decades to construct numerical models of accretion flows on to black holes. Such discs are prone to a global non-axisymmetric hydrodynamic instability, known as Papaloizou-Pringle instability (PPI), which can redistribute angular momentum and also lead to an emission of gravitational waves. It is, however, not clear yet how the development of the PPI is affected by the presence of a magnetic field and by the concurrent development of the magnetorotational instability (MRI). We present a numerical analysis using three-dimensional GRMHD simulations of the interplay between the PPI and the MRI considering, for the first time, an analytical magnetized equilibrium solution as initial condition. In the purely hydrodynamic case, the PPI selects as expected the large-scale m = 1 azimuthal mode as the fastest growing and non-linearly dominant mode. However, when the torus is threaded by a weak toroidal magnetic field, the development of the MRI leads to the suppression of large-scale modes and redistributes power across smaller scales. If the system starts with a significantly excited m = 1 mode, the PPI can be dominant in a transient phase, before being ultimately quenched by the MRI. Such dynamics may well be important in compact star mergers and tidal disruption events.

  19. Chasing discs around O-type (proto)stars: Evidence from ALMA observations

    NARCIS (Netherlands)

    Cesaroni, R.; Sánchez-Monge, Á.; Beltrán, M. T.; Johnston, K. G.; Maud, L. T.; Moscadelli, L.; Mottram, J. C.; Ahmadi, A.; Allen, V.; Beuther, H.; Csengeri, T.; Etoka, S.; Fuller, G. A.; Galli, D.; Galván-Madrid, R.; Goddi, C.; Henning, T.; Hoare, M. G.; Klaassen, P. D.; Kuiper, R.; Kumar, M. S. N.; Lumsden, S.; Peters, T.; Rivilla, V. M.; Schilke, P.; Testi, L.; van der Tak, F.; Vig, S.; Walmsley, C. M.; Zinnecker, H.

    2017-01-01

    Context. Circumstellar discs around massive stars could mediate the accretion onto the star from the infalling envelope, and could minimize the effects of radiation pressure. Despite such a crucial role, only a few convincing candidates have been provided for discs around deeply embedded O-type

  20. Dippers and dusty disc edges: new diagnostics and comparison to model predictions

    Science.gov (United States)

    Bodman, Eva H. L.; Quillen, Alice C.; Ansdell, Megan; Hippke, Michael; Boyajian, Tabetha S.; Mamajek, Eric E.; Blackman, Eric G.; Rizzuto, Aaron; Kastner, Joel H.

    2017-09-01

    We revisit the nature of large dips in flux from extinction by dusty circumstellar material that is observed by Kepler for many young stars in the Upper Sco and ρ Oph star formation regions. These young, low-mass 'dipper' stars are known to have low accretion rates and primarily host moderately evolved dusty circumstellar discs. Young low-mass stars often exhibit rotating starspots that cause quasi-periodic photometric variations. We found no evidence for periods associated with the dips that are different from the starspot rotation period in spectrograms constructed from the light curves. The material causing the dips in most of these light curves must be approximately corotating with the star. We find that disc temperatures computed at the disc corotation radius are cool enough that dust should not sublime. Crude estimates for stellar magnetic field strengths and accretion rates are consistent with magnetospheric truncation near the corotation radius. Magnetospheric truncation models can explain why the dips are associated with material near corotation and how dusty material is lifted out of the mid-plane to obscure the star that would account for the large fraction of young low-mass stars that are dippers. We propose that variations in disc orientation angle, stellar magnetic field dipole tilt axis and disc accretion rate are underlying parameters accounting for differences in the dipper light curves.

  1. Appearance of Keplerian discs orbiting Kerr superspinars

    Energy Technology Data Exchange (ETDEWEB)

    Stuchlik, Zdenek; Schee, Jan, E-mail: zdenek.stuchlik@fpf.slu.c, E-mail: jan.schee@fpf.slu.c [Institute of Physics, Faculty of Philosophy and Science, Silesian University in Opava, Bezrucovo nam. 13, Opava (Czech Republic)

    2010-11-07

    We study optical phenomena related to the appearance of Keplerian accretion discs orbiting Kerr superspinars predicted by string theory. The superspinar exterior is described by standard Kerr naked singularity geometry breaking the black hole limit on the internal angular momentum (spin). We construct local photon escape cones for a variety of orbiting sources that enable us to determine the superspinars silhouette in the case of distant observers. We show that the superspinar silhouette depends strongly on the assumed edge where the external Kerr spacetime is joined to the internal spacetime governed by string theory and significantly differs from the black hole silhouette. The appearance of the accretion disc is strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile. Apparent extension of the disc grows significantly with the growing spin, while the frequency shift grows with the descending spin. This behaviour differs substantially from the appearance of discs orbiting black holes enabling thus, at least in principle, to distinguish clearly the Kerr superspinars and black holes. In vicinity of a Kerr superspinar the non-escaped photons have to be separated to those captured by the superspinar and those being trapped in its strong gravitational field leading to self-illumination of the disc that could even influence its structure and cause self-reflection effect of radiation of the disc. The amount of trapped photons grows with descending superspinar spin. We thus can expect significant self-illumination effects in the field of Kerr superspinars with near-extreme spin a {approx} 1.

  2. Jet launching radius in low-power radio-loud AGNs in advection-dominated accretion flows

    Science.gov (United States)

    Le, Truong; Newman, William; Edge, Brinkley

    2018-06-01

    Using our theory for the production of relativistic outflows, we estimate the jet launching radius and the inferred mass accretion rate for 52 low-power radio-loud AGNs based on the observed jet powers. Our analysis indicates that (1) a significant fraction of the accreted energy is required to convert the accreted mass to relativistic energy particles for the production of the jets near the event horizon, (2) the jet's launching radius moves radially towards the horizon as the mass accretion rate or jet's power increases, and (3) no jet/outflow formation is possible beyond 44 gravitational radii.

  3. SURFACE LAYER ACCRETION IN CONVENTIONAL AND TRANSITIONAL DISKS DRIVEN BY FAR-ULTRAVIOLET IONIZATION

    International Nuclear Information System (INIS)

    Perez-Becker, Daniel; Chiang, Eugene

    2011-01-01

    Whether protoplanetary disks accrete at observationally significant rates by the magnetorotational instability (MRI) depends on how well ionized they are. Disk surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates, ranging from ∼0.01 μm sized grains to angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic carbon and sulfur produces a plasma so dense that it is immune to ion recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01-0.1 g cm -2 , behaves in the ideal magnetohydrodynamic limit and can accrete at observationally significant rates at radii ∼> 1-10 AU. Surface layer accretion driven by FUV ionization can reproduce the trend of increasing accretion rate with increasing hole size seen in transitional disks. At radii ∼<1-10 AU, FUV-ionized surface layers cannot sustain the accretion rates generated at larger distance, and unless turbulent mixing of plasma can thicken the MRI-active layer, an additional means of transport is needed. In the case of transitional disks, it could be provided by planets.

  4. Instability of warped discs

    Science.gov (United States)

    Doǧan, S.; Nixon, C. J.; King, A. R.; Pringle, J. E.

    2018-05-01

    Accretion discs are generally warped. If a warp in a disc is too large, the disc can `break' apart into two or more distinct planes, with only tenuous connections between them. Further, if an initially planar disc is subject to a strong differential precession, then it can be torn apart into discrete annuli that precess effectively independently. In previous investigations, torque-balance formulae have been used to predict where and when the disc breaks into distinct parts. In this work, focusing on discs with Keplerian rotation and where the shearing motions driving the radial communication of the warp are damped locally by turbulence (the `diffusive' regime), we investigate the stability of warped discs to determine the precise criterion for an isolated warped disc to break. We find and solve the dispersion relation, which, in general, yields three roots. We provide a comprehensive analysis of this viscous-warp instability and the emergent growth rates and their dependence on disc parameters. The physics of the instability can be understood as a combination of (1) a term that would generally encapsulate the classical Lightman-Eardley instability in planar discs (given by ∂(νΣ)/∂Σ < 0) but is here modified by the warp to include ∂(ν1|ψ|)/∂|ψ| < 0, and (2) a similar condition acting on the diffusion of the warp amplitude given in simplified form by ∂(ν2|ψ|)/∂|ψ| < 0. We discuss our findings in the context of discs with an imposed precession, and comment on the implications for different astrophysical systems.

  5. Gravitating discs around black holes

    International Nuclear Information System (INIS)

    Karas, V; Hure, J-M; Semerak, O

    2004-01-01

    Fluid discs and tori around black holes are discussed within different approaches and with the emphasis on the role of disc gravity. First reviewed are the prospects of investigating the gravitational field of a black hole-disc system using analytical solutions of stationary, axially symmetric Einstein equations. Then, more detailed considerations are focused to the middle and outer parts of extended disc-like configurations where relativistic effects are small and the Newtonian description is adequate. Within general relativity, only a static case has been analysed in detail. Results are often very inspiring. However, simplifying assumptions must be imposed: ad hoc profiles of the disc density are commonly assumed and the effects of frame-dragging are completely lacking. Astrophysical discs (e.g. accretion discs in active galactic nuclei) typically extend far beyond the relativistic domain and are fairly diluted. However, self-gravity is still essential for their structure and evolution, as well as for their radiation emission and the impact on the surrounding environment. For example, a nuclear star cluster in a galactic centre may bear various imprints of mutual star-disc interactions, which can be recognized in observational properties, such as the relation between the central mass and stellar velocity dispersion. (topical review)

  6. DZ Chamaeleontis: a bona fide photoevaporating disc

    Science.gov (United States)

    Canovas, H.; Montesinos, B.; Schreiber, M. R.; Cieza, L. A.; Eiroa, C.; Meeus, G.; de Boer, J.; Ménard, F.; Wahhaj, Z.; Riviere-Marichalar, P.; Olofsson, J.; Garufi, A.; Rebollido, I.; van Holstein, R. G.; Caceres, C.; Hardy, A.; Villaver, E.

    2018-02-01

    Context. DZ Cha is a weak-lined T Tauri star (WTTS) surrounded by a bright protoplanetary disc with evidence of inner disc clearing. Its narrow Hα line and infrared spectral energy distribution suggest that DZ Cha may be a photoevaporating disc. Aims: We aim to analyse the DZ Cha star + disc system to identify the mechanism driving the evolution of this object. Methods: We have analysed three epochs of high resolution optical spectroscopy, photometry from the UV up to the sub-mm regime, infrared spectroscopy, and J-band imaging polarimetry observations of DZ Cha. Results: Combining our analysis with previous studies we find no signatures of accretion in the Hα line profile in nine epochs covering a time baseline of 20 yr. The optical spectra are dominated by chromospheric emission lines, but they also show emission from the forbidden lines [SII] 4068 and [OI] 6300Å that indicate a disc outflow. The polarized images reveal a dust depleted cavity of 7 au in radius and two spiral-like features, and we derive a disc dust mass limit of Mdust 80 MJup) companions are detected down to 0.̋07 ( 8 au, projected). Conclusions: The negligible accretion rate, small cavity, and forbidden line emission strongly suggests that DZ Cha is currently at the initial stages of disc clearing by photoevaporation. At this point the inner disc has drained and the inner wall of the truncated outer disc is directly exposed to the stellar radiation. We argue that other mechanisms like planet formation or binarity cannot explain the observed properties of DZ Cha. The scarcity of objects like this one is in line with the dispersal timescale (≲105 yr) predicted by this theory. DZ Cha is therefore an ideal target to study the initial stages of photoevaporation. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 097.C-0536. Based on data obtained from the ESO Science Archive Facility under request number 250112.

  7. The formation of planets by disc fragmentation

    Directory of Open Access Journals (Sweden)

    Stamatellos Dimitris

    2013-04-01

    Full Text Available I discuss the role that disc fragmentation plays in the formation of gas giant and terrestrial planets, and how this relates to the formation of brown dwarfs and low-mass stars, and ultimately to the process of star formation. Protostellar discs may fragment, if they are massive enough and can cool fast enough, but most of the objects that form by fragmentation are brown dwarfs. It may be possible that planets also form, if the mass growth of a proto-fragment is stopped (e.g. if this fragment is ejected from the disc, or suppressed and even reversed (e.g by tidal stripping. I will discuss if it is possible to distinguish whether a planet has formed by disc fragmentation or core accretion, and mention of a few examples of observed exoplanets that are suggestive of formation by disc fragmentation.

  8. A truly Newtonian softening length for disc simulations

    Czech Academy of Sciences Publication Activity Database

    Huré, J.-M.; Trova, Audrey

    2015-01-01

    Roč. 447, č. 2 (2015), s. 1866-1872 ISSN 0035-8711 Institutional support: RVO:67985815 Keywords : accretion discs * gravitation * numerical methods Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.952, year: 2015

  9. ZOMG - II. Does the halo assembly history influence central galaxies and gas accretion?

    Science.gov (United States)

    Romano-Díaz, Emilio; Garaldi, Enrico; Borzyszkowski, Mikolaj; Porciani, Cristiano

    2017-08-01

    The growth rate and the internal dynamics of galaxy-sized dark-matter haloes depend on their location within the cosmic web. Haloes that sit at the nodes grow in mass till the present time and are dominated by radial orbits. Conversely, haloes embedded in prominent filaments do not change much in size and are dominated by tangential orbits. Using zoom hydrodynamical simulations including star formation and feedback, we study how gas accretes on to these different classes of objects, which, for simplicity, we dub 'accreting' and 'stalled' haloes. We find that all haloes get a fresh supply of newly accreted gas in their inner regions, although this slowly decreases with time, in particular for the stalled haloes. The inflow of new gas is always higher than (but comparable with) that of recycled material. Overall, the cold-gas fraction increases (decreases) with time for the accreting (stalled) haloes. In all cases, a stellar disc and a bulge form at the centre of the simulated haloes. The total stellar mass is in excellent agreement with expectations based on the abundance-matching technique. Many properties of the central galaxies do not seem to correlate with the large-scale environment in which the haloes reside. However, there are two notable exceptions that characterize stalled haloes with respect to their accreting counterparts: (I) The galaxy disc contains much older stellar populations. (II) Its vertical scaleheight is larger by a factor of 2 or more. This thickening is likely due to the heating of the long-lived discs by mergers and close flybys.

  10. The vortex structure and flux creep within superconducting permanent-magnet high aspect-ratio discs

    International Nuclear Information System (INIS)

    Watson, J.H.P.; Younas, I.

    1997-01-01

    Inhomogeneous type II superconducting discs magnetized by an applied field will retain some magnetization when field is switched off so the superconducting disc will behave as a permanent magnet after flux creep has reduced to a low value.This paper examines the superconducting vortex structure within superconducting permanent-magnet high aspect-ratio discs which is consistent with the calculated magnetic field distribution.The discs, with radius R, have the axis along the z-direction and the mid-plane of the disc corresponds to z = 0. These discs with large aspect ratios in the remnant state have a region between radius r l and R where the magnetic field is reversed. Surrounding the line r = r l and z = 0 there is a region where H cl which is in the Meissner state. Near r l the vortex lines are strongly curved. For radii r l vortex lines creep to larger values of r. For radii r > r l vortex lines creep to smaller values of r, meet at r l with vortex lines of opposite sign and form a continuous loop which decreases in size and is finally annihilated in the Meissner region. Flux creep induces lossless currents in the Meissner region. (author)

  11. Low-density, radiatively inefficient rotating-accretion flow on to a black hole

    Science.gov (United States)

    Inayoshi, Kohei; Ostriker, Jeremiah P.; Haiman, Zoltán; Kuiper, Rolf

    2018-05-01

    We study low-density axisymmetric accretion flows on to black holes (BHs) with two-dimensional hydrodynamical simulations, adopting the α-viscosity prescription. When the gas angular momentum is low enough to form a rotationally supported disc within the Bondi radius (RB), we find a global steady accretion solution. The solution consists of a rotational equilibrium distribution around r ˜ RB, where the density follows ρ ∝ (1 + RB/r)3/2, surrounding a geometrically thick and optically thin accretion disc at the centrifugal radius RC(accretion flows (ρ ∝ r-1/2). In the inner solution, the gas inflow rate decreases towards the centre due to convection (\\dot{M}∝ r), and the net accretion rate (including both inflows and outflows) is strongly suppressed by several orders of magnitude from the Bondi accretion rate \\dot{M}_B. The net accretion rate depends on the viscous strength, following \\dot{M}/\\dot{M}_B∝ (α /0.01)^{0.6}. This solution holds for low accretion rates of \\dot{M}_B/\\dot{M}_Edd≲ 10^{-3} having minimal radiation cooling, where \\dot{M}_Edd is the Eddington accretion rate. In a hot plasma at the bottom (r < 10-3 RB), thermal conduction would dominate the convective energy flux. Since suppression of the accretion by convection ceases, the final BH feeding rate is found to be \\dot{M}/\\dot{M}_B˜ 10^{-3}-10-2. This rate is as low as \\dot{M}/\\dot{M}_Edd˜ 10^{-7}-10-6 inferred for SgrA* and the nuclear BHs in M31 and M87, and can explain their low luminosities, without invoking any feedback mechanism.

  12. Stunted accretion growth of black holes by combined effect of the flow angular momentum and radiation feedback

    Science.gov (United States)

    Sugimura, Kazuyuki; Hosokawa, Takashi; Yajima, Hidenobu; Inayoshi, Kohei; Omukai, Kazuyuki

    2018-05-01

    Accretion on to seed black holes (BHs) is believed to play a crucial role in formation of supermassive BHs observed at high-redshift (z > 6). Here, we investigate the combined effect of gas angular momentum and radiation feedback on the accretion flow, by performing 2D axially symmetric radiation hydrodynamics simulations that solve the flow structure across the Bondi radius and the outer part of the accretion disc simultaneously. The accreting gas with finite angular momentum forms a rotationally-supported disc inside the Bondi radius, where the accretion proceeds by the angular momentum transport due to assumed α-type viscosity. We find that the interplay of radiation and angular momentum significantly suppresses accretion even if the radiative feedback is weakened in an equatorial shadowing region. The accretion rate is O(α) ˜ O(0.01 - 0.1) times the Bondi value, where α is the viscosity parameter. By developing an analytical model, we show that such a great reduction of the accretion rate persists unless the angular momentum is so small that the corresponding centrifugal radius is ≲ 0.04 times the Bondi radius. We argue that BHs are hard to grow quickly via rapid mass accretion considering the angular momentum barrier presented in this paper.

  13. Optical effects related to Keplerian discs orbiting Kehagias-Sfetsos naked singularities

    Science.gov (United States)

    Stuchlík, Zdeněk; Schee, Jan

    2014-10-01

    We demonstrate possible optical signatures of the Kehagias-Sfetsos (KS) naked singularity spacetimes representing a spherically symmetric vacuum solution of the modified Hořava gravity. In such spacetimes, accretion structures significantly different from those present in standard black hole spacetimes occur due to the ‘antigravity’ effect, which causes an internal static sphere surrounded by Keplerian discs. We focus our attention on the optical effects related to the Keplerian accretion discs, constructing the optical appearance of the Keplerian discs, the spectral continuum due to their thermal radiation, and the spectral profiled lines generated in the innermost parts of such discs. The KS naked singularity signature is strongly encoded in the characteristics of predicted optical effects, especially in cases where the spectral continuum and spectral lines are profiled by the strong gravity of the spacetimes due to the vanishing region of the angular velocity gradient influencing the effectiveness of the viscosity mechanism. We can conclude that optical signatures of KS naked singularities can be well distinguished from the signatures of standard black holes.

  14. Optical effects related to Keplerian discs orbiting Kehagias–Sfetsos naked singularities

    International Nuclear Information System (INIS)

    Stuchlík, Zdeněk; Schee, Jan

    2014-01-01

    We demonstrate possible optical signatures of the Kehagias–Sfetsos (KS) naked singularity spacetimes representing a spherically symmetric vacuum solution of the modified Hořava gravity. In such spacetimes, accretion structures significantly different from those present in standard black hole spacetimes occur due to the ‘antigravity’ effect, which causes an internal static sphere surrounded by Keplerian discs. We focus our attention on the optical effects related to the Keplerian accretion discs, constructing the optical appearance of the Keplerian discs, the spectral continuum due to their thermal radiation, and the spectral profiled lines generated in the innermost parts of such discs. The KS naked singularity signature is strongly encoded in the characteristics of predicted optical effects, especially in cases where the spectral continuum and spectral lines are profiled by the strong gravity of the spacetimes due to the vanishing region of the angular velocity gradient influencing the effectiveness of the viscosity mechanism. We can conclude that optical signatures of KS naked singularities can be well distinguished from the signatures of standard black holes. (paper)

  15. Early Hydrodynamic Escape Limits Rocky Planets to Less Than or Equal to 1.6 Earth Radii

    Science.gov (United States)

    Lehmer, O. R.; Catling, D. C.

    2017-01-01

    In the past decade thousands of exoplanet candidates and hundreds of confirmed exoplanets have been found. For sub-Neptune-sized planets, those less than approx. 10 Earth masses, we can separate planets into two broad categories: predominantly rocky planets, and gaseous planets with thick volatile sheaths. Observations and subsequent analysis of these planets show that rocky planets are only found with radii less than approx. 1.6 Earth radii. No rocky planet has yet been found that violates this limit. We propose that hydrodynamic escape of hydrogen rich protoatmospheres, accreted by forming planets, explains the limit in rocky planet size. Following the hydrodynamic escape model employed by Luger et al. (2015), we modelled the XUV driven escape from young planets (less than approx.100 Myr in age) around a Sun-like star. With a simple, first-order model we found that the rocky planet radii limit occurs consistently at approx. 1.6 Earth radii across a wide range of plausible parameter spaces. Our model shows that hydrodynamic escape can explain the observed cutoff between rocky and gaseous planets. Fig. 1 shows the results of our model for rocky planets between 0.5 and 10 Earth masses that accrete 3 wt. % H2/He during formation. The simulation was run for 100 Myr, after that time the XUV flux drops off exponentially and hydrodynamic escape drops with it. A cutoff between rocky planets and gaseous ones is clearly seen at approx. 1.5-1.6 Earth radii. We are only interested in the upper size limit for rocky planets. As such, we assumed pure hydrogen atmospheres and the highest possible isothermal atmospheric temperatures, which will produce an upper limit on the hydrodynamic loss rate. Previous work shows that a reasonable approximation for an upper temperature limit in a hydrogen rich protoatmosphere is 2000-3000 K, consistent with our assumptions. From these results, we propose that the observed dichotomy between mini-Neptunes and rocky worlds is simply explained by

  16. Advection-dominated Inflow/Outflows from Evaporating Accretion Disks.

    Science.gov (United States)

    Turolla; Dullemond

    2000-03-01

    In this Letter we investigate the properties of advection-dominated accretion flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD). In our picture, the ADAF fills the central cavity evacuated by the SSD and extends beyond the transition radius into a coronal region. We find that, because of global angular momentum conservation, a significant fraction of the hot gas flows away from the black hole, forming a transsonic wind, unless the injection rate depends only weakly on radius (if r2sigma&d2;~r-xi, xiBernoulli number of the inflowing gas is negative if the transition radius is less, similar100 Schwarzschild radii, so matter falling into the hole is gravitationally bound. The ratio of inflowing to outflowing mass is approximately 1/2, so in these solutions the accretion rate is of the same order as in standard ADAFs and much larger than in advection-dominated inflow/outflow models. The possible relevance of evaporation-fed solutions to accretion flows in black hole X-ray binaries is briefly discussed.

  17. The diverse lives of massive protoplanets in self-gravitating discs

    Science.gov (United States)

    Stamatellos, Dimitris; Inutsuka, Shu-ichiro

    2018-04-01

    Gas giant planets may form early-on during the evolution of protostellar discs, while these are relatively massive. We study how Jupiter-mass planet-seeds (termed protoplanets) evolve in massive, but gravitationally stable (Q≳1.5), discs using radiative hydrodynamic simulations. We find that the protoplanet initially migrates inwards rapidly, until it opens up a gap in the disc. Thereafter, it either continues to migrate inwards on a much longer timescale or starts migrating outwards. Outward migration occurs when the protoplanet resides within a gap with gravitationally unstable edges, as a high fraction of the accreted gas is high angular momentum gas from outside the protoplanet's orbit. The effect of radiative heating from the protoplanet is critical in determining the direction of the migration and the eccentricity of the protoplanet. Gap opening is facilitated by efficient cooling that may not be captured by the commonly used β-cooling approximation. The protoplanet initially accretes at a high rate (˜10-3MJ yr-1), and its accretion luminosity could be a few tenths of the host star's luminosity, making the protoplanet easily observable (albeit only for a short time). Due to the high gas accretion rate, the protoplanet generally grows above the deuterium-burning mass-limit. Protoplanet radiative feedback reduces its mass growth so that its final mass is near the brown dwarf-planet boundary. The fate of a young planet-seed is diverse and could vary from a gas giant planet on a circular orbit at a few AU from the central star to a brown dwarf on an eccentric, wide orbit.

  18. Disk accretion onto magnetic T Tauri stars

    International Nuclear Information System (INIS)

    Koenigl, A.

    1991-01-01

    The dynamical and radiative consequences of disk accretion onto magnetic T Tauri stars (TTS) are examined using the Ghosh and Lamb model. It is shown that a prolonged disk accretion phase is compatible with the low rotation rates measured in these stars if they possess a kilogauss strength field that disrupts the disk at a distance of a few stellar radii from the center. It is estimated that a steady state in which the net torque exerted on the star is zero can be attained on a time scale that is shorter than the age of the youngest visible TTS. Although the disk does not develop an ordinary shear boundary layer in this case, one can account for the observed UV excess and Balmer emission in terms of the shocks that form at the bottom of the high-latitude magnetic accretion columns on the stellar surface. This picture also provides a natural explanation of some of the puzzling variability properties of stars like DF Tau and RY Lup. YY Ori stars are interpreted as magnetic TTS in which the observer's line of sight is roughly parallel to an accretion column. 37 refs

  19. Long term multiwavelength studies of the corona/disc connection in AGN

    Science.gov (United States)

    Buisson, D.; Lohfink, A.; Alston, W.; Fabian, A.; Gallo, L.; Kara, E.; Zoghbi, A.; Wilkins, D.; Miller, J.; Cackett, E.

    2017-10-01

    One way of increasing our understanding of AGN is determining the nature of the connection between the optical/UV emitting accretion disc and the X-ray emitting corona. Studies of variability in these two bands are a key tool for gaining insight into the processes involved. We will present results from a sample of long-term AGN monitoring campaigns in the optical, UV and X-ray with Swift. In particular, we will explore UV/optical-X-ray correlations and associated time lags. We will compare these measurements and the UV/optical RMS spectra with theoretical reprocessing models and confront recent claims of the observed lags being longer than those which are expected for a standard thin disc. Additionally, a new Swift monitoring campaign of the z=2 quasar PG 1247+267 allows us to probe the shorter wavelengths at the peak of the accretion disc spectrum, providing information on the region of the disc closest to the black hole. However, not all AGN show such correlations, including IRAS 13224-3809, the subject of a recent 1.5 Ms XMM observation. Using this and other examples, we will explore the possible reasons for the lack of observed correlation.

  20. Synchrotron radiation from spherically accreting black holes

    International Nuclear Information System (INIS)

    Ipser, J.R.; Price, R.H.

    1982-01-01

    Spherical accretion onto a Schwartzchild black hole, of gas with frozen-in magnetic field, is studied numerically and analytically for a range of hole masses and accretion rates in which synchrotron emission is the dominant radiative mechanism. At small radii the equipartition of magnetic, kinetic, and gravitational energy is assumed to apply, and the gas is heated by dissipation of infalling magnetic energy, turbulent energy, etc. The models can be classified into three types: (a) synchrotron cooling negligible, (b) synchrotron cooling important but synchrotron self-absorption negligible, (c) synchrotron cooling and self-absorption important. In the first case gas temperatures become very high near the horizon but luminosity efficiencies (luminosity/mass-energy accretion rate) are low. In cases (b) and (c) the gas flow near the horizon is essentially isothermal and luminosity efficiencies are fairly high. The analysis and results for the isothermal cases (b) and (c) are valid only for moderate dissipative heating and synchrotron self-absorption. If self-absorption is very strong or if dissipated energy is comparable to infall energy, Comptonization effects, not included in the analysis, become important

  1. Giant planet migration during FU Orionis outbursts: 1D disc models

    Science.gov (United States)

    Dunhill, A. C.

    2018-05-01

    I present the results of semi-analytic calculations of migrating planets in young, outbursting circumstellar discs. Formed far out in the disc via gravitational fragmentation early on in its lifetime, these planets typically migrate at very slow rates and are therefore mostly expected to remain at large radii (such as is the case in HR 8799). I show that changes in the disc structure during FUor outbursts affect the planet's ability to maintain a gap and can allow a massive giant planet's semimajor axis to reduce by almost 5 per cent in a single outburst under the most optimistic conditions. Given that a single disc will likely undergo ˜10 such outbursts this process can significantly alter the expected radial distribution for GI-formed planets.

  2. The Causal Connection Between Disc and Power-Law Variability in Hard State Black Hole X-Ray Binaries

    Science.gov (United States)

    Uttley, P.; Wilkinson, T.; Cassatella, P.; Wilms, J.; Pottschimdt, K.; Hanke, M.; Boeck, M.

    2010-01-01

    We use the XMM-Newton EPIC-pn instrument in timing mode to extend spectral time-lag studies of hard state black hole X-ray binaries into the soft X-ray band. \\Ve show that variations of the disc blackbody emission substantially lead variations in the power-law emission, by tenths of a second on variability time-scales of seconds or longer. The large lags cannot be explained by Compton scattering but are consistent with time-delays due to viscous propagation of mass accretion fluctuations in the disc. However, on time-scales less than a second the disc lags the power-law variations by a few ms, consistent with the disc variations being dominated by X-ray heating by the power-law, with the short lag corresponding to the light-travel time between the power-law emitting region and the disc. Our results indicate that instabilities in the accretion disc are responsible for continuum variability on time-scales of seconds or longer and probably also on shorter time-scales.

  3. Misaligned Accretion and Jet Production

    Science.gov (United States)

    King, Andrew; Nixon, Chris

    2018-04-01

    Disk accretion onto a black hole is often misaligned from its spin axis. If the disk maintains a significant magnetic field normal to its local plane, we show that dipole radiation from Lense–Thirring precessing disk annuli can extract a significant fraction of the accretion energy, sharply peaked toward small disk radii R (as R ‑17/2 for fields with constant equipartition ratio). This low-frequency emission is immediately absorbed by surrounding matter or refracted toward the regions of lowest density. The resultant mechanical pressure, dipole angular pattern, and much lower matter density toward the rotational poles create a strong tendency to drive jets along the black hole spin axis, similar to the spin-axis jets of radio pulsars, also strong dipole emitters. The coherent primary emission may explain the high brightness temperatures seen in jets. The intrinsic disk emission is modulated at Lense–Thirring frequencies near the inner edge, providing a physical mechanism for low-frequency quasi-periodic oscillations (QPOs). Dipole emission requires nonzero hole spin, but uses only disk accretion energy. No spin energy is extracted, unlike the Blandford–Znajek process. Magnetohydrodynamic/general-relativistic magnetohydrodynamic (MHD/GRMHD) formulations do not directly give radiation fields, but can be checked post-process for dipole emission and therefore self-consistency, given sufficient resolution. Jets driven by dipole radiation should be more common in active galactic nuclei (AGN) than in X-ray binaries, and in low accretion-rate states than high, agreeing with observation. In non-black hole accretion, misaligned disk annuli precess because of the accretor’s mass quadrupole moment, similarly producing jets and QPOs.

  4. Relativistic thick discs in the Kerr-de Sitter backgrounds

    International Nuclear Information System (INIS)

    Slany, Petr; StuchlIk, Zdenek

    2005-01-01

    Perfect fluid tori with a uniform distribution of the specific angular momentum, l(r, θ) = const, orbiting the Kerr-de Sitter black holes or naked singularities are studied. It is well known that the structure of equipotential surfaces of such marginally stable tori reflects the basic properties of any tori with a general distribution of the specific angular momentum. Closed equipotential surfaces corresponding to stationary thick discs are allowed only in the spacetimes admitting stable circular geodesics. The last closed surface crosses itself in the cusp(s) enabling the outflow of matter from the torus due to the violation of hydrostatic equilibrium. The inner cusp enables an accretion onto the central object. The influence of the repulsive cosmological constant, Λ > 0, on the equipotential surfaces lies in the existence of the outer cusp (with a stabilizing effect on the thick discs) and in the strong collimation of open equipotential surfaces along the rotational axis. Both the effects take place near a so-called static radius where the gravitational attraction is just balanced by the cosmic repulsion. The outer cusp enables excretion, i.e., the outflow of matter from the torus into the outer space. The plus-family discs (which are always co-rotating in the black-hole backgrounds but can be counter-rotating, even with negative energy of the fluid elements, in some naked-singularity backgrounds) are thicker and more extended than the minus-family ones (which are always counter-rotating in all backgrounds). For co-rotating discs in the naked-singularity spacetimes, the potential well between the centre of the disc and its edges at the cusps is usually much higher than in the black-hole spacetimes. If the parameters of naked-singularity spacetimes are very close to the parameters of extreme black-hole spacetimes, the family of possible disc-like configurations includes members with two isolated discs where the inner one is always a counter-rotating accretion

  5. The case for cases B and C: intrinsic hydrogen line ratios of the broad-line region of active galactic nuclei, reddenings, and accretion disc sizes

    Science.gov (United States)

    Gaskell, C. Martin

    2017-05-01

    Low-redshift active galactic nuclei (AGNs) with extremely blue optical spectral indices are shown to have a mean, velocity-averaged, broad-line Hα/Hβ ratio of ≈2.72 ± 0.04, consistent with a Baker-Menzel Case B value. Comparison of a wide range of properties of the very bluest AGNs with those of a luminosity-matched subset of the Dong et al. blue AGN sample indicates that the only difference is the internal reddening. Ultraviolet fluxes are brighter for the bluest AGNs by an amount consistent with the flat AGN reddening curve of Gaskell et al. The lack of a significant difference in the GALEX (far-ultraviolet-near-ultraviolet) colour index strongly rules out a steep Small Magellanic Cloud-like reddening curve and also argues against an intrinsically harder spectrum for the bluest AGNs. For very blue AGNs, the Ly α/Hβ ratio is also consistent with being the Case B value. The Case B ratios provide strong support for the self-shielded broad-line model of Gaskell, Klimek & Nazarova. It is proposed that the greatly enhanced Ly α/Hβ ratio at very high velocities is a consequence of continuum fluorescence in the Lyman lines (Case C). Reddenings of AGNs mean that the far-UV luminosity is often underestimated by up to an order of magnitude. This is a major factor causing the discrepancies between measured accretion disc sizes and the predictions of simple accretion disc theory. Dust covering fractions for most AGNs are lower than has been estimated. The total mass in lower mass supermassive black holes must be greater than hitherto estimated.

  6. Accretion flow diagnostics with X-ray spectral timing: the hard state of SWIFT J1753.5-0127

    NARCIS (Netherlands)

    Cassatella, P.; Uttley, P.; Maccarone, T.

    2012-01-01

    Recent XMM-Newton studies of X-ray variability in the hard states of black hole X-ray binaries (BHXRBs) indicate that the variability is generated in the ‘standard’ optically thick accretion disc that is responsible for the multi-colour blackbody emission. The variability originates in the disc as

  7. The lamppost model: effects of photon trapping, the bottom lamp and disc truncation

    Science.gov (United States)

    Niedźwiecki, Andrzej; Zdziarski, Andrzej A.

    2018-04-01

    We study the lamppost model, in which the primary X-ray sources in accreting black-hole systems are located symmetrically on the rotation axis on both sides of the black hole surrounded by an accretion disc. We show the importance of the emission of the source on the opposite side to the observer. Due to gravitational light bending, its emission can increase the direct (i.e., not re-emitted by the disc) flux by as much as an order of magnitude. This happens for near to face-on observers when the disc is even moderately truncated. For truncated discs, we also consider effects of emission of the top source gravitationally bent around the black hole. We also present results for the attenuation of the observed radiation with respect to that emitted by the lamppost as functions of the lamppost height, black-hole spin and the degree of disc truncation. This attenuation, which is due to the time dilation, gravitational redshift and the loss of photons crossing the black-hole horizon, can be as severe as by several orders of magnitude for low lamppost heights. We also consider the contribution to the observed flux due to re-emission by optically-thick matter within the innermost stable circular orbit.

  8. Accretion-induced variability links young stellar objects, white dwarfs, and black holes.

    Science.gov (United States)

    Scaringi, Simone; Maccarone, Thomas J; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R; Aranzana, Ester; Dhillon, Vikram S; Barros, Susana C C

    2015-10-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.

  9. Tracing Planets in Circumstellar Discs

    Directory of Open Access Journals (Sweden)

    Uribe Ana L.

    2013-04-01

    Full Text Available Planets are assumed to form in circumstellar discs around young stellar objects. The additional gravitational potential of a planet perturbs the disc and leads to characteristic structures, i.e. spiral waves and gaps, in the disc density profile. We perform a large-scale parameter study on the observability of these planet-induced structures in circumstellar discs in the (submm wavelength range for the Atacama Large (SubMillimeter Array (ALMA. On the basis of hydrodynamical and magneto-hydrodynamical simulations of star-disc-planet models we calculate the disc temperature structure and (submm images of these systems. These are used to derive simulated ALMA maps. Because appropriate objects are frequent in the Taurus-Auriga region, we focus on a distance of 140 pc and a declination of ≈ 20°. The explored range of star-disc-planet configurations consists of six hydrodynamical simulations (including magnetic fields and different planet masses, nine disc sizes with outer radii ranging from 9 AU to 225 AU, 15 total disc masses in the range between 2.67·10-7 M⊙ and 4.10·10-2 M⊙, six different central stars and two different grain size distributions, resulting in 10 000 disc models. At almost all scales and in particular down to a scale of a few AU, ALMA is able to trace disc structures induced by planet-disc interaction or the influence of magnetic fields in the wavelength range between 0.4...2.0 mm. In most cases, the optimum angular resolution is limited by the sensitivity of ALMA. However, within the range of typical masses of protoplane tary discs (0.1 M⊙...0.001 M⊙ the disc mass has a minor impact on the observability. At the distance of 140 pc it is possible to resolve discs down to 2.67·10-6 M⊙ and trace gaps in discs with 2.67·10-4 M⊙ with a signal-to-noise ratio greater than three. In general, it is more likely to trace planet-induced gaps in magneto-hydrodynamical disc models, because gaps are wider in the presence of

  10. The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence

    Science.gov (United States)

    Baglio, M. C.; Campana, S.; D'Avanzo, P.; Papitto, A.; Burderi, L.; Di Salvo, T.; Muñoz-Darias, T.; Rea, N.; Torres, D. F.

    2017-04-01

    We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4 m Gran Telescopio Canarias (GTC) in August 2014. Although the source was in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced in bluer filters (I.e. the g-band). After subtracting the flares, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even when a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars, was detected. We conclude that the observed flaring could be a manifestation of the presence of an accretion disc in the system. The observed light curve variability could be explained by the presence of a superhump, which might be another proof of the formation of an accretion disc. In particular, the disc at the time of our observations was probably preparing the new outburst of the source, which occurred a few months later, in 2015. Based on observations made with the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma.

  11. SUPERNOVA LIGHT CURVES POWERED BY FALLBACK ACCRETION

    Energy Technology Data Exchange (ETDEWEB)

    Dexter, Jason; Kasen, Daniel, E-mail: jdexter@berkeley.edu [Departments of Physics and Astronomy, University of California, Berkeley, CA 94720 (United States)

    2013-07-20

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time ({approx}>days) power potentially associated with the accretion of this 'fallback' material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as M-dot {proportional_to}t{sup -5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous ({approx}> 10{sup 44} erg s{sup -1}) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  12. Non-axisymmetric line-driven disc winds - I. Disc perturbations

    Science.gov (United States)

    Dyda, Sergei; Proga, Daniel

    2018-04-01

    We study mass outflows driven from accretion discs by radiation pressure due to spectral lines. To investigate non-axisymmetric effects, we use the ATHENA++ code and develop a new module to account for radiation pressure driving. In 2D, our new simulations are consistent with previous 2D axisymmetric solutions by Proga et al., who used the ZEUS 2D code. Specifically, we find that the disc winds are time dependent, characterized by a dense stream confined to ˜45° relative to the disc mid-plane and bounded on the polar side by a less dense, fast stream. In 3D, we introduce a vertical, ϕ-dependent, subsonic velocity perturbation in the disc mid-plane. The perturbation does not change the overall character of the solution but global outflow properties such as the mass, momentum, and kinetic energy fluxes are altered by up to 100 per cent. Non-axisymmetric density structures develop and persist mainly at the base of the wind. They are relatively small, and their densities can be a few times higher than the azimuthal average. The structure of the non-axisymmetric and axisymmetric solutions differ also in other ways. Perhaps most importantly from the observational point of view are the differences in the so-called clumping factors, that serve as a proxy for emissivity due to two body processes. In particular, the spatially averaged clumping factor over the entire fast stream, while it is of a comparable value in both solutions, it varies about 10 times faster in the non-axisymmetric case.

  13. Brightening of an accretion disk due to viscous dissipation of gravitational waves during the coalescence of supermassive black holes.

    Science.gov (United States)

    Kocsis, Bence; Loeb, Abraham

    2008-07-25

    Mergers of supermassive black hole binaries release peak power of up to approximately 10(57) erg s(-1) in gravitational waves (GWs). As the GWs propagate through ambient gas, they induce shear and a small fraction of their power is dissipated through viscosity. The dissipated heat appears as electromagnetic (EM) radiation, providing a prompt EM counterpart to the GW signal. For thin accretion disks, the GW heating rate exceeds the accretion power at distances farther than approximately 10(3) Schwarzschild radii, independently of the accretion rate and viscosity coefficient.

  14. ACCRETION SHOCKS IN CLUSTERS OF GALAXIES AND THEIR SZ SIGNATURE FROM COSMOLOGICAL SIMULATIONS

    International Nuclear Information System (INIS)

    Molnar, Sandor M.; Hearn, Nathan; Haiman, Zoltan; Bryan, Greg; Evrard, August E.; Lake, George

    2009-01-01

    Cold dark matter (CDM) hierarchical structure formation models predict the existence of large-scale accretion shocks between the virial and turnaround radii of clusters of galaxies. Kocsis et al. suggest that the Sunyaev-Zel'dovich signal associated with such shocks might be observable with the next generation radio interferometer, ALMA (Atacama Large Millimeter Array). We study the three-dimensional distribution of accretion shocks around individual clusters of galaxies drawn from adaptive mesh refinement (AMR) and smoothed particle hydrodynamics simulations of ΛCDM (dark energy dominated CDM) models. In relaxed clusters, we find two distinct sets of shocks. One set ('virial shocks'), with Mach numbers of 2.5-4, is located at radii 0.9-1.3 R vir , where R vir is the spherical infall estimate of the virial radius, covering about 40%-50% of the total surface area around clusters at these radii. Another set of stronger shocks ( e xternal shocks ) is located farther out, at about 3 R vir , with large Mach numbers (∼100), covering about 40%-60% of the surface area. We simulate SZ surface brightness maps of relaxed massive galaxy clusters drawn from high-resolution AMR runs, and conclude that ALMA should be capable of detecting the virial shocks in massive clusters of galaxies. More simulations are needed to improve estimates of astrophysical noise and to determine optimal observational strategies.

  15. The disc-jet coupling in the neutron star X-ray binary 4U 1728-34

    NARCIS (Netherlands)

    Tudose, Valeriu; Tzioumis, Anastasios; Belloni, Tomaso; Altamirano, Diego; Linares, Manuel; Mendez, Mariano; Hiemstra, Beike

    2010-01-01

    The present radio proposal is part of a multi-wavelength campaign focused on the study of the accretion/ejection process in the neutron star X-ray binary system 4U 1728-34. Our intention is to study the behaviour of the inner part of the accretion disc as inferred from the X-ray observations of the

  16. Shocks in the relativistic transonic accretion with low angular momentum

    Czech Academy of Sciences Publication Activity Database

    Suková, Petra; Charzynski, S.; Janiuk, A.

    2017-01-01

    Roč. 472, č. 4 (2017), s. 4327-4342 ISSN 0035-8711 R&D Projects: GA ČR(CZ) GJ17-06962Y Institutional support: RVO:67985815 Keywords : accretion discs * hydrodynamics * shock waves Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics OBOR OECD: Astronomy (including astrophysics,space science) Impact factor: 4.961, year: 2016

  17. The hydrodynamics of accretion discs. Pt. 1,2. Stability. Turbulent models

    Energy Technology Data Exchange (ETDEWEB)

    Stewart, J

    1975-08-01

    The disc is idealized to be a stationary axisymmetric toroidal flow of a compressible fluid. The stability against linearized short wavelength perturbations is discussed. When the Rayleigh and Schwarzschild criteria are satisfied, the flow is stable against axisymmetric perturbations. However, almost all non-axisymmetric perturbations are not secularly stable, and examples of dynamically unstable modes are given. For turbulent models, two new points are made. The crucial problem for the horizontal structure of the disc, the prescription of the Reynolds stress gradient, is resolved by a direct calculation from first principles. A preliminary attempt is also made to describe the vertical structure, leading to a sandwich model. The predictions of this theory are shown to be consistent with the fine scale structure of Cyg X-1. (DE)

  18. Suppression of atmospheric recycling of planets embedded in a protoplanetary disc by buoyancy barrier

    Science.gov (United States)

    Kurokawa, Hiroyuki; Tanigawa, Takayuki

    2018-06-01

    The ubiquity of super-Earths poses a problem for planet formation theory to explain how they avoided becoming gas giants. Rapid recycling of the envelope gas of planets embedded in a protoplanetary disc has been proposed to delay the cooling and following accretion of disc gas. We compare isothermal and non-isothermal 3D hydrodynamical simulations of the gas flow past a planet to investigate the influence on the feasibility of the recycling mechanism. Radiative cooling is implemented by using the β cooling model. We find that, in either case, gas enters the Bondi sphere at high latitudes and leaves through the midplane regions, or vice versa when disc gas rotates sub-Keplerian. However, in contrast to the isothermal case where the recycling flow reaches the deeper part of the envelope, the inflow is inhibited from reaching the deep envelope in the non-isothermal case. Once the atmosphere starts cooling, buoyant force prevents the high-entropy disc gas from intruding the low-entropy atmosphere. We suggest that the buoyancy barrier isolates the lower envelope from the recycling and allows further cooling, which may lead runaway gas accretion onto the core.

  19. Formation of S0s via disc accretion around high-redshift compact ellipticals

    Science.gov (United States)

    Diaz, Jonathan; Bekki, Kenji; Forbes, Duncan A.; Couch, Warrick J.; Drinkwater, Michael J.; Deeley, Simon

    2018-06-01

    We present hydrodynamical N-body models which demonstrate that elliptical galaxies can transform into S0s by acquiring a disc. In particular, we show that the merger with a massive gas-rich satellite can lead to the formation of a baryonic disc around an elliptical. We model the elliptical as a massive, compact galaxy which could be observed as a `red nugget' in the high-z universe. This scenario contrasts with existing S0 formation scenarios in the literature in two important ways. First, the progenitor is an elliptical galaxy whereas scenarios in the literature typically assume a spiral progenitor. Secondly, the physical conditions underlying our proposed scenario can exist in low-density environments such as the field, in contrast to scenarios in the literature which typically address dense environments like clusters and groups. As a consequence, S0s in the field may be the most likely candidates to have evolved from elliptical progenitors. Our scenario also naturally explains recent observations which indicate that field S0s may have older bulges than discs, contrary to cluster S0s which seem to have older discs than bulges.

  20. Numerical Simulations of Wind Accretion in Symbiotic Binaries

    Science.gov (United States)

    de Val-Borro, M.; Karovska, M.; Sasselov, D.

    2009-08-01

    About half of the binary systems are close enough to each other for mass to be exchanged between them at some point in their evolution, yet the accretion mechanism in wind accreting binaries is not well understood. We study the dynamical effects of gravitational focusing by a binary companion on winds from late-type stars. In particular, we investigate the mass transfer and formation of accretion disks around the secondary in detached systems consisting of an asymptotic giant branch (AGB) mass-losing star and an accreting companion. The presence of mass outflows is studied as a function of mass-loss rate, wind temperature, and binary orbital parameters. A two-dimensional hydrodynamical model is used to study the stability of mass transfer in wind accreting symbiotic binary systems. In our simulations we use an adiabatic equation of state and a modified version of the isothermal approximation, where the temperature depends on the distance from the mass losing star and its companion. The code uses a block-structured adaptive mesh refinement method that allows us to have high resolution at the position of the secondary and resolve the formation of bow shocks and accretion disks. We explore the accretion flow between the components and formation of accretion disks for a range of orbital separations and wind parameters. Our results show the formation of stream flow between the stars and accretion disks of various sizes for certain orbital configurations. For a typical slow and massive wind from an AGB star the flow pattern is similar to a Roche lobe overflow with accretion rates of 10% of the mass loss from the primary. Stable disks with exponentially decreasing density profiles and masses of the order 10-4 solar masses are formed when wind acceleration occurs at several stellar radii. The disks are geometrically thin with eccentric streamlines and close to Keplerian velocity profiles. The formation of tidal streams and accretion disks is found to be weakly dependent on

  1. NUMERICAL SIMULATIONS OF WIND ACCRETION IN SYMBIOTIC BINARIES

    International Nuclear Information System (INIS)

    De Val-Borro, M.; Karovska, M.; Sasselov, D.

    2009-01-01

    About half of the binary systems are close enough to each other for mass to be exchanged between them at some point in their evolution, yet the accretion mechanism in wind accreting binaries is not well understood. We study the dynamical effects of gravitational focusing by a binary companion on winds from late-type stars. In particular, we investigate the mass transfer and formation of accretion disks around the secondary in detached systems consisting of an asymptotic giant branch (AGB) mass-losing star and an accreting companion. The presence of mass outflows is studied as a function of mass-loss rate, wind temperature, and binary orbital parameters. A two-dimensional hydrodynamical model is used to study the stability of mass transfer in wind accreting symbiotic binary systems. In our simulations we use an adiabatic equation of state and a modified version of the isothermal approximation, where the temperature depends on the distance from the mass losing star and its companion. The code uses a block-structured adaptive mesh refinement method that allows us to have high resolution at the position of the secondary and resolve the formation of bow shocks and accretion disks. We explore the accretion flow between the components and formation of accretion disks for a range of orbital separations and wind parameters. Our results show the formation of stream flow between the stars and accretion disks of various sizes for certain orbital configurations. For a typical slow and massive wind from an AGB star the flow pattern is similar to a Roche lobe overflow with accretion rates of 10% of the mass loss from the primary. Stable disks with exponentially decreasing density profiles and masses of the order 10 -4 solar masses are formed when wind acceleration occurs at several stellar radii. The disks are geometrically thin with eccentric streamlines and close to Keplerian velocity profiles. The formation of tidal streams and accretion disks is found to be weakly dependent

  2. Decoupling of magnetic fields in collapsing protostellar envelopes and disc formation and fragmentation

    Science.gov (United States)

    Zhao, Bo; Caselli, Paola; Li, Zhi-Yun; Krasnopolsky, Ruben

    2018-02-01

    Efficient magnetic braking is a formidable obstacle to the formation of rotationally supported discs (RSDs) around protostars in magnetized dense cores. We have previously shown, through 2D (axisymmetric) non-ideal magnetohydrodynamic simulations, that removing very small grains (VSGs: ∼10 Å to few 100 Å) can greatly enhance ambipolar diffusion and enable the formation of RSDs. Here, we extend the simulations of disc formation enabled by VSG removal to 3D. We find that the key to this scenario of disc formation is that the drift velocity of the magnetic field almost cancels out the infall velocity of the neutrals in the 102-103 au scale 'pseudo-disc' where the field lines are most severely pinched and most of protostellar envelope mass infall occurs. As a result, the bulk neutral envelope matter can collapse without dragging much magnetic flux into the disc-forming region, which lowers the magnetic braking efficiency. We find that the initial discs enabled by VSG removal tend to be Toomre-unstable, which leads to the formation of prominent spiral structures that function as centrifugal barriers. The piling-up of infall material near the centrifugal barrier often produces dense fragments of tens of Jupiter masses, especially in cores that are not too strongly magnetized. Some fragments accrete on to the central stellar object, producing bursts in mass accretion rate. Others are longer lived, although whether they can survive for a long term to produce multiple systems remains to be ascertained. Our results highlight the importance of dust grain evolution in determining the formation and properties of protostellar discs and potentially multiple systems.

  3. The Physical Relation between Disc and Coronal Emission in Quasars

    Directory of Open Access Journals (Sweden)

    Elisabeta Lusso

    2018-01-01

    Full Text Available We propose a modified version of the observed non-linear relation between the X-ray (2 keV and the ultraviolet (2,500 Å emission in quasars (i.e., LX∝LUVγ which involves the full width at half-maximum, FWHM, of the broad emission line, i.e., LX∝LUVγ^ FWHMβ^. By analyzing a sample of 550 optically selected non-jetted quasars in the redshift range of 0.36–2.23 from the Sloan Digital Sky Survey cross matched with the XMM-Newton catalog 3XMM-DR6, we found that the additional dependence of the observed LX − LUV correlation on the FWHM of the Mgii broad emission line is statistically significant. Our statistical analysis leads to a much tighter relation with respect to the one neglecting FWHM, and it does not evolve with redshift. We interpret this new relation within an accretion disc corona scenario where reconnection and magnetic loops above the accretion disc can account for the production of the primary X-ray radiation. For a broad line region size depending on the disc luminosity as Rblr∝Ldisc0.5, we find that LX∝LUV4/7 FWHM4/7, which is in very good agreement with the observed correlation.

  4. Cepheid radii and effective temperatures

    International Nuclear Information System (INIS)

    Fernley, J.A.; Skillen, I.; Jameson, R.F.

    1989-01-01

    New infrared photometry for the Cepheid variables T Vul, δ Cephei and XCyg is presented. Combining this with published infrared photometry of T Vul, ηAql, S Sge and XCyg and published optical photometry we use the infrared flux method to determine effective temperatures and angular radii at all phases of the pulsation cycle. These angular radii combined with published radial velocity curves then give the radii of the stars. Knowing the radii and effective temperatures we obtain the absolute magnitudes. (author)

  5. Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes

    Science.gov (United States)

    T.Dauser; Garcia, J.; Wilms, J.; Boeck, M.; Brenneman, L. W.; Falanga, M.; Fukumura, Keigo; Reynolds, C. S.

    2013-01-01

    X-ray irradiation of the accretion disc leads to strong reflection features, which are then broadened and distorted by relativistic effects. We present a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source. These geometries include the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating. Incorporating this code in the RELLINE model for relativistic line emission, the line shape for any configuration can be predicted. We study how different irradiation geometries affect the determination of the spin of the black hole. Broad emission lines are produced only for compact irradiating sources situated close to the black hole. This is the only case where the black hole spin can be unambiguously determined. In all other cases the line shape is narrower, which could either be explained by a low spin or an elongated source. We conclude that for those cases and independent of the quality of the data, no unique solution for the spin exists and therefore only a lower limit of the spin value can be given

  6. Evolution of accretion disks in tidal disruption events

    Energy Technology Data Exchange (ETDEWEB)

    Shen, Rong-Feng [Current address: Racah Institute of Physics, Hebrew University of Jerusalem, Israel. (Israel); Matzner, Christopher D., E-mail: rf.shen@mail.huji.ac.il, E-mail: matzner@astro.utoronto.ca [Department of Astronomy and Astrophysics, University of Toronto, M5S 3H4 (Canada)

    2014-04-01

    During a stellar tidal disruption event (TDE), an accretion disk forms as stellar debris returns to the disruption site and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. A spreading disk is a source of matter for re-accretion at rates that may exceed the later stellar fallback rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history or, at longer wavelengths, by its own emission. We model the evolution of TDE disk size and accretion rate by accounting for the time-dependent fallback rate, for the influence of wind losses in the early advective stage, and for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure-dominated states. The model provides a dynamic basis for modeling TDE light curves. All or part of a young TDE disk will precess as a solid body because of the Lense-Thirring effect, and precession may manifest itself as a quasi-periodic modulation of the light curve. The precession period increases with time. Applying our results to the jetted TDE candidate Swift J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which a main-sequence star was fully disrupted by an intermediate mass black hole on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t = 500 days corresponding to a disk transition from the advective state to the gas-pressure-dominated state.

  7. The Smith Cloud: surviving a high-speed transit of the Galactic disc

    Science.gov (United States)

    Tepper-García, Thor; Bland-Hawthorn, Joss

    2018-02-01

    The origin and survival of the Smith high-velocity H I cloud has so far defied explanation. This object has several remarkable properties: (i) its prograde orbit is ≈100 km s-1 faster than the underlying Galactic rotation; (ii) its total gas mass (≳ 4 × 106 M⊙) exceeds the mass of all other high-velocity clouds (HVCs) outside of the Magellanic Stream; (iii) its head-tail morphology extends to the Galactic H I disc, indicating some sort of interaction. The Smith Cloud's kinetic energy rules out models based on ejection from the disc. We construct a dynamically self-consistent, multi-phase model of the Galaxy with a view to exploring whether the Smith Cloud can be understood in terms of an infalling, compact HVC that has transited the Galactic disc. We show that while a dark-matter (DM) free HVC of sufficient mass and density can reach the disc, it does not survive the transit. The most important ingredient to survival during a transit is a confining DM subhalo around the cloud; radiative gas cooling and high spatial resolution (≲ 10pc) are also essential. In our model, the cloud develops a head-tail morphology within ∼10 Myr before and after its first disc crossing; after the event, the tail is left behind and accretes on to the disc within ∼400 Myr. In our interpretation, the Smith Cloud corresponds to a gas 'streamer' that detaches, falls back and fades after the DM subhalo, distorted by the disc passage, has moved on. We conclude that subhaloes with MDM ≲ 109 M⊙ have accreted ∼109 M⊙ of gas into the Galaxy over cosmic time - a small fraction of the total baryon budget.

  8. The wind and the magnetospheric accretion onto the T Tauri star S Coronae Australis at sub-au resolution

    Science.gov (United States)

    Gravity Collaboration; Garcia Lopez, R.; Perraut, K.; Caratti O Garatti, A.; Lazareff, B.; Sanchez-Bermudez, J.; Benisty, M.; Dougados, C.; Labadie, L.; Brandner, W.; Garcia, P. J. V.; Henning, Th.; Ray, T. P.; Abuter, R.; Amorim, A.; Anugu, N.; Berger, J. P.; Bonnet, H.; Buron, A.; Caselli, P.; Clénet, Y.; Coudé Du Foresto, V.; de Wit, W.; Deen, C.; Delplancke-Ströbele, F.; Dexter, J.; Eckart, A.; Eisenhauer, F.; Garcia Dabo, C. E.; Gendron, E.; Genzel, R.; Gillessen, S.; Haubois, X.; Haug, M.; Haussmann, F.; Hippler, S.; Hubert, Z.; Hummel, C. A.; Horrobin, M.; Jocou, L.; Kellner, S.; Kervella, P.; Kulas, M.; Kolb, J.; Lacour, S.; Le Bouquin, J.-B.; Léna, P.; Lippa, M.; Mérand, A.; Müller, E.; Ott, T.; Panduro, J.; Paumard, T.; Perrin, G.; Pfuhl, O.; Ramirez, A.; Rau, C.; Rohloff, R.-R.; Rousset, G.; Scheithauer, S.; Schöller, M.; Straubmeier, C.; Sturm, E.; Thi, W. F.; van Dishoeck, E.; Vincent, F.; Waisberg, I.; Wank, I.; Wieprecht, E.; Wiest, M.; Wiezorrek, E.; Woillez, J.; Yazici, S.; Zins, G.

    2017-12-01

    Aims: To investigate the inner regions of protoplanetary discs, we performed near-infrared interferometric observations of the classical T Tauri binary system S CrA. Methods: We present the first VLTI-GRAVITY high spectral resolution (R 4000) observations of a classical T Tauri binary, S CrA (composed of S CrA N and S CrA S and separated by 1.̋4), combining the four 8m telescopes in dual-field mode. Results: Our observations in the near-infrared K-band continuum reveal a disc around each binary component, with similar half-flux radii of about 0.1 au at d 130 pc, inclinations (i = 28 ± 3° and i = 22 ± 6°), and position angles (PA = 0°± 6° and PA = –2°± 12°), suggesting that they formed from the fragmentation of a common disc. The S CrA N spectrum shows bright He I and Brγ line emission exhibiting inverse P Cygni profiles, typically associated with infalling gas. The continuum-compensated Brγ line visibilities of S CrA N show the presence of a compact Brγ emitting region whose radius is about 0.06 au, which is twice as big as the truncation radius. This component is mostly tracing a wind. Moreover, a slight radius change between the blue- and red-shifted Brγ line components is marginally detected. Conclusions: The presence of an inverse P Cygni profile in the He I and Brγ lines, along with the tentative detection of a slightly larger size of the blue-shifted Brγ line component, hint at the simultaneous presence of a wind and magnetospheric accretion in S CrA N.

  9. Relativistic jets from accreting black holes

    International Nuclear Information System (INIS)

    Coriat, Mickael

    2010-01-01

    Matter ejection processes, more commonly called jets, are among the most ubiquitous phenomena of the universe at ail scales of size and energy and are inseparable from accretion process. This intimate link, still poorly understood, is the main focus of this thesis. Through multi-wavelength observations of X-ray binary Systems hosting a black hole, I will try to bring new constraints on the physics of relativistic jets and the accretion - ejection coupling. We strive first to compare the simultaneous infrared, optical and X-ray emissions of the binary GX 339-4 over a period of five years. We study the nature of the central accretion flow, one of the least understood emission components of X-ray binaries, both in its geometry and in term of the physical processes that take place. This component is fundamental since it is could be the jets launching area or be highly connected to it. Then we focus on the infrared emission of the jets to investigate the physical conditions close to the jets base. We finally study the influence of irradiation of the outer accretion disc by the central X-ray source. Then, we present the results of a long-term radio and X-ray study of the micro-quasar H1743- 322. This System belongs to a population of accreting black holes that display, for a given X-ray luminosity, a radio emission fainter than expected. We make several assumptions about the physical origin of this phenomenon and show in particular that these sources could have a radiatively efficient central accretion flow. We finally explore the phases of return to the hard state of GX 339-4. We follow the re-emergence of the compact jets emission and try to bring new constraints on the physics of jet formation. (author) [fr

  10. Accretion-induced luminosity spreads in young clusters: evidence from stellar rotation

    Science.gov (United States)

    Littlefair, S. P.; Naylor, Tim; Mayne, N. J.; Saunders, Eric; Jeffries, R. D.

    2011-05-01

    We present an analysis of the rotation of young stars in the associations Cepheus OB3b, NGC 2264, 2362 and the Orion Nebula Cluster (ONC). We discover a correlation between rotation rate and position in a colour-magnitude diagram (CMD) such that stars which lie above an empirically determined median pre-main sequence rotate more rapidly than stars which lie below this sequence. The same correlation is seen, with a high degree of statistical significance, in each association studied here. If position within the CMD is interpreted as being due to genuine age spreads within a cluster, then the stars above the median pre-main sequence would be the youngest stars. This would in turn imply that the most rapidly rotating stars in an association are the youngest, and hence those with the largest moments of inertia and highest likelihood of ongoing accretion. Such a result does not fit naturally into the existing picture of angular momentum evolution in young stars, where the stars are braked effectively by their accretion discs until the disc disperses. Instead, we argue that, for a given association of young stars, position within the CMD is not primarily a function of age, but of accretion history. We show that this hypothesis could explain the correlation we observe between rotation rate and position within the CMD.

  11. Correlating non-linear properties with spectral states of RXTE data: possible observational evidences for four different accretion modes around compact objects

    Science.gov (United States)

    Adegoke, Oluwashina; Dhang, Prasun; Mukhopadhyay, Banibrata; Ramadevi, M. C.; Bhattacharya, Debbijoy

    2018-05-01

    By analysing the time series of RXTE/PCA data, the non-linear variabilities of compact sources have been repeatedly established. Depending on the variation in temporal classes, compact sources exhibit different non-linear features. Sometimes they show low correlation/fractal dimension, but in other classes or intervals of time they exhibit stochastic nature. This could be because the accretion flow around a compact object is a non-linear general relativistic system involving magnetohydrodynamics. However, the more conventional way of addressing a compact source is the analysis of its spectral state. Therefore, the question arises: What is the connection of non-linearity to the underlying spectral properties of the flow when the non-linear properties are related to the associated transport mechanisms describing the geometry of the flow? This work is aimed at addressing this question. Based on the connection between observed spectral and non-linear (time series) properties of two X-ray binaries: GRS 1915+105 and Sco X-1, we attempt to diagnose the underlying accretion modes of the sources in terms of known accretion classes, namely, Keplerian disc, slim disc, advection dominated accretion flow and general advective accretion flow. We explore the possible transition of the sources from one accretion mode to others with time. We further argue that the accretion rate must play an important role in transition between these modes.

  12. Spiral density waves and vertical circulation in protoplanetary discs

    Science.gov (United States)

    Riols, A.; Latter, H.

    2018-06-01

    Spiral density waves dominate several facets of accretion disc dynamics - planet-disc interactions and gravitational instability (GI) most prominently. Though they have been examined thoroughly in two-dimensional simulations, their vertical structures in the non-linear regime are somewhat unexplored. This neglect is unwarranted given that any strong vertical motions associated with these waves could profoundly impact dust dynamics, dust sedimentation, planet formation, and the emissivity of the disc surface. In this paper, we combine linear calculations and shearing box simulations in order to investigate the vertical structure of spiral waves for various polytropic stratifications and wave amplitudes. For sub-adiabatic profiles, we find that spiral waves develop a pair of counter-rotating poloidal rolls. Particularly strong in the non-linear regime, these vortical structures issue from the baroclinicity supported by the background vertical entropy gradient. They are also intimately connected to the disc's g modes which appear to interact non-linearly with the density waves. Furthermore, we demonstrate that the poloidal rolls are ubiquitous in gravitoturbulence, emerging in the vicinity of GI spiral wakes, and potentially transporting grains off the disc mid-plane. Other than hindering sedimentation and planet formation, this phenomena may bear on observations of the disc's scattered infrared luminosity. The vortical features could also impact on the turbulent dynamo operating in young protoplanetary discs subject to GI, or possibly even galactic discs.

  13. Shocks in the relativistic transonic accretion with low angular momentum

    Science.gov (United States)

    Suková, P.; Charzyński, S.; Janiuk, A.

    2017-12-01

    We perform 1D/2D/3D relativistic hydrodynamical simulations of accretion flows with low angular momentum, filling the gap between spherically symmetric Bondi accretion and disc-like accretion flows. Scenarios with different directional distributions of angular momentum of falling matter and varying values of key parameters such as spin of central black hole, energy and angular momentum of matter are considered. In some of the scenarios the shock front is formed. We identify ranges of parameters for which the shock after formation moves towards or outwards the central black hole or the long-lasting oscillating shock is observed. The frequencies of oscillations of shock positions which can cause flaring in mass accretion rate are extracted. The results are scalable with mass of central black hole and can be compared to the quasi-periodic oscillations of selected microquasars (such as GRS 1915+105, XTE J1550-564 or IGR J17091-3624), as well as to the supermassive black holes in the centres of weakly active galaxies, such as Sgr A*.

  14. Chasing discs around O-type (proto)stars: Evidence from ALMA observations

    Science.gov (United States)

    Cesaroni, R.; Sánchez-Monge, Á.; Beltrán, M. T.; Johnston, K. G.; Maud, L. T.; Moscadelli, L.; Mottram, J. C.; Ahmadi, A.; Allen, V.; Beuther, H.; Csengeri, T.; Etoka, S.; Fuller, G. A.; Galli, D.; Galván-Madrid, R.; Goddi, C.; Henning, T.; Hoare, M. G.; Klaassen, P. D.; Kuiper, R.; Kumar, M. S. N.; Lumsden, S.; Peters, T.; Rivilla, V. M.; Schilke, P.; Testi, L.; van der Tak, F.; Vig, S.; Walmsley, C. M.; Zinnecker, H.

    2017-06-01

    Context. Circumstellar discs around massive stars could mediate the accretion onto the star from the infalling envelope, and could minimize the effects of radiation pressure. Despite such a crucial role, only a few convincing candidates have been provided for discs around deeply embedded O-type (proto)stars. Aims: In order to establish whether disc-mediated accretion is the formation mechanism for the most massive stars, we have searched for circumstellar, rotating discs around a limited sample of six luminous (>105L⊙) young stellar objects. These objects were selected on the basis of their IR and radio properties in order to maximize the likelihood of association with disc+jet systems. Methods: We used ALMA with 0.̋2 resolution to observe a large number of molecular lines typical of hot molecular cores. In this paper we limit our analysis to two disc tracers (methyl cyanide, CH3CN, and its isotopologue, 13CH3CN), and an outflow tracer (silicon monoxide, SiO). Results: We reveal many cores, although their number depends dramatically on the target. We focus on the cores that present prominent molecular line emission. In six of these a velocity gradient is seen across the core,three of which show evidence of Keplerian-like rotation. The SiO data reveal clear but poorly collimated bipolar outflow signatures towards two objects only. This can be explained if real jets are rare (perhaps short-lived) in very massive objects and/or if stellar multiplicity significantly affects the outflow structure.For all cores with velocity gradients, the velocity field is analysed through position-velocity plots to establish whether the gas is undergoing rotation with νrot ∝ R- α, as expected for Keplerian-like discs. Conclusions: Our results suggest that in three objects we are observing rotation in circumstellar discs, with three more tentative cases, and one core where no evidence for rotation is found. In all cases but one, we find that the gas mass is less than the mass of

  15. Mergers and mass accretion for infalling halos both end well outside cluster virial radii

    International Nuclear Information System (INIS)

    Behroozi, Peter S.; Wechsler, Risa H.; Lu, Yu; Busha, Michael T.; Hahn, Oliver; Klypin, Anatoly; Primack, Joel R.

    2014-01-01

    We find that infalling dark matter halos (i.e., the progenitors of satellite halos) begin losing mass well outside the virial radius of their eventual host halos. The peak mass occurs at a range of clustercentric distances, with median and 68th percentile range of 1.8 −1.0 +2.3 R vir,host for progenitors of z = 0 satellites. The peak circular velocity for infalling halos occurs at significantly larger distances (3.7 −2.2 +3.3 R vir,host at z = 0). This difference arises because different physical processes set peak circular velocity (typically, ∼1:5 and larger mergers which cause transient circular velocity spikes) and peak mass (typically, smooth accretion) for infalling halos. We find that infalling halos also stop having significant mergers well before they enter the virial radius of their eventual hosts. Mergers larger than a 1:40 ratio in halo mass end for infalling halos at similar clustercentric distances (∼1.9 R vir, host ) as the end of overall mass accretion. However, mergers larger than 1:3 typically end for infalling halos at more than four virial radial away from their eventual hosts. This limits the ability of mergers to affect quenching and morphology changes in clusters. We also note that the transient spikes which set peak circular velocity may lead to issues with abundance matching on that parameter, including unphysical galaxy stellar mass growth profiles near clusters; we propose a simple observational test to check if a better halo proxy for galaxy stellar mass exists.

  16. Mergers and mass accretion for infalling halos both end well outside cluster virial radii

    Energy Technology Data Exchange (ETDEWEB)

    Behroozi, Peter S.; Wechsler, Risa H.; Lu, Yu; Hahn, Oliver; Busha, Michael T.; Klypin, Anatoly; Primack, Joel R.

    2014-05-14

    We find that infalling dark matter halos (i.e., the progenitors of satellite halos) begin losing mass well outside the virial radius of their eventual host halos. The peak mass occurs at a range of clustercentric distances, with median and 68th percentile range of $1.8^{+2.3}_{-1.0} \\,R_\\mathrm{vir,host}$ for progenitors of z = 0 satellites. The peak circular velocity for infalling halos occurs at significantly larger distances ($3.7^{+3.3}_{-2.2} \\,R_\\mathrm{vir,host}$ at z = 0). This difference arises because different physical processes set peak circular velocity (typically, ~1:5 and larger mergers which cause transient circular velocity spikes) and peak mass (typically, smooth accretion) for infalling halos. We find that infalling halos also stop having significant mergers well before they enter the virial radius of their eventual hosts. Mergers larger than a 1:40 ratio in halo mass end for infalling halos at similar clustercentric distances (~1.9 R vir, host) as the end of overall mass accretion. However, mergers larger than 1:3 typically end for infalling halos at more than four virial radial away from their eventual hosts. This limits the ability of mergers to affect quenching and morphology changes in clusters. We also note that the transient spikes which set peak circular velocity may lead to issues with abundance matching on that parameter, including unphysical galaxy stellar mass growth profiles near clusters; we propose a simple observational test to check if a better halo proxy for galaxy stellar mass exists.

  17. Swings between rotation and accretion power in a binary millisecond pulsar.

    Science.gov (United States)

    Papitto, A; Ferrigno, C; Bozzo, E; Rea, N; Pavan, L; Burderi, L; Burgay, M; Campana, S; Di Salvo, T; Falanga, M; Filipović, M D; Freire, P C C; Hessels, J W T; Possenti, A; Ransom, S M; Riggio, A; Romano, P; Sarkissian, J M; Stairs, I H; Stella, L; Torres, D F; Wieringa, M H; Wong, G F

    2013-09-26

    It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star's rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales.

  18. Photometric Evidence for a Disc-Jet Connection in CH Cygni

    Science.gov (United States)

    Sokoloski, J. L.; Kenyon, S. J.

    2001-12-01

    We describe observations of the rapid optical variations of the symbiotic star CH Cygni on 12 nights between 1997 and 1999. The B-band differential light curves reveal an incredible diversity of flickering behavior, from large-amplitude (up to 0.5 mag) variations with a power-law power spectrum, to lower amplitude (0.1 mag) variations with both power-law and non-power-law power spectra, to the complete absence of rapid variations down to a level of mmag. The series of light curves from observations in 1997/1998 exhibit an evolution from smooth, low-amplitude variations, to high-amplitude flickering with power at all measurable time scales. This evolution may be showing us the re-creation of the inner accretion disc after its disruption in association with the jet that was produced in early 1997 (Karovska et al. 1998). We do not find any evidence for quasi-periodic oscillations in the power spectra of individual light curves, and we believe the instances in which flickering completely disappeared coincide with eclipse of the white dwarf and accretion disc. We discuss the implications of our results for magnetic propeller models of this system, as well as compare CH Cygni to other systems where disc-jet connections have been proposed, such as the Galactic microquasar GRS 1915+105. This work was funded in part by NSF grant INT-9902665 to J.L.S.

  19. CIRCUMBINARY MAGNETOHYDRODYNAMIC ACCRETION INTO INSPIRALING BINARY BLACK HOLES

    Energy Technology Data Exchange (ETDEWEB)

    Noble, Scott C.; Mundim, Bruno C.; Nakano, Hiroyuki; Campanelli, Manuela; Zlochower, Yosef [Center for Computational Relativity and Gravitation, Rochester Institute of Technology, Rochester, NY 14623 (United States); Krolik, Julian H. [Physics and Astronomy Department, Johns Hopkins University, Baltimore, MD 21218 (United States); Yunes, Nicolas, E-mail: scn@astro.rit.edu [Department of Physics, Montana State University, Bozeman, MT 59717 (United States)

    2012-08-10

    We have simulated the magnetohydrodynamic evolution of a circumbinary disk surrounding an equal-mass binary comprising two non-spinning black holes during the period in which the disk inflow time is comparable to the binary evolution time due to gravitational radiation. Both the changing spacetime and the binary orbital evolution are described by an innovative technique utilizing high-order post-Newtonian approximations. Prior to the beginning of the inspiral, the structure of the circumbinary disk is predicted well by extrapolation from Newtonian results: a gap of roughly two binary separation radii is cleared, and matter piles up at the outer edge of this gap as inflow is retarded by torques exerted by the binary; the accretion rate is roughly half its value at large radius. During inspiral, the inner edge of the disk initially moves inward in coordination with the shrinking binary, but-as the orbital evolution accelerates-the inward motion of the disk edge falls behind the rate of binary compression. In this stage, the binary torque falls substantially, but the accretion rate decreases by only 10%-20%. When the binary separation is tens of gravitational radii, the rest-mass efficiency of disk radiation is a few percent, suggesting that supermassive binary black holes could be very luminous at this stage of their evolution. Inner disk heating is modulated at a beat frequency comparable to the binary orbital frequency. However, a disk with sufficient surface density to be luminous may be optically thick, suppressing periodic modulation of the luminosity.

  20. Black hole spin dependence of general relativistic multi-transonic accretion close to the horizon

    Czech Academy of Sciences Publication Activity Database

    Das, T. K.; Nag, S.; Hedge, S.; Bhattacharya, S.; Maity, I.; Czerny, B.; Barai, P.; Wiita, P. J.; Karas, Vladimír; Naskar, T.

    2015-01-01

    Roč. 37, May (2015), s. 81-104 ISSN 1384-1076 R&D Projects: GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : black holes * accretion discs Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 1.085, year: 2015

  1. Dust discs around low-mass main-sequence stars

    International Nuclear Information System (INIS)

    Wolstencroft, R.D.; Walker, H.J.

    1988-01-01

    Current understanding of the formation of circumstellar discs as a natural accompaniment to the process of low-mass star formation is briefly reviewed. Models of the thermal emission from the dust discs around the prototype stars α Lyr, α PsA, β Pic and ε Eri are discussed, which indicate that the central regions of three of these discs are almost devoid of dust within radii ranging between 17 and 26 AU, with the temperature of the hottest dust lying between about 115 and 210 K. One possible explanation of the dust-free zones is the presence of a planet at the inner boundary of each cloud that sweeps up grains crossing its orbit. The colour, diameter and thickness of the optical image of β Pic, obtained by coronagraphic techniques, have provided further information on the size, radial distribution of number density and orbital inclination of the grains. The difference in surface brightness on the two sides of the disc is puzzling, but might be explained if the grains are elongated and aligned by the combined effects of a stellar wind and a magnetic field of spiral configuration. Finally, we discuss the orbital evolution and lifetimes of particles in these discs, which are governed primarily by radiation pressure, Poynting-Robertson drag and grain-grain collisions. (author)

  2. Mergers and mass accretion for infalling halos both end well outside cluster virial radii

    Energy Technology Data Exchange (ETDEWEB)

    Behroozi, Peter S. [Space Telescope Science Institute, Baltimore, MD 21218 (United States); Wechsler, Risa H.; Lu, Yu; Busha, Michael T. [Physics Department, Stanford University, Department of Particle and Particle Astrophysics, SLAC National Accelerator Laboratory, Kavli Institute for Particle Astrophysics and Cosmology Stanford, CA 94305 (United States); Hahn, Oliver [Institute for Astronomy, ETH Zurich, 8093-CH Zurich (Switzerland); Klypin, Anatoly [Astronomy Department, New Mexico State University, Las Cruces, NM 88003 (United States); Primack, Joel R., E-mail: behroozi@stsci.edu [Department of Physics, University of California at Santa Cruz, Santa Cruz, CA 95064 (United States)

    2014-06-01

    We find that infalling dark matter halos (i.e., the progenitors of satellite halos) begin losing mass well outside the virial radius of their eventual host halos. The peak mass occurs at a range of clustercentric distances, with median and 68th percentile range of 1.8{sub −1.0}{sup +2.3} R{sub vir,host} for progenitors of z = 0 satellites. The peak circular velocity for infalling halos occurs at significantly larger distances (3.7{sub −2.2}{sup +3.3} R{sub vir,host} at z = 0). This difference arises because different physical processes set peak circular velocity (typically, ∼1:5 and larger mergers which cause transient circular velocity spikes) and peak mass (typically, smooth accretion) for infalling halos. We find that infalling halos also stop having significant mergers well before they enter the virial radius of their eventual hosts. Mergers larger than a 1:40 ratio in halo mass end for infalling halos at similar clustercentric distances (∼1.9 R {sub vir,} {sub host}) as the end of overall mass accretion. However, mergers larger than 1:3 typically end for infalling halos at more than four virial radial away from their eventual hosts. This limits the ability of mergers to affect quenching and morphology changes in clusters. We also note that the transient spikes which set peak circular velocity may lead to issues with abundance matching on that parameter, including unphysical galaxy stellar mass growth profiles near clusters; we propose a simple observational test to check if a better halo proxy for galaxy stellar mass exists.

  3. The Ongoing Growth of the M87 Halo through Accretion Events

    Directory of Open Access Journals (Sweden)

    Alessia Longobardi

    2015-12-01

    Full Text Available Planetary nebulas (PNs offer a unique tool to investigate the outer regions of massive galaxies because their strong [OIII]λ5007Å emission line makes them detectable out to several effective radii from the galaxy’s centre. We use a deep and extended spectroscopic survey of PNs (∼300 objects to study the spatial distribution, the kinematics and the stellar populations in the extended outer halo of the bright elliptical galaxy M87 (NGC 4486 in the Virgo cluster. We show that in the Virgo core, M87 stellar halo and the intracluster light are two distinct dynamical components, with different velocity distributions. Moreover the synergy of the PN kinematical information and the deep V/B-band photometry revealed an ongoing accretion event in the outer regions of M87. This satellite accretion represents a non-negligible perturbation of the halo properties: beyond 60 kpc the M87 halo is still growing with 60% of its light being added by the accretion event at the distance where it is detected.

  4. Parsec-Scale Accretion and Winds Irradiated by a Quasar

    Science.gov (United States)

    Dorodnitsyn, A.; Kallman, T.; Proga, D.

    2016-01-01

    We present numerical simulations of properties of a parsec-scale torus exposed to illumination by the central black hole in an active galactic nucleus (AGN). Our physical model allows to investigate the balance between the formation of winds and accretion simultaneously. Radiation-driven winds are allowed by taking into account radiation pressure due to UV and IR radiation along with X-ray heating and dust sublimation. Accretion is allowed through angular momentum transport and the solution of the equations of radiative, viscous radiation hydrodynamics. Our methods adopt flux-limited diffusion radiation hydrodynamics for the dusty, infrared pressure driven part of the flow, along with X-ray heating and cooling. Angular momentum transport in the accreting part of the flow is modeled using effective viscosity. Our results demonstrate that radiation pressure on dust can play an important role in shaping AGN obscuration. For example, when the luminosity illuminating the torus exceeds L greater than 0.01 L(sub Edd), where L(sub Edd) is the Eddington luminosity, we find no episodes of sustained disk accretion because radiation pressure does not allow a disk to form. Despite the absence of the disk accretion, the flow of gas to smaller radii still proceeds at a rate 10(exp -4)-10(exp -1)M dot yr(exp -1) through the capturing of the gas from the hot evaporative flow, thus providing a mechanism to deliver gas from a radiation-pressure dominated torus to the inner accretion disk. As L L(sub edd) increases, larger radiation input leads to larger torus aspect ratios and increased obscuration of the central black hole. We also find the important role of the X-ray heated gas in shaping the obscuring torus.

  5. MHD simulations of ram pressure stripping of a disc galaxy

    Science.gov (United States)

    Ramos-Martínez, Mariana; Gómez, Gilberto C.; Pérez-Villegas, Ángeles

    2018-05-01

    The removal of the interstellar medium (ISM) of disc galaxies through ram pressure stripping (RPS) has been extensively studied in numerous simulations. Nevertheless, the role of magnetic fields (MFs) on the gas dynamics in this process has been hardly studied, although the MF influence on the large-scale disc structure is well established. With this in mind, we present a 3D magnetohydrodynamic simulation of face-on RPS of a disc galaxy to study the impact of the galactic MF in the gas stripping. The main effect of including a galactic MF is a flared disc. When the intracluster medium wind hits this flared disc, oblique shocks are produced at the interaction interface, where the ISM is compressed, generating a gas inflow from large radii towards the central regions of the galaxy. This inflow is observed for {˜ } 150 {Myr} and may supply the central parts of the galaxy with material for star formation while the outskirts of the disc are being stripped of gas, thus the oblique shocks can induce and enhance the star formation in the remaining disc. We also observed that the MF alters the shape and structure of the swept gas, giving a smooth appearance in the magnetized case and clumpier and filamentary-like morphology in the hydro case. Finally, we estimated the truncation radius expected for our models using the Gunn-Gott criterion and found that that is in agreement with the simulations.

  6. Radii of radioactive nuclei

    International Nuclear Information System (INIS)

    Mittig, W.; Plagnol, E.; Schutz, Y.

    1989-11-01

    A new simple direct method for the measurement of the total reaction cross section (σ R ) for several light radioactive nuclei (A≤40) is developed. From that, the reduced strong absorption radii (r o 2 ) are obtained. A comparison is made with data obtained by other techniques. A strong isospin dependence of the nuclear radii is observed. (L.C.) [pt

  7. Confrontation of the Magnetically Arrested Disc Scenario with Observations of FR II Sources

    Energy Technology Data Exchange (ETDEWEB)

    Rusinek, Katarzyna; Sikora, Marek, E-mail: krusinek@camk.edu.pl [Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warsaw (Poland)

    2017-10-12

    The main aim of our work was to check whether powers of jets in FR II radio galaxies (RGs) and quasars (QSOs) can be reproduced by the Magnetically Arrested Disc (MAD) scenario. Assuming that established in the recent numerical simulations of the MAD scenario the (H/R){sup 2} dependence of the jet production efficiency is correct, we demonstrate that in order to reproduce the observed jet powers in FR II sources: (i) accretion discs must be geometrically much thicker than the standard ones; (ii) and/or that the jet production is strongly modulated.

  8. Bowen emission from Aquila X-1: evidence for multiple components and constraint on the accretion disc vertical structure

    Science.gov (United States)

    Jiménez-Ibarra, F.; Muñoz-Darias, T.; Wang, L.; Casares, J.; Mata Sánchez, D.; Steeghs, D.; Armas Padilla, M.; Charles, P. A.

    2018-03-01

    We present a detailed spectroscopic study of the optical counterpart of the neutron star X-ray transient Aquila X-1 during its 2011, 2013 and 2016 outbursts. We use 65 intermediate resolution GTC-10.4 m spectra with the aim of detecting irradiation-induced Bowen blend emission from the donor star. While Gaussian fitting does not yield conclusive results, our full phase coverage allows us to exploit Doppler mapping techniques to independently constrain the donor star radial velocity. By using the component N III 4640.64/4641.84 Å, we measure Kem = 102 ± 6 km s-1. This highly significant detection (≳13σ) is fully compatible with the true companion star radial velocity obtained from near-infrared spectroscopy during quiescence. Combining these two velocities we determine, for the first time, the accretion disc opening angle and its associated error from direct spectroscopic measurements and detailed modelling, obtaining α = 15.5 ^{+ 2.5}_{-5} deg. This value is consistent with theoretical work if significant X-ray irradiation is taken into account and is important in the light of recent observations of GX339-4, where discrepant results were obtained between the donor's intrinsic radial velocity and the Bowen-inferred value. We also discuss the limitations of the Bowen technique when complete phase coverage is not available.

  9. Accretion and ejection in resistive GR-MHD

    Energy Technology Data Exchange (ETDEWEB)

    Qian, Qian

    2017-05-10

    In this thesis, the accretion and ejection processes from a black hole accretion system is investigated by means of resistive general relativistic magnetohydrodynamic simulations. As a supplement to the results from prior research with non-relativistic simulations, my results confirm that the winds and outflows originated from thin accretion disks can also be observed in general relativistic simulations. In the first part, the execution of the implementation of resistivity, namely magnetic diffusivity, into the existing non-resistive general relativistic magnetohydrodynamic code HARM is illustrated. The test simulations of the new code rHARM include the comparison with analytical solution of the diffusion equation and a classic shock tube test. rHARM shows reliable performances in these tests. In the second part, rHARM is applied to investigate the evolution of magnetized tori. The results show that the existence of resistivity leads to inefficient accretions of matter from tori onto black holes by weakening the magnetorotational instability inside the tori. An indication for a critical magnetic diffusivity in this simulation setup is found beyond which no magnetorotational instability develops in the linear regime. In the third part, as the main purpose of this PhD project, rHARM is used to perform simulations of magnetically diffusive thin accretion disks that are threaded by a large-scale poloidal magnetic field around non-rotating and rotating black holes. These long-term simulations last 3000 code time units, which are about 195 rotation periods at the disk inner boundary, correspondingly. Their computational domains extend from black hole horizon to 80 Schwarzschild radii. Outflows driven from the accretion disk are clearly seen. These outflows have the typical radial velocity of 0.1 speed of light. In my analyses, I argue that these outflows are driven by the magnetic pressure gradient from the toroidal magnetic field generated by the rotation of the disk

  10. Radii of nuclei off stability

    International Nuclear Information System (INIS)

    Sugimoto, Kenzo

    1982-01-01

    An experiment is proposed to determine systematically the radii of the nuclei produced through the projectile fragmentation process in high energy heavy-ion collision. The measurement of total reaction cross section using the projectile fragments of a single nuclide on a target give information about nuclear radii. The production cross section of the fragments is appreciable for many nuclides. Therefore, it is possible to map systematically the reaction radii of the nuclei which can be produced as the projectile fragments. In an experiment using the projectile fragments as the incident beam, the cross section can be expressed as a function of the radii of a projectile and a target. An experiment with He-8 produced by the fragmentation of C-12 is proposed. The He-8 has four neutrons in the p-3/2 orbit outside the He-4 core. Proton and neutron distributions for He isotopes were calculated on the basis of the Hartree-Fock method. The information related to this kind of distribution can be obtained by the proposed experiment. The nuclear structure effect is seen in the nuclear radii of other unstable nuclei. The experimental examples of the isotope shift measurement and the excitation energy are presented. (Kato, T.)

  11. Chaotic cold accretion on to black holes in rotating atmospheres

    Science.gov (United States)

    Gaspari, M.; Brighenti, F.; Temi, P.

    2015-07-01

    The fueling of black holes is one key problem in the evolution of baryons in the universe. Chaotic cold accretion (CCA) profoundly differs from classic accretion models, as Bondi and thin disc theories. Using 3D high-resolution hydrodynamic simulations, we now probe the impact of rotation on the hot and cold accretion flow in a typical massive galaxy. In the hot mode, with or without turbulence, the pressure-dominated flow forms a geometrically thick rotational barrier, suppressing the black hole accretion rate to ~1/3 of the spherical case value. When radiative cooling is dominant, the gas loses pressure support and quickly circularizes in a cold thin disk; the accretion rate is decoupled from the cooling rate, although it is higher than that of the hot mode. In the more common state of a turbulent and heated atmosphere, CCA drives the dynamics if the gas velocity dispersion exceeds the rotational velocity, i.e., turbulent Taylor number Tat 1), the broadening of the distribution and the efficiency of collisions diminish, damping the accretion rate ∝ Tat-1, until the cold disk drives the dynamics. This is exacerbated by the increased difficulty to grow TI in a rotating halo. The simulated sub-Eddington accretion rates cover the range inferred from AGN cavity observations. CCA predicts inner flat X-ray temperature and r-1 density profiles, as recently discovered in M 87 and NGC 3115. The synthetic Hα images reproduce the main features of cold gas observations in massive ellipticals, as the line fluxes and the filaments versus disk morphology. Such dichotomy is key for the long-term AGN feedback cycle. As gas cools, filamentary CCA develops and boosts AGN heating; the cold mode is thus reduced and the rotating disk remains the sole cold structure. Its consumption leaves the atmosphere in hot mode with suppressed accretion and feedback, reloading the cycle.

  12. Two-fluid dusty shocks: simple benchmarking problems and applications to protoplanetary discs

    Science.gov (United States)

    Lehmann, Andrew; Wardle, Mark

    2018-05-01

    The key role that dust plays in the interstellar medium has motivated the development of numerical codes designed to study the coupled evolution of dust and gas in systems such as turbulent molecular clouds and protoplanetary discs. Drift between dust and gas has proven to be important as well as numerically challenging. We provide simple benchmarking problems for dusty gas codes by numerically solving the two-fluid dust-gas equations for steady, plane-parallel shock waves. The two distinct shock solutions to these equations allow a numerical code to test different forms of drag between the two fluids, the strength of that drag and the dust to gas ratio. We also provide an astrophysical application of J-type dust-gas shocks to studying the structure of accretion shocks on to protoplanetary discs. We find that two-fluid effects are most important for grains larger than 1 μm, and that the peak dust temperature within an accretion shock provides a signature of the dust-to-gas ratio of the infalling material.

  13. Schwarzschild black hole encircled by a rotating thin disc: Properties of perturbative solution

    Science.gov (United States)

    Kotlařík, P.; Semerák, O.; Čížek, P.

    2018-04-01

    Will [Astrophys. J. 191, 521 (1974), 10.1086/152992] solved the perturbation of a Schwarzschild black hole due to a slowly rotating light concentric thin ring, using Green's functions expressed as infinite-sum expansions in multipoles and in the small mass and rotational parameters. In a previous paper [P. Čížek and O. Semerák, Astrophys. J. Suppl. Ser. 232, 14 (2017), 10.3847/1538-4365/aa876b], we expressed the Green functions in closed form containing elliptic integrals, leaving just summation over the mass expansion. Such a form is more practical for numerical evaluation, but mainly for generalizing the problem to extended sources where the Green functions have to be integrated over the source. We exemplified the method by computing explicitly the first-order perturbation due to a slowly rotating thin disc lying between two finite radii. After finding basic parameters of the system—mass and angular momentum of the black hole and of the disc—we now add further properties, namely those which reveal how the disc gravity influences geometry of the black-hole horizon and those of circular equatorial geodesics (specifically, radii of the photon, marginally bound and marginally stable orbits). We also realize that, in the linear order, no ergosphere occurs and the central singularity remains pointlike, and check the implications of natural physical requirements (energy conditions and subluminal restriction on orbital speed) for the single-stream as well as counter-rotating double-stream interpretations of the disc.

  14. A tool to separate optical/infrared disc and jet emission in X-ray transient outbursts: the colour-magnitude diagrams of XTE J1550-564

    NARCIS (Netherlands)

    Russell, D.M.; Maitra, D.; Dunn, R.J.H.; Fender, R.P.

    2011-01-01

    It is now established that thermal disc emission and non-thermal jet emission can both play a role at optical/infrared (OIR) wavelengths in X-ray transients. The spectra of the jet and disc components differ, as do their dependence on mass accretion properties. Here we demonstrate that the OIR

  15. The late inspiral of supermassive black hole binaries with circumbinary gas discs in the LISA band

    Science.gov (United States)

    Tang, Yike; Haiman, Zoltán; MacFadyen, Andrew

    2018-05-01

    We present the results of 2D, moving-mesh, viscous hydrodynamical simulations of an accretion disc around a merging supermassive black hole binary (SMBHB). The simulation is pseudo-Newtonian, with the BHs modelled as point masses with a Paczynski-Wiita potential, and includes viscous heating, shock heating, and radiative cooling. We follow the gravitational inspiral of an equal-mass binary with a component mass Mbh = 106 M⊙ from an initial separation of 60rg (where rg ≡ GMbh/c2 is the gravitational radius) to the merger. We find that a central, low-density cavity forms around the binary, as in previous work, but that the BHs capture gas from the circumbinary disc and accrete efficiently via their own minidiscs, well after their inspiral outpaces the viscous evolution of the disc. The system remains luminous, displaying strong periodicity at twice the binary orbital frequency throughout the entire inspiral process, all the way to the merger. In the soft X-ray band, the thermal emission is dominated by the inner edge of the circumbinary disc with especially clear periodicity in the early inspiral. By comparison, harder X-ray emission is dominated by the minidiscs, and the light curve is initially more noisy but develops a clear periodicity in the late inspiral stage. This variability pattern should help identify the electromagnetic counterparts of SMBHBs detected by the space-based gravitational-wave detector LISA.

  16. Origin of chemically distinct discs in the Auriga cosmological simulations

    Science.gov (United States)

    Grand, Robert J. J.; Bustamante, Sebastián; Gómez, Facundo A.; Kawata, Daisuke; Marinacci, Federico; Pakmor, Rüdiger; Rix, Hans-Walter; Simpson, Christine M.; Sparre, Martin; Springel, Volker

    2018-03-01

    The stellar disc of the Milky Way shows complex spatial and abundance structure that is central to understanding the key physical mechanisms responsible for shaping our Galaxy. In this study, we use six very high resolution cosmological zoom-in simulations of Milky Way-sized haloes to study the prevalence and formation of chemically distinct disc components. We find that our simulations develop a clearly bimodal distribution in the [α/Fe]-[Fe/H] plane. We find two main pathways to creating this dichotomy, which operate in different regions of the galaxies: (a) an early (z > 1) and intense high-[α/Fe] star formation phase in the inner region (R ≲ 5 kpc) induced by gas-rich mergers, followed by more quiescent low-[α/Fe] star formation; and (b) an early phase of high-[α/Fe] star formation in the outer disc followed by a shrinking of the gas disc owing to a temporarily lowered gas accretion rate, after which disc growth resumes. In process (b), a double-peaked star formation history around the time and radius of disc shrinking accentuates the dichotomy. If the early star formation phase is prolonged (rather than short and intense), chemical evolution proceeds as per process (a) in the inner region, but the dichotomy is less clear. In the outer region, the dichotomy is only evident if the first intense phase of star formation covers a large enough radial range before disc shrinking occurs; otherwise, the outer disc consists of only low-[α/Fe] sequence stars. We discuss the implication that both processes occurred in the Milky Way.

  17. Massive star formation by accretion. II. Rotation: how to circumvent the angular momentum barrier?

    Science.gov (United States)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.; Klessen, R. S.

    2017-06-01

    Context. Rotation plays a key role in the star-formation process, from pre-stellar cores to pre-main-sequence (PMS) objects. Understanding the formation of massive stars requires taking into account the accretion of angular momentum during their PMS phase. Aims: We study the PMS evolution of objects destined to become massive stars by accretion, focusing on the links between the physical conditions of the environment and the rotational properties of young stars. In particular, we look at the physical conditions that allow the production of massive stars by accretion. Methods: We present PMS models computed with a new version of the Geneva Stellar Evolution code self-consistently including accretion and rotation according to various accretion scenarios for mass and angular momentum. We describe the internal distribution of angular momentum in PMS stars accreting at high rates and we show how the various physical conditions impact their internal structures, evolutionary tracks, and rotation velocities during the PMS and the early main sequence. Results: We find that the smooth angular momentum accretion considered in previous studies leads to an angular momentum barrier and does not allow the formation of massive stars by accretion. A braking mechanism is needed in order to circumvent this angular momentum barrier. This mechanism has to be efficient enough to remove more than two thirds of the angular momentum from the inner accretion disc. Due to the weak efficiency of angular momentum transport by shear instability and meridional circulation during the accretion phase, the internal rotation profiles of accreting stars reflect essentially the angular momentum accretion history. As a consequence, careful choice of the angular momentum accretion history allows circumvention of any limitation in mass and velocity, and production of stars of any mass and velocity compatible with structure equations.

  18. SIZES AND TEMPERATURE PROFILES OF QUASAR ACCRETION DISKS FROM CHROMATIC MICROLENSING

    International Nuclear Information System (INIS)

    Blackburne, Jeffrey A.; Pooley, David; Rappaport, Saul; Schechter, Paul L.

    2011-01-01

    Microlensing perturbations to the flux ratios of gravitationally lensed quasar images can vary with wavelength because of the chromatic dependence of the accretion disk's apparent size. Multiwavelength observations of microlensed quasars can thus constrain the temperature profiles of their accretion disks, a fundamental test of an important astrophysical process which is not currently possible using any other method. We present single-epoch broadband flux ratios for 12 quadruply lensed quasars in 8 bands ranging from 0.36 to 2.2 μm, as well as Chandra 0.5-8 keV flux ratios for five of them. We combine the optical/IR and X-ray ratios, together with X-ray ratios from the literature, using a Bayesian approach to constrain the half-light radii of the quasars in each filter. Comparing the overall disk sizes and wavelength slopes to those predicted by the standard thin accretion disk model, we find that on average the disks are larger than predicted by nearly an order of magnitude, with sizes that grow with wavelength with an average slope of ∼0.2 rather than the slope of 4/3 predicted by the standard thin disk theory. Though the error bars on the slope are large for individual quasars, the large sample size lends weight to the overall result. Our results present severe difficulties for a standard thin accretion disk as the main source of UV/optical radiation from quasars.

  19. The response of relativistic outflowing gas to the inner accretion disk of a black hole.

    Science.gov (United States)

    Parker, Michael L; Pinto, Ciro; Fabian, Andrew C; Lohfink, Anne; Buisson, Douglas J K; Alston, William N; Kara, Erin; Cackett, Edward M; Chiang, Chia-Ying; Dauser, Thomas; De Marco, Barbara; Gallo, Luigi C; Garcia, Javier; Harrison, Fiona A; King, Ashley L; Middleton, Matthew J; Miller, Jon M; Miniutti, Giovanni; Reynolds, Christopher S; Uttley, Phil; Vasudevan, Ranjan; Walton, Dominic J; Wilkins, Daniel R; Zoghbi, Abderahmen

    2017-03-01

    The brightness of an active galactic nucleus is set by the gas falling onto it from the galaxy, and the gas infall rate is regulated by the brightness of the active galactic nucleus; this feedback loop is the process by which supermassive black holes in the centres of galaxies may moderate the growth of their hosts. Gas outflows (in the form of disk winds) release huge quantities of energy into the interstellar medium, potentially clearing the surrounding gas. The most extreme (in terms of speed and energy) of these-the ultrafast outflows-are the subset of X-ray-detected outflows with velocities higher than 10,000 kilometres per second, believed to originate in relativistic (that is, near the speed of light) disk winds a few hundred gravitational radii from the black hole. The absorption features produced by these outflows are variable, but no clear link has been found between the behaviour of the X-ray continuum and the velocity or optical depth of the outflows, owing to the long timescales of quasar variability. Here we report the observation of multiple absorption lines from an extreme ultrafast gas flow in the X-ray spectrum of the active galactic nucleus IRAS 13224-3809, at 0.236 ± 0.006 times the speed of light (71,000 kilometres per second), where the absorption is strongly anti-correlated with the emission of X-rays from the inner regions of the accretion disk. If the gas flow is identified as a genuine outflow then it is in the fastest five per cent of such winds, and its variability is hundreds of times faster than in other variable winds, allowing us to observe in hours what would take months in a quasar. We find X-ray spectral signatures of the wind simultaneously in both low- and high-energy detectors, suggesting a single ionized outflow, linking the low- and high-energy absorption lines. That this disk wind is responding to the emission from the inner accretion disk demonstrates a connection between accretion processes occurring on very different

  20. MAGNETOROTATIONAL-INSTABILITY-DRIVEN ACCRETION IN PROTOPLANETARY DISKS

    International Nuclear Information System (INIS)

    Bai Xuening

    2011-01-01

    Non-ideal MHD effects play an important role in the gas dynamics in protoplanetary disks (PPDs). This paper addresses the influence of non-ideal MHD effects on the magnetorotational instability (MRI) and angular momentum transport in PPDs using the most up-to-date results from numerical simulations. We perform chemistry calculations using a complex reaction network with standard prescriptions for X-ray and cosmic-ray ionizations. We first show that whether or not grains are included, the recombination time is at least one order of magnitude less than the orbital time within five disk scale heights, justifying the validity of local ionization equilibrium and strong coupling limit in PPDs. The full conductivity tensor at different disk radii and heights is evaluated, with the MRI active region determined by requiring that (1) the Ohmic Elsasser number Λ be greater than 1 and (2) the ratio of gas to magnetic pressure β be greater than β min (Am) as identified in the recent study by Bai and Stone, where Am is the Elsasser number for ambipolar diffusion. With full flexibility as to the magnetic field strength, we provide a general framework for estimating the MRI-driven accretion rate M-dot and the magnetic field strength in the MRI active layer. We find that the MRI active layer always exists at any disk radius as long as the magnetic field in PPDs is sufficiently weak. However, the optimistically predicted M-dot in the inner disk (r = 1-10 AU) appears insufficient to account for the observed range of accretion rates in PPDs (around 10 -8 M sun yr -1 ) even in the grain-free calculation, and the presence of solar abundance sub-micron grains further reduces M-dot by one to two orders of magnitude. Moreover, we find that the predicted M-dot increases with radius in the inner disk where accretion is layered, which would lead to runaway mass accumulation if disk accretion is solely driven by the MRI. Our results suggest that stronger sources of ionization and

  1. Theoretical Calculation of Absolute Radii of Atoms and Ions. Part 1. The Atomic Radii

    Directory of Open Access Journals (Sweden)

    Raka Biswas

    2002-02-01

    Full Text Available Abstract. A set of theoretical atomic radii corresponding to the principal maximum in the radial distribution function, 4πr2R2 for the outermost orbital has been calculated for the ground state of 103 elements of the periodic table using Slater orbitals. The set of theoretical radii are found to reproduce the periodic law and the Lother Meyer’s atomic volume curve and reproduce the expected vertical and horizontal trend of variation in atomic size in the periodic table. The d-block and f-block contractions are distinct in the calculated sizes. The computed sizes qualitatively correlate with the absolute size dependent properties like ionization potentials and electronegativity of elements. The radii are used to calculate a number of size dependent periodic physical properties of isolated atoms viz., the diamagnetic part of the atomic susceptibility, atomic polarizability and the chemical hardness. The calculated global hardness and atomic polarizability of a number of atoms are found to be close to the available experimental values and the profiles of the physical properties computed in terms of the theoretical atomic radii exhibit their inherent periodicity. A simple method of computing the absolute size of atoms has been explored and a large body of known material has been brought together to reveal how many different properties correlate with atomic size.

  2. Radiative, two-temperature simulations of low-luminosity black hole accretion flows in general relativity

    Science.gov (United States)

    Sądowski, Aleksander; Wielgus, Maciek; Narayan, Ramesh; Abarca, David; McKinney, Jonathan C.; Chael, Andrew

    2017-04-01

    We present a numerical method that evolves a two-temperature, magnetized, radiative, accretion flow around a black hole, within the framework of general relativistic radiation magnetohydrodynamics. As implemented in the code KORAL, the gas consists of two sub-components - ions and electrons - which share the same dynamics but experience independent, relativistically consistent, thermodynamical evolution. The electrons and ions are heated independently according to a prescription from the literature for magnetohydrodynamical turbulent dissipation. Energy exchange between the particle species via Coulomb collisions is included. In addition, electrons gain and lose energy and momentum by absorbing and emitting synchrotron and bremsstrahlung radiation and through Compton scattering. All evolution equations are handled within a fully covariant framework in the relativistic fixed-metric space-time of the black hole. Numerical results are presented for five models of low-luminosity black hole accretion. In the case of a model with a mass accretion rate dot{M}˜ 4× 10^{-8} dot{M}_Edd, we find that radiation has a negligible effect on either the dynamics or the thermodynamics of the accreting gas. In contrast, a model with a larger dot{M}˜ 4× 10^{-4} dot{M}_Edd behaves very differently. The accreting gas is much cooler and the flow is geometrically less thick, though it is not quite a thin accretion disc.

  3. Formation of massive seed black holes via collisions and accretion

    Science.gov (United States)

    Boekholt, T. C. N.; Schleicher, D. R. G.; Fellhauer, M.; Klessen, R. S.; Reinoso, B.; Stutz, A. M.; Haemmerlé, L.

    2018-05-01

    Models aiming to explain the formation of massive black hole seeds, and in particular the direct collapse scenario, face substantial difficulties. These are rooted in rather ad hoc and fine-tuned initial conditions, such as the simultaneous requirements of extremely low metallicities and strong radiation backgrounds. Here, we explore a modification of such scenarios where a massive primordial star cluster is initially produced. Subsequent stellar collisions give rise to the formation of massive (104-105 M⊙) objects. Our calculations demonstrate that the interplay among stellar dynamics, gas accretion, and protostellar evolution is particularly relevant. Gas accretion on to the protostars enhances their radii, resulting in an enhanced collisional cross-section. We show that the fraction of collisions can increase from 0.1 to 1 per cent of the initial population to about 10 per cent when compared to gas-free models or models of protostellar clusters in the local Universe. We conclude that very massive objects can form in spite of initial fragmentation, making the first massive protostellar clusters viable candidate birth places for observed supermassive black holes.

  4. Local models of astrophysical discs

    Science.gov (United States)

    Latter, Henrik N.; Papaloizou, John

    2017-12-01

    Local models of gaseous accretion discs have been successfully employed for decades to describe an assortment of small-scale phenomena, from instabilities and turbulence, to dust dynamics and planet formation. For the most part, they have been derived in a physically motivated but essentially ad hoc fashion, with some of the mathematical assumptions never made explicit nor checked for consistency. This approach is susceptible to error, and it is easy to derive local models that support spurious instabilities or fail to conserve key quantities. In this paper we present rigorous derivations, based on an asympototic ordering, and formulate a hierarchy of local models (incompressible, Boussinesq and compressible), making clear which is best suited for a particular flow or phenomenon, while spelling out explicitly the assumptions and approximations of each. We also discuss the merits of the anelastic approximation, emphasizing that anelastic systems struggle to conserve energy unless strong restrictions are imposed on the flow. The problems encountered by the anelastic approximation are exacerbated by the disc's differential rotation, but also attend non-rotating systems such as stellar interiors. We conclude with a defence of local models and their continued utility in astrophysical research.

  5. The impact of feedback and the hot halo on the rates of gas accretion onto galaxies

    Science.gov (United States)

    Correa, Camila A.; Schaye, Joop; van de Voort, Freeke; Duffy, Alan R.; Wyithe, J. Stuart B.

    2018-04-01

    We investigate the physics that drives the gas accretion rates onto galaxies at the centers of dark matter haloes using the EAGLE suite of hydrodynamical cosmological simulations. We find that at redshifts z ≤ 2 the accretion rate onto the galaxy increases with halo mass in the halo mass range 1010 - 1011.7 M⊙, flattens between the halo masses 1011.7 - 1012.7 M⊙, and increases again for higher-mass haloes. However, the galaxy gas accretion does not flatten at intermediate halo masses when AGN feedback is switched off. To better understand these trends, we develop a physically motivated semi-analytic model of galaxy gas accretion. We show that the flattening is produced by the rate of gas cooling from the hot halo. The ratio of the cooling radius and the virial radius does not decrease continuously with increasing halo mass as generally thought. While it decreases up to ˜1013 M⊙ haloes, it increases for higher halo masses, causing an upturn in the galaxy gas accretion rate. This may indicate that in high-mass haloes AGN feedback is not sufficiently efficient. When there is no AGN feedback, the density of the hot halo is higher, the ratio of the cooling and virial radii does not decrease as much and the cooling rate is higher. Changes in the efficiency of stellar feedback can also increase or decrease the accretion rates onto galaxies. The trends can plausibly be explained by the re-accretion of gas ejected by progenitor galaxies and by the suppression of black hole growth, and hence AGN feedback, by stellar feedback.

  6. Isothermal Bondi Accretion in Jaffe and Hernquist Galaxies with a Central Black Hole: Fully Analytical Solutions

    Energy Technology Data Exchange (ETDEWEB)

    Ciotti, Luca; Pellegrini, Silvia, E-mail: luca.ciotti@unibo.it [Department of Physics and Astronomy, University of Bologna, via Piero Gobetti 93/2, I-40129 Bologna (Italy)

    2017-10-10

    One of the most active fields of research of modern-day astrophysics is that of massive black hole formation and coevolution with the host galaxy. In these investigations, ranging from cosmological simulations, to semi-analytical modeling, to observational studies, the Bondi solution for accretion on a central point-mass is widely adopted. In this work we generalize the classical Bondi accretion theory to take into account the effects of the gravitational potential of the host galaxy, and of radiation pressure in the optically thin limit. Then, we present the fully analytical solution, in terms of the Lambert–Euler W -function, for isothermal accretion in Jaffe and Hernquist galaxies with a central black hole. The flow structure is found to be sensitive to the shape of the mass profile of the host galaxy. These results and the formulae that are provided, most importantly, the one for the critical accretion parameter, allow for a direct evaluation of all flow properties, and are then useful for the abovementioned studies. As an application, we examine the departure from the true mass accretion rate of estimates obtained using the gas properties at various distances from the black hole, under the hypothesis of classical Bondi accretion. An overestimate is obtained from regions close to the black hole, and an underestimate outside a few Bondi radii; the exact position of the transition between the two kinds of departure depends on the galaxy model.

  7. New Insights on the Accretion Disk-Winds Connection in Radio-Loud AGNs from Suzaku

    Science.gov (United States)

    Tombesi, F.; Sambruna, R. M.; Reeves, J. N.; Braito, V.; Cappi, M.; Reynolds, S.; Mushotzky, R. F.

    2011-01-01

    From the spectral analysis of long Suzaku observations of five radio-loud AGNs we have been able to discover the presence of ultra-fast outflows with velocities ,,approx.0.1 c in three of them, namely 3C III, 3C 120 and 3C 390.3. They are consistent with being accretion disk winds/outflows. We also performed a follow-up on 3C III to monitor its outflow on approx.7 days time-scales and detected an anti-correlated variability of a possible relativistic emission line with respect to blue-shifted Fe K features, following a flux increase. This provides the first direct evidence for an accretion disc-wind connection in an AGN. The mass outflow rate of these outflows can be comparable to the accretion rate and their mechanical power can correspond to a significant fraction of the bolometric luminosity and is comparable to their typical jet power. Therefore, they can possibly play a significant role in the expected feedback from AGNs and can give us further clues on the relation between the accretion disk and the formation of winds/jets.

  8. The Accretion Disk Wind in the Black Hole GRS 1915 + 105

    Science.gov (United States)

    Miller, J.M.; Raymond, J.; Fabian, A. C.; Gallo, E.; Kaastra, J.; Kallman, T.; King, A. L.; Proga, D.; Reynolds, C. S.; Zoghbi, A.

    2016-01-01

    We report on a 120 kiloseconds Chandra/HETG spectrum of the black hole GRS 1915+105. The observation was made during an extended and bright soft state in 2015 June. An extremely rich disk wind absorption spectrum is detected, similar to that observed at lower sensitivity in 2007. The very high resolution of the third-order spectrum reveals four components to the disk wind in the Fe K band alone; the fastest has a blueshift of v = 0.03 c (velocity equals 0.03 the speed of light). Broadened reemission from the wind is also detected in the first-order spectrum, giving rise to clear accretion disk P Cygni profiles. Dynamical modeling of the re-emission spectrum gives wind launching radii of r approximately equal to 10 (sup 2-4) GM (Gravitational constant times Mass) divided by c (sup 2) (the speed of light squared). Wind density values of n approximately equal to 10 (sup 13-16) per cubic centimeter are then required by the ionization parameter formalism. The small launching radii, high density values, and inferred high mass outflow rates signal a role for magnetic driving. With simple, reasonable assumptions, the wind properties constrain the magnitude of the emergent magnetic field to be B approximately equal to 10 (sup 3-4) G (Gravitational constant) if the wind is driven via magnetohydrodynamic (MHD) pressure from within the disk and B approximately equal to 10 (sup 4-5) G (Gravitational constant) if the wind is driven by magnetocentrifugal acceleration. The MHD estimates are below upper limits predicted by the canonical alpha-disk model. We discuss these results in terms of fundamental disk physics and black hole accretion modes.

  9. SURFACE LAYER ACCRETION IN TRANSITIONAL AND CONVENTIONAL DISKS: FROM POLYCYCLIC AROMATIC HYDROCARBONS TO PLANETS

    International Nuclear Information System (INIS)

    Perez-Becker, Daniel; Chiang, Eugene

    2011-01-01

    'Transitional' T Tauri disks have optically thin holes with radii ∼>10 AU, yet accrete up to the median T Tauri rate. Multiple planets inside the hole can torque the gas to high radial speeds over large distances, reducing the local surface density while maintaining accretion. Thus multi-planet systems, together with reductions in disk opacity due to grain growth, can explain how holes can be simultaneously transparent and accreting. There remains the problem of how outer disk gas diffuses into the hole. Here it has been proposed that the magnetorotational instability (MRI) erodes disk surface layers ionized by stellar X-rays. In contrast to previous work, we find that the extent to which surface layers are MRI-active is limited not by ohmic dissipation but by ambipolar diffusion, the latter measured by Am: the number of times a neutral hydrogen molecule collides with ions in a dynamical time. Simulations by Hawley and Stone showed that Am ∼ 100 is necessary for ions to drive MRI turbulence in neutral gas. We calculate that in X-ray-irradiated surface layers, Am typically varies from ∼10 -3 to 1, depending on the abundance of charge-adsorbing polycyclic aromatic hydrocarbons, whose properties we infer from Spitzer observations. We conclude that ionization of H 2 by X-rays and cosmic rays can sustain, at most, only weak MRI turbulence in surface layers 1-10 g cm -2 thick, and that accretion rates in such layers are too small compared to observed accretion rates for the majority of disks.

  10. First results from the use of the relativistic and slim disc model SLIMULX in XSPEC

    Czech Academy of Sciences Publication Activity Database

    Caballero-García, María Dolores; Bursa, Michal; Dovčiak, Michal; Fabrika, S.; Castro-Tirado, A.J.; Karas, Vladimír

    2017-01-01

    Roč. 47, č. 2 (2017), s. 84-93 ISSN 1335-1842 R&D Projects: GA MŠk(CZ) 7E13012 EU Projects: European Commission(XE) 312789 - STRONGGRAVITY Institutional support: RVO:67985815 Keywords : accretion-discs * black hole physics * relativistic processes Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics OBOR OECD: Astronomy (including astrophysics,space science) Impact factor: 0.336, year: 2016

  11. The role of zonal flows in disc gravito-turbulence

    Science.gov (United States)

    Vanon, R.

    2018-04-01

    The work presented here focuses on the role of zonal flows in the self-sustenance of gravito-turbulence in accretion discs. The numerical analysis is conducted using a bespoke pseudo-spectral code in fully compressible, non-linear conditions. The disc in question, which is modelled using the shearing sheet approximation, is assumed to be self-gravitating, viscous, and thermally diffusive; a constant cooling timescale is also considered. Zonal flows are found to emerge at the onset of gravito-turbulence and they remain closely linked to the turbulent state. A cycle of zonal flow formation and destruction is established, mediated by a slow mode instability (which allows zonal flows to grow) and a non-axisymmetric instability (which disrupts the zonal flow), which is found to repeat numerous times. It is in fact the disruptive action of the non-axisymmetric instability to form new leading and trailing shearing waves, allowing energy to be extracted from the background flow and ensuring the self-sustenance of the gravito-turbulent regime.

  12. The role of zonal flows in disc gravito-turbulence

    Science.gov (United States)

    Vanon, R.

    2018-07-01

    The work presented here focuses on the role of zonal flows in the self-sustenance of gravito-turbulence in accretion discs. The numerical analysis is conducted using a bespoke pseudo-spectral code in fully compressible, non-linear conditions. The disc in question, which is modelled using the shearing sheet approximation, is assumed to be self-gravitating, viscous, and thermally diffusive; a constant cooling time-scale is also considered. Zonal flows are found to emerge at the onset of gravito-turbulence and they remain closely linked to the turbulent state. A cycle of zonal flow formation and destruction is established, mediated by a slow mode instability (which allows zonal flows to grow) and a non-axisymmetric instability (which disrupts the zonal flow), which is found to repeat numerous times. It is in fact the disruptive action of the non-axisymmetric instability to form new leading and trailing shearing waves, allowing energy to be extracted from the background flow and ensuring the self-sustenance of the gravito-turbulent regime.

  13. Enhancing the rate of tidal disruptions of stars by a self-gravitating disc around a massive central black hole

    Directory of Open Access Journals (Sweden)

    Šubr L.

    2012-12-01

    Full Text Available We further study the idea that a self-gravitating accretion disc around a supermassive black hole can increase the rate of gradual orbital decay of stellar trajectories (and hence tidal disruption events by setting some stars on eccentric trajectories. Cooperation between the gravitational field of the disc and the dissipative environment can provide a mechanism explaining the origin of stars that become bound tightly to the central black hole. We examine this process as a function of the black hole mass and conclude that it is most efficient for intermediate central masses of the order of ∼ 104Mʘ. Members of the cluster experience the stage of orbital decay via collisions with an accretion disc and by other dissipative processes, such as tidal effects, dynamical friction and the emission of gravitational waves. Our attention is concentrated on the region of gravitational dominance of the central body. Mutual interaction between stars and the surrounding environment establishes a non-spherical shape and anisotropy of the nuclear cluster. In some cases, the stellar sub-system acquires ring-type geometry. Stars of the nuclear cluster undergo a tidal disruption event as they plunge below the tidal radius of the supermassive black hole.

  14. On the structure of circumbinary accretion disks and the tidal evolution of commensurable satellites

    International Nuclear Information System (INIS)

    Lin, D.N.C.; Papaloizou, J.

    1979-01-01

    The investigation is continued of tidal torques on accretion disk flows in the vicinity of close binary systems. It is shown that the tidal effect can truncate the inner edge of circumbinary accretion discs. If the viscous dissipation is weak in such disks, density enhancement can be produced at the outer Lindblad resonance. The results are applied to contact binaries and the formation of commensurable satellites in the solar system. In order to determine whether the present configurations are a result of formation, or subsequent tidal evolution, the forced eccentricity of resonant satellites is related to the Q values of the planet and satellites. It is found that while the Galilean satellites may owe their present configuration, in part, to tidal effects, this is unlikely for other commensurable pairs. (author)

  15. Predictions for the reverberating spectral line signal from a newly formed black hole accretion disk: case of tidal disruption flares

    Czech Academy of Sciences Publication Activity Database

    Zhang, W.; Yu, W.; Karas, Vladimír; Dovčiak, Michal

    2015-01-01

    Roč. 807, č. 89 (2015), s. 1-12 ISSN 0004-637X R&D Projects: GA MŠk(CZ) LH14049; GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : black holes * accretion discs Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.909, year: 2015

  16. Magnetism, X-rays and accretion rates in WD 1145+017 and other polluted white dwarf systems

    Science.gov (United States)

    Farihi, J.; Fossati, L.; Wheatley, P. J.; Metzger, B. D.; Mauerhan, J.; Bachman, S.; Gänsicke, B. T.; Redfield, S.; Cauley, P. W.; Kochukhov, O.; Achilleos, N.; Stone, N.

    2018-02-01

    This paper reports circular spectropolarimetry and X-ray observations of several polluted white dwarfs including WD 1145+017, with the aim to constrain the behaviour of disc material and instantaneous accretion rates in these evolved planetary systems. Two stars with previously observed Zeeman splitting, WD 0322-019 and WD 2105-820, are detected above 5σ and 〈Bz〉 > 1 kG, while WD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this minimum level of confidence. For these latter three stars, high-resolution spectra and atmospheric modelling are used to obtain limits on magnetic field strengths via the absence of Zeeman splitting, finding B* Earth composition material falling on to the magnetic polar regions of white dwarfs, where X-rays and cyclotron radiation may contribute to accretion luminosity. This analysis is applied to X-ray data for WD 1145+017, WD 1729+371, and WD 2326+049, and the upper bound count rates are modelled with spectra for a range of plasma kT = 1-10 keV in both the magnetic and non-magnetic accretion regimes. The results for all three stars are consistent with a typical dusty white dwarf in a steady state at 108-109 g s-1. In particular, the non-magnetic limits for WD 1145+017 are found to be well below previous estimates of up to 1012 g s-1, and likely below 1010 g s-1, thus suggesting the star-disc system may be average in its evolutionary state, and only special in viewing geometry.

  17. WISE J080822.18-644357.3 - a 45 Myr-old accreting M dwarf hosting a primordial disc

    Science.gov (United States)

    Murphy, Simon J.; Mamajek, Eric E.; Bell, Cameron P. M.

    2018-05-01

    WISE J080822.18-644357.3 (WISE J0808-6443) was recently identified as a new M dwarf debris disc system and a candidate member of the 45 Myr-old Carina association. Given that the strength of its infrared excess (LIR/L⋆ ≃ 0.1) appears to be more consistent with a young protoplanetary disc, we present the first optical spectra of the star and reassess its evolutionary and membership status. We find WISE J0808-6443 to be a Li-rich M5 star with strong H α emission (-125 main sequence evolution.

  18. Rings and gaps in the disc around Elias 24 revealed by ALMA

    Science.gov (United States)

    Dipierro, G.; Ricci, L.; Pérez, L.; Lodato, G.; Alexander, R. D.; Laibe, G.; Andrews, S.; Carpenter, J. M.; Chandler, C. J.; Greaves, J. A.; Hall, C.; Henning, T.; Kwon, W.; Linz, H.; Mundy, L.; Sargent, A.; Tazzari, M.; Testi, L.; Wilner, D.

    2018-04-01

    We present Atacama Large Millimeter/sub-millimeter Array (ALMA) Cycle 2 observations of the 1.3-mm dust continuum emission of the protoplanetary disc surrounding the T Tauri star Elias 24 with an angular resolution of ˜0.2 arcsec (˜28 au). The dust continuum emission map reveals a dark ring at a radial distance of 0.47 arcsec (˜65 au) from the central star, surrounded by a bright ring at 0.58 arcsec (˜81 au). In the outer disc, the radial intensity profile shows two inflection points at 0.71 and 0.87 arcsec (˜99 and 121 au, respectively). We perform global three-dimensional smoothed particle hydrodynamic gas/dust simulations of discs hosting a migrating and accreting planet. Combining the dust density maps of small and large grains with three-dimensional radiative transfer calculations, we produce synthetic ALMA observations of a variety of disc models in order to reproduce the gap- and ring-like features observed in Elias 24. We find that the dust emission across the disc is consistent with the presence of an embedded planet with a mass of ˜0.7 MJ at an orbital radius of ˜ 60 au. Our model suggests that the two inflection points in the radial intensity profile are due to the inward radial motion of large dust grains from the outer disc. The surface brightness map of our disc model provides a reasonable match to the gap- and ring-like structures observed in Elias 24, with an average discrepancy of ˜5 per cent of the observed fluxes around the gap region.

  19. Structural changes in the hot Algol OGLE-LMC-DPV-097 and its disc related to its long cycle

    Science.gov (United States)

    Garcés L, J.; Mennickent, R. E.; Djurašević, G.; Poleski, R.; Soszyński, I.

    2018-06-01

    Double Periodic Variables (DPVs) are hot Algols showing a long photometric cycle of uncertain origin. We report the discovery of changes in the orbital light curve of OGLE-LMC-DPV-097 which depend on the phase of its long photometric cycle. During the ascending branch of the long cycle the brightness at the first quadrature is larger than during the second quadrature, during the maximum of the long cycle the brightness is basically the same at both quadratures, during the descending branch the brightness at the second quadrature is larger than during the first quadrature, and during the minimum of the long cycle the secondary minimum disappears. We model the light curve at different phases of the long cycle and find that the data are consistent with changes in the properties of the accretion disc and two disc spots. The disc's size and temperature change with the long-cycle period. We find a smaller and hotter disc at minimum, and larger and cooler disc at maximum. The spot temperatures, locations, and angular sizes also show variability during the long cycle.

  20. Consistent van der Waals radii for the whole main group.

    Science.gov (United States)

    Mantina, Manjeera; Chamberlin, Adam C; Valero, Rosendo; Cramer, Christopher J; Truhlar, Donald G

    2009-05-14

    Atomic radii are not precisely defined but are nevertheless widely used parameters in modeling and understanding molecular structure and interactions. The van der Waals radii determined by Bondi from molecular crystals and data for gases are the most widely used values, but Bondi recommended radius values for only 28 of the 44 main-group elements in the periodic table. In the present Article, we present atomic radii for the other 16; these new radii were determined in a way designed to be compatible with Bondi's scale. The method chosen is a set of two-parameter correlations of Bondi's radii with repulsive-wall distances calculated by relativistic coupled-cluster electronic structure calculations. The newly determined radii (in A) are Be, 1.53; B, 1.92; Al, 1.84; Ca, 2.31; Ge, 2.11; Rb, 3.03; Sr, 2.49; Sb, 2.06; Cs, 3.43; Ba, 2.68; Bi, 2.07; Po, 1.97; At, 2.02; Rn, 2.20; Fr, 3.48; and Ra, 2.83.

  1. The disc-jet coupling in the neutron star X-ray binary 4U 1728-34

    Science.gov (United States)

    Tudose, Valeriu; Tzioumis, Anastasios; Belloni, Tomaso; Altamirano, Diego; Linares, Manuel; Mendez, Mariano; Hiemstra, Beike

    2010-10-01

    The present radio proposal is part of a multi-wavelength campaign focused on the study of the accretion/ejection process in the neutron star X-ray binary system 4U 1728-34. Our intention is to study the behaviour of the inner part of the accretion disc as inferred from the X-ray observations of the Fe emission line and the kHz quasi-periodic oscillations, and to link it to the properties of the radio jet. To achieve this goal we request 5 × 11h of observing time with ATCA, scheduled at regular intervals in the period 2010 August 27- October 13, the visibility window of the granted X-ray observations with RXTE (PI: Mendez) and Suzaku (PI: Linares).

  2. Disc defect classification for optical disc drives

    NARCIS (Netherlands)

    Helvoirt, van J.; Leenknegt, G.A.L.; Steinbuch, M.; Goossens, H.J.

    2005-01-01

    Optical disc drives are subject to various disturbances and faults. A special type of fault is the so-called disc defect. In this paper we present an approach for disc defect classification. It is based on hierarchical clustering of measured signals that are affected by disc defects. The

  3. Shadow of a Large Disc Casts New Light on the Formation of High Mass Stars

    Science.gov (United States)

    2004-05-01

    Massive Star Observed that Forms through a Rotating Accretion Disc Summary Based on a large observational effort with different telescopes and instruments, mostly from the European Southern Observatory (ESO), a team of European astronomers [1] has shown that in the M 17 nebula a high mass star [2] forms via accretion through a circumstellar disc, i.e. through the same channel as low-mass stars. To reach this conclusion, the astronomers used very sensitive infrared instruments to penetrate the south-western molecular cloud of M 17 so that faint emission from gas heated up by a cluster of massive stars, partly located behind the molecular cloud, could be detected through the dust. Against the background of this hot region a large opaque silhouette, which resembles a flared disc seen nearly edge-on, is found to be associated with an hour-glass shaped reflection nebula. This system complies perfectly with a newly forming high-mass star surrounded by a huge accretion disc and accompanied by an energetic bipolar mass outflow. The new observations corroborate recent theoretical calculations which claim that stars up to 40 times more massive than the Sun can be formed by the same processes that are active during the formation of stars of smaller masses. PR Photo 15a/04: Stellar cluster and star-forming region M 17 (also available without text inside photo) PR Photo 15b/04: Silhouette disc seen in M 17 PR Photo 15c/04: Rotation of the disc in M 17. PR Photo 15d/04: Bipolar reflection nebula and silhouette disc of a young, massive star in M 17 PR Photo 15e/04: Optical spectrum of the bipolar nebula. PR Video 03/04: Zooming in onto the disc. The M 17 region ESO PR Photo 15a/04 ESO PR Photo 15a/04 [Preview - JPEG: 400 x 497 pix - 271k] [Normal - JPEG: 800 x 958 pix - 604k] ESO PR Photo 15a1/04 ESO PR Photo 15a/04 (without text within photo) [Preview - JPEG: 400 x 480 pix - 275k] [Normal - JPEG: 800 x 959 pix - 634k] [High-Res - JPEG: 3000 x 3597 pix - 3.8M] [Full-Res - JPEG

  4. Magnetically gated accretion in an accreting 'non-magnetic' white dwarf.

    Science.gov (United States)

    Scaringi, S; Maccarone, T J; D'Angelo, C; Knigge, C; Groot, P J

    2017-12-13

    White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15 per cent of these binaries, the magnetic field of the white dwarf is strong enough (at 10 6 gauss or more) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as 'non-magnetic', because until now there has been no evidence that they have a magnetic field that is strong enough to affect the accretion dynamics. Here we report an analysis of archival optical observations of the 'non-magnetic' accreting white dwarf in the binary system MV Lyrae, whose light curve displays quasi-periodic bursts of about 30 minutes duration roughly every 2 hours. The timescale and amplitude of these bursts indicate the presence of an unstable, magnetically regulated accretion mode, which in turn implies the existence of magnetically gated accretion, in which disk material builds up around the magnetospheric boundary (at the co-rotation radius) and then accretes onto the white dwarf, producing bursts powered by the release of gravitational potential energy. We infer a surface magnetic field strength for the white dwarf in MV Lyrae of between 2 × 10 4 gauss and 1 × 10 5 gauss, too low to be detectable by other current methods. Our discovery provides a new way of studying the strength and evolution of magnetic fields in accreting white dwarfs and extends the connections between accretion onto white dwarfs, young stellar objects and neutron stars, for which similar magnetically gated accretion cycles have been identified.

  5. Observing the On-going Formation of Planets and its Effects on Their Parent Discs

    Science.gov (United States)

    Willson, Matthew Alexander

    2017-06-01

    immediately outside of a ring of dusty material largely responsible for the NIR comment of the disc SED, similar to TW Hya b located in a shallow gap in the dust disc outside another ring of over-dense dusty material which bounds a deep but narrow gap. Both of these companion candidates maybe migrating cores which are feeding from the enriched ring of material. I conducted a more extensive study of the pre-transitional disc, V1247 Ori, covering three epochs and the H-, K- and L-wavebands. Complementary observations with VLT/SPHERE in Hα and continuum plus SMA observations in CO (2-1) and continuum were performed. The orientation and geometry of the outer disc was recovered with the SMA data and determine the direction of rotation. We image the inner rim of the outer disc in L-band SAM data, recovering the rim in all three epochs. Combining all three data sets together we form a detailed image of the rim. In H- and K-band SAM data we observe the motion of a close-in companion candidate. This motion was found to be too large to be adequately explained through a near-circular Keplerian orbit within the plane of the disc around the central star. Hence an alternate hypothesis had to be developed. I postulated that the fitted position of the companion maybe influenced by the emission from the disc rim seen in the L-band SAM data. I constructed a suite of model SAM data sets of a companion and a disc rim and found that under the right conditions the fitted separation of a companion will be larger than the true separation. Under these conditions we find the motion of the companion candidate to be consistent with a near-circular Keplerian orbit within the plane of the disc at a semi-major axis of ˜6 au. The Hα data lack the necessary resolution to confirm the companion as an accreting body, but through the high contrast sensitivities enabled by the state of the art SPHERE instrument I was able to rule out any other accreting body within the gap, unless deeply embedded by the sparse

  6. Partial dust obscuration in active galactic nuclei as a cause of broad-line profile and lag variability, and apparent accretion disc inhomogeneities

    Science.gov (United States)

    Gaskell, C. Martin; Harrington, Peter Z.

    2018-04-01

    The profiles of the broad emission lines of active galactic nuclei (AGNs) and the time delays in their response to changes in the ionizing continuum ("lags") give information about the structure and kinematics of the inner regions of AGNs. Line profiles are also our main way of estimating the masses of the supermassive black holes (SMBHs). However, the profiles often show ill-understood, asymmetric structure and velocity-dependent lags vary with time. Here we show that partial obscuration of the broad-line region (BLR) by outflowing, compact, dusty clumps produces asymmetries and velocity-dependent lags similar to those observed. Our model explains previously inexplicable changes in the ratios of the hydrogen lines with time and velocity, the lack of correlation of changes in line profiles with variability of the central engine, the velocity dependence of lags, and the change of lags with time. We propose that changes on timescales longer than the light-crossing time do not come from dynamical changes in the BLR, but are a natural result of the effect of outflowing dusty clumps driven by radiation pressure acting on the dust. The motion of these clumps offers an explanation of long-term changes in polarization. The effects of the dust complicate the study of the structure and kinematics of the BLR and the search for sub-parsec SMBH binaries. Partial obscuration of the accretion disc can also provide the local fluctuations in luminosity that can explain sizes deduced from microlensing.

  7. ON THE LAMPPOST MODEL OF ACCRETING BLACK HOLES

    Energy Technology Data Exchange (ETDEWEB)

    Niedźwiecki, Andrzej; Szanecki, Michał [Łódź University, Department of Physics, Pomorska 149/153, 90-236 Łódź (Poland); Zdziarski, Andrzej A. [Centrum Astronomiczne im. M. Kopernika, Bartycka 18, 00-716 Warszawa (Poland)

    2016-04-10

    We study the lamppost model, in which the X-ray source in accreting black hole (BH) systems is located on the rotation axis close to the horizon. We point out a number of inconsistencies in the widely used lamppost model relxilllp, e.g., neglecting the redshift of the photons emitted by the lamppost that are directly observed. They appear to invalidate those model fitting results for which the source distances from the horizon are within several gravitational radii. Furthermore, if those results were correct, most of the photons produced in the lamppost would be trapped by the BH, and the luminosity generated in the source as measured at infinity would be much larger than that observed. This appears to be in conflict with the observed smooth state transitions between the hard and soft states of X-ray binaries. The required increase of the accretion rate and the associated efficiency reduction also present a problem for active galactic nuclei. Then, those models imply the luminosity measured in the local frame is much higher than that produced in the source and measured at infinity, due to the additional effects of time dilation and redshift, and the electron temperature is significantly higher than that observed. We show that these conditions imply that the fitted sources would be out of the e{sup ±} pair equilibrium. On the other hand, the above issues pose relatively minor problems for sources at large distances from the BH, where relxilllp can still be used.

  8. Comparison of Animal Discs Used in Disc Research to Human Lumbar Disc: Torsion Mechanics and Collagen Content

    Science.gov (United States)

    Showalter, Brent L.; Beckstein, Jesse C.; Martin, John T.; Beattie, Elizabeth E.; Orías, Alejandro A. Espinoza; Schaer, Thomas P.; Vresilovic, Edward J.; Elliott, Dawn M.

    2012-01-01

    Study Design Experimental measurement and normalization of in vitro disc torsion mechanics and collagen content for several animal species used in intervertebral disc research and comparing these to the human disc. Objective To aid in the selection of appropriate animal models for disc research by measuring torsional mechanical properties and collagen content. Summary of Background Data There is lack of data and variability in testing protocols for comparing animal and human disc torsion mechanics and collagen content. Methods Intervertebral disc torsion mechanics were measured and normalized by disc height and polar moment of inertia for 11 disc types in 8 mammalian species: the calf, pig, baboon, goat, sheep, rabbit, rat, and mouse lumbar, and cow, rat, and mouse caudal. Collagen content was measured and normalized by dry weight for the same discs except the rat and mouse. Collagen fiber stretch in torsion was calculated using an analytical model. Results Measured torsion parameters varied by several orders of magnitude across the different species. After geometric normalization, only the sheep and pig discs were statistically different from human. Fiber stretch was found to be highly dependent on the assumed initial fiber angle. The collagen content of the discs was similar, especially in the outer annulus where only the calf and goat discs were statistically different from human. Disc collagen content did not correlate with torsion mechanics. Conclusion Disc torsion mechanics are comparable to human lumbar discs in 9 of 11 disc types after normalization by geometry. The normalized torsion mechanics and collagen content of the multiple animal discs presented is useful for selecting and interpreting results for animal models of the disc. Structural composition of the disc, such as initial fiber angle, may explain the differences that were noted between species after geometric normalization. PMID:22333953

  9. Accreting CO material onto ONe white dwarfs towards accretion-induced collapse

    Science.gov (United States)

    Wu, Cheng-Yuan; Wang, Bo

    2018-03-01

    The final outcomes of accreting ONe white dwarfs (ONe WDs) have been studied for several decades, but there are still some issues that are not resolved. Recently, some studies suggested that the deflagration of oxygen would occur for accreting ONe WDs with Chandrasekhar masses. In this paper, we aim to investigate whether ONe WDs can experience accretion-induced collapse (AIC) or explosions when their masses approach the Chandrasekhar limit. Employing the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA), we simulate the long-term evolution of ONe WDs with accreting CO material. The ONe WDs undergo weak multicycle carbon flashes during the mass-accretion process, leading to mass increase of the WDs. We found that different initial WD masses and mass-accretion rates influence the evolution of central density and temperature. However, the central temperature cannot reach the explosive oxygen ignition temperature due to neutrino cooling. This work implies that the final outcome of accreting ONe WDs is electron-capture induced collapse rather than thermonuclear explosion.

  10. On the Observability of Individual Population III Stars and Their Stellar-mass Black Hole Accretion Disks through Cluster Caustic Transits

    Science.gov (United States)

    Windhorst, Rogier A.; Timmes, F. X.; Wyithe, J. Stuart B.; Alpaslan, Mehmet; Andrews, Stephen K.; Coe, Daniel; Diego, Jose M.; Dijkstra, Mark; Driver, Simon P.; Kelly, Patrick L.; Kim, Duho

    2018-02-01

    We summarize panchromatic Extragalactic Background Light data to place upper limits on the integrated near-infrared surface brightness (SB) that may come from Population III stars and possible accretion disks around their stellar-mass black holes (BHs) in the epoch of First Light, broadly taken from z ≃ 7–17. Theoretical predictions and recent near-infrared power spectra provide tighter constraints on their sky signal. We outline the physical properties of zero-metallicity Population III stars from MESA stellar evolution models through helium depletion and of BH accretion disks at z≳ 7. We assume that second-generation non-zero-metallicity stars can form at higher multiplicity, so that BH accretion disks may be fed by Roche-lobe overflow from lower-mass companions. We use these near-infrared SB constraints to calculate the number of caustic transits behind lensing clusters that the James Webb Space Telescope and the next-generation ground-based telescopes may observe for both Population III stars and their BH accretion disks. Typical caustic magnifications can be μ ≃ {10}4{--}{10}5, with rise times of hours and decline times of ≲ 1 year for cluster transverse velocities of {v}T≲ 1000 km s‑1. Microlensing by intracluster-medium objects can modify transit magnifications but lengthen visibility times. Depending on BH masses, accretion-disk radii, and feeding efficiencies, stellar-mass BH accretion-disk caustic transits could outnumber those from Population III stars. To observe Population III caustic transits directly may require monitoring 3–30 lensing clusters to {AB}≲ 29 mag over a decade.

  11. Diquark correlations from nucleon charge radii

    International Nuclear Information System (INIS)

    Carlson, Carl E.; Carone, Christopher D.; Kwee, Herry J.; Lebed, Richard F.

    2006-01-01

    We argue that precise measurements of charge and magnetic radii can meaningfully constrain diquark models of the nucleon. We construct properly symmetrized, nonrelativistic three-quark wave functions that interpolate between the limits of a pointlike diquark pair and no diquark correlation. We find that good fits to the data can be obtained for a wide range of diquark sizes, provided that the diquark wave functions are close to those that reduce to a purely scalar state in the pointlike limit. A modest improvement in the experimental uncertainties will render a fit to the charge radii a more telling diagnostic for the presence of spatially correlated quark pairs within the nucleon

  12. Radii of Redox Components from Absolute Redox Potentials Compared with Covalent and Aqueous Ionic Radii

    Czech Academy of Sciences Publication Activity Database

    Heyrovská, Raji

    2010-01-01

    Roč. 22, č. 9 (2010), s. 903-907 ISSN 1040-0397 Institutional support: RVO:68081707 Keywords : Electrochemistry * Absolute redox potentials * Radii of redox components Subject RIV: BO - Biophysics Impact factor: 2.721, year: 2010

  13. GLOBAL STRUCTURE OF THREE DISTINCT ACCRETION FLOWS AND OUTFLOWS AROUND BLACK HOLES FROM TWO-DIMENSIONAL RADIATION-MAGNETOHYDRODYNAMIC SIMULATIONS

    International Nuclear Information System (INIS)

    Ohsuga, Ken; Mineshige, Shin

    2011-01-01

    We present the detailed global structure of black hole accretion flows and outflows through newly performed two-dimensional radiation-magnetohydrodynamic simulations. By starting from a torus threaded with weak toroidal magnetic fields and by controlling the central density of the initial torus, ρ 0 , we can reproduce three distinct modes of accretion flow. In model A, which has the highest central density, an optically and geometrically thick supercritical accretion disk is created. The radiation force greatly exceeds the gravity above the disk surface, thereby driving a strong outflow (or jet). Because of mild beaming, the apparent (isotropic) photon luminosity is ∼22L E (where L E is the Eddington luminosity) in the face-on view. Even higher apparent luminosity is feasible if we increase the flow density. In model B, which has moderate density, radiative cooling of the accretion flow is so efficient that a standard-type, cold, and geometrically thin disk is formed at radii greater than ∼7 R S (where R S is the Schwarzschild radius), while the flow is radiatively inefficient otherwise. The magnetic-pressure-driven disk wind appears in this model. In model C, the density is too low for the flow to be radiatively efficient. The flow thus becomes radiatively inefficient accretion flow, which is geometrically thick and optically thin. The magnetic-pressure force, together with the gas-pressure force, drives outflows from the disk surface, and the flow releases its energy via jets rather than via radiation. Observational implications are briefly discussed.

  14. Artificial Disc Replacement

    Science.gov (United States)

    ... Spondylolisthesis BLOG FIND A SPECIALIST Treatments Artificial Disc Replacement (ADR) Patient Education Committee Jamie Baisden The disc ... Disc An artificial disc (also called a disc replacement, disc prosthesis or spine arthroplasty device) is a ...

  15. Evolution of double white dwarf binaries undergoing direct-impact accretion: Implications for gravitational wave astronomy

    Science.gov (United States)

    Kremer, Kyle; Breivik, Katelyn; Larson, Shane L.; Kalogera, Vassiliki

    2017-01-01

    For close double white dwarf binaries, the mass-transfer phenomenon known as direct-impact accretion (when the mass transfer stream impacts the accretor directly rather than forming a disc) may play a pivotal role in the long-term evolution of the systems. In this analysis, we explore the long-term evolution of white dwarf binaries accreting through direct-impact and explore implications of such systems to gravitational wave astronomy. We cover a broad range of parameter space which includes initial component masses and the strength of tidal coupling, and show that these systems, which lie firmly within the LISA frequency range, show strong negative chirps which can last as long as several million years. Detections of double white dwarf systems in the direct-impact phase by detectors such as LISA would provide astronomers with unique ways of probing the physics governing close compact object binaries.

  16. STANDING SHOCK INSTABILITY IN ADVECTION-DOMINATED ACCRETION FLOWS

    Energy Technology Data Exchange (ETDEWEB)

    Le, Truong [Department of Physics, Astronomy and Geology, Berry College, Mount Berry, GA 30149 (United States); Wood, Kent S.; Wolff, Michael T. [High Energy Space Environment Branch, Space Science Division, Naval Research Laboratory, Washington, DC 20375 (United States); Becker, Peter A. [Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030 (United States); Putney, Joy, E-mail: tle@berry.edu [Department of Physics and Engineering, Washington and Lee University, Lexington, VA 24450 (United States)

    2016-03-10

    Depending on the values of the energy and angular momentum per unit mass in the gas supplied at large radii, inviscid advection-dominated accretion flows can display velocity profiles with either preshock deceleration or preshock acceleration. Nakayama has shown that these two types of flow configurations are expected to have different stability properties. By employing the Chevalier and Imamura linearization method and the Nakayama instability boundary conditions, we discover that there are regions of parameter space where disks/shocks with outflows can be stable or unstable. In regions of instability, we find that preshock deceleration is always unstable to the zeroth mode with zero frequency of oscillation, but is always stable to the fundamental mode and overtones. Furthermore, we also find that preshock acceleration is always unstable to the zeroth mode and that the fundamental mode and overtones become increasingly less stable as the shock location moves away from the horizon when the disk half-height expands above ∼12 gravitational radii at the shock radius. In regions of stability, we demonstrate the zeroth mode to be stable for the velocity profiles that exhibit preshock acceleration and deceleration. Moreover, for models that are linearly unstable, our model suggests the possible existence of quasi-periodic oscillations (QPOs) with ratios 2:3 and 3:5. These ratios are believed to occur in stellar and supermassive black hole candidates, for example, in GRS 1915+105 and Sgr A*, respectively. We expect that similar QPO ratios also exist in regions of stable shocks.

  17. Interaction cross-sections and matter radii of A = 20 isobars

    International Nuclear Information System (INIS)

    Chulkov, L.; Bochkarev, O.; Geissel, H.; Golovkov, M.; Janas, Z.; Keller, H.; Kobayashi, T.; Muenzenberg, G.; Nickel, F.; Ogloblin, A.; Patra, S.; Piechaczek, A.; Roeckl, E.; Schwab, W.; Suemmerer, K.; Suzuki, T.; Tanihata, I.; Yoshida, K.

    1995-11-01

    High-energy interaction cross-sections of A=20 nuclei ( 20 N, 20 O, 20 F, 20 Ne, 20 Na, 20 Mg) on carbon were measured with accuracies of ∼1%. The nuclear matter rms radii derived from the measured cross-sections show an irregular dependence on isospin projection. The largest difference in radii, which amounts to approximately 0.2 fm, has been obtained for the mirror nuclei 20 O and 20 Mg. The influenc of nuclear deformation and binding energy on the radii is discussed. By evaluating the difference in rms radii of neutron and proton distributions, evidence has been found for the existence of a proton skin for 20 Mg and of a neutron skin for 20 N. (orig.)

  18. THERMAL AND DYNAMICAL PROPERTIES OF GAS ACCRETING ONTO A SUPERMASSIVE BLACK HOLE IN AN ACTIVE GALACTIC NUCLEUS

    International Nuclear Information System (INIS)

    Mościbrodzka, M.; Proga, D.

    2013-01-01

    We study stability of gas accretion in active galactic nuclei (AGNs). Our grid-based simulations cover a radial range from 0.1 to 200 pc, which may enable linking the galactic/cosmological simulations with small-scale black hole (BH) accretion models within a few hundreds of Schwarzschild radii. Here, as in previous studies by our group, we include gas radiative cooling as well as heating by a sub-Eddington X-ray source near the central supermassive BH of 10 8 M ☉ . Our theoretical estimates and simulations show that for the X-ray luminosity, L X ∼ 0.008 L Edd , the gas is thermally and convectively unstable within the computational domain. In the simulations, we observe that very tiny fluctuations in an initially smooth, spherically symmetric, accretion flow, grow first linearly and then nonlinearly. Consequently, an initially one-phase flow relatively quickly transitions into a two-phase/cold-hot accretion flow. For L X = 0.015 L Edd or higher, the cold clouds continue to accrete but in some regions of the hot phase, the gas starts to move outward. For L X Edd , the cold phase contribution to the total mass accretion rate only moderately dominates over the hot phase contribution. This result might have some consequences for cosmological simulations of the so-called AGN feedback problem. Our simulations confirm the previous results of Barai et al. who used smoothed particle hydrodynamic (SPH) simulations to tackle the same problem. Here, however, because we use a grid-based code to solve equations in one dimension and two dimensions, we are able to follow the gas dynamics at much higher spacial resolution and for longer time compared with the three-dimensional SPH simulations. One of the new features revealed by our simulations is that the cold condensations in the accretion flow initially form long filaments, but at the later times, those filaments may break into smaller clouds advected outward within the hot outflow. Therefore, these simulations may serve as

  19. Table of nuclear root mean square charge radii. Summary

    International Nuclear Information System (INIS)

    Paviotti-Corcuera, R.; McLaughlin, P.K.

    1999-01-01

    This document describes a table of nuclear root-mean-square (rms) charge radii evaluated by two different procedures. The data are available from the IAEA Nuclear Data Section via INTERNET or on PC diskettes upon request. This document supersedes the previous IAEA-NDS-163, 1990, 'Nuclear Charge Radii'. (author)

  20. Feeding supermassive black holes by collisional cascades

    Science.gov (United States)

    Faber, Christian; Dehnen, Walter

    2018-05-01

    The processes driving gas accretion on to supermassive black holes (SMBHs) are still poorly understood. Angular momentum conservation prevents gas within ˜10 pc of the black hole from reaching radii ˜10-3 pc where viscous accretion becomes efficient. Here we present simulations of the collapse of a clumpy shell of swept-up isothermal gas, which is assumed to have formed as a result of feedback from a previous episode of AGN activity. The gas falls towards the SMBH forming clumps and streams, which intersect, collide, and often form a disc. These collisions promote partial cancellations of angular momenta, resulting in further infall and more collisions. This continued collisional cascade generates a tail of gas with sufficiently small angular momenta and provides a viable route for gas inflow to sub-parsec scales. The efficiency of this process hardly depends on details, such as gas temperature, initial virial ratio and power spectrum of the gas distribution, as long as it is not strongly rotating. Adding star formation to this picture might explain the near-simultaneous formation of the S-stars (from tidally disrupted binaries formed in plunging gas streams) and the sub-parsec young stellar disc around Sgr A⋆.

  1. CONSTRAINTS ON COMPTON-THICK WINDS FROM BLACK HOLE ACCRETION DISKS: CAN WE SEE THE INNER DISK?

    International Nuclear Information System (INIS)

    Reynolds, Christopher S.

    2012-01-01

    Strong evidence is emerging that winds can be driven from the central regions of accretion disks in both active galactic nuclei and Galactic black hole binaries. Direct evidence for highly ionized, Compton-thin inner-disk winds comes from observations of blueshifted (v ∼ 0.05-0.1c) iron-K X-ray absorption lines. However, it has been suggested that the inner regions of black hole accretion disks can also drive Compton-thick winds—such winds would enshroud the inner disk, preventing us from seeing direct signatures of the accretion disk (i.e., the photospheric thermal emission, or the Doppler/gravitationally broadened iron Kα line). Here, we show that, provided the source is sub-Eddington, the well-established wind-driving mechanisms fail to launch a Compton-thick wind from the inner disk. For the accelerated region of the wind to be Compton-thick, the momentum carried in the wind must exceed the available photon momentum by a factor of at least 2/λ, where λ is the Eddington ratio of the source, ruling out radiative acceleration unless the source is very close to the Eddington limit. Compton-thick winds also carry large mass fluxes, and a consideration of the connections between the wind and the disk shows this to be incompatible with magneto-centrifugal driving. Finally, thermal driving of the wind is ruled out on the basis of the large Compton radii that typify black hole systems. In the absence of some new acceleration mechanisms, we conclude that the inner regions of sub-Eddington accretion disks around black holes are indeed naked.

  2. THE EFFECTS OF ACCRETION FLOW DYNAMICS ON THE BLACK HOLE SHADOW OF SAGITTARIUS A*

    Energy Technology Data Exchange (ETDEWEB)

    Pu, Hung-Yi; Asada, Keiichi [Institute of Astronomy and Astrophysics, Academia Sinica, 11F of Astronomy-Mathematics Building, AS/NTU No. 1, Taipei 10617, Taiwan (China); Akiyama, Kazunori [Massachusetts Institute of Technology, Haystack Observatory, Route 40, Westford, MA 01886 (United States)

    2016-11-01

    A radiatively inefficient accretion flow (RIAF), which is commonly characterized by its sub-Keplerian nature, is a favored accretion model for the supermassive black hole at the Galactic center, Sagittarius A*. To investigate the observable features of an RIAF, we compare the modeled shadow images, visibilities, and spectra of three flow models with dynamics characterized by (i) a Keplerian shell that is rigidly rotating outside the innermost stable circular orbit (ISCO) and infalling with a constant angular momentum inside ISCO, (ii) a sub-Keplerian motion, and (iii) a free-falling motion with zero angular momentum at infinity. At near-millimeter wavelengths, the emission is dominated by the flow within several Schwarzschild radii. The energy shift due to these flow dynamics becomes important and distinguishable, suggesting that the flow dynamics are an important model parameter for interpreting the millimeter/sub-millimeter very long baseline interferometric observations with the forthcoming, fully assembled Event Horizon Telescope (EHT). As an example, we demonstrate that structural variations of Sagittarius A* on event horizon-scales detected in previous EHT observations can be explained by the non-stationary dynamics of an RIAF.

  3. Angular momentum transfer in primordial discs and the rotation of the first stars

    Science.gov (United States)

    Hirano, Shingo; Bromm, Volker

    2018-05-01

    We investigate the rotation velocity of the first stars by modelling the angular momentum transfer in the primordial accretion disc. Assessing the impact of magnetic braking, we consider the transition in angular momentum transport mode at the Alfvén radius, from the dynamically dominated free-fall accretion to the magnetically dominated solid-body one. The accreting protostar at the centre of the primordial star-forming cloud rotates with close to breakup speed in the case without magnetic fields. Considering a physically motivated model for small-scale turbulent dynamo amplification, we find that stellar rotation speed quickly declines if a large fraction of the initial turbulent energy is converted to magnetic energy (≳ 0.14). Alternatively, if the dynamo process were inefficient, for amplification due to flux freezing, stars would become slow rotators if the pre-galactic magnetic field strength is above a critical value, ≃10-8.2 G, evaluated at a scale of nH = 1 cm-3, which is significantly higher than plausible cosmological seed values (˜10-15 G). Because of the rapid decline of the stellar rotational speed over a narrow range in model parameters, the first stars encounter a bimodal fate: rapid rotation at almost the breakup level, or the near absence of any rotation.

  4. General Relativistic Magnetohydrodynamic Simulations of Magnetically Choked Accretion Flows around Black Holes

    Energy Technology Data Exchange (ETDEWEB)

    McKinney, Jonathan C.; Tchekhovskoy, Alexander; Blandford, Roger D.

    2012-04-26

    Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height H to cylindrical radius R ratio of |H/R| {approx} 0.2-1) accretion flows around BHs with various dimensionless spins (a/M, with BH mass M) and with initially toroidally-dominated ({phi}-directed) and poloidally-dominated (R-z directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough |a/M|, coherent large-scale (i.e. >> H) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. While models with |H/R| {approx} 1 and |a/M| {le} 0.5 do not launch jets due to quenching by mass infall, for sufficiently high |a/M| or low |H/R| the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric 'magnetically choked accretion flow' (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with and 100% efficiency for |a/M| {approx}> 0.9. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk oscillation (JDO) type of quasi-periodic oscillation (QPO) mechanism. The high-frequency QPO has spherical harmonic |m| = 1 mode period of {tau} {approx} 70GM/c{sup 3} for a/M {approx} 0.9 with coherence quality factors Q {approx}> 10. Overall, our models are qualitatively distinct from most prior MHD simulations (typically, |H/R| << 1 and poloidal flux is

  5. The Observational and Theoretical Tidal Radii of Globular Clusters in M87

    Science.gov (United States)

    Webb, Jeremy J.; Sills, Alison; Harris, William E.

    2012-02-01

    Globular clusters have linear sizes (tidal radii) which theory tells us are determined by their masses and by the gravitational potential of their host galaxy. To explore the relationship between observed and expected radii, we utilize the globular cluster population of the Virgo giant M87. Unusually deep, high signal-to-noise images of M87 are used to measure the effective and limiting radii of approximately 2000 globular clusters. To compare with these observations, we simulate a globular cluster population that has the same characteristics as the observed M87 cluster population. Placing these simulated clusters in the well-studied tidal field of M87, the orbit of each cluster is solved and the theoretical tidal radius of each cluster is determined. We compare the predicted relationship between cluster size and projected galactocentric distance to observations. We find that for an isotropic distribution of cluster velocities, theoretical tidal radii are approximately equal to observed limiting radii for R gc < 10 kpc. However, the isotropic simulation predicts a steep increase in cluster size at larger radii, which is not observed in large galaxies beyond the Milky Way. To minimize the discrepancy between theory and observations, we explore the effects of orbital anisotropy on cluster sizes, and suggest a possible orbital anisotropy profile for M87 which yields a better match between theory and observations. Finally, we suggest future studies which will establish a stronger link between theoretical tidal radii and observed radii.

  6. THE OBSERVATIONAL AND THEORETICAL TIDAL RADII OF GLOBULAR CLUSTERS IN M87

    International Nuclear Information System (INIS)

    Webb, Jeremy J.; Sills, Alison; Harris, William E.

    2012-01-01

    Globular clusters have linear sizes (tidal radii) which theory tells us are determined by their masses and by the gravitational potential of their host galaxy. To explore the relationship between observed and expected radii, we utilize the globular cluster population of the Virgo giant M87. Unusually deep, high signal-to-noise images of M87 are used to measure the effective and limiting radii of approximately 2000 globular clusters. To compare with these observations, we simulate a globular cluster population that has the same characteristics as the observed M87 cluster population. Placing these simulated clusters in the well-studied tidal field of M87, the orbit of each cluster is solved and the theoretical tidal radius of each cluster is determined. We compare the predicted relationship between cluster size and projected galactocentric distance to observations. We find that for an isotropic distribution of cluster velocities, theoretical tidal radii are approximately equal to observed limiting radii for R gc < 10 kpc. However, the isotropic simulation predicts a steep increase in cluster size at larger radii, which is not observed in large galaxies beyond the Milky Way. To minimize the discrepancy between theory and observations, we explore the effects of orbital anisotropy on cluster sizes, and suggest a possible orbital anisotropy profile for M87 which yields a better match between theory and observations. Finally, we suggest future studies which will establish a stronger link between theoretical tidal radii and observed radii.

  7. Accreting Black Holes

    OpenAIRE

    Begelman, Mitchell C.

    2014-01-01

    I outline the theory of accretion onto black holes, and its application to observed phenomena such as X-ray binaries, active galactic nuclei, tidal disruption events, and gamma-ray bursts. The dynamics as well as radiative signatures of black hole accretion depend on interactions between the relatively simple black-hole spacetime and complex radiation, plasma and magnetohydrodynamical processes in the surrounding gas. I will show how transient accretion processes could provide clues to these ...

  8. [Research progress of intervertebral disc endogenous stem cells for intervertebral disc regeneration].

    Science.gov (United States)

    Liang, Hang; Deng, Xiangyu; Shao, Zengwu

    2017-10-01

    To summarize the research progress of intervertebral disc endogenous stem cells for intervertebral disc regeneration and deduce the therapeutic potential of endogenous repair for intervertebral disc degeneration. The original articles about intervertebral disc endogenous stem cells for intervertebral disc regeneration were extensively reviewed; the reparative potential in vivo and the extraction and identification in vitro of intervertebral disc endogenous stem cells were analyzed; the prospect of endogenous stem cells for intervertebral disc regeneration was predicted. Stem cell niche present in the intervertebral discs, from which stem cells migrate to injured tissues and contribute to tissues regeneration under certain specific microenvironment. Moreover, the migration of stem cells is regulated by chemokines system. Tissue specific progenitor cells have been identified and successfully extracted and isolated. The findings provide the basis for biological therapy of intervertebral disc endogenous stem cells. Intervertebral disc endogenous stem cells play a crucial role in intervertebral disc regeneration. Therapeutic strategy of intervertebral disc endogenous stem cells is proven to be a promising biological approach for intervertebral disc regeneration.

  9. Accretion from an inhomogeneous medium

    International Nuclear Information System (INIS)

    Livio, M.; Soker, N.; Koo, M. de; Savonije, G.J.

    1986-01-01

    The problem of accretion by a compact object from an inhomogeneous medium is studied in the general γnot=1 case. The mass accretion rate is found to decrease with increasing γ. The rate of accretion of angular momentum is found to be significantly lower than the rate at which angular momentum is deposited into the Bondi-Hoyle, symmetrical, accretion cylinder. The consequences of the results are studied for the cases of neutron stars accreting from the winds of early-type companions and white dwarfs and main-sequence stars accreting from winds of cool giants. (author)

  10. Hot-spot model for accretion disc variability as random process II. Mathematics of the power-spectrum break frequency

    Czech Academy of Sciences Publication Activity Database

    Pecháček, Tomáš; Goosmann, R. W.; Karas, Vladimír; Czerny, B.; Dovčiak, Michal

    2013-01-01

    Roč. 556, August (2013), A77/1-A77/12 ISSN 0004-6361 R&D Projects: GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : accretion * black holes Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.479, year: 2013

  11. Fouling in a MBR system with rotating membrane discs

    DEFF Research Database (Denmark)

    Jørgensen, Mads Koustrup; Bentzen, Thomas Ruby; Christensen, Morten Lykkegaard

    concentrations and a clear effluent with no bacteria present in the permeate [1]. However, the process performance is limited by membrane fouling, which results in a lower productivity and higher energy demand and hence places demands for limitation of fouling and/or cleaning of the membranes. One way to do...... uses rotating ceramic membrane discs for creation of shear, which can be changed by controlling the membrane rotation speed of the membrane. Furthermore, the influence of shear on fouling is studied at different radii from the center of rotation, by dividing membranes into different concentric rings......Membrane bioreactors (MBR) are an attractive alternative solution for municipal and industrial wastewater treatment. The MBR, which is a combination of a bioreactor for sludge degradation and a membrane for separation, has the advantages of a low footprint, ability to handle high sludge...

  12. Strong gravity effects in accreting black-hole systems

    International Nuclear Information System (INIS)

    Niedzwiecki, A.

    2006-01-01

    I briefly review current status of studying effects of strong gravity in X-ray astronomy. Matter accreting onto a black hole probes the relativistic region of space-time and the high-energy radiation it produces should contain signatures of strong gravity effects. Current X-ray observations provide the evidence that the observed emission originates, in some cases, at a distance of a few gravitational radii from a black hole. Moreover, certain observations invoke interpretations favouring rapid rotation of the black hole. Some observational properties of black hole systems are supposed to result from the lack of a material surface in these objects. I consider further effects, specific for the black hole environment, which can be studied in X-ray data. Bulk motion Comptonization, which would directly reveal converging flow of matter plunging into a black hole, is unlikely to be important in formation of X-ray spectra. Similarly, Penrose processes are unlikely to give observational effects, although this issue has not been thoroughly studied so far for all plausible radiative mechanisms. (author)

  13. Small radii of neutron stars as an indication of novel in-medium effects

    International Nuclear Information System (INIS)

    Jiang, Wei-Zhou; Li, Bao-An; Fattoyev, F.J.

    2015-01-01

    At present, neutron star radii from both observations and model predictions remain very uncertain. Whereas different models can predict a wide range of neutron star radii, it is not possible for most models to predict radii that are smaller than about 10 km, thus if such small radii are established in the future they will be very difficult to reconcile with model estimates. By invoking a new term in the equation of state that enhances the energy density, but leaves the pressure unchanged we simulate the current uncertainty in the neutron star radii. This new term can be possibly due to the exchange of the weakly interacting light U-boson with appropriate in-medium parameters, which does not compromise the success of the conventional nuclear models. The validity of this new scheme will be tested eventually by more precise measurements of neutron star radii. (orig.)

  14. EVIDENCE FOR AN ACCRETION ORIGIN FOR THE OUTER HALO GLOBULAR CLUSTER SYSTEM OF M31

    International Nuclear Information System (INIS)

    Mackey, A. D.; Huxor, A. P.; Ferguson, A. M. N.; Irwin, M. J.; Chapman, S. C.; Tanvir, N. R.; McConnachie, A. W.; Ibata, R. A.; Lewis, G. F.

    2010-01-01

    We use a sample of newly discovered globular clusters from the Pan-Andromeda Archaeological Survey (PAndAS) in combination with previously cataloged objects to map the spatial distribution of globular clusters in the M31 halo. At projected radii beyond ∼30 kpc, where large coherent stellar streams are readily distinguished in the field, there is a striking correlation between these features and the positions of the globular clusters. Adopting a simple Monte Carlo approach, we test the significance of this association by computing the probability that it could be due to the chance alignment of globular clusters smoothly distributed in the M31 halo. We find that the likelihood of this possibility is low, below 1%, and conclude that the observed spatial coincidence between globular clusters and multiple tidal debris streams in the outer halo of M31 reflects a genuine physical association. Our results imply that the majority of the remote globular cluster system of M31 has been assembled as a consequence of the accretion of cluster-bearing satellite galaxies. This constitutes the most direct evidence to date that the outer halo globular cluster populations in some galaxies are largely accreted.

  15. RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

    International Nuclear Information System (INIS)

    Cunningham, Andrew J.; Klein, Richard I.; McKee, Christopher F.; Krumholz, Mark R.; Teyssier, Romain

    2012-01-01

    We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of ∼2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of ∼0.2 β 1/2 compared to the Bondi value, where β is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

  16. RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

    Energy Technology Data Exchange (ETDEWEB)

    Cunningham, Andrew J.; Klein, Richard I. [Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States); McKee, Christopher F. [Department of Astronomy, University of California Berkeley, Berkeley, CA 94720 (United States); Krumholz, Mark R. [Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA 94560 (United States); Teyssier, Romain, E-mail: ajcunn@gmail.com [Service d' Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette (France)

    2012-01-10

    We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of {approx}2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of {approx}0.2 {beta}{sup 1/2} compared to the Bondi value, where {beta} is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

  17. The Emerging Paradigm of Pebble Accretion

    NARCIS (Netherlands)

    Ormel, C.W.; Pessah, M.; Gressel, O.

    2017-01-01

    Pebble accretion is the mechanism in which small particles ("pebbles") accrete onto big bodies big (planetesimals or planetary embryos) in gas-rich environments. In pebble accretion accretion , accretion occurs by settling and depends only on the mass of the gravitating body gravitating , not its

  18. The comparative effect of FUV, EUV and X-ray disc photoevaporation on gas giant separations

    Science.gov (United States)

    Jennings, Jeff; Ercolano, Barbara; Rosotti, Giovanni P.

    2018-04-01

    Gas giants' early (≲ 5 Myr) orbital evolution occurs in a disc losing mass in part to photoevaporation driven by high energy irradiance from the host star. This process may ultimately overcome viscous accretion to disperse the disc and halt migrating giants by starving their orbits of gas, imprinting on giant planet separations in evolved systems. Inversion of this distribution could then give insight into whether stellar FUV, EUV or X-ray flux dominates photoevaporation, constraining planet formation and disc evolution models. We use a 1D hydrodynamic code in population syntheses for gas giants undergoing Type II migration in a viscously evolving disc subject to either a primarily FUV, EUV or X-ray flux from a pre-solar T Tauri star. The photoevaporative mass loss profile's unique peak location and width in each energetic regime produces characteristic features in the distribution of giant separations: a severe dearth of ≲ 2 MJ planets interior to 5 AU in the FUV scenario, a sharp concentration of ≲ 3 MJ planets between ≈1.5 - 2 AU in the EUV case, and a relative abundance of ≈2 - 3.5 MJ giants interior to 0.5 AU in the X-ray model. These features do not resemble the observational sample of gas giants with mass constraints, though our results do show some weaker qualitative similarities. We thus assess how the differing photoevaporative profiles interact with migrating giants and address the effects of large model uncertainties as a step to better connect disc models with trends in the exoplanet population.

  19. The measurement of dynamic radii for passenger car tyre

    Science.gov (United States)

    Anghelache, G.; Moisescu, R.

    2017-10-01

    The tyre dynamic rolling radius is an extremely important parameter for vehicle dynamics, for operation of safety systems as ESP, ABS, TCS, etc., for road vehicle research and development, as well as for validation or as an input parameter of automotive simulations and models. The paper investigates the dynamic rolling radii of passenger car tyre and the influence of rolling speed and inflation pressure on their magnitude. The measurement of dynamic rolling radii has been performed on a chassis dynamometer test rig. The dynamic rolling radii have been measured indirectly, using longitudinal rolling speed and angular velocity of wheel. Due to the subtle effects that the parameters have on rolling radius magnitude, very accurate equipment has to be used. Two different methods have been chosen for measuring the wheel angular velocity: the stroboscopic lamp and the incremental rotary encoder. The paper shows that the stroboscopic lamp has an insufficient resolution, therefore it was no longer used for experimental investigation. The tyre dynamic rolling radii increase with rolling speed and with tyre inflation pressure, but the effect of pressure is more significant. The paper also makes considerations on the viability of simplified formulae from literature for calculating the tyre dynamic rolling radius.

  20. Testing asteroseismic radii of dwarfs and subgiants with Kepler and Gaia

    Science.gov (United States)

    Sahlholdt, C. L.; Silva Aguirre, V.; Casagrande, L.; Mosumgaard, J. R.; Bojsen-Hansen, M.

    2018-05-01

    We test asteroseismic radii of Kepler main-sequence and subgiant stars by deriving their parallaxes which are compared with those of the first Gaia data release. We compute radii based on the asteroseismic scaling relations as well as by fitting observed oscillation frequencies to stellar models for a subset of the sample, and test the impact of using effective temperatures from either spectroscopy or the infrared flux method. An offset of 3 per cent, showing no dependency on any stellar parameters, is found between seismic parallaxes derived from frequency modelling and those from Gaia. For parallaxes based on radii from the scaling relations, a smaller offset is found on average; however, the offset becomes temperature dependent which we interpret as problems with the scaling relations at high stellar temperatures. Using the hotter infrared flux method temperature scale, there is no indication that radii from the scaling relations are inaccurate by more than about 5 per cent. Taking the radii and masses from the modelling of individual frequencies as reference values, we seek to correct the scaling relations for the observed temperature trend. This analysis indicates that the scaling relations systematically overestimate radii and masses at high temperatures, and that they are accurate to within 5 per cent in radius and 13 per cent in mass for main-sequence stars with temperatures below 6400 K. However, further analysis is required to test the validity of the corrections on a star-by-star basis and for more evolved stars.

  1. Systematics of experimental charge radii of elements and elementary particles

    Energy Technology Data Exchange (ETDEWEB)

    Antony, M.S.; Britz, J.

    1987-02-01

    The systematics of experimental charge radii of elements and elementary particles ..pi../sup -/, K/sup -/, K/sup 0/, p and n is discussed. The root-meansquare charge radius of a quark core in nucleous derived from the systematics is estimated to be 0.3 fm. Charge radii evaluated from Coulomb displacement energies are also tabulated.

  2. The 3H–3He Charge Radii Difference

    Directory of Open Access Journals (Sweden)

    Myers L. S.

    2016-01-01

    Full Text Available The upcoming E12-14-009 [1] experiment at Jefferson Lab will determine the ratio of the electric form factors for the A=3 mirror nuclei 3He and 3H. The measurement will use a 1.1 GeV electron beam, a special collimator plate to allow for simultaneous optics measurements, and the low-activity tritium target being prepared for Jefferson Lab. By observing the dependence of the form factor ratio as a function of Q2 over 0.05–0.09 GeV2, the dependence of the radii extraction on the shape of the form factors is minimized. As a result, we anticipate the uncertainty of the extracted charge radii difference to be 0.03 fm, a reduction of 70% from the current measurement. Using precise measurements of the 3He charge radius from isotopic shift or μHe measurements [2–4], we can deduce the absolute 3H charge radius. The results will provide a direct comparison to recent calculations of the charge radii.

  3. Measurements of interaction cross sections and nuclear radii of Li isotopes

    International Nuclear Information System (INIS)

    Tanihata, I.; Hamagaki, H.; Hashimoto, O.; Shida, Y.; Yoshikawa, N.; Sugimoto, K.; Yamakawa, O.; Kobayashi, T.; Takahashi, N.

    1985-08-01

    Interaction cross sections(σ sub(I)) for all known Li isotopes ( 6 Li - 11 Li) and 9 Be on targets Be, C, and Al have been measured at 790 MeV/nucleon. Nuclear radii(R sub(I)) of these isotopes have been deduced from the σ sub(I). The differences of radii among isobars( 6 He - 6 Li, 8 He - 8 Li, and 9 Li - 9 Be) have been found for the first time. A comparison of R sub(I) with the rms radii obtained from electron-scattering is presented. (author)

  4. New methods to benchmark simulations of accreting black holes systems against observations

    Science.gov (United States)

    Markoff, Sera; Chatterjee, Koushik; Liska, Matthew; Tchekhovskoy, Alexander; Hesp, Casper; Ceccobello, Chiara; Russell, Thomas

    2017-08-01

    The field of black hole accretion has been significantly advanced by the use of complex ideal general relativistic magnetohydrodynamics (GRMHD) codes, now capable of simulating scales from the event horizon out to ~10^5 gravitational radii at high resolution. The challenge remains how to test these simulations against data, because the self-consistent treatment of radiation is still in its early days, and is complicated by dependence on non-ideal/microphysical processes not yet included in the codes. On the other extreme, a variety of phenomenological models (disk, corona, jet, wind) can well-describe spectra or variability signatures in a particular waveband, although often not both. To bring these two methodologies together, we need robust observational “benchmarks” that can be identified and studied in simulations. I will focus on one example of such a benchmark, from recent observational campaigns on black holes across the mass scale: the jet break. I will describe new work attempting to understand what drives this feature by searching for regions that share similar trends in terms of dependence on accretion power or magnetisation. Such methods can allow early tests of simulation assumptions and help pinpoint which regions will dominate the light production, well before full radiative processes are incorporated, and will help guide the interpretation of, e.g. Event Horizon Telescope data.

  5. Deformation of the Galactic Centre stellar cusp due to the gravity of a growing gas disc

    Science.gov (United States)

    Kaur, Karamveer; Sridhar, S.

    2018-06-01

    The nuclear star cluster surrounding the massive black hole at the Galactic Centre consists of young and old stars, with most of the stellar mass in an extended, cuspy distribution of old stars. The compact cluster of young stars was probably born in situ in a massive accretion disc around the black hole. We investigate the effect of the growing gravity of the disc on the orbits of the old stars, using an integrable model of the deformation of a spherical star cluster with anisotropic velocity dispersions. A formula for the perturbed phase-space distribution function is derived using linear theory, and new density and surface density profiles are computed. The cusp undergoes a spheroidal deformation with the flattening increasing strongly at smaller distances from the black hole; the intrinsic axis ratio ˜0.8 at ˜0.15 pc. Stellar orbits are deformed such that they spend more time near the disc plane and sample the dense inner parts of the disc; this could result in enhanced stripping of the envelopes of red giant stars. Linear theory accounts only for orbits whose apsides circulate. The non-linear theory of adiabatic capture into resonance is needed to understand orbits whose apsides librate. The mechanism is a generic dynamical process, and it may be common in galactic nuclei.

  6. Wind accretion: Theory and observations

    Science.gov (United States)

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.; Sidoli, L.; Paizis, A.

    2015-07-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass

  7. Disc operational system

    International Nuclear Information System (INIS)

    Veretenov, V.Yu.; Volkov, A.I.; Gurevich, M.I.; Kozik, V.S.; Pod'yachev, E.I.; Shapiro, M.L.

    1974-01-01

    A disc operational system is proposed, which is based on the file structure and designed for use in a BESM-6 computer with the software system comprising a dispatcher DD-73 and a monitor 'Dubna'. The main distinguishing feature of the disc operational system is the decentralization of the file system. Each disc package is an independent file unaffected by the state of the other disc packages. The use of several disc packages is allowed. The above feature of the disc operational system makes it possible to simplify the language of communication with the system, to give the user the opportunity of controlling the file quite independently, and to simplify the maintenance of the discs by the computer personnel. One and the same disc can be simultaneously addressed by all problems in the processor (both mathematical and service). A single file, however, may be used in the recording mode by only one problem. The description presented is the instruction for users. It also describes special possibilities open to the system programmers [ru

  8. Towards a population synthesis model of self-gravitating disc fragmentation and tidal downsizing II: the effect of fragment-fragment interactions

    Science.gov (United States)

    Forgan, D. H.; Hall, C.; Meru, F.; Rice, W. K. M.

    2018-03-01

    It is likely that most protostellar systems undergo a brief phase where the protostellar disc is self-gravitating. If these discs are prone to fragmentation, then they are able to rapidly form objects that are initially of several Jupiter masses and larger. The fate of these disc fragments (and the fate of planetary bodies formed afterwards via core accretion) depends sensitively not only on the fragment's interaction with the disc, but also with its neighbouring fragments. We return to and revise our population synthesis model of self-gravitating disc fragmentation and tidal downsizing. Amongst other improvements, the model now directly incorporates fragment-fragment interactions while the disc is still present. We find that fragment-fragment scattering dominates the orbital evolution, even when we enforce rapid migration and inefficient gap formation. Compared to our previous model, we see a small increase in the number of terrestrial-type objects being formed, although their survival under tidal evolution is at best unclear. We also see evidence for disrupted fragments with evolved grain populations - this is circumstantial evidence for the formation of planetesimal belts, a phenomenon not seen in runs where fragment-fragment interactions are ignored. In spite of intense dynamical evolution, our population is dominated by massive giant planets and brown dwarfs at large semimajor axis, which direct imaging surveys should, but only rarely, detect. Finally, disc fragmentation is shown to be an efficient manufacturer of free-floating planetary mass objects, and the typical multiplicity of systems formed via gravitational instability will be low.

  9. Testing Asteroseismic Radii of Dwarfs and Subgiants with Kepler and Gaia

    DEFF Research Database (Denmark)

    Sahlholdt, C. L.; Silva Aguirre, V.; Casagrande, L.

    2018-01-01

    We test asteroseismic radii of Kepler main-sequence and subgiant stars by deriving their parallaxes which are compared with those of the first Gaia data release. We compute radii based on the asteroseismic scaling relations as well as by fitting observed oscillation frequencies to stellar models...... overestimate radii and masses at high temperatures, and that they are accurate to within 5% in radius and 13% in mass for main-sequence stars with temperatures below 6400 K. However, further analysis is required to test the validity of the corrections on a star-by-star basis and for more evolved stars....

  10. Nuclear charge radii of 8,9Li determined by laser spectroscopy

    International Nuclear Information System (INIS)

    Ewald, G.; Dax, A.; Goette, S.; Kirchner, R.; Kluge, H.J.; Kuehl, T.; Sanchez, R.; Wojtaszek, A.; Noertershaeuser, W.; Drake, G.W.F.; Yan, Z.C.; Zimmermann, C.

    2004-06-01

    The 2S → 3S transition of 6,7,8,9 Li was studied by high-resolution laser spectroscopy using two-photon Doppler-free excitation and resonance-ionization detection. The hyperfine structure splitting and the isotope shift were determined with precision at the 100 kHz level. Combined with recent theoretical work, the changes in nuclear charge radii of 8,9 Li were determined. These are now the lightest short-lived isotopes for which the charge radii have been measured. It is found that the charge radii monotonically decrease with increasing neutron number from 6 Li to 9 Li. (orig.)

  11. Accretion-Ejection Instability in magnetized accretion disk around compact objects

    International Nuclear Information System (INIS)

    Varniere, Peggy

    2002-01-01

    The major problem in accretion physics come from the origin of angular momentum transfer in the disk. My PhD deal with a mechanism (the Accretion-Ejection Instability, AEI) able to explain and link together accretion in the inner region of the disk and ejection. This instability occurs in magnetized accretion disk near equipartition with gas pressure. We first study the impact of some relativistic effects on the instability, particularly on the m = 1 mode. And compared the results with the Quasi-Periodic Oscillation (QPO) observed in micro-quasars. In the second part we study analytically and numerically the Alfven wave emission mechanism which re-emit the angular momentum and energy taken from the inner region of the disk into the corona. The last part deals with MHD numerical simulation. First of all a 2D non-linear disk simulation which contribute to QPO modelization. The last chapter is about a beginning collaboration on 3D simulation in order to study the Alfven wave emission in the corona. (author) [fr

  12. Variation of nuclear radii in the drip line regions

    CERN Document Server

    Beiner, M; Mas, D

    1976-01-01

    The authors are concerned with predictions of the energy density method with respect to the nuclear sizes (RMS radii). It is known that the commonly accepted A/sup 1/3/-type laws are only approximative and deviations are expected to grow significantly as one goes away from the beta -stability region. Particular attention is paid to the variation of nuclear radii in the drip line regions. Implications of the resulting large total Coulomb energy variations between neighbouring nuclei will be emphasized.

  13. On accretion from an inhomogeneous medium

    International Nuclear Information System (INIS)

    Davies, R.E.; Pringle, J.E.

    1980-01-01

    Hypersonic accretion flow in two dimensions from an infinite medium which contains a small density and/or velocity gradient is considered. To first order in rsub(a)/h, where rsub(a) is the accretion radius and h the scale of the gradient, the accretion rate is unaffected and the accreted angular momentum is zero. Thus previous estimates of the amount of angular momentum accreted may severely overestimate the actual value. (author)

  14. The ionizing effect of low-energy cosmic rays from a class II object on its protoplanetary disc

    Science.gov (United States)

    Rodgers-Lee, D.; Taylor, A. M.; Ray, T. P.; Downes, T. P.

    2017-11-01

    We investigate the ionizing effect of low-energy cosmic rays (CRs) from a young star on its protoplanetary disc (PPD). We consider specifically the effect of ∼3 GeV protons injected at the inner edge of the PPD. An increase in the ionization fraction as a result of these CRs could allow the magnetorotational instability to operate in otherwise magnetically dead regions of the disc. For the typical values assumed we find an ionization rate of ζCR ∼ 10-17 s-1 at 1 au. The transport equation is solved by treating the propagation of the CRs as diffusive. We find for increasing diffusion coefficients the CRs penetrate further in the PPD, while varying the mass density profile of the disc is found to have little effect. We investigate the effect of an energy spectrum of CRs. The influence of a disc wind is examined by including an advective term. For advective wind speeds between 1 and 100 km s-1 diffusion dominates at all radii considered here (out to 10 au) for reasonable diffusion coefficients. Overall, we find that low-energy CRs can significantly ionize the mid-plane of PPDs out to ∼1 au. By increasing the luminosity or energy of the CRs, within plausible limits, their radial influence could increase to ∼2 au at the mid-plane but it remains challenging to significantly ionize the mid-plane further out.

  15. Snow accretion on overhead wires

    Energy Technology Data Exchange (ETDEWEB)

    Sakamoto, Y. [Meteorological Research Inst. for Technology Co. Ltd., Tokyo (Japan); Tachizaki, S.; Sudo, N. [Tohoku Electric Power Co. Ltd., Miyagi (Japan)

    2005-07-01

    Wet snow accretion can cause extensive damage to transmission systems. This paper reviewed some of the difficulties faced by researchers in the study of wet snow accretion on overhead lines in Japan. The study of snow accretion phenomena is complicated by the range of phase changes in water. Snowflakes produced in an upper atmospheric layer with a temperature below freezing do not melt when they go through a lower atmospheric layer with a temperature above freezing, but are in a mixed state of solid and liquid due to the latent heat of melting. The complicated properties of water make studies of snow accretion difficult, as well as the fact that snow changes its physical properties rapidly, due to the effects of ambient temperature, rainfall, and solar radiation. The adhesive forces that cause snow accretion include freezing; bonding through freezing; sintering; condensation and freezing of vapor in the air; mechanical intertwining of snowflakes; capillary action due to liquids; coherent forces between ice particles and water formed through the metamorphosis of snowflakes. In addition to these complexities, differences in laboratory room environments and natural snow environments can also pose difficulties for researchers. Equations describing the relationship between the density of accreted snow and the meteorological parameters involved were presented, as well as empirical equations which suggested that snow accretion efficiency has a dependency on air temperature. An empirical model for estimating snow loads in Japan was outlined, as well as various experiments observing show shedding. Correlations for wet snow accretion included precipitation intensity; duration of precipitation; air temperature; wind speed and wind direction in relation to the overhead line. Issues concerning topography and wet snow accretion were reviewed. It was concluded that studies of snow accretion will benefit by the collection of data in each matrix of the relevant parameters. 12 refs

  16. Massive disc formation in the tidal disruption of a neutron star by a nearly extremal black hole

    International Nuclear Information System (INIS)

    Lovelace, Geoffrey; Kidder, Lawrence E; Duez, Matthew D; Foucart, Francois; Pfeiffer, Harald P; Scheel, Mark A; Szilágyi, Béla

    2013-01-01

    Black hole–neutron star (BHNS) binaries are important sources of gravitational waves for second-generation interferometers, and BHNS mergers are also a proposed engine for short, hard gamma-ray bursts. The behavior of both the spacetime (and thus the emitted gravitational waves) and the neutron-star matter in a BHNS merger depend strongly and nonlinearly on the black hole's spin. While there is a significant possibility that astrophysical black holes could have spins that are nearly extremal (i.e. near the theoretical maximum), to date fully relativistic simulations of BHNS binaries have included black-hole spins only up to S/M 2 = 0.9, which corresponds to the black hole having approximately half as much rotational energy as possible, given the black hole's mass. In this paper, we present a new simulation of a BHNS binary with a mass ratio q = 3 and black-hole spin S/M 2 = 0.97, the highest simulated to date. We find that the black hole's large spin leads to the most massive accretion disc and the largest tidal tail outflow of any fully relativistic BHNS simulations to date, even exceeding the results implied by extrapolating results from simulations with lower black-hole spin. The disc appears to be remarkably stable. We also find that the high black-hole spin persists until shortly before the time of merger; afterward, both merger and accretion spin down the black hole. (paper)

  17. EVOLUTION OF MASSIVE PROTOSTARS VIA DISK ACCRETION

    International Nuclear Information System (INIS)

    Hosokawa, Takashi; Omukai, Kazuyuki; Yorke, Harold W.

    2010-01-01

    Mass accretion onto (proto-)stars at high accretion rates M-dot * > 10 -4 M sun yr -1 is expected in massive star formation. We study the evolution of massive protostars at such high rates by numerically solving the stellar structure equations. In this paper, we examine the evolution via disk accretion. We consider a limiting case of 'cold' disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated evolution via spherically symmetric accretion, the opposite limit, whereby the material accreting onto the star contains the entropy produced in the accretion shock front. We examine how different accretion geometries affect the evolution of massive protostars. For cold disk accretion at 10 -3 M sun yr -1 , the radius of a protostar is initially small, R * ≅ a few R sun . After several solar masses have accreted, the protostar begins to bloat up and for M * ≅ 10 M sun the stellar radius attains its maximum of 30-400 R sun . The large radius ∼100 R sun is also a feature of spherically symmetric accretion at the same accreted mass and accretion rate. Hence, expansion to a large radius is a robust feature of accreting massive protostars. At later times, the protostar eventually begins to contract and reaches the zero-age main sequence (ZAMS) for M * ≅ 30 M sun , independent of the accretion geometry. For accretion rates exceeding several 10 -3 M sun yr -1 , the protostar never contracts to the ZAMS. The very large radius of several hundreds R sun results in the low effective temperature and low UV luminosity of the protostar. Such bloated protostars could well explain the existence of bright high-mass protostellar objects, which lack detectable H II regions.

  18. Nearly collisionless spherical accretion

    International Nuclear Information System (INIS)

    Begelman, M.C.

    1977-01-01

    A fluid-like gas accretes much more efficiently than a collisionless gas. The ability of an accreting gas to behave like a fluid depends on the relationship of the mean free path of a gas particle at r → infinity lambdasub(infinity), to the typical length scales associated with the star-gas system. This relationship is examined in detail. For constant collision cross-section evidence is found for a rapid changeover from collisionless to fluid-like accretion flow when lambdasub(infinity) drops below a certain value, but for hard Coulomb collisions, the transition is more gradual, and is sensitive to the adiabatic index of the gas at r→ infinity. To these results must be added the effects of the substantial cusp of bound particles, which always develops in a system with arbitrarily small but non-zero cross-section. The density run in such a cusp depends on the collision properties of the particles. 'Loss-cone' accretion from the cusp may in some cases exceed the predicted accretion rate. (author)

  19. Charge radii of neutron-deficient Ca isotopes

    Science.gov (United States)

    Miller, A. J.; Minamisono, K.; Klose, A.; Everett, N.; Kalman, C.; Powel, R. C.; Watkins, J.; Garand, D.; Sumithrarachchi, C.; Krämer, J.; Maa, B.; Nörtershäuser, W.; Rossi, D. M.; Kujawa, C.; Pineda, S.; Lantis, J.; Liu, Y.; Mantica, P. F.; Pearson, M. R.

    2017-09-01

    Nucleon shell closures are generally associated with a local minimum in mean-square charge radii, 〈r2 〉 , along an isotopic chain. The 〈r2 〉 of 18Ar and 19K isotopes, however, do not show this signature at the N = 20 neutron shell closure. To gain a microscopic understanding of this abnormal behavior, measurements of 〈r2 〉 of neutron-deficient Ca isotopes below N = 20 have been proposed at the BEam COoling and LAser spectroscopy (BECOLA) facility at NSCL/MSU. Preliminary results will be presented and the deduced charge radii will be compared to theoretical calculations and the trends in the nearby isotopic chains. Work supported in part by NSF Grant PHY-15-65546, U.S. DOE Grant DE-NA0002924 and by the Deutsche Forschungsgemeinschaft through Grant SFB 1245.

  20. Iron K and Compton hump reverberation in SWIFT J2127.4+5654 and NGC 1365 revealed by NuSTAR and XMM–Newton

    DEFF Research Database (Denmark)

    Kara, E.; Zoghbi, A.; Marinucci, A.

    2015-01-01

    In the past five years, a flurry of X-ray reverberation lag measurements of accreting supermassive black holes have been made using the XMM–Newton telescope in the 0.3–10 keV energy range. In this work, we use the NuSTAR (Nuclear Spectroscopic Telescope Array) telescope to extend the lag analysis...... up to higher energies for two Seyfert galaxies, SWIFT J2127.4+5654 and NGC 1365. X-ray reverberation lags are due to the light travel time delays between the direct continuum emission and the reprocessed emission from the inner radii of an ionized accretion disc. XMM–Newton has been particularly...... evidence for Compton reflection, known as the Compton ‘hump’. The XMM–Newton data show Fe K lags in both SWIFT J2127.4+5654 and NGC 1365. The NuSTAR data provide independent confirmation of these Fe K lags, and also show evidence for the corresponding Compton hump lags, especially in SWIFT J2127...

  1. Magnetohydrodynamics of accretion disks

    International Nuclear Information System (INIS)

    Torkelsson, U.

    1994-04-01

    The thesis consists of an introduction and summary, and five research papers. The introduction and summary provides the background in accretion disk physics and magnetohydrodynamics. The research papers describe numerical studies of magnetohydrodynamical processes in accretion disks. Paper 1 is a one-dimensional study of the effect of magnetic buoyancy on a flux tube in an accretion disk. The stabilizing influence of an accretion disk corona on the flux tube is demonstrated. Paper 2-4 present numerical simulations of mean-field dynamos in accretion disks. Paper 11 verifies the correctness of the numerical code by comparing linear models to previous work by other groups. The results are also extended to somewhat modified disk models. A transition from an oscillatory mode of negative parity for thick disks to a steady mode of even parity for thin disks is found. Preliminary results for nonlinear dynamos at very high dynamo numbers are also presented. Paper 3 describes the bifurcation behaviour of the nonlinear dynamos. For positive dynamo numbers it is found that the initial steady solution is replaced by an oscillatory solution of odd parity. For negative dynamo numbers the solution becomes chaotic at sufficiently high dynamo numbers. Paper 4 continues the studies of nonlinear dynamos, and it is demonstrated that a chaotic solution appears even for positive dynamo numbers, but that it returns to a steady solution of mixed parity at very high dynamo numbers. Paper 5 describes a first attempt at simulating the small-scale turbulence of an accretion disk in three dimensions. There is only find cases of decaying turbulence, but this is rather due to limitations of the simulations than that turbulence is really absent in accretion disks

  2. Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

    Science.gov (United States)

    Nandi, Anuj; Mandal, S.; Sreehari, H.; Radhika, D.; Das, Santabrata; Chattopadhyay, I.; Iyer, N.; Agrawal, V. K.; Aktar, R.

    2018-05-01

    We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data (˜166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband (3-150 keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasi-periodic Oscillation (QPO) frequencies using the two-component advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (\\dot{m}d), sub-Keplerian accretion rate (\\dot{m}h), shock location (rs) and black hole mass (M_{bh}) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as L^{obs}_{jet} ˜3-6 ×10^{37} erg s^{-1} during one of the observed radio flares which indicates that jet power corresponds to 8-16% mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate (˜14%) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of 5.2-7.9 M_{⊙} with 90% confidence.

  3. Accretion onto CO White Dwarfs using MESA

    Science.gov (United States)

    Feng, Wanda; Starrfield, Sumner

    2018-06-01

    The nature of type Ia Supernovae (SNe Ia) progenitor systems and their underlying mechanism are not well understood. There are two competing progenitor scenarios: the single-degenerate scenario wherein a white dwarf (WD) star accretes material from a companion star, reaching the Chandrasekhar mass limit; and, the double-degenerate scenario wherein two WDs merge. In this study, we investigate the single-degenerate scenario by accretion onto carbon-oxygen (CO) WDs using the Modules for Experiments in Stellar Astrophysics (MESA). We vary the WD mass, composition of the accreting material, and accretion rate in our models. Mixing between the accreted material and the WD core is informed by multidimensional studies that suggest occurance after thermonuclear runaway (TNR) ensues. We compare the accretion of solar composition material onto CO WDs with the accretion of mixed solar and core material after TNR. As many of our models eject less material than accreted, our study supports that accretion onto CO WDs is a feasible channel for SNe I progenitors.

  4. Relativistic, accreting disks

    International Nuclear Information System (INIS)

    Abramowicz, M.A; Jaroszynski, M.; Sikora, M.

    1978-01-01

    An analytic theory of the hydrodynamical structure of accreting disks (without self-gravitation but with pressure) orbiting around and axially symmetric, stationary, compact body (e.g. black hole) is presented. The inner edge of the marginally stable accreting disk (i.e. disk with constant angular momentum density) has a sharp cusp located on the equatorial plane between rsub(ms) and rsub(mb). The existence of the cusp is also typical for any angular momentum distribution. The physical importance of the cusp follows from the close analogy with the case of a close binary system (L 1 Lagrange point on the Roche lobe). The existence of the cusp is thus a crucial phenomenon in such problems as boundary condition for the viscous stresses, accretion rate etc. (orig.) [de

  5. MR imaging findings of a sequestered disc in the lumbar spine: a comparison with an extruded disc

    International Nuclear Information System (INIS)

    Sim, Su Youn; Park, Ji Seon; Ryu, Kyung Nam; Jin, Wook

    2007-01-01

    To compare the MR findings of a sequestered disc with an extruded disc. MR images of 28 patients with a sequestered disc and 18 patients with an extruded disc were retrospectively reviewed. Patients with sequestered discs were divided into two groups whether definite separation from the parent disc was or was not seen. In the latter group (definite separation not seen) and the extruded disc group of patients, the signal intensities of the herniated discs were compared with the signal intensities of the parent discs and were evaluated on T1-and T2-weighted images. We also assessed the presence of a notch within the herniated disc. In the sequestered disc group of patients (28 discs), only 5 discs (18%) showed obvious separation from the parent disc. Among the remaining 23 discs with indefinite separation, the notch was visible in 14 discs (61%) and 9 discs (39%) had no notch. In the extruded disc group (18 discs), the notch was visible in 2 (11%) discs and the difference between the two groups was statistically significant (ρ 0.0002). The signal intensities of the herniated discs on T1-weighted images were isointense in both the sequestered and extruded discs. The difference of incidence of high signal intensities on T2-weighted images was not statistically significant (ρ = 0.125). It is necessary to consider the possibility of the presence of a sequestered disc when a herniated disc material shows a notch

  6. LACK OF INFLATED RADII FOR KEPLER GIANT PLANET CANDIDATES RECEIVING MODEST STELLAR IRRADIATION

    International Nuclear Information System (INIS)

    Demory, Brice-Olivier; Seager, Sara

    2011-01-01

    The most irradiated transiting hot Jupiters are characterized by anomalously inflated radii, sometimes exceeding Jupiter's size by more than 60%. While different theoretical explanations have been applied, none of them provide a universal resolution to this observation, despite significant progress in the past years. We refine the photometric transit light curve analysis of 115 Kepler giant planet candidates based on public Q0-Q2 photometry. We find that 14% of them are likely false positives, based on their secondary eclipse depth. We report on planet radii versus stellar flux. We find an increase in planet radii with increased stellar irradiation for the Kepler giant planet candidates, in good agreement with existing hot Jupiter systems. We find that in the case of modest irradiation received from the stellar host, giant planets do not have inflated radii, and appear to have radii independent of the host star incident flux. This finding suggests that the physical mechanisms inflating hot Jupiters become ineffective below a given orbit-averaged stellar irradiation level of ∼2 × 10 8 erg s –1 cm –2 .

  7. A differential equation for the Generalized Born radii.

    Science.gov (United States)

    Fogolari, Federico; Corazza, Alessandra; Esposito, Gennaro

    2013-06-28

    The Generalized Born (GB) model offers a convenient way of representing electrostatics in complex macromolecules like proteins or nucleic acids. The computation of atomic GB radii is currently performed by different non-local approaches involving volume or surface integrals. Here we obtain a non-linear second-order partial differential equation for the Generalized Born radius, which may be solved using local iterative algorithms. The equation is derived under the assumption that the usual GB approximation to the reaction field obeys Laplace's equation. The equation admits as particular solutions the correct GB radii for the sphere and the plane. The tests performed on a set of 55 different proteins show an overall agreement with other reference GB models and "perfect" Poisson-Boltzmann based values.

  8. Spins, moments and charge radii beyond $^{48}$Ca

    CERN Multimedia

    Neyens, G; Rajabali, M M; Hammen, M; Blaum, K; Froemmgen, N E; Garcia ruiz, R F; Kreim, K D; Budincevic, I

    Laser spectroscopy of $^{49-54}$Ca is proposed as a continuation of the experimental theme initiated with IS484 “Ground-state properties of K-isotopes from laser and $\\beta$-NMR spectroscopy” and expanded in INTC-I-117 “Moments, Spins and Charge Radii Beyond $^{48}$Ca.” It is anticipated that the charge radii of these isotopes can show strong evidence for the existence of a sub-shell closure at N=32 and could provide a first tentative investigation into the existence of a shell effect at N=34. Furthermore the proposed experiments will simultaneously provide model-independent measurements of the spins, magnetic moments and quadrupole moments of $^{51,53}$Ca permitting existing and future excitation spectra to be pinned to firm unambiguous ground states.

  9. Accretion onto a Kiselev black hole

    Energy Technology Data Exchange (ETDEWEB)

    Jiao, Lei [Hebei University, College of Physical Science and Technology, Baoding (China); Yang, Rongjia [Hebei University, College of Physical Science and Technology, Baoding (China); Hebei University, Hebei Key Lab of Optic-Electronic Information and Materials, Baoding (China)

    2017-05-15

    We consider accretion onto a Kiselev black hole. We obtain the fundamental equations for accretion without the back-reaction. We determine the general analytic expressions for the critical points and the mass accretion rate and find the physical conditions the critical points should fulfill. The case of a polytropic gas are discussed in detail. It turns out that the quintessence parameter plays an important role in the accretion process. (orig.)

  10. Gravity signatures of terrane accretion

    Science.gov (United States)

    Franco, Heather; Abbott, Dallas

    1999-01-01

    In modern collisional environments, accreted terranes are bracketed by forearc gravity lows, a gravitational feature which results from the abandonment of the original trench and the initiation of a new trench seaward of the accreted terrane. The size and shape of the gravity low depends on the type of accreted feature and the strength of the formerly subducting plate. Along the Central American trench, the accretion of Gorgona Island caused a seaward trench jump of 48 to 66 km. The relict trench axes show up as gravity lows behind the trench with minimum values of -78 mgal (N of Gorgona) and -49 mgal (S of Gorgona) respectively. These forearc gravity lows have little or no topographic expression. The active trench immediately seaward of these forearc gravity lows has minimum gravity values of -59 mgal (N of Gorgona) and -58 mgal (S of Gorgona), respectively. In the north, the active trench has a less pronounced gravity low than the sediment covered forearc. In the Mariana arc, two Cretaceous seamounts have been accreted to the Eocene arc. The northern seamount is most likely a large block, the southern seamount may be a thrust slice. These more recent accretion events have produced modest forearc topographic and gravity lows in comparison with the topographic and gravity lows within the active trench. However, the minimum values of the Mariana forearc gravity lows are modest only by comparison to the Mariana Trench (-216 mgal); their absolute values are more negative than at Gorgona Island (-145 to -146 mgal). We speculate that the forearc gravity lows and seaward trench jumps near Gorgona Island were produced by the accretion of a hotspot island from a strong plate. The Mariana gravity lows and seaward trench jumps (or thrust slices) were the result of breaking a relatively weak plate close to the seamount edifice. These gravity lows resulting from accretion events should be preserved in older accreted terranes.

  11. Relativistic, accreting disks

    Energy Technology Data Exchange (ETDEWEB)

    Abramowicz, M A; Jaroszynski, M; Sikora, M [Polska Akademia Nauk, Warsaw

    1978-02-01

    An analytic theory of the hydrodynamical structure of accreting disks (without self-gravitation but with pressure) orbiting around an axially symmetric, stationary, compact body (e.g. black hole) is presented. The inner edge of the marginally stable accreting disk (i.e. disk with constant angular momentum density) has a sharp cusp located on the equatorial plane between r/sub ms/ and r/sub mb/. The existence of the cusp is also typical for any angular momentum distribution. The physical importance of the cusp follows from the close analogy with the case of a close binary system (L/sub 1/ Lagrange point on the Roche lobe). The existence of the cusp is thus a crucial phenomenon in such problems as boundary condition for the viscous stresses, accretion rate, etc.

  12. X-ray photoevaporation's limited success in the formation of planetesimals by the streaming instability

    Science.gov (United States)

    Ercolano, Barbara; Jennings, Jeff; Rosotti, Giovanni; Birnstiel, Tilman

    2017-12-01

    The streaming instability is often invoked as solution to the fragmentation and drift barriers in planetesimal formation, catalysing the aggregation of dust on kyr time-scales to grow km-sized cores. However, there remains a lack of consensus on the physical mechanism(s) responsible for initiating it. One potential avenue is disc photoevaporation, wherein the preferential removal of relatively dust-free gas increases the disc metallicity. Late in the disc lifetime, photoevaporation dominates viscous accretion, creating a gradient in the depleted gas surface density near the location of the gap. This induces a local pressure maximum that collects drifting dust particles, which may then become susceptible to the streaming instability. Using a one-dimensional viscous evolution model of a disc subject to internal X-ray photoevaporation, we explore the efficacy of this process to build planetesimals. Over a range of parameters, we find that the amount of dust mass converted into planetesimals is often planetary cores. Our results are in contrast to a recent, similar investigation that considered an far-ultra-violet (FUV)-driven photoevaporation model and reported the formation of tens of M⊕ at large (>100 au) disc radii. The discrepancies are primarily a consequence of the different photoevaporation profiles assumed. Until observations more tightly constrain photoevaporation models, the relevance of this process to the formation of planets remains uncertain.

  13. What makes the family of barred disc galaxies so rich: damping stellar bars in spinning haloes

    Science.gov (United States)

    Collier, Angela; Shlosman, Isaac; Heller, Clayton

    2018-05-01

    We model and analyse the secular evolution of stellar bars in spinning dark matter (DM) haloes with the cosmological spin λ ˜ 0-0.09. Using high-resolution stellar and DM numerical simulations, we focus on angular momentum exchange between stellar discs and DM haloes of various axisymmetric shapes - spherical, oblate, and prolate. We find that stellar bars experience a diverse evolution that is guided by the ability of parent haloes to absorb angular momentum, J, lost by the disc through the action of gravitational torques, resonant and non-resonant. We confirm that dynamical bar instability is accelerated via resonant J-transfer to the halo. Our main findings relate to the long-term secular evolution of disc-halo systems: with an increasing λ, bars experience less growth and basically dissolve after they pass through vertical buckling instability. Specifically, with increasing λ, (1) the vertical buckling instability in stellar bars colludes with inability of the inner halo to absorb J - this emerges as the main factor weakening or destroying bars in spinning haloes; (2) bars lose progressively less J, and their pattern speeds level off; (3) bars are smaller, and for λ ≳ 0.06 cease their growth completely following buckling; (4) bars in λ > 0.03 haloes have ratio of corotation-to-bar radii, RCR/Rb > 2, and represent so-called slow bars without offset dust lanes. We provide a quantitative analysis of J-transfer in disc-halo systems, and explain the reasons for absence of growth in fast spinning haloes and its observational corollaries. We conclude that stellar bar evolution is substantially more complex than anticipated, and bars are not as resilient as has been considered so far.

  14. WIND-ACCRETION DISKS IN WIDE BINARIES, SECOND-GENERATION PROTOPLANETARY DISKS, AND ACCRETION ONTO WHITE DWARFS

    International Nuclear Information System (INIS)

    Perets, Hagai B.; Kenyon, Scott J.

    2013-01-01

    Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long-term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind accretion leads to long-lived stable disks over the lifetime of the asymptotic giant branch donor star. The disks have masses of a few times 10 –5 -10 –3 M ☉ , with surface density and temperature profiles that follow broken power laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly, 50%-80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M ☉ . When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks, suggesting that coordinated observational programs might improve our understanding of uncertain disk physics.

  15. Mixed ice accretion on aircraft wings

    Science.gov (United States)

    Janjua, Zaid A.; Turnbull, Barbara; Hibberd, Stephen; Choi, Kwing-So

    2018-02-01

    Ice accretion is a problematic natural phenomenon that affects a wide range of engineering applications including power cables, radio masts, and wind turbines. Accretion on aircraft wings occurs when supercooled water droplets freeze instantaneously on impact to form rime ice or runback as water along the wing to form glaze ice. Most models to date have ignored the accretion of mixed ice, which is a combination of rime and glaze. A parameter we term the "freezing fraction" is defined as the fraction of a supercooled droplet that freezes on impact with the top surface of the accretion ice to explore the concept of mixed ice accretion. Additionally we consider different "packing densities" of rime ice, mimicking the different bulk rime densities observed in nature. Ice accretion is considered in four stages: rime, primary mixed, secondary mixed, and glaze ice. Predictions match with existing models and experimental data in the limiting rime and glaze cases. The mixed ice formulation however provides additional insight into the composition of the overall ice structure, which ultimately influences adhesion and ice thickness, and shows that for similar atmospheric parameter ranges, this simple mixed ice description leads to very different accretion rates. A simple one-dimensional energy balance was solved to show how this freezing fraction parameter increases with decrease in atmospheric temperature, with lower freezing fraction promoting glaze ice accretion.

  16. Early UV emission from disc-originated matter (DOM) in Type Ia supernovae in the double-degenerate scenario

    Science.gov (United States)

    Levanon, Naveh; Soker, Noam

    2017-09-01

    We show that the blue and UV excess emission in the first few days of some Type Ia supernovae (SNe Ia) can be accounted in the double-degenerate (DD) scenario by the collision of the SN ejecta with circumstellar matter that was blown by the accretion disc formed during the merger process of the two white dwarfs (WDs). We assume that in cases of excess early light, the disc blows the circumstellar matter, that we term disc-originated matter (DOM), hours to days before explosion. To perform our analysis, we first provide a model-based definition for early excess light, replacing the definition of excess light relative to a power-law fit to the rising luminosity. We then examine the light curves of the SNe Ia iPTF14atg and SN 2012cg, and find that the collision of the ejecta with a DOM in the frame of the DD scenario can account for their early excess emission. Thus, early excess light does not necessarily imply the presence of a stellar companion in the frame of the single-degenerate scenario. Our findings further increase the variety of phenomena that the DD scenario can account for, and emphasize the need to consider all different SN Ia scenarios when interpreting observations.

  17. On the Determination of the Spin of the Black Hole in Cyg X-1 from X-Ray Reflection Spectra

    Science.gov (United States)

    Fabian, A. C.; Wilkins, D.; Miller, J. M.; Reis, R. C.; Reynolds, C. S.; Cackett, E. M.; Nowak, M. A.; Pooley, G.; Pottschmidt, K.; Sanders, J. S.; hide

    2012-01-01

    The spin of Cygnus X-I is measured by fitting reflection models to Suzaku data covering the energy band 0.9-400 keY. The inner radius of the accretion disc is found to lie within 2 gravitational radii (rg = GM/c(exp 2)) and a value for the dimensionless black hole spin is obtained of 0.97(sup .0.14) (sup -0.02). This agrees with recent measurements using the continuum fitting method by Gou et al. and of the broad iron line by Duro et al. The disc inclination is measured at 23.7(sup +6.7) (sup -5.4) deg. which is consistent with the recent optical measurement of the binary system inclination by Orosz et al of 27+/- 0.8 deg. We pay special attention to the emissivity profile caused by irradiation of the inner disc by the hard power-law source. 1be X-ray observations and simulations show that the index q of that profile deviates from the commonly used, Newtonian, value of 3 within 3r(sub g), steepening considerably within 2r(sub g). as expected in the strong gravity regime.

  18. Foundations of Black Hole Accretion Disk Theory

    Directory of Open Access Journals (Sweden)

    Marek A. Abramowicz

    2013-01-01

    Full Text Available This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks, Shakura-Sunyaev (thin disks, slim disks, and advection-dominated accretion flows (ADAFs. After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs.

  19. Foundations of Black Hole Accretion Disk Theory.

    Science.gov (United States)

    Abramowicz, Marek A; Fragile, P Chris

    2013-01-01

    This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).

  20. Pebble Accretion in Turbulent Protoplanetary Disks

    Science.gov (United States)

    Xu, Ziyan; Bai, Xue-Ning; Murray-Clay, Ruth A.

    2017-09-01

    It has been realized in recent years that the accretion of pebble-sized dust particles onto planetary cores is an important mode of core growth, which enables the formation of giant planets at large distances and assists planet formation in general. The pebble accretion theory is built upon the orbit theory of dust particles in a laminar protoplanetary disk (PPD). For sufficiently large core mass (in the “Hill regime”), essentially all particles of appropriate sizes entering the Hill sphere can be captured. However, the outer regions of PPDs are expected to be weakly turbulent due to the magnetorotational instability (MRI), where turbulent stirring of particle orbits may affect the efficiency of pebble accretion. We conduct shearing-box simulations of pebble accretion with different levels of MRI turbulence (strongly turbulent assuming ideal magnetohydrodynamics, weakly turbulent in the presence of ambipolar diffusion, and laminar) and different core masses to test the efficiency of pebble accretion at a microphysical level. We find that accretion remains efficient for marginally coupled particles (dimensionless stopping time {τ }s˜ 0.1{--}1) even in the presence of strong MRI turbulence. Though more dust particles are brought toward the core by the turbulence, this effect is largely canceled by a reduction in accretion probability. As a result, the overall effect of turbulence on the accretion rate is mainly reflected in the changes in the thickness of the dust layer. On the other hand, we find that the efficiency of pebble accretion for strongly coupled particles (down to {τ }s˜ 0.01) can be modestly reduced by strong turbulence for low-mass cores.

  1. The radii of the Wolf-Rayet stars and the extent of their chromosphere-corona formation

    Energy Technology Data Exchange (ETDEWEB)

    Sahade, J [Instituto de Astronomia y Fisica del Espacio, Buenos Aires, Argentina; Zorec, J [College de France, Paris, France

    1981-03-01

    The radii of 14 Wolf-Rayet stars are computed on the basis of previously reported absolute fluxes in the region from 4150 to 8000 A for 10 WN stars and from 3650 to 8000 A for four WC stars. For comparison, the radii of 11 Of stars are also calculated. The Wolf-Rayet radii are found to range from about 10 to 35 solar radii, values that do not appear to provide supporting evidence for the hypothesis that Of stars evolve into late WN stars. Available UV observations of Gamma-2 Vel are used to investigate the extent of the chromosphere-corona structure in Wolf-Rayet stars. It is suggested that the second electron-temperature maximum in a recently proposed model for the extended envelopes of Wolf-Rayet stars should be further than about 300 solar radii from the center of a star.

  2. WIND-ACCRETION DISKS IN WIDE BINARIES, SECOND-GENERATION PROTOPLANETARY DISKS, AND ACCRETION ONTO WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Perets, Hagai B. [Technion-Israel Institute of Technology, Haifa (Israel); Kenyon, Scott J., E-mail: hperets@physics.technion.ac.il [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

    2013-02-20

    Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long-term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind accretion leads to long-lived stable disks over the lifetime of the asymptotic giant branch donor star. The disks have masses of a few times 10{sup -5}-10{sup -3} M {sub Sun }, with surface density and temperature profiles that follow broken power laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly, 50%-80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M {sub Sun }. When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks, suggesting that coordinated observational programs might improve our understanding of uncertain disk physics.

  3. The rise and fall of stellar across the peak of cosmic star formation history: effects of mergers versus diffuse stellar mass acquisition

    Science.gov (United States)

    Welker, C.; Dubois, Y.; Devriendt, J.; Pichon, C.; Kaviraj, S.; Peirani, S.

    2017-02-01

    Building galaxy merger trees from a state-of-the-art cosmological hydrodynamical simulation, Horizon-AGN, we perform a statistical study of how mergers and diffuse stellar mass acquisition processes drive galaxy morphologic properties above z > 1. By diffuse mass acquisition here, we mean both accretion of stars by unresolved mergers (relative stellar mass growth smaller than 4.5 per cent) as well as in situ star formation when no resolved mergers are detected along the main progenitor branch of a galaxy. We investigate how stellar densities, galaxy sizes and galaxy morphologies (defined via shape parameters derived from the inertia tensor of the stellar density) depend on mergers of different mass ratios. We investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that diffuse stellar accretion and in situ formation tend to flatten small galaxies over cosmic time, leading to the formation of discs. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar discs, confirming the origin of elliptical galaxies. We confirm that mergers grow galaxy sizes more efficiently than diffuse processes (r_{0.5}∝ M_s^{0.85} and r_{0.5}∝ M_s^{0.1} on average, respectively) and we also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size-mass evolution r_{0.5}∝ M_s^{1.2} instead of r_{0.5}∝ M_s^{-0.5}-M^{0.5} for discs depending on the merger mass ratio. The gas content drives the size-mass evolution due to merger with a faster size growth for gas-poor galaxies r_{0.5}∝ M_s2 than for gas-rich galaxies r0.5 ∝ Ms.

  4. Multi-time-scale X-ray reverberation mapping of accreting black holes

    Science.gov (United States)

    Mastroserio, Guglielmo; Ingram, Adam; van der Klis, Michiel

    2018-04-01

    Accreting black holes show characteristic reflection features in their X-ray spectrum, including an iron Kα line, resulting from hard X-ray continuum photons illuminating the accretion disc. The reverberation lag resulting from the path-length difference between direct and reflected emission provides a powerful tool to probe the innermost regions around both stellar-mass and supermassive black holes. Here, we present for the first time a reverberation mapping formalism that enables modelling of energy-dependent time lags and variability amplitude for a wide range of variability time-scales, taking the complete information of the cross-spectrum into account. We use a pivoting power-law model to account for the spectral variability of the continuum that dominates over the reverberation lags for longer time-scale variability. We use an analytic approximation to self-consistently account for the non-linear effects caused by this continuum spectral variability, which have been ignored by all previous reverberation studies. We find that ignoring these non-linear effects can bias measurements of the reverberation lags, particularly at low frequencies. Since our model is analytic, we are able to fit simultaneously for a wide range of Fourier frequencies without prohibitive computational expense. We also introduce a formalism of fitting to real and imaginary parts of our cross-spectrum statistic, which naturally avoids some mistakes/inaccuracies previously common in the literature. We perform proof-of-principle fits to Rossi X-ray Timing Explorer data of Cygnus X-1.

  5. The signature of the magnetorotational instability in the Reynolds and Maxwell stress tensors in accretion discs

    DEFF Research Database (Denmark)

    Pessah, Martin Elias; Chan, Chi-kwan; Psaltis, Dimitrios

    2006-01-01

    stresses during the late times of the exponential growth of the instability is determined only by the local shear and does not depend on the initial spectrum of perturbations or the strength of the seed magnetic. Even though we derived these properties of the stress tensors for the exponential growth...... of the instability, the mean (averaged over the disc scale-height) Reynolds stress is always positive, the mean Maxwell stress is always negative, and hence the mean total stress is positive and leads to a net outward flux of angular momentum. More importantly, we show that the ratio of the Maxwell to the Reynolds...

  6. Distribution of correlation radii in disordered ferroelectrics

    Czech Academy of Sciences Publication Activity Database

    Glinchuk, M. D.; Eliseev, E. A.; Stepanovich, V. A.; Jastrabík, Lubomír

    2002-01-01

    Roč. 81, č. 25 (2002), s. 4808-4810 ISSN 0003-6951 R&D Projects: GA MŠk LN00A015 Institutional research plan: CEZ:AV0Z1010914 Keywords : disordered ferroelectrics * distribution of correlation radii * polar nanoregions Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 4.207, year: 2002

  7. Isospin dependence of nuclear charge radii and its microscopic demonstration

    International Nuclear Information System (INIS)

    Lei Yian; Zeng Jinyan

    2007-01-01

    The analysis of experimental nuclear charge radii R c indicates that R c deviates systematically from the A 1/3 law, i.e., R c /A 1/3 gradually decreases with increasing A, whereas R c /Z 1/3 remains almost a constant. This statement is also supported by the analysis of a large amount of experimental nuclear giant monopole resonance energy data E x ∝R -1 . The deviation of nuclear charge radii from the A 1/3 law is basically caused by the isospin independence of A 1/3 law, and the isospin dependence has been partly included in Z 1/3 law. In the frame of nuclear shell model, a microscopic demonstration of the Z 1/3 law is given. The difference in the harmonic oscillator potential strength between proton and neutron (ω p and ω n ) can be accounted for by the Z 1/3 law. Similar to Wigner's nuclear isobaric multiplet mass equation (IMME), a modified Z 1/3 law for nuclear charge radii is proposed. (authors)

  8. Super-Eddington Accretion in Tidal Disruption Events: the Impact of Realistic Fallback Rates on Accretion Rates

    Science.gov (United States)

    Wu, Samantha; Coughlin, Eric R.; Nixon, Chris

    2018-04-01

    After the tidal disruption of a star by a massive black hole, disrupted stellar debris can fall back to the hole at a rate significantly exceeding its Eddington limit. To understand how black hole mass affects the duration of super-Eddington accretion in tidal disruption events, we first run a suite of simulations of the disruption of a Solar-like star by a supermassive black hole of varying mass to directly measure the fallback rate onto the hole, and we compare these fallback rates to the analytic predictions of the "frozen-in" model. Then, adopting a Zero-Bernoulli Accretion flow as an analytic prescription for the accretion flow around the hole, we investigate how the accretion rate onto the black hole evolves with the more accurate fallback rates calculated from the simulations. We find that numerically-simulated fallback rates yield accretion rates onto the hole that can, depending on the black hole mass, be nearly an order of magnitude larger than those predicted by the frozen-in approximation. Our results place new limits on the maximum black hole mass for which super-Eddington accretion occurs in tidal disruption events.

  9. Hot Accretion onto Black Holes with Outflow

    Directory of Open Access Journals (Sweden)

    Park Myeong-Gu

    2018-01-01

    Full Text Available Classic Bondi accretion flow can be generalized to rotating viscous accretion flow. Study of hot accretion flow onto black holes show that its physical charateristics change from Bondi-like for small gas angular momentum to disk-like for Keperian gas angular momentum. Especially, the mass accretion rate divided by the Bondi accretion rate is proportional to the viscosity parameter alpha and inversely proportional to the gas angular momentum divided by the Keplerian angular momentum at the Bondi radius for gas angular momentum comparable to the Keplerian value. The possible presence of outflow will increase the mass inflow rate at the Bondi radius but decrease the mass accretion rate across the black hole horizon by many orders of magnitude. This implies that the growth history of supermassive black holes and their coevolution with host galaxies will be dramatically changed when the accreted gas has angular momentum or develops an outflow.

  10. Double rupture disc experience

    International Nuclear Information System (INIS)

    1979-01-01

    Result of these observations, comparisons and evaluations can be summarized in the following list of concerns regarding the use of double rupture discs coupled to the liquid space of a steam generator that is subjected to a large leak sodium water reaction event. Single rupture disc show delayed collapse characteristics in LLTR Series I and double disc assemblies are presumed to be more complex with additional delay before opening to give pressure relief. Delayed failure increases pressures in the IHTS and must be adequately covered by design requirements. With CRBR design, the first disc may fail only partially reducing the loading on the second disc with the result that relief performance may not meet requirements

  11. SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS

    International Nuclear Information System (INIS)

    Matt, Sean P.; Pinzón, Giovanni; Greene, Thomas P.; Pudritz, Ralph E.

    2012-01-01

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh and Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

  12. Source to Accretion Disk Tilt

    OpenAIRE

    Montgomery, M. M.; Martin, E. L.

    2010-01-01

    Many different system types retrogradely precess, and retrograde precession could be from a tidal torque by the secondary on a misaligned accretion disk. However, a source to cause and maintain disk tilt is unknown. In this work, we show that accretion disks can tilt due to a force called lift. Lift results from differing gas stream supersonic speeds over and under an accretion disk. Because lift acts at the disk's center of pressure, a torque is applied around a rotation axis passing through...

  13. Radially truncated galactic discs

    NARCIS (Netherlands)

    Grijs, R. de; Kregel, M.; Wesson, K H

    2000-01-01

    Abstract: We present the first results of a systematic analysis of radially truncatedexponential discs for four galaxies of a sample of disc-dominated edge-onspiral galaxies. Edge-on galaxies are very useful for the study of truncatedgalactic discs, since we can follow their light distributions out

  14. Accretion disks in active galactic nuclei

    International Nuclear Information System (INIS)

    Begelman, M.C.

    1985-01-01

    The innermost regions of the central engines in active galactic nuclei are examined, and it is shown how different modes of accretion with angular momentum may account for the diverse manifestations of activity in the nuclei of galaxies. These modes are subsequently compared with the observed properties of quasars, Type I Seyferts, and radio galaxies. It was found that the qualitative features of an accretion flow orbiting a massive black hole depend principally on the ratio of the actual accretion rate to the Eddington accretion rate. For a value of this ratio much less than one, the flow may become an ion torus supported by gas pressure; for a value much greater than one, the flow traps its radiative output and becomes an inefficient radiation torus. At intermediate values, the flow may settle into a thin accretion disk. 62 references

  15. Fitted HBT radii versus space-time variances in flow-dominated models

    International Nuclear Information System (INIS)

    Lisa, Mike; Frodermann, Evan; Heinz, Ulrich

    2007-01-01

    The inability of otherwise successful dynamical models to reproduce the 'HBT radii' extracted from two-particle correlations measured at the Relativistic Heavy Ion Collider (RHIC) is known as the 'RHIC HBT Puzzle'. Most comparisons between models and experiment exploit the fact that for Gaussian sources the HBT radii agree with certain combinations of the space-time widths of the source which can be directly computed from the emission function, without having to evaluate, at significant expense, the two-particle correlation function. We here study the validity of this approach for realistic emission function models some of which exhibit significant deviations from simple Gaussian behaviour. By Fourier transforming the emission function we compute the 2-particle correlation function and fit it with a Gaussian to partially mimic the procedure used for measured correlation functions. We describe a novel algorithm to perform this Gaussian fit analytically. We find that for realistic hydrodynamic models the HBT radii extracted from this procedure agree better with the data than the values previously extracted from the space-time widths of the emission function. Although serious discrepancies between the calculated and measured HBT radii remain, we show that a more 'apples-to-apples' comparison of models with data can play an important role in any eventually successful theoretical description of RHIC HBT data. (author)

  16. On the odd-even effect in the charge radii of isotopes

    International Nuclear Information System (INIS)

    Talmi, I.

    1984-01-01

    Core polarization by valence neutrons is suggested as a possible mechanism for producing odd-even variation in the charge radii of isotopes. The nuclei considered have closed proton shells and neutrons in states with lowest seniority or generalized seniority. Simple expressions are derived for jsup(n) neutron configurations and various multipole terms of the pn interaction. The resulting expressions give a good fit to the radii of calcium isotopes and also of lead isotopes for which these expressions are only approximate. (orig.)

  17. Double-disc gate valve

    International Nuclear Information System (INIS)

    Wheatley, S.J.

    1979-01-01

    The invention relates to an improvement in a conventional double-disc gate valve having a vertically movable gate assembly including a wedge, spreaders slidably engaged therewith, a valve disc carried by the spreaders. When the gate assembly is lowered to a selected point in the valve casing, the valve discs are moved transversely outward to close inlet and outlet ports in the casing. The valve includes hold-down means for guiding the disc-and-spreader assemblies as they are moved transversely outward and inward. If such valves are operated at relatively high differential pressures, they sometimes jam during opening. Such jamming has been a problem for many years in gate valves used in gaseous diffusion plants for the separation of uranium isotopes. The invention is based on the finding that the above-mentioned jamming results when the outlet disc tilts about its horizontal axis in a certain way during opening of the valve. In accordance with the invention, tilting of the outlet disc is maintained at a tolerable value by providing the disc with a rigid downwardly extending member and by providing the casing with a stop for limiting inward arcuate movement of the member to a preselected value during opening of the valve

  18. Optic disc oedema

    DEFF Research Database (Denmark)

    Nielsen, Marianne Kromann; Hamann, Steffen

    2014-01-01

    Optic disc oedema describes the nonspecific, localized swelling of the optic nerve head regardless of aetiology. Therefore, differentiating among the various aetiologies depends on a thorough history and knowledge of the clinical characteristics of the underlying conditions. Papilloedema strictly...... refers to optic disc oedema as a consequence of elevated intracranial pressure. It is usually a bilateral condition and visual function is preserved until late. Optic disc oedema caused by an anterior optic neuropathy is usually unilateral and accompanied by the loss of visual function....

  19. MAKE SUPER-EARTHS, NOT JUPITERS: ACCRETING NEBULAR GAS ONTO SOLID CORES AT 0.1 AU AND BEYOND

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Eve J.; Chiang, Eugene; Ormel, Chris W., E-mail: evelee@berkeley.edu, E-mail: echiang@astro.berkeley.edu, E-mail: ormel@berkeley.edu [Department of Astronomy, University of California Berkeley, Berkeley, CA 94720-3411 (United States)

    2014-12-20

    Close-in super-Earths having radii 1-4 R {sub ⊕} may possess hydrogen atmospheres comprising a few percent by mass of their rocky cores. We determine the conditions under which such atmospheres can be accreted by cores from their parent circumstellar disks. Accretion from the nebula is problematic because it is too efficient: we find that 10 M {sub ⊕} cores embedded in solar metallicity disks tend to undergo runaway gas accretion and explode into Jupiters, irrespective of orbital location. The threat of runaway is especially dire at ∼0.1 AU, where solids may coagulate on timescales orders of magnitude shorter than gas clearing times; thus nascent atmospheres on close-in orbits are unlikely to be supported against collapse by planetesimal accretion. The time to runaway accretion is well approximated by the cooling time of the atmosphere's innermost convective zone, whose extent is controlled by where H{sub 2} dissociates. Insofar as the temperatures characterizing H{sub 2} dissociation are universal, timescales for core instability tend not to vary with orbital distance—and to be alarmingly short for 10 M {sub ⊕} cores. Nevertheless, in the thicket of parameter space, we identify two scenarios, not mutually exclusive, that can reproduce the preponderance of percent-by-mass atmospheres for super-Earths at ∼0.1 AU, while still ensuring the formation of Jupiters at ≳ 1 AU. Scenario (a): planets form in disks with dust-to-gas ratios that range from ∼20× solar at 0.1 AU to ∼2× solar at 5 AU. Scenario (b): the final assembly of super-Earth cores from mergers of proto-cores—a process that completes quickly at ∼0.1 AU once begun—is delayed by gas dynamical friction until just before disk gas dissipates completely. Both scenarios predict that the occurrence rate for super-Earths versus orbital distance, and the corresponding rate for Jupiters, should trend in opposite directions, as the former population is transformed into the latter: as

  20. Thermonuclear process and accretion onto neutron star envelopes: x-ray burst and transient sources

    International Nuclear Information System (INIS)

    Starrfield, S.; Kenyon, S.; Sparks, W.M.; Truran, J.W.; Theoretical Division, Los Alamos National Laboratory)

    1982-01-01

    We have used a Lagrangian, fully implicit, one-dimensional, hydrodynamic computer code to investigate the evolution of thermonuclear runaways in the thick, accreted, hydrogen-rich envelopes of 1.0 M/sub sun/ neutron stars with radii of 10 km and 20 km. Our simulations produce outbursts which range in time scale from about 2000 seconds to longer than 1 day. Peak effective temperature was 3.3 x 10 7 K (kTapprox.2.91 keV), and peak luminosity was 2 x 10 5 L/sub sun/ for the 10 km study. The 20 km neutron star produced a peak effective temperature and luminosity of 5.3 x 10 6 K and 5.9 x 10 2 L/sub sun/, respectively. We also investigated the effects of changes in the rates of the 14 O(α,p) and 15 O(α,ν) reactions on the evolution. Hydrodynamic expansion on the 10 km neutron star produced a precursor lasting about 10 - 6 seconds

  1. The magnetic field of the equatorial magnetotail from 10 to 40 earth radii

    Science.gov (United States)

    Fairfield, D. H.

    1986-01-01

    A statistical study of IMP 6, 7, and 8 magnetotail magnetic field measurements near the equatorial plane reveals new information about various aspects of magnetospheric structure. More magnetic flux crosses the equatorial plane on the dawn and dusk flanks of the tail than near midnight, but no evidence is found for a dependence on the interplanetary magnetic field sector polarity. Field magnitudes within 3 earth radii of the equatorial plane near dawn are more than twice as large as those near dusk for Xsm = -20 to -10 earth radii. The frequency of occurrence of southward fields is greatest near midnight, and such fields are seen almost twice as often for Xsm = -20 to -10 earth radii as for Xsm beyond -20 earth radii. This latter result supports the idea that the midnight region of the tail between 10 and 20 is a special location where neutral lines are particularly apt to form. Such a neutral line will approach nearest the earth in the midnight and premidnight region, where substorms are thought to have their onset.

  2. CT discography for cervical soft disc hernia

    Energy Technology Data Exchange (ETDEWEB)

    Iwasa, Kenichi; Mizutani, Shigeru; Morimoto, Hiroyuki; Yamada, Hidehito; Iwasa, Satoru

    1985-03-01

    In this study the effectiveness of computed tomographic discography (CTD) in diagnosing cervical soft disc hernia was evaluated. Twenty-five intervertebral discs of 15 cases with cervical soft disc hernia were examined with a discography and then a CT scan. Results of the CT scan were as follows: three discs were protruded, 12 discs were prolapsed, 6 discs were extruded, and 4 discs were sequestrated. The findings were helpful in determining the location of soft disc hernias between the median and posterolateral discs. They were also valuable in classifying types of hernias and surgical approaches.

  3. CT discography for cervical soft disc hernia

    International Nuclear Information System (INIS)

    Iwasa, Kenichi; Mizutani, Shigeru; Morimoto, Hiroyuki; Yamada, Hidehito; Iwasa, Satoru

    1985-01-01

    In this study the effectiveness of computed tomographic discography (CTD) in diagnosing cervical soft disc hernia was evaluated. Twenty-five interververtebral discs of 15 cases with cervical soft disc hernia were examined with a discography and then a CT scan. Results of the CT scan were as follows: three discs were protruded, 12 discs were prolapsed, 6 discs were extruded, and 4 discs were sequestrated. The findings were helpful in determining the location of soft disc hernians between the median and posterolateral discs. They were also valuable in classifying types of hernians and surgical aproaches. (author)

  4. RADII OF RAPIDLY ROTATING STARS, WITH APPLICATION TO TRANSITING-PLANET HOSTS

    International Nuclear Information System (INIS)

    Brown, Timothy M.

    2010-01-01

    The currently favored method for estimating radii and other parameters of transiting-planet host stars is to match theoretical models to observations of the stellar mean density ρ * , the effective temperature T eff , and the composition parameter [Z]. This explicitly model-dependent approach is based on readily available observations, and results in small formal errors. Its performance will be central to the reliability of results from ground-based transit surveys such as TrES, HAT, and SuperWASP, as well as to the space-borne missions MOST, CoRoT, and Kepler. Here, I use two calibration samples of stars (eclipsing binaries (EBs) and stars for which asteroseismic analyses are available) having well-determined masses and radii to estimate the accuracy and systematic errors inherent in the ρ * method. When matching to the Yonsei-Yale stellar evolution models, I find the most important systematic error results from selection bias favoring rapidly rotating (hence probably magnetically active) stars among the EB sample. If unaccounted for, this bias leads to a mass-dependent underestimate of stellar radii by as much as 4% for stars of 0.4 M sun , decreasing to zero for masses above about 1.4 M sun . Relative errors in estimated stellar masses are three times larger than those in radii. The asteroseismic sample suggests (albeit with significant uncertainty) that systematic errors are small for slowly rotating, inactive stars. Systematic errors arising from failings of the Yonsei-Yale models of inactive stars probably exist, but are difficult to assess because of the small number of well-characterized comparison stars having low mass and slow rotation. Poor information about [Z] is an important source of random error, and may be a minor source of systematic error as well. With suitable corrections for rotation, it is likely that systematic errors in the ρ * method can be comparable to or smaller than the random errors, yielding radii that are accurate to about 2% for

  5. Simulating X-ray bursts during a transient accretion event

    Science.gov (United States)

    Johnston, Zac; Heger, Alexander; Galloway, Duncan K.

    2018-06-01

    Modelling of thermonuclear X-ray bursts on accreting neutron stars has to date focused on stable accretion rates. However, bursts are also observed during episodes of transient accretion. During such events, the accretion rate can evolve significantly between bursts, and this regime provides a unique test for burst models. The accretion-powered millisecond pulsar SAX J1808.4-3658 exhibits accretion outbursts every 2-3 yr. During the well-sampled month-long outburst of 2002 October, four helium-rich X-ray bursts were observed. Using this event as a test case, we present the first multizone simulations of X-ray bursts under a time-dependent accretion rate. We investigate the effect of using a time-dependent accretion rate in comparison to constant, averaged rates. Initial results suggest that using a constant, average accretion rate between bursts may underestimate the recurrence time when the accretion rate is decreasing, and overestimate it when the accretion rate is increasing. Our model, with an accreted hydrogen fraction of X = 0.44 and a CNO metallicity of ZCNO = 0.02, reproduces the observed burst arrival times and fluences with root mean square (rms) errors of 2.8 h, and 0.11× 10^{-6} erg cm^{-2}, respectively. Our results support previous modelling that predicted two unobserved bursts and indicate that additional bursts were also missed by observations.

  6. Charge radii and electromagnetic moments of At-211195

    Science.gov (United States)

    Cubiss, J. G.; Barzakh, A. E.; Seliverstov, M. D.; Andreyev, A. N.; Andel, B.; Antalic, S.; Ascher, P.; Atanasov, D.; Beck, D.; Bieroń, J.; Blaum, K.; Borgmann, Ch.; Breitenfeldt, M.; Capponi, L.; Cocolios, T. E.; Day Goodacre, T.; Derkx, X.; De Witte, H.; Elseviers, J.; Fedorov, D. V.; Fedosseev, V. N.; Fritzsche, S.; Gaffney, L. P.; George, S.; Ghys, L.; Heßberger, F. P.; Huyse, M.; Imai, N.; Kalaninová, Z.; Kisler, D.; Köster, U.; Kowalska, M.; Kreim, S.; Lane, J. F. W.; Liberati, V.; Lunney, D.; Lynch, K. M.; Manea, V.; Marsh, B. A.; Mitsuoka, S.; Molkanov, P. L.; Nagame, Y.; Neidherr, D.; Nishio, K.; Ota, S.; Pauwels, D.; Popescu, L.; Radulov, D.; Rapisarda, E.; Revill, J. P.; Rosenbusch, M.; Rossel, R. E.; Rothe, S.; Sandhu, K.; Schweikhard, L.; Sels, S.; Truesdale, V. L.; Van Beveren, C.; Van den Bergh, P.; Wakabayashi, Y.; Van Duppen, P.; Wendt, K. D. A.; Wienholtz, F.; Whitmore, B. W.; Wilson, G. L.; Wolf, R. N.; Zuber, K.

    2018-05-01

    Hyperfine-structure parameters and isotope shifts of At-211195 have been measured for the first time at CERN-ISOLDE, using the in-source resonance-ionization spectroscopy method. The hyperfine structures of isotopes were recorded using a triad of experimental techniques for monitoring the photo-ion current. The Multi-Reflection Time-of-Flight Mass Spectrometer, in connection with a high-resolution electron multiplier, was used as an ion-counting setup for isotopes that either were affected by strong isobaric contamination or possessed a long half-life; the ISOLDE Faraday cups were used for cases with high-intensity beams; and the Windmill decay station was used for short-lived, predominantly α -decaying nuclei. The electromagnetic moments and changes in the mean-square charge radii of the astatine nuclei have been extracted from the measured hyperfine-structure constants and isotope shifts. This was only made possible by dedicated state-of-the-art large-scale atomic computations of the electronic factors and the specific mass shift of atomic transitions in astatine that are needed for these extractions. By comparison with systematics, it was possible to assess the reliability of the results of these calculations and their ascribed uncertainties. A strong deviation in the ground-state mean-square charge radii of the lightest astatine isotopes, from the trend of the (spherical) lead isotopes, is interpreted as the result of an onset of deformation. This behavior bears a resemblance to the deviation observed in the isotonic polonium isotopes. Cases for shape coexistence have been identified in At,199197, for which a significant difference in the charge radii for ground (9 /2- ) and isomeric (1 /2+ ) states has been observed.

  7. Cold gas accretion in galaxies

    NARCIS (Netherlands)

    Sancisi, Renzo; Fraternali, Filippo; Oosterloo, Tom; van der Hulst, Thijs

    Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: (1) A large number of galaxies are accompanied by

  8. Precise Masses & Radii of the Planets Orbiting K2-3 and GJ3470

    Science.gov (United States)

    Kosiarek, Molly; Crossfield, Ian; Hardegree-Ullman, Kevin; Livingston, John; Howard, Andrew; Fulton, Benjamin; Hirsch, Lea; Isaacson, Howard; Petigura, Erik; Sinukoff, Evan; Weiss, Lauren; Knutson, Heather; Bonfils, Xavier; Benneke, Björn; Beichman, Charles; Dressing, Courtney

    2018-01-01

    We report improved masses, radii, and densities for two planetary systems, K2-3 and GJ3470, derived from a combination of new radial velocity and transit observations. Both stars are nearby, early M dwarfs. K2-3 hosts three super-Earth planets between 1.5 and 2 Earth-radii at orbital periods between 10 and 45 days, while GJ 3470 hosts one 4 Earth-radii planet with a period of 3.3 days. Furthermore, we confirmed GJ3470's rotation period through multi-year ground-based photometry; RV analysis must account for this rotation signature. Due to the planets' low densities (all stars, they are among the best candidates for transmission spectroscopy with JWST and HST in order to characterize their atmospheric compositions.

  9. Moments and mean square charge radii of short-lived argon isotopes

    CERN Document Server

    Klein, A; Georg, U; Keim, M; Lievens, P; Neugart, R; Neuroth, M; Silverans, R E; Vermeeren, L

    1996-01-01

    We report on the measurement of optical isotope shifts for $^{32-40}$Ar and for $^{46}$Ar from which the changes in mean square nuclear charge radii across the N = 20 neutron shell closure are deducted. The investigations were carried out by collinear laser spectroscopy in fast beams of neutral argon atoms. The ultra-sensitive detection combines optical pumping, state-selective collisional ionization and counting of $\\beta$-radioactivity. By reaching far into the sd-shell, the results add new information to the systematics of radii in the calcium region (Z $\\approx$ 20). Contrary to all major neutron shell closures with N $\\geq$ 28, the N = 20 shell closure causes no significant slope change in the development of the radii. Information from the hyperfine structure of the odd-A isotopes includes includes the magnetic moments of $^{33}$Ar (I=1/2) and $^{39}$Ar (I=7/2), and the quadrupole moments of $^{35}$Ar, $^{37}$Ar (I=3/2) and $^{39}$Ar. The electromagnetic moments are compared to shell-model predictions fo...

  10. Tracking the Iron Kα line and the Ultra Fast Outflow in NGC 2992 at different accretion states

    Science.gov (United States)

    Marinucci, A.; Bianchi, S.; Braito, V.; Matt, G.; Nardini, E.; Reeves, J.

    2018-06-01

    The Seyfert 2 galaxy NGC 2992 has been monitored eight times by XMM-Newton in 2010 and then observed again in 2013, while in 2015 it was simultaneously targeted by Swift and NuSTAR. XMM-Newton always caught the source in a faint state (2-10 keV fluxes ranging from 0.3 to 1.6× 10-11 erg cm-2 s-1) but NuSTAR showed an increase in the 2-10 keV flux up to 6× 10-11 erg cm-2 s-1. We find possible evidence of an Ultra Fast Outflow with velocity v1 = 0.21 ± 0.01c (detected at about 99% confidence level) in such a flux state. The UFO in NGC 2992 is consistent with being ejected at a few tens of gravitational radii only at accretion rates greater than 2% of the Eddington luminosity. The analysis of the low flux 2010/2013 XMM data allowed us to determine that the Iron Kα emission line complex in this object is likely the sum of three distinct components: a constant, narrow one due to reflection from cold, distant material (likely the molecular torus); a narrow, but variable one which is more intense in brighter observations and a broad relativistic one emitted in the innermost regions of the accretion disk, which has been detected only in the 2003 XMM observation.

  11. Increases to Inferred Rates of Planetesimal Accretion due to Thermohaline Mixing in Metal-accreting White Dwarfs

    Science.gov (United States)

    Bauer, Evan B.; Bildsten, Lars

    2018-06-01

    Many isolated, old white dwarfs (WDs) show surprising evidence of metals in their photospheres. Given that the timescale for gravitational sedimentation is astronomically short, this is taken as evidence for ongoing accretion, likely of tidally disrupted planetesimals. The rate of such accretion, {\\dot{M}}acc}, is important to constrain, and most modeling of this process relies on assuming an equilibrium between diffusive sedimentation and metal accretion supplied to the WD’s surface convective envelope. Building on the earlier work of Deal and collaborators, we show that high {\\dot{M}}acc} models with only diffusive sedimentation are unstable to thermohaline mixing and that models that account for the enhanced mixing from the active thermohaline instability require larger accretion rates, sometimes reaching {\\dot{M}}acc}≈ {10}13 {{g}} {{{s}}}-1 to explain observed calcium abundances. We present results from a grid of MESA models that include both diffusion and thermohaline mixing. These results demonstrate that both mechanisms are essential for understanding metal pollution across the range of polluted WDs with hydrogen atmospheres. Another consequence of active thermohaline mixing is that the observed metal abundance ratios are identical to accreted material.

  12. Topics in the physics of accretion onto black holes

    International Nuclear Information System (INIS)

    Stoeger, W.R.

    1977-06-01

    The subject is covered in chapters, entitled: introduction and overview; boundary-condition modification of accretion-disk models; standard assumptions and nonkeplerian inner-disk models; the 'inner edge' of accretion disks and spiral orbits; a review of comptonization in accretion disks and a criterion for Lightman-Eardley stability; the thickening of accretion disks and flows; radial pressure gradients and low-angular-momentum accretion; accretion-disk scenarios for X-ray transient and burst sources; photon pair-creation processes in transrelativistic plasmas; and the astrophysical consequences of Rosen's bi-metric theory of gravity. (U.K.)

  13. Fas ligand exists on intervertebral disc cells: a potential molecular mechanism for immune privilege of the disc.

    Science.gov (United States)

    Takada, Toru; Nishida, Kotaro; Doita, Minoru; Kurosaka, Masahiro

    2002-07-15

    Rat and human intervertebral disc specimens were examined immunohistochemically. Reverse transcription polymerase chain reaction (RT-PCR) analysis was also performed on rat disc tissue to demonstrate the existence of Fas ligand. To clarify the existence of Fas ligand on intact intervertebral disc cells. The nucleus pulposus has been reported to be an immune-privileged site. The immune-privileged characteristic in other tissues such as the retina and testis has been attributed to the local expression of Fas ligand, which acts by inducing apoptosis of invading Fas-positive T-cells. The existence of Fas ligand in normal disc cells has not yet been addressed. Skeletally mature SD male rats were killed, and the coccygeal discs were harvested. Human disc specimens were obtained from idiopathic scoliosis patients during surgical procedures. Immunohistochemical staining for Fas ligand was performed for cross-sections of the discs by standard procedures. Reverse transcription polymerase chain reaction analysis was also carried out to demonstrate Fas ligand mRNA expression on rat intervertebral discs. Testes of the rats were used for positive controls, and muscles were used for negative controls. The sections were observed by light microscopy. The nucleus pulposus cells exhibited intense positive immune staining for Fas ligand. The outer anulus fibrosus cells and notochordal cells exhibited little immunopositivity. The positive controls exhibited positive immune staining, and the negative control showed no immunopositivity. The result of RT-PCR confirmed the existence of Fas ligand in disc cells. The human nucleus pulposus cells showed a similar predilection to rat disc cells. We demonstrated the existence of Fas ligand on disc cells, which should play a key role in the potential molecular mechanism to maintain immune privilege of the disc. Immune privilege and Fas ligand expression of the intervertebral disc may provide a new insight for basic science research as well as

  14. Numerical study of nonspherical black hole accretion

    International Nuclear Information System (INIS)

    Hawley, J.F.

    1984-01-01

    This thesis describes in detail a two-dimensional, axisymmetric computer code for calculating fully relativistic ideal gas hydrodynamics around a Kerr black hole. The aim is to study fully dynamic inviscid fluid accretion onto black holes, as well as to study the evolution and development of nonlinear instabilities in pressure supported accretion disks. In order to fully calibrate and document the code, certain analytic solutions for shock tubes and special accretion flows are derived; these solutions form the basis for code testing. The numerical techniques used are developed and discussed. A variety of alternate differencing schemes are compared on an analytic test bed. Some discussion is devoted to general issues in finite differencing. The working code is calibrated using analytically solvable accretion problems, including the radial accretion of dust and of fluid with pressure (Bondi accretion). Two dimensional test problems include the spiraling infall of low angular momentum fluid, the formation of a pressure supported torus, and the stable evolution of a torus. A series of numerical models are discussed and illustrated with selected plots

  15. Optic disc and peripapillary retinal nerve fiber layer characteristics associated with glaucomatous optic disc in young myopia.

    Science.gov (United States)

    Lee, Jong Eun; Sung, Kyung Rim; Park, Ji Min; Yoon, Joo Young; Kang, Sung Yong; Park, Sung Bae; Koo, Hyung Jin

    2017-03-01

    To explore optic disc and peripapillary retinal nerve fiber layer (RNFL) features associated with glaucomatous optic disc (GOD) in young myopia. Presence of GOD, optic disc tilt, and disc torsion were determined using fundus photographs. If the measured disc tilt ratio was >1.3, the optic disc was classified as tilted. Optic disc torsion was defined as a >15° deviation in the long axis of the optic disc from the vertical meridian. The average and four quadrants RNFL thicknesses were assessed using spectral domain optical coherence tomography (SD-OCT). Logistic regression analyses were performed to identify factors associated with the presence of GOD. Nine hundred and sixty myopic subjects were recruited from four refractive surgery clinic databases. The mean age was 26.6 ± 5.7 years and spherical equivalent (SE) was -5.5 ± 2.5 diopters. Among 960 eyes, 26 (2.7%) received GOD group classification. Among 934 normal eyes, 290 (31.0%) had titled optic discs. Eighteen eyes (69.2%) in the GOD group had tilted optic discs. When compared to normal eyes, the GOD group had significantly higher tilt ratios (1.4 ± 0.2 vs. 1.2 ± 0.1, p Optic disc tilt was found in approximately one-third of young myopic eyes and was independently associated with the presence of GOD.

  16. Structures formation through self-organized accretion on cosmic strings

    International Nuclear Information System (INIS)

    Murdzek, R.

    2009-01-01

    In this paper, we shall show that the formation of structures through accretion by a cosmic string is driven by a natural feed-back mechanism: a part of the energy radiated by accretions creates a pressure on the accretion disk itself. This phenomenon leads to a nonlinear evolution of the accretion process. Thus, the formation of structures results as a consequence of a self-organized growth of the accreting central object.

  17. Accretion and evolution of ∼2.5 M ⊕ planets with voluminous H/He envelopes

    International Nuclear Information System (INIS)

    Bodenheimer, Peter; Lissauer, Jack J.

    2014-01-01

    Formation of planets in the Neptune size range with low-mass, but voluminous, H 2 /He gaseous envelopes is modeled by detailed numerical simulations according to the core-nucleated accretion scenario. Formation locations ranging from 0.5 to 4 AU from a star of 1 M ☉ are considered. The final planets have heavy-element cores of 2.2-2.5 M ⊕ and envelopes in the range 0.037-0.16 M ⊕ . After the formation process, which lasts 2 Myr or less, the planets evolve at constant mass up to an age of several Gyr. For assumed equilibrium temperatures of 250, 500, and 1000 K, their calculated final radii are compared with those observed by the Kepler spacecraft. For the particular case of Kepler-11 f, we address the question whether it could have formed in situ or whether migration from a formation location farther out in the disk is required.

  18. Retina image–based optic disc segmentation

    Directory of Open Access Journals (Sweden)

    Ching-Lin Wang

    2016-05-01

    Full Text Available The change of optic disc can be used to diagnose many eye diseases, such as glaucoma, diabetic retinopathy and macular degeneration. Moreover, retinal blood vessel pattern is unique for human beings even for identical twins. It is a highly stable pattern in biometric identification. Since optic disc is the beginning of the optic nerve and main blood vessels in retina, it can be used as a reference point of identification. Therefore, optic disc segmentation is an important technique for developing a human identity recognition system and eye disease diagnostic system. This article hence presents an optic disc segmentation method to extract the optic disc from a retina image. The experimental results show that the optic disc segmentation method can give impressive results in segmenting the optic disc from a retina image.

  19. Prognosis of intervertebral disc loss from diagnosis of degenerative disc disease

    Science.gov (United States)

    Li, S.; Lin, A.; Tay, K.; Romano, W.; Osman, Said

    2015-03-01

    Degenerative Disc Disease (DDD) is one of the most common causes of low back pain, and is a major factor in limiting the quality of life of an individual usually as they enter older stages of life, the disc degeneration reduces the shock absorption available which in turn causes pain. Disc loss is one of the central processes in the pathogenesis of DDD. In this study, we investigated whether the image texture features quantified from magnetic resonance imaging (MRI) could be appropriate markers for diagnosis of DDD and prognosis of inter-vertebral disc loss. The main objective is to use simple image based biomarkers to perform prognosis of spinal diseases using non-invasive procedures. Our results from 65 subjects proved the higher success rates of the combination marker compared to the individual markers and in the future, we will extend the study to other spine regions to allow prognosis and diagnosis of DDD for a wider region.

  20. Physiological pattern of lumbar disc height

    International Nuclear Information System (INIS)

    Biggemann, M.; Frobin, W.; Brinckmann, P.

    1997-01-01

    Purpose of this study is to present a new method of quantifying objectively the height of all discs in lateral radiographs of the lumbar spine and of analysing the normal craniocaudal sequence pattern of lumbar disc heights. Methods: The new parameter is the ventrally measured disc height corrected for the dependence on the angle of lordosis by normalisation to mean angles observed in the erect posture of healthy persons. To eliminate radiographic magnification, the corrected ventral height is related to the mean depth of the cranially adjoining vertebra. In this manner lumbar disc heights were objectively measured in young, mature and healthy persons (146 males and 65 females). The craniocaudal sequence pattern was analysed by mean values from all persons and by height differences of adjoining discs in each individual lumbar spine. Results: Mean normative values demonstrated an increase in disc height between L1/L2 and L4/L5 and a constant or decreasing disc height between L4/L5 and L5/S1. However, this 'physiological sequence of disc height in the statistical mean' was observed in only 36% of normal males and 55% of normal females. Conclusion: The radiological pattern of the 'physiological sequence of lumbar disc height' leads to a relevant portion of false positive pathological results especially at L4/L5. An increase of disc height from L4/L5 to L5/S1 may be normal. The recognition of decreased disc height should be based on an abrupt change in the heights of adjoining discs and not on a deviation from a craniocaudal sequence pattern. (orig.) [de

  1. CT morphometry of adult thoracic intervertebral discs.

    Science.gov (United States)

    Fletcher, Justin G R; Stringer, Mark D; Briggs, Christopher A; Davies, Tilman M; Woodley, Stephanie J

    2015-10-01

    Despite being commonly affected by degenerative disorders, there are few data on normal thoracic intervertebral disc dimensions. A morphometric analysis of adult thoracic intervertebral discs was, therefore, undertaken. Archival computed tomography scans of 128 recently deceased individuals (70 males, 58 females, 20-79 years) with no known spinal pathology were analysed to determine thoracic disc morphometry and variations with disc level, sex and age. Reliability was assessed by intraclass correlation coefficients (ICCs). Anterior and posterior intervertebral disc heights and axial dimensions were significantly greater in men (anterior disc height 4.0±1.4 vs 3.6±1.3 mm; posterior disc height 3.6±0.90 vs 3.4±0.93 mm; p<0.01). Disc heights and axial dimensions at T4-5 were similar or smaller than at T2-3, but thereafter increased caudally (mean anterior disc height T4-5 and T10-11, 2.7±0.7 and 5.4±1.2 mm, respectively, in men; 2.6±0.8 and 5.1±1.3 mm, respectively, in women; p<0.05). Except at T2-3, anterior disc height decreased with advancing age and anteroposterior and transverse disc dimensions increased; posterior and middle disc heights and indices of disc shape showed no consistent statistically significant changes. Most parameters showed substantial to almost perfect agreement for intra- and inter-rater reliability. Thoracic disc morphometry varies significantly and consistently with disc level, sex and age. This study provides unique reference data on adult thoracic intervertebral disc morphometry, which may be useful when interpreting pathological changes and for future biomechanical and functional studies.

  2. Thin accretion disks in stationary axisymmetric wormhole spacetimes

    International Nuclear Information System (INIS)

    Harko, Tiberiu; Kovacs, Zoltan; Lobo, Francisco S. N.

    2009-01-01

    In this paper, we study the physical properties and the equilibrium thermal radiation emission characteristics of matter forming thin accretion disks in stationary axially symmetric wormhole spacetimes. The thin disk models are constructed by taking different values of the wormhole's angular velocity, and the time averaged energy flux, the disk temperature, and the emission spectra of the accretion disks are obtained. Comparing the mass accretion in a rotating wormhole geometry with the one of a Kerr black hole, we verify that the intensity of the flux emerging from the disk surface is greater for wormholes than for rotating black holes with the same geometrical mass and accretion rate. We also present the conversion efficiency of the accreting mass into radiation, and show that the rotating wormholes provide a much more efficient engine for the transformation of the accreting mass into radiation than the Kerr black holes. Therefore specific signatures appear in the electromagnetic spectrum of thin disks around rotating wormholes, thus leading to the possibility of distinguishing wormhole geometries by using astrophysical observations of the emission spectra from accretion disks.

  3. : Nuclear Spirals and Mass Accretion to Supermassive Black Holes in Weakly-Barred Galaxies

    Science.gov (United States)

    Kim, Woong-Tae; Elmegreen, Bruce

    2018-01-01

    Disk galaxies, especially barred-spiral galaxies, abound with rings and spirals in their nuclear regions. Nuclear spirals existing even in weakly barred galaxies are thought to channel gas inflows to supermassive black holes residing at the centers. We use high-resolution hydrodynamic simulations to study the properties of nuclear gas spirals driven by weak bar-like or oval potentials. The amplitude of the spirals increases toward the center by a geometric effect, readily developing into shocks at small radii even for very weak potentials. The shape of the spirals and shocks depends rather sensitively on the background shear. When shear is low, the nuclear spirals are loosely wound and the shocks are almost straight, resulting in large mass inflows toward the center. When shear is high, on the other hand, the spirals are tightly wound and the shocks are oblique, forming a circumnuclear disk through which gas flows inward at a relatively lower rate. The induced mass inflow rates are enough to power black hole accretion in various types of Seyfert galaxies.

  4. Are Collapsed Cervical Discs Amenable to Total Disc Arthroplasty?: Analysis of Prospective Clinical Data With 2-Year Follow Up.

    Science.gov (United States)

    Patwardhan, Avinash G; Carandang, Gerard; Voronov, Leonard I; Havey, Robert M; Paul, Gary A; Lauryssen, Carl; Coric, Domagoj; Dimmig, Thomas; Musante, David

    2016-12-15

    Analysis of prospectively collected radiographic data. To investigate the influence of preoperative index-level range of motion (ROM) and disc height on postoperative ROM after cervical total disc arthroplasty (TDA) using compressible disc prostheses. Clinical studies demonstrate benefits of motion preservation over fusion; however, questions remain unanswered as to which preoperative factors have the ability to identify patients who are most likely to have good postoperative motion, which is the primary rationale for TDA. We analyzed prospectively collected data from a single-arm, multicenter study with 2-year follow up of 30 patients with 48 implanted levels. All received compressible cervical disc prostheses of 6 mm-height (M6C, Spinal Kinetics, Sunnyvale, CA). The influence of index-level preoperative disc height and ROM (each with two levels: below-median and above-median) on postoperative ROM was analyzed using 2 x 2 ANOVA. We further analyzed the radiographic outcomes of a subset of discs with preoperative height less than 3 mm, the so-called "collapsed" discs. Shorter (3.0 ± 0.4 mm) discs were significantly less mobile preoperatively than taller (4.4 ± 0.5 mm) discs (6.7° vs. 10.5°, P = 0.01). The postoperative ROM did not differ between the shorter and taller discs (5.6° vs. 5.0°, P = 0.63). Tall discs that were less mobile preoperatively had significantly smaller postoperative ROM than short discs with above-median preoperative mobility (P < 0.05). The "collapsed discs" (n = 8) were less mobile preoperatively compared with all discs combined (5.1° vs. 8.6°, P < 0.01). These discs were distracted to more than two times the preoperative height, from 2.6 to 5.7 mm, and had significantly greater postoperative ROM than all discs combined (7.6° vs. 5.3°, P < 0.05). We observed a significant interaction between preoperative index-level disc height and ROM in influencing postoperative ROM. Although limited by small sample

  5. Disc-halo interactions in ΛCDM

    Science.gov (United States)

    Bauer, Jacob S.; Widrow, Lawrence M.; Erkal, Denis

    2018-05-01

    We present a new method for embedding a stellar disc in a cosmological dark matter halo and provide a worked example from a Λ cold dark matter zoom-in simulation. The disc is inserted into the halo at a redshift z = 3 as a zero-mass rigid body. Its mass and size are then increased adiabatically while its position, velocity, and orientation are determined from rigid-body dynamics. At z = 1, the rigid disc (RD) is replaced by an N-body disc whose particles sample a three-integral distribution function (DF). The simulation then proceeds to z = 0 with live disc (LD) and halo particles. By comparison, other methods assume one or more of the following: the centre of the RD during the growth phase is pinned to the minimum of the halo potential, the orientation of the RD is fixed, or the live N-body disc is constructed from a two rather than three-integral DF. In general, the presence of a disc makes the halo rounder, more centrally concentrated, and smoother, especially in the innermost regions. We find that methods in which the disc is pinned to the minimum of the halo potential tend to overestimate the amount of adiabatic contraction. Additionally, the effect of the disc on the subhalo distribution appears to be rather insensitive to the disc insertion method. The LD in our simulation develops a bar that is consistent with the bars seen in late-type spiral galaxies. In addition, particles from the disc are launched or `kicked up' to high galactic latitudes.

  6. The comparative analysis of rocks' resistance to forward-slanting disc cutters and traditionally installed disc cutters

    Science.gov (United States)

    Zhang, Zhao-Huang; Fei, Sun; Liang, Meng

    2016-08-01

    At present, disc cutters of a full face rock tunnel boring machine are mostly mounted in the traditional way. Practical use in engineering projects reveals that this installation method not only heavily affects the operation life of disc cutters, but also increases the energy consumption of a full face rock tunnel boring machine. To straighten out this issue, therefore, a rock-breaking model is developed for disc cutters' movement after the research on the rock breaking of forward-slanting disc cutters. Equations of its displacement are established based on the analysis of velocity vector of a disc cutter's rock-breaking point. The functional relations then are brought forward between the displacement parameters of a rock-breaking point and its coordinate through the analysis of micro displacement of a rock-breaking point. Thus, the geometric equations of rock deformation are derived for the forward-slanting installation of disc cutters. With a linear relationship remaining between the acting force and its deformation either before or after the leap breaking, the constitutive relation of rock deformation can be expressed in the form of generalized Hooke law, hence the comparative analysis of the variation in the resistance of rock to the disc cutters mounted in the forward-slanting way with that in the traditional way. It is discovered that with the same penetration, strain of the rock in contact with forward-slanting disc cutters is apparently on the decline, in other words, the resistance of rock to disc cutters is reduced. Thus wear of disc cutters resulted from friction is lowered and energy consumption is correspondingly decreased. It will be useful for the development of installation and design theory of disc cutters, and significant for the breakthrough in the design of full face rock tunnel boring machine.

  7. A Simulation Model of Focus and Radial Servos in Compact Disc Players with Disc Surface Defects

    DEFF Research Database (Denmark)

    Odgaard, Peter Fogh; Stoustrup, Jakob; Andersen, Palle

    2004-01-01

    Compact Disc players have been on the market in more than two decades.As a consequence most of the control servo problems have been solved. A large remaining problem to solve is the handling of Compact Discs with severe surface defects like scratches and fingerprints. This paper introduces a method...... for making the design of controllers handling surface defects easier. A simulation model of Compact Disc players playing discs with surface defects is presented. The main novel element in the model is a model of the surface defects. That model is based on data from discs with surface defects. This model...

  8. Dependence of Shear and Concentration on Fouling in a Membrane Bioreactor with Rotating Membrane Discs

    DEFF Research Database (Denmark)

    Jørgensen, Mads Koustrup; Pedersen, Malene Thostrup; Christensen, Morten Lykkegaard

    2014-01-01

    Rotating ceramic membrane discs were fouled with lab-scale membrane bioreactors (MBR) sludge. Sludge filtrations were performed at varying rotation speeds and in different concentric rings of the membranes on different sludge concentrations. Data showed that the back transport expressed by limiting...... flux increased with rotation speed and distance from membrane center as an effect of shear. Further, the limiting flux decreased with increasing sludge concentration. A model was developed to link the sludge concentration and shear stress to the limiting flux. The model was able to simulate the effect...... of shear stress and sludge concentration on the limiting flux. The model was developed by calculating the shear rate at laminar flow regime at different rotation speeds and radii on the membrane. Furthermore, through the shear rate and shear stress, the non-Newtonian behavior of MBR sludge was addressed...

  9. Intratracheal Seal Disc

    DEFF Research Database (Denmark)

    Christiansen, Karen J; Moeslund, Niels; Lauridsen, Henrik

    2017-01-01

    . The device consisted of an intratracheal silicone seal disc fixated by a cord through the stoma to an external part. At day 14, computed tomography (CT) was performed before the device was extracted. With the pulling of a cord, the disc unraveled into a thin thread and was extracted through the stoma. At day...

  10. The inflated radii of M dwarfs in the Pleiades

    Science.gov (United States)

    Jackson, R. J.; Deliyannis, Constantine P.; Jeffries, R. D.

    2018-05-01

    Rotation periods obtained with the Kepler satellite have been combined with precise measurements of projected rotation velocity from the WIYN 3.5-m telescope to determine the distribution of projected radii for several hundred low-mass (0.1 ≤ M/M⊙ ≤ 0.8), fast-rotating members of the Pleiades cluster. A maximum likelihood modelling technique, that takes account of observational uncertainties, selection effects and censored data, and considers the effects of differential rotation and unresolved binarity, has been used to find that the average radius of these stars is 14 ± 2 per cent larger at a given luminosity than predicted by current evolutionary models of Dotter et al. and Baraffe et al. The same models are a reasonable match to the interferometric radii of older, magnetically inactive field M dwarfs, suggesting that the over-radius may be associated with the young, magnetically active nature of the Pleiades objects. No evidence is found for any change in this over-radius above and below the boundary marking the transition to full convection. Published evolutionary models that incorporate either the effects of magnetic inhibition of convection or the blocking of flux by dark star-spots do not individually explain the radius inflation, but a combination of the two effects might. The distribution of projected radii is consistent with the adopted hypothesis of a random spatial orientation of spin axes; strong alignments of the spin vectors into cones with an opening semi-angle <30° can be ruled out. Any plausible but weaker alignment would increase the inferred over-radius.

  11. Gas accretion onto galaxies

    CERN Document Server

    Davé, Romeel

    2017-01-01

    This edited volume presents the current state of gas accretion studies from both observational and theoretical perspectives, and charts our progress towards answering the fundamental yet elusive question of how galaxies get their gas. Understanding how galaxies form and evolve has been a central focus in astronomy for over a century. These studies have accelerated in the new millennium, driven by two key advances: the establishment of a firm concordance cosmological model that provides the backbone on which galaxies form and grow, and the recognition that galaxies grow not in isolation but within a “cosmic ecosystem” that includes the vast reservoir of gas filling intergalactic space. This latter aspect in which galaxies continually exchange matter with the intergalactic medium via inflows and outflows has been dubbed the “baryon cycle”. The topic of this book is directly related to the baryon cycle, in particular its least well constrained aspect, namely gas accretion. Accretion is a rare area of ast...

  12. MR image assessment of disc configuration and degree of anterior disc displacement in internal derangement related to age

    International Nuclear Information System (INIS)

    Igarashi, Chinami; Kobayashi, Kaoru; Imanaka, Masahiro; Yuasa, Masao; Yamamoto, Akira

    1999-01-01

    This study was designed to evaluate the configuration of the articular disc and degree of anterior disc displacement on magnetic resonance (MR) imagings in temporomandibular joints (TMJs) with internal derangement. A total of 363 joints diagnosed as having anterior disc displacement with reduction (ADD w R) and 523 joints diagnosed as having anterior disc displacement without reduction (ADD wo R) by MR imaging were examined. These joints did not show severe osseous changes on the condylar head or glenoid fossa. We assessed the configuration of the articular disc and degree of anterior disc displacement. In the ADD w R group, 82.6% of the articular discs showed biconcave configuration; enlargement of the posterior band in 4.6%, biconvex configuration in 0.5%, and others in 10.7%. Moreover 62.5% of the discs showed a slight degree of anterior disc displacement; were 27.2% moderately displaced and were 10.2% severe displaced. The prevalence of slightly displaced discs was higher in the TMJs of cases over 50 years of age than in cases under 30 years in the ADD w R group. On the other hand, in the ADD wo R group 35.9% of the articular discs showed biconcave configuration; enlargement of the posterior band in 12.6%, biconvex configuration in 25.4%, and others in 22.3%. Furthermore, 4.4% of the discs were slightly displaced; 43.9% moderately displaced and 51.6% were severely displaced. The prevalence of severely displaced and deformed discs in joints of cases over 40 years of age was high in the ADD wo R group. The prevalence of slightly displaced biconcave discs was higher in the ADD w R group. The other hand, the prevalence of severely displaced deformed discs was higher in the ADD wo R group. MR findings of internal derangement of the TMJ were found to be significantly correlated with age. (author)

  13. Discovery of new dipper stars with K2: a window into the inner disc region of T Tauri stars

    Science.gov (United States)

    Hedges, Christina; Hodgkin, Simon; Kennedy, Grant

    2018-05-01

    In recent years, a new class of young stellar object (YSO) has been defined, referred to as dippers, where large transient drops in flux are observed. These dips are too large to be attributed to stellar variability, last from hours to days and can reduce the flux of a star by 10-50 per cent. This variability has been attributed to occultations by warps or accretion columns near the inner edge of circumstellar discs. Here, we present 95 dippers in the Upper Scorpius association and ρ Ophiuchus cloud complex found in K2 Campaign 2 data using supervised machine learning with a random forest classifier. We also present 30 YSOs that exhibit brightening events on the order of days, known as bursters. Not all dippers and bursters are known members, but all exhibit infrared excesses and are consistent with belonging to either of the two young star-forming regions. We find 21.0 ± 5.5 per cent of stars with discs are dippers for both regions combined. Our entire dipper sample consists only of late-type (KM) stars, but we show that biases limit dipper discovery for earlier spectral types. Using the dipper properties as a proxy, we find that the temperature at the inner disc edge is consistent with interferometric results for similar and earlier type stars.

  14. Hvad enhver kordreng skal kunne. Betragtning af motetten Ut Phebi radiis af Josquin Desprez

    DEFF Research Database (Denmark)

    Christoffersen, Peter Woetmann

    2003-01-01

    Josquin Desprez, Ut Phebi radiis, motet, prayer mode, hexachord, Ockeghem, Brumel, Isaac, Compère, sound, udtryk......Josquin Desprez, Ut Phebi radiis, motet, prayer mode, hexachord, Ockeghem, Brumel, Isaac, Compère, sound, udtryk...

  15. Lumbar disc excision through fenestration

    Directory of Open Access Journals (Sweden)

    Sangwan S

    2006-01-01

    Full Text Available Background : Lumbar disc herniation often causes sciatica. Many different techniques have been advocated with the aim of least possible damage to other structures while dealing with prolapsed disc surgically in the properly selected and indicated cases. Methods : Twenty six patients with clinical symptoms and signs of prolapsed lumbar intervertebral disc having radiological correlation by MRI study were subjected to disc excision by interlaminar fenestration method. Results : The assessment at follow-up showed excellent results in 17 patients, good in 6 patients, fair in 2 patients and poor in 1 patient. The mean preoperative and postoperative Visual Analogue Scores were 9.34 ±0.84 and 2.19 ±0.84 on scale of 0-10 respectively. These were statistically significant (p value< 0.001, paired t test. No significant complications were recorded. Conclusion : Procedures of interlaminar fenestration and open disc excision under direct vision offers sufficient adequate exposure for lumbar disc excision with a smaller incision, lesser morbidity, shorter convalescence, early return to work and comparable overall results in the centers where recent laser and endoscopy facilities are not available.

  16. Metrical analysis of disc-condyle relation with different splint treatment positions in patients with TMJ disc displacement

    Directory of Open Access Journals (Sweden)

    Mu-Qing Liu

    Full Text Available Abstract Objective: To evaluate the effect of bite positions characterizing different splint treatments (anterior repositioning and stabilization splints on the disc-condyle relation in patients with TMJ disc displacement with reduction (DDwR, using magnetic resonance imaging (MRI. Material and Methods: 37 patients, with a mean age of 18.8±4.3 years (7 male and 30 females and diagnosed with DDwR based on the RDC/TMD, were recruited. MRI metrical analysis of the spatial changes of the disc/condyle, as well as their relationships, was done in three positions: maximum intercuspation (Position 1, anterior repositioning splint position (Position 2, and stabilization splint position (Position 3. Disc/condyle coordinate measurements and disc condyle angles were determined and compared. Results: In Position 1, the average disc-condyle angle was 53.4° in the 60 joints with DDwR, while it was −13.3° with Position 2 and 30.1° with Position 3. The frequency of successful "disc recapture" with Position 2 was significantly higher (58/60, 96.7% than Position 3 (20/60, 33.3%. In Positions 2 and 3, the condyle moved forward and downward while the disc moved backward. The movements were, however, more remarkable with Position 2. Conclusions: Anterior repositioning of the mandible improves the spatial relationship between the disc and condyle in patients with DDwR. In addition to anterior and inferior movement of the condyle, transitory posterior movement of the disc also occurred.

  17. Theory of Disk Accretion onto Magnetic Stars

    Directory of Open Access Journals (Sweden)

    Lai Dong

    2014-01-01

    Full Text Available Disk accretion onto magnetic stars occurs in a variety of systems, including accreting neutron stars (with both high and low magnetic fields, white dwarfs, and protostars. We review some of the key physical processes in magnetosphere-disk interaction, highlighting the theoretical uncertainties. We also discuss some applications to the observations of accreting neutron star and protostellar systems, as well as possible connections to protoplanetary disks and exoplanets.

  18. Influence of piezoceramic to fused silica plate thickness on the radii of curvature of piezoelectric bimorph mirror

    International Nuclear Information System (INIS)

    Libu, M.; Susanth, S.; Vasanthakumari, K. G.; Dileep Kumar, C. J.; Raghu, N.

    2012-01-01

    Piezoelectric based bimorph mirrors (PBM) find extensive use in focusing of x-ray beams. Many optical instruments require use of PBM whose radii of curvature can be tuned precisely. The 100 mm and 300 mm PBMs were fabricated with varying piezoelectric to fused silica plate thicknesses. The radii of curvature of free standing mirrors were measured as a function of voltage and it was found to decrease with increasing voltage. For a given piezoelectric plate thickness, as the fused silica thickness increases, the radii of curvature was found to increase owing to increase in stiffness of the mirror. On the other hand, for a given fused silica plate thickness, when the piezoelectric plate thickness is increased, the radii of curvature are decreased for a given electric field, due to increase in generated force. This study brings out the influence of piezoceramic to fused silica plate thickness on the radii of curvature of PBM.

  19. Gravitomagnetic acceleration from black hole accretion disks

    International Nuclear Information System (INIS)

    Poirier, J; Mathews, G J

    2016-01-01

    We demonstrate how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near an accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism contributing to the production of jets, it presents a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet. (note)

  20. Gravitomagnetic acceleration from black hole accretion disks

    Science.gov (United States)

    Poirier, J.; Mathews, G. J.

    2016-05-01

    We demonstrate how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near an accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism contributing to the production of jets, it presents a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  1. The magnetic field of the equatorial magnetotail - AMPTE/CCE observations at R less than 8.8 earth radii

    Science.gov (United States)

    Fairfield, D. H.; Acuna, M. H.; Zanetti, L. J.; Potemra, T. A.

    1987-01-01

    The MPTE/CCE magnetic field experiment has been used to obtain a quantitative evaluation of the frequency and extent of magnetic field distortion in the near-tail region at less than 8.8 earth radii. The variation of this distortion with Kp, radial distance, longitude, and near-equatorial latitude is reported. It has been found that taillike distortions from the dipole field direction may reach 80 deg near the MPTE/CE apogee of 8.8 earth radii. The Bz field component in dipole coordinates was always positive within 0.5 earth radii of the equatorial current sheet, indicating the neutral lines were never seen inside of 8.8 earth radii. Fields were most taillike near midnight and during times of high Kp. At 8.5 earth radii the equatorial field magnitude depressions were roughly half the dipole field strength of 51 nT. These depressions are larger at lesser distances, reaching -40 nT at 3.4 earth radii for Kp of 2- or less and -80 nT and Kp of 3+ and greater.

  2. Form factors and charge radii in a quantum chromodynamics ...

    Indian Academy of Sciences (India)

    tic form factors and charge radii of D, Ds,B,Bs and Bc mesons in a quantum chromodynamics. (QCD)-inspired ... as pointed out in [12,13], one can expect a similar success here too. .... 0 were large and the formalism failed to account for large ...

  3. Outflow and Accretion Physics in Active Galactic Nuclei

    Science.gov (United States)

    McGraw, Sean Michael

    2016-09-01

    intervals are associated with high-ionization species such as C IV and N V, low-ionization lines including Mg II and Al III, and ground and excited states from Fe II multiplets. The detected BAL and mini-BAL variations in a subset of sources provide evidence supporting scenarios involving either transverse motions of gas or ionization changes within the absorbers. We conclude that some outflows in our samples likely exist on the order of 0.01-1 pc from the SMBH, and the possibility remains that we are tracing outflowing gas on larger scales within limits ranging from ≤10 pc to ≤1 kpc from the central source. We estimate outflow kinetic luminosities between ˜10 6 and 1 times the bolometric luminosity of the quasar, indicating that the BAL outflows we probe likely possess a range of energies and only some absorber energies are likely sufficient for AGN feedback processes. We estimate the SMBH mass in the LLAGN in NGC 4203 to be ˜1.1x10 7 solar masses within a factor of ˜2. This mass estimate in conjunction with theoretical predictions is consistent with the existence of a two-component accretion flow in the nucleus of NGC 4203, consisting of a hot, advection-dominated torus at small radii connected with a thin, radiatively efficient disk at larger scales. These results provide a significant increase in the information available for quasar outflow properties and the conditions in low-luminosity accretion disks, and will inform future observational and theoretical studies that attempt to construct a more complete picture of AGN and their effects on the surrounding environments.

  4. Disc Golf: Teaching a Lifetime Activity

    Science.gov (United States)

    Eastham, Susan L.

    2015-01-01

    Disc golf is a lifetime activity that can be enjoyed by students of varying skill levels and abilities. Disc golf follows the principles of ball golf but is generally easier for students to play and enjoy success. The object of disc golf is similar to ball golf and involves throwing a disc from the teeing area to the target in as few throws as…

  5. Early Results from NICER Observations of Accreting Neutron Stars

    Science.gov (United States)

    Chakrabarty, Deepto; Ozel, Feryal; Arzoumanian, Zaven; Gendreau, Keith C.; Bult, Peter; Cackett, Ed; Chenevez, Jerome; Fabian, Andy; Guillot, Sebastien; Guver, Tolga; Homan, Jeroen; Keek, Laurens; Lamb, Frederick; Ludlam, Renee; Mahmoodifar, Simin; Markwardt, Craig B.; Miller, Jon M.; Psaltis, Dimitrios; Strohmayer, Tod E.; Wilson-Hodge, Colleen A.; Wolff, Michael T.

    2018-01-01

    The Neutron Star Interior Composition Explorer (NICER) offers significant new capabilities for the study of accreting neuton stars relative to previous X-ray missions including large effective area, low background, and greatly improved low-energy response. The NICER Burst and Accretion Working Group has designed a 2 Ms observation program to study a number of phenomena in accreting neutron stars including type-I X-ray bursts, superbursts, accretion-powered pulsations, quasi-periodic oscillations, and accretion disk reflection spectra. We present some early results from the first six months of the NICER mission.

  6. Accretion and evolution of ∼2.5 M {sub ⊕} planets with voluminous H/He envelopes

    Energy Technology Data Exchange (ETDEWEB)

    Bodenheimer, Peter [UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); Lissauer, Jack J., E-mail: peter@ucolick.org, E-mail: Jack.J.Lissauer@nasa.gov [Space Science and Astrobiology Division, NASA-Ames Research Center, Moffett Field, CA 94035 (United States)

    2014-08-20

    Formation of planets in the Neptune size range with low-mass, but voluminous, H{sub 2}/He gaseous envelopes is modeled by detailed numerical simulations according to the core-nucleated accretion scenario. Formation locations ranging from 0.5 to 4 AU from a star of 1 M {sub ☉} are considered. The final planets have heavy-element cores of 2.2-2.5 M {sub ⊕} and envelopes in the range 0.037-0.16 M {sub ⊕}. After the formation process, which lasts 2 Myr or less, the planets evolve at constant mass up to an age of several Gyr. For assumed equilibrium temperatures of 250, 500, and 1000 K, their calculated final radii are compared with those observed by the Kepler spacecraft. For the particular case of Kepler-11 f, we address the question whether it could have formed in situ or whether migration from a formation location farther out in the disk is required.

  7. Unexpectedly large charge radii of neutron-rich calcium isotopes

    CERN Document Server

    Garcia Ruiz, R F; Blaum, K; Ekström, A; Frömmgen, N; Hagen, G; Hammen, M; Hebeler, K; Holt, J D; Jansen, G R; Kowalska, M; Kreim, K; Nazarewicz, W; Neugart, R; Neyens, G; Nörtershäuser, W; Papenbrock, T; Papuga, J; Schwenk, A; Simonis, J; Wendt, K A; Yordanov, D T

    2016-01-01

    Despite being a complex many-body system, the atomic nucleus exhibits simple structures for certain ‘magic’ numbers of protons and neutrons. The calcium chain in particular is both unique and puzzling: evidence of doubly magic features are known in 40,48Ca, and recently suggested in two radioactive isotopes, 52,54Ca. Although many properties of experimentally known calcium isotopes have been successfully described by nuclear theory, it is still a challenge to predict the evolution of their charge radii. Here we present the first measurements of the charge radii of 49,51,52Ca, obtained from laser spectroscopy experiments at ISOLDE, CERN. The experimental results are complemented by state-of-the-art theoretical calculations. The large and unexpected increase of the size of the neutron-rich calcium isotopes beyond N = 28 challenges the doubly magic nature of 52Ca and opens new intriguing questions on the evolution of nuclear sizes away from stability, which are of importance for our understanding of neutron-...

  8. Systematics of nuclear RMS charge radii

    International Nuclear Information System (INIS)

    Brown, B.A.; Bronk, C.; Hodgson, P.E.

    1984-01-01

    The experimental RMS charge radii of isotopic sequences of nuclei are compared with calculations based on the spherical droplet model and spherical single-particle potential models. Harmonic-oscillator, Woods-Saxon and Skyrme Hartree-Fock single-particle potentials are considered. Deviations between experiment and theory are discussed in terms of the model parameters and in terms of the fundamental inadequacies of the models. The experimental B(E2) values connecting the ground states to the lowest 2 + states are used to estimate the increase in RMS radius due to the effects of deformation and zero-point vibrational motion. (author)

  9. Measuring the spins of accreting black holes

    International Nuclear Information System (INIS)

    McClintock, Jeffrey E; Narayan, Ramesh; Gou, Lijun; Kulkarni, Akshay; Penna, Robert F; Steiner, James F; Davis, Shane W; Orosz, Jerome A; Remillard, Ronald A

    2011-01-01

    A typical galaxy is thought to contain tens of millions of stellar-mass black holes, the collapsed remnants of once massive stars, and a single nuclear supermassive black hole. Both classes of black holes accrete gas from their environments. The accreting gas forms a flattened orbiting structure known as an accretion disk. During the past several years, it has become possible to obtain measurements of the spins of the two classes of black holes by modeling the x-ray emission from their accretion disks. Two methods are employed, both of which depend upon identifying the inner radius of the accretion disk with the innermost stable circular orbit, whose radius depends only on the mass and spin of the black hole. In the Fe Kα method, which applies to both classes of black holes, one models the profile of the relativistically broadened iron line with a special focus on the gravitationally redshifted red wing of the line. In the continuum-fitting (CF) method, which has so far only been applied to stellar-mass black holes, one models the thermal x-ray continuum spectrum of the accretion disk. We discuss both methods, with a strong emphasis on the CF method and its application to stellar-mass black holes. Spin results for eight stellar-mass black holes are summarized. These data are used to argue that the high spins of at least some of these black holes are natal, and that the presence or absence of relativistic jets in accreting black holes is not entirely determined by the spin of the black hole.

  10. [Partial nucleotomy of the ovine disc as an in vivo model for disc degeneration].

    Science.gov (United States)

    Guder, E; Hill, S; Kandziora, F; Schnake, K J

    2009-01-01

    The aim of this study was to develop a suitable animal model for the clinical situation of progressive disc degeneration after microsurgical nucleotomy. Twenty sheep underwent standardised partial anterolateral nucleotomy at lumbar segment 3/4. After randomisation, 10 animals were sacrificed after 12 weeks (group 1). The remainder was sacrificed after 48 weeks (group 2). For radiological examination X-rays, MRI and post-mortem CT scans were performed. Lumbar discs L 3/4 with adjacent subchondral trabecular bone were harvested and analysed macroscopically and histologically. An image-analysing computer program was used to measure histomorphometric indices of bone structure. 17 segments could be evaluated. After 12 weeks (group 1) histological and radiological degenerative disc changes were noted. After 48 weeks (group 2), radiological signs in MRI reached statistical significance. Furthermore, group 2 showed significantly more osteophyte formations in CT scans. Histomorphometric changes of the disc and the adjacent vertebral bone structure suggest a significant progressive degenerative remodelling. The facet joints did not show any osteoarthrosis after 48 weeks. Partial nucleotomy of the ovine lumbar disc leads to radiological and histological signs of disc degeneration similar to those seen in humans after microsurgical nucleotomy. The presented in vivo model may be useful to evaluate new orthopaedic treatment strategies.

  11. A spin-down mechanism for accreting neutron stars

    International Nuclear Information System (INIS)

    Illarionov, A.F.; AN SSSR, Moscow. Fizicheskij Inst.); Kompaneets, D.A.

    1990-01-01

    We propose a new spin-down mechanism for accreting neutron stars that explains the existence of a number of long-period (p≅100-1000 s) X-ray pulsars in wide binaries with OB-stars. The spin-down is a result of efficient angular momentum transfer from the rotating magnetosphere of the accreting star to an outflowing stream of magnetized matter. The outflow is formed within a limited solid angle, and the outflow rate is less than the accretion rate. The outflow formation is connected with the anisotropy and intensity of the hard X-ray emission of the neutron star. X-rays from the pulsar heat through Compton scattering the accreting matter anisotropically. The heated matter has a lower density than the surrounding accreting matter and flows up by the action of the buoyancy force. We find the criterion for the outflow to form deep in the accretion flow (i.e., close to the neutron star magnetosphere). The neutron star loses angular momentum when the outflow forms so deep as to capture the magnetic field lines from the rotating magnetosphere. The balance between angular momentum gain by accreting gas and loss by outflowing matter takes place at a particular value of the period of the spinning neutron star. (orig.)

  12. Radiological assessment of loss of disc height during acute and chronic degenerative lumbar disc alterations

    International Nuclear Information System (INIS)

    Zoellner, J.; Sancaktaroglu, T.; Nafe, B.; Eysel, P.; Loew, R.

    2001-01-01

    Aim of the study: A loss of disc height with increasing segmental mobility is an important reason for low back pain. The measurement of hyaluronic acid content of the nucleus pulposus prolaps shows a difference between acute (group 1) and chronic (group 2) disc degeneration. The purpose of the present investigation was to determine the decreasing of disc height between these two groups and the no-symptomatic segments of these patients. Methods: 20 human lateral preoperative X-ray measurements according to Frobin et al. were taken; group 1 with 7 patients (mean age 41 years) and group 2 with 13 patients (mean age 44 years). Results: There was a significant tendency (p=0.091) to a reduction of disc height in group 2 between symptomatic and asymptomatic discs. Conclusion: The used method is not suitable to answer the present question conclusively. (orig.) [de

  13. Actuator disc edge singularity. The key to a revised actuator disc concept and momentum theory

    Energy Technology Data Exchange (ETDEWEB)

    Kuik, G.A.M. van (The Wind Energy Group of the Technical University Eindhoven (NL))

    1989-01-01

    Since the beginning of rotor aerodynamics the actuator disc momentum theory occupies a prominant place in almost any textbook on this subject. Specially in axial flow the theory provides an easy and rather accurate performance prediction. The results first obtained by Lanchester for the induced power of a hovering rotor and the maximum power of a wind turbine are still used as guidelines for complicated calculations. On the other hand, experimental results for propellers are known to deviate systematically (some 10%) from the momentum theory results. This is commonly attributed to the differences between a real rotor and an actuator disc. However, some actuator disc- and actuator strip (the 2-dimensional version) experiments are described in literature, showing the same deviations from momentum theory results. Therefore, apart from the question how representative an actuator disc is for a real rotor, the actuator disc concept itself may be inadequate. This problem is the subject of the work describe here. It will be shown that the classical actuator disc concept ignores discrete forces resulting from a flow singularity at the edge of the disc. The (extended) momentum theory, applied to this actuator strip model, shows a shift of the results towards the experimental data, and for the static case (hover) even a quantitative agreement is obtained. (author) 12 refs.

  14. Vascular complications of prosthetic inter-vertebral discs.

    Science.gov (United States)

    Daly, Kevin J; Ross, E Raymond S; Norris, Heather; McCollum, Charles N

    2006-10-01

    Five consecutive cases of prosthetic inter-vertebral disc displacement with severe vascular complications on revisional surgery are described. The objective of this case report is to warn spinal surgeons that major vascular complications are likely with anterior displacement of inter-vertebral discs. We have not been able to find a previous report on vascular complications associated with anterior displacement of prosthetic inter-vertebral discs. In all five patients the prosthetic disc had eroded into the bifurcation of the inferior vena cava and the left common iliac vein. In three cases the aortic bifurcation was also involved. The fibrosis was so severe that dissecting out the arteries and veins to provide access to the relevant disc proved impossible. Formal division of the left common iliac vein and artery with subsequent repair was our solution. Anterior inter-vertebral disc displacement was associated with severe vascular injury. Preventing anterior disc displacement is essential in disc design. In the event of anterior displacement, disc removal should be planned with a Vascular Surgeon.

  15. Accretion of Ghost Condensate by Black Holes

    Energy Technology Data Exchange (ETDEWEB)

    Frolov, A

    2004-06-02

    The intent of this letter is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as tenth of a solar mass per second for 10MeV-scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.

  16. Review of gravitomagnetic acceleration from accretion disks

    Science.gov (United States)

    Poirier, J.; Mathews, G. J.

    2015-11-01

    We review the development of the equations of gravitoelectromagnetism and summarize how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near the accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism to produce collimated jets, it is a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  17. Black hole spin from wobbling and rotation of the M87 jet and a sign of a magnetically arrested disc

    Science.gov (United States)

    Sob'yanin, Denis Nikolaevich

    2018-06-01

    New long-term Very Long Baseline Array observations of the well-known jet in the M87 radio galaxy at 43 GHz show that the jet experiences a sideways shift with an approximately 8-10 yr quasi-periodicity. Such jet wobbling can be indicative of a relativistic Lense-Thirring precession resulting from a tilted accretion disc. The wobbling period together with up-to-date kinematic data on jet rotation opens up the possibility for estimating angular momentum of the central supermassive black hole. In the case of a test-particle precession, the specific angular momentum is J/Mc = (2.7 ± 1.5) × 1014 cm, implying moderate dimensionless spin parameters a = 0.5 ± 0.3 and 0.31 ± 0.17 for controversial gas-dynamic and stellar-dynamic black hole masses. However, in the case of a solid-body-like precession, the spin parameter is much smaller for both masses, 0.15 ± 0.05. Rejecting this value on the basis of other independent spin estimations requires the existence of a magnetically arrested disc in M87.

  18. Deep-down ionization of protoplanetary discs

    Science.gov (United States)

    Glassgold, A. E.; Lizano, S.; Galli, D.

    2017-12-01

    The possible occurrence of dead zones in protoplanetary discs subject to the magneto-rotational instability highlights the importance of disc ionization. We present a closed-form theory for the deep-down ionization by X-rays at depths below the disc surface dominated by far-ultraviolet radiation. Simple analytic solutions are given for the major ion classes, electrons, atomic ions, molecular ions and negatively charged grains. In addition to the formation of molecular ions by X-ray ionization of H2 and their destruction by dissociative recombination, several key processes that operate in this region are included, e.g. charge exchange of molecular ions and neutral atoms and destruction of ions by grains. Over much of the inner disc, the vertical decrease in ionization with depth into the disc is described by simple power laws, which can easily be included in more detailed modelling of magnetized discs. The new ionization theory is used to illustrate the non-ideal magnetohydrodynamic effects of Ohmic, Hall and Ambipolar diffusion for a magnetic model of a T Tauri star disc using the appropriate Elsasser numbers.

  19. Fitted Hanbury-Brown Twiss radii versus space-time variances in flow-dominated models

    Science.gov (United States)

    Frodermann, Evan; Heinz, Ulrich; Lisa, Michael Annan

    2006-04-01

    The inability of otherwise successful dynamical models to reproduce the Hanbury-Brown Twiss (HBT) radii extracted from two-particle correlations measured at the Relativistic Heavy Ion Collider (RHIC) is known as the RHIC HBT Puzzle. Most comparisons between models and experiment exploit the fact that for Gaussian sources the HBT radii agree with certain combinations of the space-time widths of the source that can be directly computed from the emission function without having to evaluate, at significant expense, the two-particle correlation function. We here study the validity of this approach for realistic emission function models, some of which exhibit significant deviations from simple Gaussian behavior. By Fourier transforming the emission function, we compute the two-particle correlation function, and fit it with a Gaussian to partially mimic the procedure used for measured correlation functions. We describe a novel algorithm to perform this Gaussian fit analytically. We find that for realistic hydrodynamic models the HBT radii extracted from this procedure agree better with the data than the values previously extracted from the space-time widths of the emission function. Although serious discrepancies between the calculated and the measured HBT radii remain, we show that a more apples-to-apples comparison of models with data can play an important role in any eventually successful theoretical description of RHIC HBT data.

  20. Fitted Hanbury-Brown-Twiss radii versus space-time variances in flow-dominated models

    International Nuclear Information System (INIS)

    Frodermann, Evan; Heinz, Ulrich; Lisa, Michael Annan

    2006-01-01

    The inability of otherwise successful dynamical models to reproduce the Hanbury-Brown-Twiss (HBT) radii extracted from two-particle correlations measured at the Relativistic Heavy Ion Collider (RHIC) is known as the RHIC HBT Puzzle. Most comparisons between models and experiment exploit the fact that for Gaussian sources the HBT radii agree with certain combinations of the space-time widths of the source that can be directly computed from the emission function without having to evaluate, at significant expense, the two-particle correlation function. We here study the validity of this approach for realistic emission function models, some of which exhibit significant deviations from simple Gaussian behavior. By Fourier transforming the emission function, we compute the two-particle correlation function, and fit it with a Gaussian to partially mimic the procedure used for measured correlation functions. We describe a novel algorithm to perform this Gaussian fit analytically. We find that for realistic hydrodynamic models the HBT radii extracted from this procedure agree better with the data than the values previously extracted from the space-time widths of the emission function. Although serious discrepancies between the calculated and the measured HBT radii remain, we show that a more apples-to-apples comparison of models with data can play an important role in any eventually successful theoretical description of RHIC HBT data

  1. ELLIPTICAL GALAXY MASSES OUT TO FIVE EFFECTIVE RADII: THE REALM OF DARK MATTER

    International Nuclear Information System (INIS)

    Deason, A. J; Belokurov, V.; Evans, N. W.; McCarthy, I. G.

    2012-01-01

    We estimate the masses of elliptical galaxies out to five effective radii using planetary nebulae and globular clusters as tracers. A sample of 15 elliptical galaxies with a broad variation in mass is compiled from the literature. A distribution function-maximum likelihood analysis is used to estimate the overall potential slope, normalization, and velocity anisotropy of the tracers. We assume power-law profiles for the potential and tracer density and a constant velocity anisotropy. The derived potential power-law indices lie in between the isothermal and Keplerian regime and vary with mass: there is tentative evidence that the less massive galaxies have steeper potential profiles than the more massive galaxies. We use stellar mass-to-light ratios appropriate for either a Chabrier/KTG (Kroupa, Tout and Gilmore) or Salpeter initial mass function to disentangle the stellar and dark matter components. The fraction of dark matter within five effective radii increases with mass, in agreement with several other studies. We employ simple models to show that a combination of star formation efficiency and baryon extent are able to account for this trend. These models are in good agreement with both our measurements out to five effective radii and recent Sloan Lens ACS Survey measurements within one effective radii when a universal Chabrier/KTG initial mass function is adopted.

  2. A Solution to the Protostellar Accretion Problem

    OpenAIRE

    Padoan, Paolo; Kritsuk, Alexei; Norman, Michael L.; Nordlund, Ake

    2004-01-01

    Accretion rates of order 10^-8 M_\\odot/yr are observed in young protostars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for protostars of smaller mass, approximately proportional to the second power of the stellar mass, \\dot{M}_accr\\propto M^2. The traditional view is that the observed accretion is the consequence of the angular momentum transport in isolated protostellar disks, controlled by disk turbulence or self--gravity. Ho...

  3. Pulsed Accretion in the T Tauri Binary TWA 3A

    Energy Technology Data Exchange (ETDEWEB)

    Tofflemire, Benjamin M.; Mathieu, Robert D. [Department of Astronomy, University of Wisconsin–Madison, 475 North Charter Street, Madison, WI 53706 (United States); Herczeg, Gregory J. [The Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871 (China); Akeson, Rachel L.; Ciardi, David R. [NASA Exoplanet Science Institute, IPAC/Caltech, Pasadena, CA 91125 (United States)

    2017-06-20

    TWA 3A is the most recent addition to a small group of young binary systems that both actively accrete from a circumbinary disk and have spectroscopic orbital solutions. As such, it provides a unique opportunity to test binary accretion theory in a well-constrained setting. To examine TWA 3A’s time-variable accretion behavior, we have conducted a two-year, optical photometric monitoring campaign, obtaining dense orbital phase coverage (∼20 observations per orbit) for ∼15 orbital periods. From U -band measurements we derive the time-dependent binary mass accretion rate, finding bursts of accretion near each periastron passage. On average, these enhanced accretion events evolve over orbital phases 0.85 to 1.05, reaching their peak at periastron. The specific accretion rate increases above the quiescent value by a factor of ∼4 on average but the peak can be as high as an order of magnitude in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in good agreement with numerical simulations of binary accretion with similar orbital parameters. In these simulations, periastron accretion bursts are fueled by periodic streams of material from the circumbinary disk that are driven by the binary orbit. We find that TWA 3A’s average accretion behavior is remarkably similar to DQ Tau, another T Tauri binary with similar orbital parameters, but with significantly less variability from orbit to orbit. This is only the second clear case of orbital-phase-dependent accretion in a T Tauri binary.

  4. Focused Wind Mass Accretion in Mira AB

    Science.gov (United States)

    Karovska, Margarita; de Val-Borro, M.; Hack, W.; Raymond, J.; Sasselov, D.; Lee, N. P.

    2011-05-01

    At a distance of about only 100pc, Mira AB is the nearest symbiotic system containing an Asymptotic Giant Branch (AGB) star (Mira A), and a compact accreting companion (Mira B) at about 0.5" from Mira A. Symbiotic systems are interacting binaries with a key evolutionary importance as potential progenitors of a fraction of asymmetric Planetary Nebulae, and SN type Ia, cosmological distance indicators. The region of interaction has been studied using high-angular resolution, multiwavelength observations ranging from radio to X-ray wavelengths. Our results, including high-angular resolution Chandra imaging, show a "bridge" between Mira A and Mira B, indicating gravitational focusing of the Mira A wind, whereby components exchange matter directly in addition to the wind accretion. We carried out a study using 2-D hydrodynamical models of focused wind mass accretion to determine the region of wind acceleration and the characteristics of the accretion in Mira AB. We highlight some of our results and discuss the impact on our understanding of accretion processes in symbiotic systems and other detached and semidetached interacting systems.

  5. Comparison of Heidelberg Retina Tomograph with disc-macula distance to disc diameter ratio in diagnosing optic nerve hypoplasia.

    Science.gov (United States)

    Pang, Yi; Frantz, Kelly A

    2016-05-01

    To evaluate whether Heidelberg Retinal Tomograph (HRT) is a valid test for diagnosing congenital optic nerve hypoplasia (CONH) compared to the ratio of the distance between the centre of the optic disc and the centre of the macula and the mean optic disc diameter (DM:DD ratio). Furthermore, to determine the optimal cut-off value of HRT disc area to differentiate a hypoplastic disc from a normal optic disc. A total of 33 subjects with CONH (4-67 years old) and 160 normal subjects (5-65 years old) were recruited and underwent comprehensive eye examinations, fundus photography and HRT. Receiver operating characteristic curves for DM:DD ratio and HRT disc area were constructed based on data from the 46 CONH eyes and 160 control eyes. Mean (±S.D.) HRT disc area was 1.94 (±0.54) mm(2) for the control eyes and 0.84 (±0.35) mm(2) for the CONH eyes (p < 0.0001). The area under the curve (AUC) for DM:DD ratio was 0.83 (95% confidence interval: 0.76-0.90). The AUC for HRT disc area was 0.96 (95% confidence interval: 0.94-0.99). A statistically significant difference was found between AUC for HRT disc area and that for DM:DD ratio (p = 0.0004). The optimal cut-off value for HRT disc area was 1.42 mm(2) with 95% sensitivity and 85% specificity. The optimal cut-off value for DM:DD ratio was 3.20 with 78% sensitivity and 78% specificity. Both HRT and the DM:DD ratio are valid tests to aid diagnosis of CONH. HRT is superior to DM:DD ratio in diagnosing CONH with higher sensitivity and specificity. We suggest the optimal cut-off value for HRT disc area as 1.42 mm(2) in order to discriminate a hypoplastic disc from a normal optic disc. © 2016 The Authors Ophthalmic & Physiological Optics © 2016 The College of Optometrists.

  6. CT findings of lumbar intervertebral disc: II. Disc herniation (HNP)

    International Nuclear Information System (INIS)

    Yang, W. J.; Lee, J. M.; Bahk, Y. W.

    1984-01-01

    In lumbar region the epidural fat pad is relatively abundant so that CT can provides sufficient information in diagnosis of lumbar HNP. Many authors have reported on the CT findings of HNP such as focal nodular protrusion of the posterior disc margin, obliteration of epidural fat pad, impingement of dural sac and nerve root, swelling of nerve root, soft tissue density in the spinal canal and calcification of disc. However there was so previous report describing incidence and reliability of the findings. It is the purpose of the present study to survey the frequency, reliability, and limitation of these CT findings. The clinical material was consisted of 30 operatively proven cases of HNP of the lumbar spine. Each lumbar CT scan was reviewed retrospectively and the findings were analysed by two radiologists independently. There were 20 males and 10 females and the mean age was 36.7 years. Involvement of L4-S5 level was 2.3 times more frequent than that of L5-S1 level. Of 30 cases, 22 were unilateral posterolateral types and 8 cases central or unilateral far lateral types. CT findings observed were nodular protrusion of the posterior margin of the disc, obliteration of epidural fat pad, impingement of dural sac or nerve root, soft tissue density in the spinal canal and calcification in the posterior portion of the protruded disc, in order of decreasing frequency. The conclusions are follows: 1. Nodular protrusion of the posterior disc margin accompanied by obliteration of epidural fat pad was observed in every case. The former findings was designated as direct sign and the latter indirect. 2. Obliteration of the epidural fat appears to be significant in lateral recesses especially when it occurs unilaterally. This was not true, however, in the centrally located fat pad. 3. Impingement of the dural sac and nerve root were observed in 90% and 67%, respectively, and were very helpful in establishing HNP diagnosis when the direct and indirect signs were equivocal

  7. The Galactic stellar disc

    International Nuclear Information System (INIS)

    Feltzing, S; Bensby, T

    2008-01-01

    The study of the Milky Way stellar discs in the context of galaxy formation is discussed. In particular, we explore the properties of the Milky Way disc using a new sample of about 550 dwarf stars for which we have recently obtained elemental abundances and ages based on high-resolution spectroscopy. For all the stars we also have full kinematic information as well as information about their stellar orbits. We confirm results from previous studies that the thin and the thick discs have distinct abundance patterns. But we also explore a larger range of orbital parameters than what has been possible in our previous studies. Several new results are presented. We find that stars that reach high above the Galactic plane and have eccentric orbits show remarkably tight abundance trends. This implies that these stars formed out of well-mixed gas that had been homogenized over large volumes. We find some evidence that suggest that the event that most likely caused the heating of this stellar population happened a few billion years ago. Through a simple, kinematic exploration of stars with super-solar [Fe/H], we show that the solar neighbourhood contains metal-rich, high velocity stars that are very likely associated with the thick disc. Additionally, the HR1614 moving group and the Hercules and Arcturus stellar streams are discussed and it is concluded that, probably, a large fraction of the groups and streams so far identified in the disc are the result of evolution and interactions within the stellar disc rather than being dissolved stellar clusters or engulfed dwarf galaxies.

  8. Björk-Shiley strut fracture and disc escape: literature review and a method of disc retrieval.

    Science.gov (United States)

    Hendel, P N

    1989-03-01

    Embolization of a prosthetic valve poppet is a rare but life-threatening event. It was reported sporadically before the introduction of the Björk-Shiley 70-degree convexoconcave prosthesis in 1980. Since that time, there have been a large number of reported mechanical failures with disc escape. The rate for the 29-mm to 33-mm mitral valves is estimated as 5.2%. In 29 of 35 patients (including the 2 presented here) in whom the site of disc lodgment could be determined, the disc was in the descending or abdominal aorta. Fifteen of these patients died. Six survivors had the disc removed at the same operation and 6 at a later operation. In 2 patients, the disc was not removed. In 2 patients in whom the disc was not removed initially, it was thought to contribute to postoperative complications. Two more cases of structural failure of the Björk-Shiley convexoconcave prosthesis are presented. A transpericardial approach to the descending aorta on bypass is described. It allows easy removal of the disc and eliminates the need for a second operation.

  9. Average configuration of the distant (less than 220-earth-radii) magnetotail - Initial ISEE-3 magnetic field results

    Science.gov (United States)

    Slavin, J. A.; Tsurutani, B. T.; Smith, E. J.; Jones, D. E.; Sibeck, D. G.

    1983-01-01

    Magnetic field measurements from the first two passes of the ISEE-3 GEOTAIL Mission have been used to study the structure of the trans-lunar tail. Good agreement was found between the ISEE-3 magnetopause crossings and the Explorer 33, 35 model of Howe and Binsack (1972). Neutral sheet location was well ordered by the hinged current sheet models based upon near earth measurements. Between X = -20 and -120 earth radii the radius of the tail increases by about 30 percent while the lobe field strength decreases by approximately 60 percent. Beyond X = -100 to -1200 earth radii the tail diameter and lobe field magnitude become nearly constant at terminal values of approximately 60 earth radii and 9 nT, respectively. The distance at which the tail was observed to cease flaring, 100-120 earth radii, is in close agreement with the predictions of the analytic tail model of Coroniti and Kennel (1972). Overall, the findings of this study suggest that the magnetotail retains much of its near earth structure out to X = -220 earth radii.

  10. Accretion and evaporation of modified Hayward black hole

    International Nuclear Information System (INIS)

    Debnath, Ujjal

    2015-01-01

    We assume the most general static spherically symmetric black hole metric. The accretion of any general kind of fluid flow around the black hole is investigated. The accretion of the fluid flow around the modified Hayward black hole is analyzed, and we then calculate the critical point, the fluid's four-velocity, and the velocity of sound during the accretion process. Also the nature of the dynamical mass of the black hole during accretion of the fluid flow, taking into consideration Hawking radiation from the black hole, i.e., evaporation of the black hole, is analyzed. (orig.)

  11. Bondi-Hoyle-Lyttleton Accretion onto Binaries

    Science.gov (United States)

    Antoni, Andrea; MacLeod, Morgan; Ramírez-Ruiz, Enrico

    2018-01-01

    Binary stars are not rare. While only close binary stars will eventually interact with one another, even the widest binary systems interact with their gaseous surroundings. The rates of accretion and the gaseous drag forces arising in these interactions are the key to understanding how these systems evolve. This poster examines accretion flows around a binary system moving supersonically through a background gas. We perform three-dimensional hydrodynamic simulations of Bondi-Hoyle-Lyttleton accretion using the adaptive mesh refinement code FLASH. We simulate a range of values of semi-major axis of the orbit relative to the gravitational focusing impact parameter of the pair. On large scales, gas is gravitationally focused by the center-of-mass of the binary, leading to dynamical friction drag and to the accretion of mass and momentum. On smaller scales, the orbital motion imprints itself on the gas. Notably, the magnitude and direction of the forces acting on the binary inherit this orbital dependence. The long-term evolution of the binary is determined by the timescales for accretion, slow down of the center-of-mass, and decay of the orbit. We use our simulations to measure these timescales and to establish a hierarchy between them. In general, our simulations indicate that binaries moving through gaseous media will slow down before the orbit decays.

  12. He stars and He-accreting CO white dwarfs

    International Nuclear Information System (INIS)

    Limongi, M.; Tornambe, A.

    1991-01-01

    He star models in the mass range 0.4-1.0 solar mass have been evolved until the red giant phase or, depending on their mass, until crystallization on the white-dwarf cooling sequence. Some of the degenerate structures obtained in these computations have been successively accreted at various He accretion rates in order to better define the fate of the accreting dwarf versus its mass and accretion rate for a fixed degeneracy level of the accreting dwarf. He stars have been further induced to transfer mass to a degenerate companion through Roche lobe overflow, in conditions of large gravitational wave radiation by the system. CO dwarfs in binary systems with He stars are found to experience a thermal behavior whose effects are such to locate the structure on the verge of obtaining a strong SN-like explosive event. 22 refs

  13. Accretion onto a noncommutative geometry inspired black hole

    Energy Technology Data Exchange (ETDEWEB)

    Kumar, Rahul [Jamia Millia Islamia, Centre for Theoretical Physics, New Delhi (India); Ghosh, Sushant G. [Jamia Millia Islamia, Centre for Theoretical Physics, New Delhi (India); Jamia Millia Islamia, Multidisciplinary Centre for Advanced Research and Studies (MCARS), New Delhi (India); University of KwaZulu-Natal, Astrophysics and Cosmology Research Unit, School of Mathematics, Statistics and Computer Science, Durban (South Africa)

    2017-09-15

    The spherically symmetric accretion onto a noncommutative (NC) inspired Schwarzschild black hole is treated for a polytropic fluid. The critical accretion rate M, sonic speed a{sub s} and other flow parameters are generalized for the NC inspired static black hole and compared with the results obtained for the standard Schwarzschild black holes. Also explicit expressions for gas compression ratios and temperature profiles below the accretion radius and at the event horizon are derived. This analysis is a generalization of Michel's solution to the NC geometry. Owing to the NC corrected black hole, the accretion flow parameters also have been modified. It turns out that M ∼ M{sup 2} is still achievable but r{sub s} seems to be substantially decreased due to the NC effects. They in turn do affect the accretion process. (orig.)

  14. Debris disc constraints on planetesimal formation

    Science.gov (United States)

    Krivov, Alexander V.; Ide, Aljoscha; Löhne, Torsten; Johansen, Anders; Blum, Jürgen

    2018-02-01

    Two basic routes for planetesimal formation have been proposed over the last decades. One is a classical `slow-growth' scenario. Another one is particle concentration models, in which small pebbles are concentrated locally and then collapse gravitationally to form planetesimals. Both types of models make certain predictions for the size spectrum and internal structure of newly born planetesimals. We use these predictions as input to simulate collisional evolution of debris discs left after the gas dispersal. The debris disc emission as a function of a system's age computed in these simulations is compared with several Spitzer and Herschel debris disc surveys around A-type stars. We confirm that the observed brightness evolution for the majority of discs can be reproduced by classical models. Further, we find that it is equally consistent with the size distribution of planetesimals predicted by particle concentration models - provided the objects are loosely bound `pebble piles' as these models also predict. Regardless of the assumed planetesimal formation mechanism, explaining the brightest debris discs in the samples uncovers a `disc mass problem'. To reproduce such discs by collisional simulations, a total mass of planetesimals of up to ˜1000 Earth masses is required, which exceeds the total mass of solids available in the protoplanetary progenitors of debris discs. This may indicate that stirring was delayed in some of the bright discs, that giant impacts occurred recently in some of them, that some systems may be younger than previously thought or that non-collisional processes contribute significantly to the dust production.

  15. T1 hyperintense disc in alkaptonuria.

    Science.gov (United States)

    Sag, Alan A; Silbergleit, Richard; Olson, Rick E; Wilson, Jon; Krishnan, Anant

    2012-10-01

    Case report. To report a rare case of alkaptonuria presenting as a T1-hyperintense disc herniation. A 46-year-old man without previous diagnosis of alkaptonuria underwent evaluation for progressive back pain revealing a T1-hyperintense disc herniation at the L3-L4 level. Discectomy recovered a blackened disc that was pathologically confirmed to be nucleus pulposus with alkaptonuric involvement. The differential diagnosis of a T1-hyperintense, T2-hypointense disc on magnetic resonance imaging is discussed, with emphasis on the pathophysiology of alkaptonuria. A single patient is reported. Pathologically proven patient presentation with radiological and pathological images. We report a rare case of alkaptonuria presenting as a T1-hyperintense disc herniation.

  16. Energy transport in radially accreting white dwarf stars

    Energy Technology Data Exchange (ETDEWEB)

    Thompson, A.M.

    1986-10-01

    Some of the non-thermal energy transport processes which may be present in a white dwarf accretion column are examined and it is determined whether these could in any way contribute to a resolution of the soft X-ray puzzle. The first two Chapters of this Thesis constitute a review of the observations and proposed models for white dwarf accretion columns. In Chapter 3 we show that in Kuijpers and Pringle's original bombardment model of white dwarf accretion columns, in which the energy of the accreting material is deposited uniformly into a static atmosphere which then radiates the energy away as optically thin bremsstrahlung/line radiation, an incorrect Coulomb collisional timescale was used. In Chapter 4 we extend the calculations of Chapter 3 to include the effect of cyclotron radiation. It is concluded that a cyclotron cooled bombardment solution for a white dwarf accretion column may exist. We extend this calculation to derive a simple piecewise uniform temperature structure for such an accretion column, incorporating the effect of thermal conduction. In Chaper 5 we examine two of the non thermal emission mechanisms that might be present in white dwarf accretion columns:- non thermal Lyman-{alpha} emission and non thermal inverse bremsstrahlung emission. It is shown that neither would actually be sufficiently large to be detectable. In Chapter 6 some possible extensions to the work presented are suggested. (author).

  17. Coherence of burst oscillations and accretion-powered pulsations in the accreting millisecond pulsar XTE J1814-338

    NARCIS (Netherlands)

    Watts, A.L.; Patruno, A.; van der Klis, M.

    2008-01-01

    X-ray timing of the accretion-powered pulsations during the 2003 outburst of the accreting millisecond pulsar XTE J1814-338 has revealed variation in the pulse time of arrival residuals. These can be interpreted in several ways, including spin-down and wandering of the fuel impact point around the

  18. The accretion of migrating giant planets

    Science.gov (United States)

    Dürmann, Christoph; Kley, Wilhelm

    2017-02-01

    Aims: Most studies concerning the growth and evolution of massive planets focus either on their accretion or their migration only. In this work we study both processes concurrently to investigate how they might mutually affect one another. Methods: We modeled a two-dimensional disk with a steady accretion flow onto the central star and embedded a Jupiter mass planet at 5.2 au. The disk is locally isothermal and viscosity is modeled using a constant α. The planet is held on a fixed orbit for a few hundred orbits to allow the disk to adapt and carve a gap. After this period, the planet is released and free to move according to the gravitational interaction with the gas disk. The mass accretion onto the planet is modeled by removing a fraction of gas from the inner Hill sphere, and the removed mass and momentum can be added to the planet. Results: Our results show that a fast migrating planet is able to accrete more gas than a slower migrating planet. Utilizing a tracer fluid we analyzed the origin of the accreted gas originating predominantly from the inner disk for a fast migrating planet. In the case of slower migration, the fraction of gas from the outer disk increases. We also found that even for very high accretion rates, in some cases gas crosses the planetary gap from the inner to the outer disk. Our simulations show that the crossing of gas changes during the migration process as the migration rate slows down. Therefore, classical type II migration where the planet migrates with the viscous drift rate and no gas crosses the gap is no general process but may only occur for special parameters and at a certain time during the orbital evolution of the planet.

  19. CHALLENGES IN FORMING PLANETS BY GRAVITATIONAL INSTABILITY: DISK IRRADIATION AND CLUMP MIGRATION, ACCRETION, AND TIDAL DESTRUCTION

    International Nuclear Information System (INIS)

    Zhu Zhaohuan; Hartmann, Lee; Nelson, Richard P.; Gammie, Charles F.

    2012-01-01

    We present two-dimensional hydrodynamic simulations of self-gravitating protostellar disks subject to axisymmetric, continuing mass loading from an infalling envelope and irradiation from the central star to explore the growth of gravitational instability (GI) and disk fragmentation. We assume that the disk is built gradually and smoothly by the infall, resulting in good numerical convergence. We confirm that for disks around solar-mass stars, infall at high rates at radii beyond ∼50 AU leads to disk fragmentation. At lower infall rates, however, irradiation suppresses fragmentation. We find that, once formed, the fragments or clumps migrate inward on typical type I timescales of ∼2 × 10 3 yr initially, but with a stochastic component superimposed due to their interaction with the GI-induced spiral arms. Migration begins to deviate from the type I timescale when the clump becomes more massive than the local disk mass, and/or when the clump begins to form a gap in the disk. As they migrate, clumps accrete from the disk at a rate between 10 –3 and 10 –1 M J yr –1 , consistent with analytic estimates that assume a 1-2 Hill radii cross section. The eventual fates of these clumps, however, diverge depending on the migration speed: 3 out of 13 clumps in our simulations become massive enough (brown dwarf mass range) to open gaps in the disk and essentially stop migrating; 4 out of 13 are tidally destroyed during inward migration; and 6 out of 13 migrate across the inner boundary of the simulated disks. A simple analytic model for clump evolution explains the different fates of the clumps and reveals some limitations of two-dimensional simulations. Overall, our results indicate that fast migration, accretion, and tidal destruction of the clumps pose challenges to the scenario of giant planet formation by GI in situ, although we cannot address whether or not remnant solid cores can survive after tidal stripping. The models where the massive clumps are not

  20. The presence and absence of lymphatic vessels in the adult human intervertebral disc: relation to disc pathology

    International Nuclear Information System (INIS)

    Kliskey, Karolina; Williams, Kelly; Yu, J.; Urban, Jill; Athanasou, Nick; Jackson, David

    2009-01-01

    Although the normal adult human intervertebral disc is considered to be avascular, vascularised cellular fibrous tissue can be found in pathological conditions involving the disc such as disc herniation. Whether lymphatics vessels form a component of this reparative tissue is not known as the presence or absence of lymphatics in herniated and normal disc tissue is not known. We examined spinal tissues and discectomy specimens for the presence of lymphatics. The examination used immunohistochemistry to identify the specific lymphatic endothelial cell markers, podoplanin and LYVE1. Lymphatic vessels were not found in the nucleus pulposus or annulus fibrosus of intact, non-herniated lumbar and thoracic discs but were present in the surrounding ligaments. Ingrowth of fibrous tissue was seen in 73% of herniated disc specimens of which 36% contained LYVE1+/podoplanin + lymphatic vessels. Lymphatic vessels were not seen in the sacrum and coccyx or biopsies of four sacrococcygeal chordomas, but they were noted in surrounding extra-osseous fat and fibrous tissue at the edge of the infiltrating tumour. Our findings indicate that lymphatic vessels are not present in the normal adult intervertebral disc but that, when there is extrusion of disc material into surrounding soft tissue, there is ingrowth of reparative fibrous tissue containing lymphatic vessels. Our findings also indicate that chordoma, a tumour of notochordal origin, spreads to regional lymph nodes via lymphatics in para-spinal soft tissues. (orig.)

  1. Stability of black hole accretion disks

    Directory of Open Access Journals (Sweden)

    Czerny B.

    2012-12-01

    Full Text Available We discuss the issues of stability of accretion disks that may undergo the limit-cycle oscillations due to the two main types of thermal-viscous instabilities. These are induced either by the domination of radiation pressure in the innermost regions close to the central black hole, or by the partial ionization of hydrogen in the zone of appropriate temperatures. These physical processes may lead to the intermittent activity in AGN on timescales between hundreds and millions of years. We list a number of observational facts that support the idea of the cyclic activity in high accretion rate sources. We conclude however that the observed features of quasars may provide only indirect signatures of the underlying instabilities. Also, the support from the sources with stellar mass black holes, whose variability timescales are observationally feasible, is limited to a few cases of the microquasars. Therefore we consider a number of plausible mechanisms of stabilization of the limit cycle oscillations in high accretion rate accretion disks. The newly found is the stabilizing effect of the stochastic viscosity fluctuations.

  2. Accretion onto a charged higher-dimensional black hole

    International Nuclear Information System (INIS)

    Sharif, M.; Iftikhar, Sehrish

    2016-01-01

    This paper deals with the steady-state polytropic fluid accretion onto a higher-dimensional Reissner-Nordstroem black hole. We formulate the generalized mass flux conservation equation, energy flux conservation and relativistic Bernoulli equation to discuss the accretion process. The critical accretion is investigated by finding the critical radius, the critical sound velocity, and the critical flow velocity. We also explore gas compression and temperature profiles to analyze the asymptotic behavior. It is found that the results for the Schwarzschild black hole are recovered when q = 0 in four dimensions. We conclude that the accretion process in higher dimensions becomes slower in the presence of charge. (orig.)

  3. Accretion onto a charged higher-dimensional black hole

    Energy Technology Data Exchange (ETDEWEB)

    Sharif, M.; Iftikhar, Sehrish [University of the Punjab, Department of Mathematics, Lahore (Pakistan)

    2016-03-15

    This paper deals with the steady-state polytropic fluid accretion onto a higher-dimensional Reissner-Nordstroem black hole. We formulate the generalized mass flux conservation equation, energy flux conservation and relativistic Bernoulli equation to discuss the accretion process. The critical accretion is investigated by finding the critical radius, the critical sound velocity, and the critical flow velocity. We also explore gas compression and temperature profiles to analyze the asymptotic behavior. It is found that the results for the Schwarzschild black hole are recovered when q = 0 in four dimensions. We conclude that the accretion process in higher dimensions becomes slower in the presence of charge. (orig.)

  4. Journal of Astrophysics and Astronomy | Indian Academy of Sciences

    Indian Academy of Sciences (India)

    We study the radiation properties of an accretion disc around a rotating black hole. We solve the hydrodynamic equations and calculate the transonic solutions of accretion disc in the presence of shocks. Then we use these solutions to generate the radiation spectrum in the presence of radiative heating and ...

  5. Archival-grade optical disc design and international standards

    Science.gov (United States)

    Fujii, Toru; Kojyo, Shinichi; Endo, Akihisa; Kodaira, Takuo; Mori, Fumi; Shimizu, Atsuo

    2015-09-01

    Optical discs currently on the market exhibit large variations in life span among discs, making them unsuitable for certain business applications. To assess and potentially mitigate this problem, we performed accelerated degradation testing under standard ISO conditions, determined the probable disc failure mechanisms, and identified the essential criteria necessary for a stable disc composition. With these criteria as necessary conditions, we analyzed the physical and chemical changes that occur in the disc components, on the basis of which we determined technological measures to reduce these degradation processes. By applying these measures to disc fabrication, we were able to develop highly stable optical discs.

  6. EFFECTS OF SPIN ON HIGH-ENERGY RADIATION FROM ACCRETING BLACK HOLES

    Energy Technology Data Exchange (ETDEWEB)

    O’ Riordan, Michael; Pe’er, Asaf [Physics Department, University College Cork, Cork (Ireland); McKinney, Jonathan C., E-mail: michael_oriordan@umail.ucc.ie [Department of Physics and Joint Space-Science Institute, University of Maryland, College Park, MD 20742 (United States)

    2016-11-01

    Observations of jets in X-ray binaries show a correlation between radio power and black hole spin. This correlation, if confirmed, points toward the idea that relativistic jets may be powered by the rotational energy of black holes. In order to examine this further, we perform general relativistic radiative transport calculations on magnetically arrested accretion flows, which are known to produce powerful jets via the Blandford–Znajek (BZ) mechanism. We find that the X-ray and γ -ray emission strongly depend on spin and inclination angle. Surprisingly, the high-energy power does not show the same dependence on spin as the BZ jet power, but instead can be understood as a redshift effect. In particular, photons observed perpendicular to the spin axis suffer little net redshift until originating from close to the horizon. Such observers see deeper into the hot, dense, highly magnetized inner disk region. This effect is largest for rapidly rotating black holes due to a combination of frame dragging and decreasing horizon radius. While the X-ray emission is dominated by the near horizon region, the near-infrared (NIR) radiation originates at larger radii. Therefore, the ratio of X-ray to NIR power is an observational signature of black hole spin.

  7. Pre-main-sequence disk accretion in Z Canis Majoris

    International Nuclear Information System (INIS)

    Hartmann, L.; Kenyon, S.J.; Hewett, R.; Edwards, S.; Strom, K.M.; Strom, S.E.; Stauffer, J.R.

    1989-01-01

    It is suggested that the pre-main-sequence object Z CMa is a luminous accretion disk, similar in many respects to the FU Orionis variables. Z CMa shows the broad, doubled optical absorption lines expected from a rapidly rotating accretion disk. The first overtone CO absorption detected in Z CMa is blue-shifted, suggesting line formation in a disk wind. Accretion at rates about 0.001 solar mass/yr over 100 yr is required to explain the luminosity of Z CMa. The large amount of material accreted (0.1 solar mass/yr) indicates that Z CMa is in a very early stage of stellar evolution, possibly in an initial phase of massive disk accretion. 41 references

  8. Pre-main-sequence disk accretion in Z Canis Majoris

    Science.gov (United States)

    Hartmann, L.; Kenyon, S. J.; Hewett, R.; Edwards, S.; Strom, K. M.; Strom, S. E.; Stauffer, J. R.

    1989-01-01

    It is suggested that the pre-main-sequence object Z CMa is a luminous accretion disk, similar in many respects to the FU Orionis variables. Z CMa shows the broad, doubled optical absorption lines expected from a rapidly rotating accretion disk. The first overtone CO absorption detected in Z CMa is blue-shifted, suggesting line formation in a disk wind. Accretion at rates about 0.001 solar mass/yr over 100 yr is required to explain the luminosity of Z CMa. The large amount of material accreted (0.1 solar mass/yr) indicates that Z CMa is in a very early stage of stellar evolution, possibly in an initial phase of massive disk accretion.

  9. Accretion in Radiative Equipartition (AiRE) Disks

    Energy Technology Data Exchange (ETDEWEB)

    Yazdi, Yasaman K.; Afshordi, Niayesh, E-mail: yyazdi@pitp.ca, E-mail: nafshordi@pitp.ca [Perimeter Institute for Theoretical Physics, 31 Caroline Street N, Waterloo, ON N2L 2Y5 (Canada)

    2017-07-01

    Standard accretion disk theory predicts that the total pressure in disks at typical (sub-)Eddington accretion rates becomes radiation pressure dominated. However, radiation pressure dominated disks are thermally unstable. Since these disks are observed in approximate steady state over the instability timescale, our accretion models in the radiation-pressure-dominated regime (i.e., inner disk) need to be modified. Here, we present a modification to the Shakura and Sunyaev model, where the radiation pressure is in equipartition with the gas pressure in the inner region. We call these flows accretion in radiative equipartition (AiRE) disks. We introduce the basic features of AiRE disks and show how they modify disk properties such as the Toomre parameter and the central temperature. We then show that the accretion rate of AiRE disks is limited from above and below, by Toomre and nodal sonic point instabilities, respectively. The former leads to a strict upper limit on the mass of supermassive black holes as a function of cosmic time (and spin), while the latter could explain the transition between hard and soft states of X-ray binaries.

  10. Time-dependent Models of Magnetospheric Accretion onto Young Stars

    Energy Technology Data Exchange (ETDEWEB)

    Robinson, C. E.; Espaillat, C. C. [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States); Owen, J. E. [Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540 (United States); Adams, F. C., E-mail: connorr@bu.edu [Physics Department, University of Michigan, Ann Arbor, MI 48109 (United States)

    2017-04-01

    Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star’s surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner, and the source of this variability remains an open problem, particularly on the shortest (∼day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ∼day-long timescales (e.g., due to turbulence), this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star, the observed time dependence will be a rapid increase in accretion luminosity, followed by a slower decline, and could be an explanation for some of the short-period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated multi-dimensional simulations and observations.

  11. Time-dependent Models of Magnetospheric Accretion onto Young Stars

    International Nuclear Information System (INIS)

    Robinson, C. E.; Espaillat, C. C.; Owen, J. E.; Adams, F. C.

    2017-01-01

    Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star’s surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner, and the source of this variability remains an open problem, particularly on the shortest (∼day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ∼day-long timescales (e.g., due to turbulence), this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star, the observed time dependence will be a rapid increase in accretion luminosity, followed by a slower decline, and could be an explanation for some of the short-period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated multi-dimensional simulations and observations.

  12. Equilibrium star formation in a constant Q disc: model optimization and initial tests

    Science.gov (United States)

    Zheng, Zheng; Meurer, Gerhardt R.; Heckman, Timothy M.; Thilker, David A.; Zwaan, Martin A.

    2013-10-01

    We develop a model for the distribution of the interstellar medium (ISM) and star formation in galaxies based on recent studies that indicate that galactic discs stabilize to a constant stability parameter, which we combine with prescriptions of how the phases of the ISM are determined and for the star formation law (SFL). The model predicts the gas surface mass density and star formation intensity of a galaxy given its rotation curve, stellar surface mass density and the gas velocity dispersion. This model is tested on radial profiles of neutral and molecular ISM surface mass density and star formation intensity of 12 galaxies selected from the H I Nearby Galaxy Survey sample. Our tests focus on intermediate radii (0.3 to 1 times the optical radius) because there are insufficient data to test the outer discs and the fits are less accurate in detail in the centre. Nevertheless, the model produces reasonable agreement with the ISM mass and star formation rate integrated over the central region in all but one case. To optimize the model, we evaluate four recipes for the stability parameter, three recipes for apportioning the ISM into molecular and neutral components, and eight versions of the SFL. We find no clear-cut best prescription for the two-fluid (gas and stars) stability parameter Q2f and therefore for simplicity, we use the Wang and Silk approximation (QWS). We found that an empirical scaling between the molecular-to-neutral ISM ratio (Rmol) and the stellar surface mass density proposed by Leroy et al. works marginally better than the other two prescriptions for this ratio in predicting the ISM profiles, and noticeably better in predicting the star formation intensity from the ISM profiles produced by our model with the SFLs we tested. Thus, in the context of our modelled ISM profiles, the linear molecular SFL and the two-component SFL work better than the other prescriptions we tested. We incorporate these relations into our `constant Q disc' model.

  13. ON THE RADII OF BROWN DWARFS MEASURED WITH AKARI NEAR-INFRARED SPECTROSCOPY

    International Nuclear Information System (INIS)

    Sorahana, S.; Yamamura, I.; Murakami, H.

    2013-01-01

    We derive the radii of 16 brown dwarfs observed by AKARI using their parallaxes and the ratios of observed to model fluxes. We find that the brown dwarf radius ranges between 0.64-1.13 R J with an average radius of 0.83 R J . We find a trend in the relation between radii and T eff ; the radius is at a minimum at T eff ∼ 1600 K, which corresponds to the spectral types of mid- to late-L. The result is interpreted by a combination of radius-mass and radius-age relations that are theoretically expected for brown dwarfs older than 10 8 yr.

  14. Centrality dependence of pion freeze-out radii in Pb-Pb collisions at $\\sqrt{\\mathbf{s_{NN}}}$=2.76 TeV

    OpenAIRE

    Adam, Jaroslav; Adamova, Dagmar; Aggarwal, Madan Mohan; Aglieri Rinella, Gianluca; Agnello, Michelangelo; Agrawal, Neelima; Ahammed, Zubayer; Ahn, Sang Un; Aimo, Ilaria; Aiola, Salvatore; Ajaz, Muhammad; Akindinov, Alexander; Alam, Sk Noor; Aleksandrov, Dmitry; Alessandro, Bruno

    2015-01-01

    We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at $\\sqrt{s_{\\rm NN}}=2.76$ TeV as a function of collision centrality and the average transverse momentum of the pair $k_{\\rm T}$. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with $k_{\\rm T}$, following a power-law behavior. This is qualitatively consistent with expectations from ...

  15. Thermochemical modelling of brown dwarf discs

    NARCIS (Netherlands)

    Greenwood, A. J.; Kamp, I.; Waters, L. B. F. M.; Woitke, P.; Thi, W.-F.; Rab, Ch.; Aresu, G.; Spaans, M.

    The physical properties of brown dwarf discs, in terms of their shapes and sizes, are still largely unexplored by observations. ALMA has by far the best capabilities to observe these discs in sub-mm CO lines and dust continuum, while also spatially resolving some discs. To what extent brown dwarf

  16. Regimes of mini black hole abandoned to accretion

    Science.gov (United States)

    Paik, Biplab

    2018-01-01

    Being inspired by the Eddington’s idea, along with other auxiliary arguments, it is unveiled that there exist regimes of a black hole that would prohibit accretion of ordinary energy. In explicit words, there exists a lower bound to black hole mass below which matter accretion process does not run for black holes. Not merely the baryonic matter, but, in regimes, also the massless photons could get prohibited from rushing into a black hole. However, unlike the baryon accretion abandoned black hole regime, the mass-regime of a black hole prohibiting accretion of radiation could vary along with its ambient temperature. For example, we discuss that earlier to 10‑8 s after the big-bang, as the cosmological temperature of the Universe grew above ˜ 1014 K, the mass range of black hole designating the radiation accretion abandoned regime, had to be in varying state being connected with the instantaneous age of the evolving Universe by an “one half” power law. It happens to be a fact that a black hole holding regimes prohibiting accretion of energy is gigantic by its size in comparison to the Planck length-scale. Hence the emergence of these regimes demands mini black holes for not being viable as profound suckers of energy. Consideration of accretion abandoned regimes could be crucial for constraining or judging the evolution of primordial black holes over the age of the Universe.

  17. X-ray pulsars: accretion flow deceleration

    International Nuclear Information System (INIS)

    Miller, G.S.

    1987-01-01

    X-ray pulsars are thought to be neutron stars that derive the energy for their x-ray emission by accreting material onto their magnetic polar caps. The accreting material and the x-ray pulsar atmospheres were idealized as fully ionized plasmas consisting only of electrons and protons. A high magnetic field (∼ 5 x 10 12 Gauss) permeates the atmospheric plasma, and causes the motion of atmospheric electrons perpendicular to the field to be quantized into discrete Landau levels. All atmospheric electrons initially lie in the Landau ground state, but in the author's calculations of Coulomb collisions between atmospheric electrons and accreting protons, he allows for processes that leave the electrons in the first excited Landau level. He also considers interactions between accreting protons and the collective modes of the atmospheric plasma. Division of the electromagnetic interaction of a fast proton with a magnetized plasma into single particle and collective effects is described in detail in Chapter 2. Deceleration of the accretion flow due to Coulomb collisions with atmospheric electrons and collective plasma effects was studied in a number of computer simulations. These simulations, along with a discussion of the physical state of the atmospheric plasma and its interactions with a past proton, are presented in Chapter 3. Details of the atmospheric model and a description of the results of the simulations are given in Chapter 4. Chapter 5 contains some brief concluding remarks, and some thoughts on future research

  18. Centrality dependence of pion freeze-out radii in Pb-Pb collisions at $\\sqrt{\\mathbf{s_{NN}}}$=2.76 TeV

    CERN Document Server

    Adam, Jaroslav; Aggarwal, Madan Mohan; Aglieri Rinella, Gianluca; Agnello, Michelangelo; Agrawal, Neelima; Ahammed, Zubayer; Ahn, Sang Un; Aimo, Ilaria; Aiola, Salvatore; Ajaz, Muhammad; Akindinov, Alexander; Alam, Sk Noor; Aleksandrov, Dmitry; Alessandro, Bruno; Alexandre, Didier; Alfaro Molina, Jose Ruben; Alici, Andrea; Alkin, Anton; Millan Almaraz, Jesus Roberto; Alme, Johan; Alt, Torsten; Altinpinar, Sedat; Altsybeev, Igor; Alves Garcia Prado, Caio; Andrei, Cristian; Andronic, Anton; Anguelov, Venelin; Anielski, Jonas; Anticic, Tome; Antinori, Federico; Antonioli, Pietro; Aphecetche, Laurent Bernard; Appelshaeuser, Harald; Arcelli, Silvia; Armesto Perez, Nestor; Arnaldi, Roberta; Arsene, Ionut Cristian; Arslandok, Mesut; Audurier, Benjamin; Augustinus, Andre; Averbeck, Ralf Peter; Azmi, Mohd Danish; Bach, Matthias Jakob; Badala, Angela; Baek, Yong Wook; Bagnasco, Stefano; Bailhache, Raphaelle Marie; Bala, Renu; Baldisseri, Alberto; Baltasar Dos Santos Pedrosa, Fernando; Baral, Rama Chandra; Barbano, Anastasia Maria; Barbera, Roberto; Barile, Francesco; Barnafoldi, Gergely Gabor; Barnby, Lee Stuart; Ramillien Barret, Valerie; Bartalini, Paolo; Barth, Klaus; Bartke, Jerzy Gustaw; Bartsch, Esther; Basile, Maurizio; Bastid, Nicole; Basu, Sumit; Bathen, Bastian; Batigne, Guillaume; Batista Camejo, Arianna; Batyunya, Boris; Batzing, Paul Christoph; Bearden, Ian Gardner; Beck, Hans; Bedda, Cristina; Behera, Nirbhay Kumar; Belikov, Iouri; Bellini, Francesca; Bello Martinez, Hector; Bellwied, Rene; Belmont Iii, Ronald John; Belmont Moreno, Ernesto; Belyaev, Vladimir; Bencedi, Gyula; Beole, Stefania; Berceanu, Ionela; Bercuci, Alexandru; Berdnikov, Yaroslav; Berenyi, Daniel; Bertens, Redmer Alexander; Berzano, Dario; Betev, Latchezar; Bhasin, Anju; Bhat, Inayat Rasool; Bhati, Ashok Kumar; Bhattacharjee, Buddhadeb; Bhom, Jihyun; Bianchi, Livio; Bianchi, Nicola; Bianchin, Chiara; Bielcik, Jaroslav; Bielcikova, Jana; Bilandzic, Ante; Biswas, Rathijit; Biswas, Saikat; Bjelogrlic, Sandro; Blair, Justin Thomas; Blanco, Fernando; Blau, Dmitry; Blume, Christoph; Bock, Friederike; Bogdanov, Alexey; Boggild, Hans; Boldizsar, Laszlo; Bombara, Marek; Book, Julian Heinz; Borel, Herve; Borissov, Alexander; Borri, Marcello; Bossu, Francesco; Botta, Elena; Boettger, Stefan; Braun-Munzinger, Peter; Bregant, Marco; Breitner, Timo Gunther; Broker, Theo Alexander; Browning, Tyler Allen; Broz, Michal; Brucken, Erik Jens; Bruna, Elena; Bruno, Giuseppe Eugenio; Budnikov, Dmitry; Buesching, Henner; Bufalino, Stefania; Buncic, Predrag; Busch, Oliver; Buthelezi, Edith Zinhle; Bashir Butt, Jamila; Buxton, Jesse Thomas; Caffarri, Davide; Cai, Xu; Caines, Helen Louise; Calero Diaz, Liliet; Caliva, Alberto; Calvo Villar, Ernesto; Camerini, Paolo; Carena, Francesco; Carena, Wisla; Carnesecchi, Francesca; Castillo Castellanos, Javier Ernesto; Castro, Andrew John; Casula, Ester Anna Rita; Cavicchioli, Costanza; Ceballos Sanchez, Cesar; Cepila, Jan; Cerello, Piergiorgio; Cerkala, Jakub; Chang, Beomsu; Chapeland, Sylvain; Chartier, Marielle; Charvet, Jean-Luc Fernand; Chattopadhyay, Subhasis; Chattopadhyay, Sukalyan; Chelnokov, Volodymyr; Cherney, Michael Gerard; Cheshkov, Cvetan Valeriev; Cheynis, Brigitte; Chibante Barroso, Vasco Miguel; Dobrigkeit Chinellato, David; Chochula, Peter; Choi, Kyungeon; Chojnacki, Marek; Choudhury, Subikash; Christakoglou, Panagiotis; Christensen, Christian Holm; Christiansen, Peter; Chujo, Tatsuya; Chung, Suh-Urk; Zhang, Chunhui; Cicalo, Corrado; Cifarelli, Luisa; Cindolo, Federico; Cleymans, Jean Willy Andre; Colamaria, Fabio Filippo; Colella, Domenico; Collu, Alberto; Colocci, Manuel; Conesa Balbastre, Gustavo; Conesa Del Valle, Zaida; Connors, Megan Elizabeth; Contreras Nuno, Jesus Guillermo; Cormier, Thomas Michael; Corrales Morales, Yasser; Cortes Maldonado, Ismael; Cortese, Pietro; Cosentino, Mauro Rogerio; Costa, Filippo; Crochet, Philippe; Cruz Albino, Rigoberto; Cuautle Flores, Eleazar; Cunqueiro Mendez, Leticia; Dahms, Torsten; Dainese, Andrea; Danu, Andrea; Das, Debasish; Das, Indranil; Das, Supriya; Dash, Ajay Kumar; Dash, Sadhana; De, Sudipan; De Caro, Annalisa; De Cataldo, Giacinto; De Cuveland, Jan; De Falco, Alessandro; De Gruttola, Daniele; De Marco, Nora; De Pasquale, Salvatore; Deisting, Alexander; Deloff, Andrzej; Denes, Ervin Sandor; D'Erasmo, Ginevra; Di Bari, Domenico; Di Mauro, Antonio; Di Nezza, Pasquale; Diaz Corchero, Miguel Angel; Dietel, Thomas; Dillenseger, Pascal; Divia, Roberto; Djuvsland, Oeystein; Dobrin, Alexandru Florin; Dobrowolski, Tadeusz Antoni; Domenicis Gimenez, Diogenes; Donigus, Benjamin; Dordic, Olja; Drozhzhova, Tatiana; Dubey, Anand Kumar; Dubla, Andrea; Ducroux, Laurent; Dupieux, Pascal; Ehlers Iii, Raymond James; Elia, Domenico; Engel, Heiko; Erazmus, Barbara Ewa; Erdemir, Irem; Erhardt, Filip; Eschweiler, Dominic; Espagnon, Bruno; Estienne, Magali Danielle; Esumi, Shinichi; Eum, Jongsik; Evans, David; Evdokimov, Sergey; Eyyubova, Gyulnara; Fabbietti, Laura; Fabris, Daniela; Faivre, Julien; Fantoni, Alessandra; Fasel, Markus; Feldkamp, Linus; Felea, Daniel; Feliciello, Alessandro; Feofilov, Grigorii; Ferencei, Jozef; Fernandez Tellez, Arturo; Gonzalez Ferreiro, Elena; Ferretti, Alessandro; Festanti, Andrea; Feuillard, Victor Jose Gaston; Figiel, Jan; Araujo Silva Figueredo, Marcel; Filchagin, Sergey; Finogeev, Dmitry; Fiore, Enrichetta Maria; Fleck, Martin Gabriel; Floris, Michele; Foertsch, Siegfried Valentin; Foka, Panagiota; Fokin, Sergey; Fragiacomo, Enrico; Francescon, Andrea; Frankenfeld, Ulrich Michael; Fuchs, Ulrich; Furget, Christophe; Furs, Artur; Fusco Girard, Mario; Gaardhoeje, Jens Joergen; Gagliardi, Martino; Gago Medina, Alberto Martin; Gallio, Mauro; Gangadharan, Dhevan Raja; Ganoti, Paraskevi; Gao, Chaosong; Garabatos Cuadrado, Jose; Garcia-Solis, Edmundo Javier; Gargiulo, Corrado; Gasik, Piotr Jan; Germain, Marie; Gheata, Andrei George; Gheata, Mihaela; Ghosh, Premomoy; Ghosh, Sanjay Kumar; Gianotti, Paola; Giubellino, Paolo; Giubilato, Piero; Gladysz-Dziadus, Ewa; Glassel, Peter; Gomez Coral, Diego Mauricio; Gomez Ramirez, Andres; Gonzalez Zamora, Pedro; Gorbunov, Sergey; Gorlich, Lidia Maria; Gotovac, Sven; Grabski, Varlen; Graczykowski, Lukasz Kamil; Graham, Katie Leanne; Grelli, Alessandro; Grigoras, Alina Gabriela; Grigoras, Costin; Grigoryev, Vladislav; Grigoryan, Ara; Grigoryan, Smbat; Grynyov, Borys; Grion, Nevio; Grosse-Oetringhaus, Jan Fiete; Grossiord, Jean-Yves; Grosso, Raffaele; Guber, Fedor; Guernane, Rachid; Guerzoni, Barbara; Gulbrandsen, Kristjan Herlache; Gulkanyan, Hrant; Gunji, Taku; Gupta, Anik; Gupta, Ramni; Haake, Rudiger; Haaland, Oystein Senneset; Hadjidakis, Cynthia Marie; Haiduc, Maria; Hamagaki, Hideki; Hamar, Gergoe; Hansen, Alexander; Harris, John William; Hartmann, Helvi; Harton, Austin Vincent; Hatzifotiadou, Despina; Hayashi, Shinichi; Heckel, Stefan Thomas; Heide, Markus Ansgar; Helstrup, Haavard; Herghelegiu, Andrei Ionut; Herrera Corral, Gerardo Antonio; Hess, Benjamin Andreas; Hetland, Kristin Fanebust; Hilden, Timo Eero; Hillemanns, Hartmut; Hippolyte, Boris; Hosokawa, Ritsuya; Hristov, Peter Zahariev; Huang, Meidana; Humanic, Thomas; Hussain, Nur; Hussain, Tahir; Hutter, Dirk; Hwang, Dae Sung; Ilkaev, Radiy; Ilkiv, Iryna; Inaba, Motoi; Ippolitov, Mikhail; Irfan, Muhammad; Ivanov, Marian; Ivanov, Vladimir; Izucheev, Vladimir; Jacobs, Peter Martin; Jadlovska, Slavka; Jahnke, Cristiane; Jang, Haeng Jin; Janik, Malgorzata Anna; Pahula Hewage, Sandun; Jena, Chitrasen; Jena, Satyajit; Jimenez Bustamante, Raul Tonatiuh; Jones, Peter Graham; Jung, Hyungtaik; Jusko, Anton; Kalinak, Peter; Kalweit, Alexander Philipp; Kamin, Jason Adrian; Kang, Ju Hwan; Kaplin, Vladimir; Kar, Somnath; Karasu Uysal, Ayben; Karavichev, Oleg; Karavicheva, Tatiana; Karayan, Lilit; Karpechev, Evgeny; Kebschull, Udo Wolfgang; Keidel, Ralf; Keijdener, Darius Laurens; Keil, Markus; Khan, Kamal; Khan, Mohammed Mohisin; Khan, Palash; Khan, Shuaib Ahmad; Khanzadeev, Alexei; Kharlov, Yury; Kileng, Bjarte; Kim, Beomkyu; Kim, Do Won; Kim, Dong Jo; Kim, Hyeonjoong; Kim, Jinsook; Kim, Mimae; Kim, Minwoo; Kim, Se Yong; Kim, Taesoo; Kirsch, Stefan; Kisel, Ivan; Kiselev, Sergey; Kisiel, Adam Ryszard; Kiss, Gabor; Klay, Jennifer Lynn; Klein, Carsten; Klein, Jochen; Klein-Boesing, Christian; Kluge, Alexander; Knichel, Michael Linus; Knospe, Anders Garritt; Kobayashi, Taiyo; Kobdaj, Chinorat; Kofarago, Monika; Kollegger, Thorsten; Kolozhvari, Anatoly; Kondratev, Valerii; Kondratyeva, Natalia; Kondratyuk, Evgeny; Konevskikh, Artem; Kopcik, Michal; Kour, Mandeep; Kouzinopoulos, Charalampos; Kovalenko, Oleksandr; Kovalenko, Vladimir; Kowalski, Marek; Koyithatta Meethaleveedu, Greeshma; Kral, Jiri; Kralik, Ivan; Kravcakova, Adela; Krelina, Michal; Kretz, Matthias; Krivda, Marian; Krizek, Filip; Kryshen, Evgeny; Krzewicki, Mikolaj; Kubera, Andrew Michael; Kucera, Vit; Kugathasan, Thanushan; Kuhn, Christian Claude; Kuijer, Paulus Gerardus; Kulakov, Igor; Kumar, Ajay; Kumar, Jitendra; Lokesh, Kumar; Kurashvili, Podist; Kurepin, Alexander; Kurepin, Alexey; Kuryakin, Alexey; Kushpil, Svetlana; Kweon, Min Jung; Kwon, Youngil; La Pointe, Sarah Louise; La Rocca, Paola; Lagana Fernandes, Caio; Lakomov, Igor; Langoy, Rune; Lara Martinez, Camilo Ernesto; Lardeux, Antoine Xavier; Lattuca, Alessandra; Laudi, Elisa; Lea, Ramona; Leardini, Lucia; Lee, Graham Richard; Lee, Seongjoo; Legrand, Iosif; Lehas, Fatiha; Lemmon, Roy Crawford; Lenti, Vito; Leogrande, Emilia; Leon Monzon, Ildefonso; Leoncino, Marco; Levai, Peter; Li, Shuang; Li, Xiaomei; Lien, Jorgen Andre; Lietava, Roman; Lindal, Svein; Lindenstruth, Volker; Lippmann, Christian; Lisa, Michael Annan; Ljunggren, Hans Martin; Lodato, Davide Francesco; Lonne, Per-Ivar; Loginov, Vitaly; Loizides, Constantinos; Lopez, Xavier Bernard; Lopez Torres, Ernesto; Lowe, Andrew John; Luettig, Philipp Johannes; Lunardon, Marcello; Luparello, Grazia; Ferreira Natal Da Luz, Pedro Hugo; Maevskaya, Alla; Mager, Magnus; Mahajan, Sanjay; Mahmood, Sohail Musa; Maire, Antonin; Majka, Richard Daniel; Malaev, Mikhail; Maldonado Cervantes, Ivonne Alicia; Malinina, Liudmila; Mal'Kevich, Dmitry; Malzacher, Peter; Mamonov, Alexander; Manko, Vladislav; Manso, Franck; Manzari, Vito; Marchisone, Massimiliano; Mares, Jiri; Margagliotti, Giacomo Vito; Margotti, Anselmo; Margutti, Jacopo; Marin, Ana Maria; Markert, Christina; Marquard, Marco; Martin, Nicole Alice; Martin Blanco, Javier; Martinengo, Paolo; Martinez Hernandez, Mario Ivan; Martinez-Garcia, Gines; Martinez Pedreira, Miguel; Martynov, Yevgen; Mas, Alexis Jean-Michel; Masciocchi, Silvia; Masera, Massimo; Masoni, Alberto; Massacrier, Laure Marie; Mastroserio, Annalisa; Masui, Hiroshi; Matyja, Adam Tomasz; Mayer, Christoph; Mazer, Joel Anthony; Mazzoni, Alessandra Maria; Mcdonald, Daniel; Meddi, Franco; Melikyan, Yuri; Menchaca-Rocha, Arturo Alejandro; Meninno, Elisa; Mercado-Perez, Jorge; Meres, Michal; Miake, Yasuo; Mieskolainen, Matti Mikael; Mikhaylov, Konstantin; Milano, Leonardo; Milosevic, Jovan; Minervini, Lazzaro Manlio; Mischke, Andre; Mishra, Aditya Nath; Miskowiec, Dariusz Czeslaw; Mitra, Jubin; Mitu, Ciprian Mihai; Mohammadi, Naghmeh; Mohanty, Bedangadas; Molnar, Levente; Montano Zetina, Luis Manuel; Montes Prado, Esther; Morando, Maurizio; Moreira De Godoy, Denise Aparecida; Moretto, Sandra; Morreale, Astrid; Morsch, Andreas; Muccifora, Valeria; Mudnic, Eugen; Muhlheim, Daniel Michael; Muhuri, Sanjib; Mukherjee, Maitreyee; Mulligan, James Declan; Gameiro Munhoz, Marcelo; Murray, Sean; Musa, Luciano; Musinsky, Jan; Nandi, Basanta Kumar; Nania, Rosario; Nappi, Eugenio; Naru, Muhammad Umair; Nattrass, Christine; Nayak, Kishora; Nayak, Tapan Kumar; Nazarenko, Sergey; Nedosekin, Alexander; Nellen, Lukas; Ng, Fabian; Nicassio, Maria; Niculescu, Mihai; Niedziela, Jeremi; Nielsen, Borge Svane; Nikolaev, Sergey; Nikulin, Sergey; Nikulin, Vladimir; Noferini, Francesco; Nomokonov, Petr; Nooren, Gerardus; Cabanillas Noris, Juan Carlos; Norman, Jaime; Nyanin, Alexander; Nystrand, Joakim Ingemar; Oeschler, Helmut Oskar; Oh, Saehanseul; Oh, Sun Kun; Ohlson, Alice Elisabeth; Okatan, Ali; Okubo, Tsubasa; Olah, Laszlo; Oleniacz, Janusz; Oliveira Da Silva, Antonio Carlos; Oliver, Michael Henry; Onderwaater, Jacobus; Oppedisano, Chiara; Orava, Risto; Ortiz Velasquez, Antonio; Oskarsson, Anders Nils Erik; Otwinowski, Jacek Tomasz; Oyama, Ken; Ozdemir, Mahmut; Pachmayer, Yvonne Chiara; Pagano, Paola; Paic, Guy; Pajares Vales, Carlos; Pal, Susanta Kumar; Pan, Jinjin; Pandey, Ashutosh Kumar; Pant, Divyash; Papcun, Peter; Papikyan, Vardanush; Pappalardo, Giuseppe; Pareek, Pooja; Park, Woojin; Parmar, Sonia; Passfeld, Annika; Paticchio, Vincenzo; Patra, Rajendra Nath; Paul, Biswarup; Peitzmann, Thomas; Pereira Da Costa, Hugo Denis Antonio; Pereira De Oliveira Filho, Elienos; Peresunko, Dmitry Yurevich; Perez Lara, Carlos Eugenio; Perez Lezama, Edgar; Peskov, Vladimir; Pestov, Yury; Petracek, Vojtech; Petrov, Viacheslav; Petrovici, Mihai; Petta, Catia; Piano, Stefano; Pikna, Miroslav; Pillot, Philippe; Pinazza, Ombretta; Pinsky, Lawrence; Piyarathna, Danthasinghe; Ploskon, Mateusz Andrzej; Planinic, Mirko; Pluta, Jan Marian; Pochybova, Sona; Podesta Lerma, Pedro Luis Manuel; Poghosyan, Martin; Polishchuk, Boris; Poljak, Nikola; Poonsawat, Wanchaloem; Pop, Amalia; Porteboeuf, Sarah Julie; Porter, R Jefferson; Pospisil, Jan; Prasad, Sidharth Kumar; Preghenella, Roberto; Prino, Francesco; Pruneau, Claude Andre; Pshenichnov, Igor; Puccio, Maximiliano; Puddu, Giovanna; Pujahari, Prabhat Ranjan; Punin, Valery; Putschke, Jorn Henning; Qvigstad, Henrik; Rachevski, Alexandre; Raha, Sibaji; Rajput, Sonia; Rak, Jan; Rakotozafindrabe, Andry Malala; Ramello, Luciano; Raniwala, Rashmi; Raniwala, Sudhir; Rasanen, Sami Sakari; Rascanu, Bogdan Theodor; Rathee, Deepika; Read, Kenneth Francis; Real, Jean-Sebastien; Redlich, Krzysztof; Reed, Rosi Jan; Rehman, Attiq Ur; Reichelt, Patrick Simon; Reidt, Felix; Ren, Xiaowen; Renfordt, Rainer Arno Ernst; Reolon, Anna Rita; Reshetin, Andrey; Rettig, Felix Vincenz; Revol, Jean-Pierre; Reygers, Klaus Johannes; Riabov, Viktor; Ricci, Renato Angelo; Richert, Tuva Ora Herenui; Richter, Matthias Rudolph; Riedler, Petra; Riegler, Werner; Riggi, Francesco; Ristea, Catalin-Lucian; Rivetti, Angelo; Rocco, Elena; Rodriguez Cahuantzi, Mario; Rodriguez Manso, Alis; Roeed, Ketil; Rogochaya, Elena; Rohr, David Michael; Roehrich, Dieter; Romita, Rosa; Ronchetti, Federico; Ronflette, Lucile; Rosnet, Philippe; Rossi, Andrea; Roukoutakis, Filimon; Roy, Ankhi; Roy, Christelle Sophie; Roy, Pradip Kumar; Rubio Montero, Antonio Juan; Rui, Rinaldo; Russo, Riccardo; Ryabinkin, Evgeny; Ryabov, Yury; Rybicki, Andrzej; Sadovskiy, Sergey; Safarik, Karel; Sahlmuller, Baldo; Sahoo, Pragati; Sahoo, Raghunath; Sahoo, Sarita; Sahu, Pradip Kumar; Saini, Jogender; Sakai, Shingo; Saleh, Mohammad Ahmad; Salgado Lopez, Carlos Alberto; Salzwedel, Jai Samuel Nielsen; Sambyal, Sanjeev Singh; Samsonov, Vladimir; Sanchez Castro, Xitzel; Sandor, Ladislav; Sandoval, Andres; Sano, Masato; Sarkar, Debojit; Scapparone, Eugenio; Scarlassara, Fernando; Scharenberg, Rolf Paul; Schiaua, Claudiu Cornel; Schicker, Rainer Martin; Schmidt, Christian Joachim; Schmidt, Hans Rudolf; Schuchmann, Simone; Schukraft, Jurgen; Schulc, Martin; Schuster, Tim Robin; Schutz, Yves Roland; Schwarz, Kilian Eberhard; Schweda, Kai Oliver; Scioli, Gilda; Scomparin, Enrico; Scott, Rebecca Michelle; Seger, Janet Elizabeth; Sekiguchi, Yuko; Sekihata, Daiki; Selyuzhenkov, Ilya; Senosi, Kgotlaesele; Seo, Jeewon; Serradilla Rodriguez, Eulogio; Sevcenco, Adrian; Shabanov, Arseniy; Shabetai, Alexandre; Shadura, Oksana; Shahoyan, Ruben; Shangaraev, Artem; Sharma, Ankita; Sharma, Mona; Sharma, Monika; Sharma, Natasha; Shigaki, Kenta; Shtejer Diaz, Katherin; Sibiryak, Yury; Siddhanta, Sabyasachi; Sielewicz, Krzysztof Marek; Siemiarczuk, Teodor; Silvermyr, David Olle Rickard; Silvestre, Catherine Micaela; Simatovic, Goran; Simonetti, Giuseppe; Singaraju, Rama Narayana; Singh, Ranbir; Singha, Subhash; Singhal, Vikas; Sinha, Bikash; Sarkar - Sinha, Tinku; Sitar, Branislav; Sitta, Mario; Skaali, Bernhard; Slupecki, Maciej; Smirnov, Nikolai; Snellings, Raimond; Snellman, Tomas Wilhelm; Soegaard, Carsten; Soltz, Ron Ariel; Song, Jihye; Song, Myunggeun; Song, Zixuan; Soramel, Francesca; Sorensen, Soren Pontoppidan; Spacek, Michal; Spiriti, Eleuterio; Sputowska, Iwona Anna; Spyropoulou-Stassinaki, Martha; Srivastava, Brijesh Kumar; Stachel, Johanna; Stan, Ionel; Stefanek, Grzegorz; Steinpreis, Matthew Donald; Stenlund, Evert Anders; Steyn, Gideon Francois; Stiller, Johannes Hendrik; Stocco, Diego; Strmen, Peter; Alarcon Do Passo Suaide, Alexandre; Sugitate, Toru; Suire, Christophe Pierre; Suleymanov, Mais Kazim Oglu; Sultanov, Rishat; Sumbera, Michal; Symons, Timothy; Szabo, Alexander; Szanto De Toledo, Alejandro; Szarka, Imrich; Szczepankiewicz, Adam; Szymanski, Maciej Pawel; Takahashi, Jun; Tambave, Ganesh Jagannath; Tanaka, Naoto; Tangaro, Marco-Antonio; Tapia Takaki, Daniel Jesus; Tarantola Peloni, Attilio; Tarhini, Mohamad; Tariq, Mohammad; Tarzila, Madalina-Gabriela; Tauro, Arturo; Tejeda Munoz, Guillermo; Telesca, Adriana; Terasaki, Kohei; Terrevoli, Cristina; Teyssier, Boris; Thaeder, Jochen Mathias; Thomas, Deepa; Tieulent, Raphael Noel; Timmins, Anthony Robert; Toia, Alberica; Trogolo, Stefano; Trubnikov, Victor; Trzaska, Wladyslaw Henryk; Tsuji, Tomoya; Tumkin, Alexandr; Turrisi, Rosario; Tveter, Trine Spedstad; Ullaland, Kjetil; Uras, Antonio; Usai, Gianluca; Utrobicic, Antonija; Vajzer, Michal; Vala, Martin; Valencia Palomo, Lizardo; Vallero, Sara; Van Der Maarel, Jasper; Van Hoorne, Jacobus Willem; Van Leeuwen, Marco; Vanat, Tomas; Vande Vyvre, Pierre; Varga, Dezso; Diozcora Vargas Trevino, Aurora; Vargyas, Marton; Varma, Raghava; Vasileiou, Maria; Vasiliev, Andrey; Vauthier, Astrid; Vechernin, Vladimir; Veen, Annelies Marianne; Veldhoen, Misha; Velure, Arild; Venaruzzo, Massimo; Vercellin, Ermanno; Vergara Limon, Sergio; Vernet, Renaud; Verweij, Marta; Vickovic, Linda; Viesti, Giuseppe; Viinikainen, Jussi Samuli; Vilakazi, Zabulon; Villalobos Baillie, Orlando; Vinogradov, Alexander; Vinogradov, Leonid; Vinogradov, Yury; Virgili, Tiziano; Vislavicius, Vytautas; Viyogi, Yogendra; Vodopyanov, Alexander; Volkl, Martin Andreas; Voloshin, Kirill; Voloshin, Sergey; Volpe, Giacomo; Von Haller, Barthelemy; Vorobyev, Ivan; Vranic, Danilo; Vrlakova, Janka; Vulpescu, Bogdan; Vyushin, Alexey; Wagner, Boris; Wagner, Jan; Wang, Hongkai; Wang, Mengliang; Wang, Yifei; Watanabe, Daisuke; Watanabe, Yosuke; Weber, Michael; Weber, Steffen Georg; Wessels, Johannes Peter; Westerhoff, Uwe; Wiechula, Jens; Wikne, Jon; Wilde, Martin Rudolf; Wilk, Grzegorz Andrzej; Wilkinson, Jeremy John; Williams, Crispin; Windelband, Bernd Stefan; Winn, Michael Andreas; Yaldo, Chris G; Yang, Hongyan; Yang, Ping; Yano, Satoshi; Yin, Zhongbao; Yokoyama, Hiroki; Yoo, In-Kwon; Yurchenko, Volodymyr; Yushmanov, Igor; Zaborowska, Anna; Zaccolo, Valentina; Zaman, Ali; Zampolli, Chiara; Correia Zanoli, Henrique Jose; Zaporozhets, Sergey; Zardoshti, Nima; Zarochentsev, Andrey; Zavada, Petr; Zavyalov, Nikolay; Zbroszczyk, Hanna Paulina; Zgura, Sorin Ion; Zhalov, Mikhail; Zhang, Haitao; Zhang, Xiaoming; Zhang, Yonghong; Zhao, Chengxin; Zhigareva, Natalia; Zhou, Daicui; Zhou, You; Zhou, Zhuo; Zhu, Hongsheng; Zhu, Jianhui; Zhu, Xiangrong; Zichichi, Antonino; Zimmermann, Alice; Zimmermann, Markus Bernhard; Zinovjev, Gennady; Zyzak, Maksym

    2016-02-04

    We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at $\\sqrt{s_{\\rm NN}}=2.76$ TeV as a function of collision centrality and the average transverse momentum of the pair $k_{\\rm T}$. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with $k_{\\rm T}$, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with $\\left^{1/3}$. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller $\\sqrt{s_{\\rm NN}}$, a decrease in the $R_{\\rm out}/R_{\\rm side}$ ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly c...

  19. Total disc replacement using tissue-engineered intervertebral discs in the canine cervical spine.

    Directory of Open Access Journals (Sweden)

    Yu Moriguchi

    Full Text Available The most common reason that adults in the United States see their physician is lower back or neck pain secondary to degenerative disc disease. To date, approaches to treat degenerative disc disease are confined to purely mechanical devices designed to either eliminate or enable flexibility of the diseased motion segment. Tissue engineered intervertebral discs (TE-IVDs have been proposed as an alternative approach and have shown promise in replacing native IVD in the rodent tail spine. Here we demonstrate the efficacy of our TE-IVDs in the canine cervical spine. TE-IVD components were constructed using adult canine annulus fibrosis and nucleus pulposus cells seeded into collagen and alginate hydrogels, respectively. Seeded gels were formed into a single disc unit using molds designed from the geometry of the canine spine. Skeletally mature beagles underwent discectomy with whole IVD resection at levels between C3/4 and C6/7, and were then divided into two groups that received only discectomy or discectomy followed by implantation of TE-IVD. Stably implanted TE-IVDs demonstrated significant retention of disc height and physiological hydration compared to discectomy control. Both 4-week and 16-week histological assessments demonstrated chondrocytic cells surrounded by proteoglycan-rich matrices in the NP and by fibrocartilaginous matrices in the AF portions of implanted TE-IVDs. Integration into host tissue was confirmed over 16 weeks without any signs of immune reaction. Despite the significant biomechanical demands of the beagle cervical spine, our stably implanted TE-IVDs maintained their position, structure and hydration as well as disc height over 16 weeks in vivo.

  20. Thermal structure of accreting neutron stars and strange stars

    International Nuclear Information System (INIS)

    Miralda-Escude, J.; Paczynski, B.; Haensel, P.

    1990-01-01

    Steady-state models of accreting neutron stars and strange stars are presented, and their properties as a function of accretion rate are analyzed. The models have steady-state envelopes, with stationary hydrogen burning taken into account, the helium shell flashes artificially suppressed, and the crust with a large number of secondary heat sources. The deep interiors are almost isothermal and are close to thermal equilibrium. A large number of models were calculated for many values of the accretion rates, with ordinary, pion-condensed, and strange cores, with and without secondary heat sources in the crust, and with the heavy element content of the accreting matter in the range Z = 0.0002-0.02. All models show a similar pattern of changes as the accretion rate is varied. For low accretion rates, the hydrogen burning shell is unstable; for intermediate rates, the hydrogen burning shell is stable, but helium burning is not; for high rates, the two shell sources burn together and are unstable. 60 refs

  1. Protoplanetary disc truncation mechanisms in stellar clusters: comparing external photoevaporation and tidal encounters

    Science.gov (United States)

    Winter, A. J.; Clarke, C. J.; Rosotti, G.; Ih, J.; Facchini, S.; Haworth, T. J.

    2018-04-01

    Most stars form and spend their early life in regions of enhanced stellar density. Therefore the evolution of protoplanetary discs (PPDs) hosted by such stars are subject to the influence of other members of the cluster. Physically, PPDs might be truncated either by photoevaporation due to ultraviolet flux from massive stars, or tidal truncation due to close stellar encounters. Here we aim to compare the two effects in real cluster environments. In this vein we first review the properties of well studied stellar clusters with a focus on stellar number density, which largely dictates the degree of tidal truncation, and far ultraviolet (FUV) flux, which is indicative of the rate of external photoevaporation. We then review the theoretical PPD truncation radius due to an arbitrary encounter, additionally taking into account the role of eccentric encounters that play a role in hot clusters with a 1D velocity dispersion σv ≳ 2 km/s. Our treatment is then applied statistically to varying local environments to establish a canonical threshold for the local stellar density (nc ≳ 104 pc-3) for which encounters can play a significant role in shaping the distribution of PPD radii over a timescale ˜3 Myr. By combining theoretical mass loss rates due to FUV flux with viscous spreading in a PPD we establish a similar threshold for which a massive disc is completely destroyed by external photoevaporation. Comparing these thresholds in local clusters we find that if either mechanism has a significant impact on the PPD population then photoevaporation is always the dominating influence.

  2. Variable accretion of stellar winds onto Sgr A*

    Science.gov (United States)

    Cuadra, Jorge; Nayakshin, Sergei

    2006-12-01

    We report a 3-dimensional numerical study of the accretion of stellar winds onto Sgr A*, the super-massive black hole at the centre of our Galaxy. Compared with previous investigations, we allow the stars to be on realistic orbits, include the recently discovered slow wind sources, and allow for optically thin radiative cooling. We frst show the strong inflience of the stellar dynamics on the accretion onto the central black hole. We then present more realistic simulations of Sgr A* accretion and frid that the slow winds shock and rapidly cool, forming cold gas clumps and flaments that coexist with the hot X-ray emitting gas. The accretion rate in this case is highly variable on time-scales of tens to hundreds of years. Such variability can in principle lead to a strongly non-linear response through accretion fbw physics not resolved here, making Sgr A* an important energy source for the Galactic centre.

  3. Variable accretion of stellar winds onto Sgr A*

    Energy Technology Data Exchange (ETDEWEB)

    Cuadra, Jorge [Max-Planck-Institut fuer Astrophysik, D-85741 Garching (Germany); Nayakshin, Sergei [Department of Physics and Astronomy, University of Leicester, LEI 7RH (United Kingdom)

    2006-12-15

    We report a 3-dimensional numerical study of the accretion of stellar winds onto Sgr A*, the super-massive black hole at the centre of our Galaxy. Compared with previous investigations, we allow the stars to be on realistic orbits, include the recently discovered slow wind sources, and allow for optically thin radiative cooling. We frst show the strong inflience of the stellar dynamics on the accretion onto the central black hole. We then present more realistic simulations of Sgr A* accretion and frid that the slow winds shock and rapidly cool, forming cold gas clumps and flaments that coexist with the hot X-ray emitting gas. The accretion rate in this case is highly variable on time-scales of tens to hundreds of years. Such variability can in principle lead to a strongly non-linear response through accretion fbw physics not resolved here, making Sgr A* an important energy source for the Galactic centre.

  4. Variable accretion of stellar winds onto Sgr A*

    International Nuclear Information System (INIS)

    Cuadra, Jorge; Nayakshin, Sergei

    2006-01-01

    We report a 3-dimensional numerical study of the accretion of stellar winds onto Sgr A*, the super-massive black hole at the centre of our Galaxy. Compared with previous investigations, we allow the stars to be on realistic orbits, include the recently discovered slow wind sources, and allow for optically thin radiative cooling. We frst show the strong inflience of the stellar dynamics on the accretion onto the central black hole. We then present more realistic simulations of Sgr A* accretion and frid that the slow winds shock and rapidly cool, forming cold gas clumps and flaments that coexist with the hot X-ray emitting gas. The accretion rate in this case is highly variable on time-scales of tens to hundreds of years. Such variability can in principle lead to a strongly non-linear response through accretion fbw physics not resolved here, making Sgr A* an important energy source for the Galactic centre

  5. Notochord Cells in Intervertebral Disc Development and Degeneration

    Science.gov (United States)

    McCann, Matthew R.; Séguin, Cheryle A.

    2016-01-01

    The intervertebral disc is a complex structure responsible for flexibility, multi-axial motion, and load transmission throughout the spine. Importantly, degeneration of the intervertebral disc is thought to be an initiating factor for back pain. Due to a lack of understanding of the pathways that govern disc degeneration, there are currently no disease-modifying treatments to delay or prevent degenerative disc disease. This review presents an overview of our current understanding of the developmental processes that regulate intervertebral disc formation, with particular emphasis on the role of the notochord and notochord-derived cells in disc homeostasis and how their loss can result in degeneration. We then describe the role of small animal models in understanding the development of the disc and their use to interrogate disc degeneration and associated pathologies. Finally, we highlight essential development pathways that are associated with disc degeneration and/or implicated in the reparative response of the tissue that might serve as targets for future therapeutic approaches. PMID:27252900

  6. Notochord Cells in Intervertebral Disc Development and Degeneration

    Directory of Open Access Journals (Sweden)

    Matthew R. McCann

    2016-01-01

    Full Text Available The intervertebral disc is a complex structure responsible for flexibility, multi-axial motion, and load transmission throughout the spine. Importantly, degeneration of the intervertebral disc is thought to be an initiating factor for back pain. Due to a lack of understanding of the pathways that govern disc degeneration, there are currently no disease-modifying treatments to delay or prevent degenerative disc disease. This review presents an overview of our current understanding of the developmental processes that regulate intervertebral disc formation, with particular emphasis on the role of the notochord and notochord-derived cells in disc homeostasis and how their loss can result in degeneration. We then describe the role of small animal models in understanding the development of the disc and their use to interrogate disc degeneration and associated pathologies. Finally, we highlight essential development pathways that are associated with disc degeneration and/or implicated in the reparative response of the tissue that might serve as targets for future therapeutic approaches.

  7. The life cycles of Be viscous decretion discs: fundamental disc parameters of 54 SMC Be stars

    Science.gov (United States)

    Rímulo, L. R.; Carciofi, A. C.; Vieira, R. G.; Rivinius, Th; Faes, D. M.; Figueiredo, A. L.; Bjorkman, J. E.; Georgy, C.; Ghoreyshi, M. R.; Soszyński, I.

    2018-05-01

    Be stars are main-sequence massive stars with emission features in their spectrum, which originates in circumstellar gaseous discs. Even though the viscous decretion disc model can satisfactorily explain most observations, two important physical ingredients, namely the magnitude of the viscosity (α) and the disc mass injection rate, remain poorly constrained. The light curves of Be stars that undergo events of disc formation and dissipation offer an opportunity to constrain these quantities. A pipeline was developed to model these events that use a grid of synthetic light curves, computed from coupled hydrodynamic and radiative transfer calculations. A sample of 54 Be stars from the OGLE survey of the Small Magellanic Cloud (SMC) was selected for this study. Because of the way our sample was selected (bright stars with clear disc events), it likely represents the densest discs in the SMC. Like their siblings in the Galaxy, the mass of the disc in the SMC increases with the stellar mass. The typical mass and angular momentum loss rates associated with the disc events are of the order of ˜10-10 M⊙ yr-1 and ˜5 × 1036 g cm2 s-2, respectively. The values of α found in this work are typically of a few tenths, consistent with recent results in the literature and with the ones found in dwarf novae, but larger than current theory predicts. Considering the sample as a whole, the viscosity parameter is roughly two times larger at build-up ( = 0.63) than at dissipation ( = 0.26). Further work is necessary to verify whether this trend is real or a result of some of the model assumptions.

  8. RINGED ACCRETION DISKS: INSTABILITIES

    Energy Technology Data Exchange (ETDEWEB)

    Pugliese, D.; Stuchlík, Z., E-mail: d.pugliese.physics@gmail.com, E-mail: zdenek.stuchlik@physics.cz [Institute of Physics and Research Centre of Theoretical Physics and Astrophysics, Faculty of Philosophy and Science, Silesian University in Opava, Bezručovo náměstí 13, CZ-74601 Opava (Czech Republic)

    2016-04-01

    We analyze the possibility that several instability points may be formed, due to the Paczyński mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider a recently proposed model of a ringed accretion disk, made up by several tori (rings) that can be corotating or counter-rotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  9. Accretion outbursts in self-gravitating protoplanetary disks

    Energy Technology Data Exchange (ETDEWEB)

    Bae, Jaehan; Hartmann, Lee [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48105 (United States); Zhu, Zhaohuan [Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Peyton Hall, Princeton, NJ 08544 (United States); Nelson, Richard P., E-mail: jaehbae@umich.edu, E-mail: lhartm@umich.edu, E-mail: zhuzh@astro.princeton.edu, E-mail: r.p.nelson@qmul.ac.uk [Astronomy Unit, Queen Mary University of London, Mile End Road, London E1 4NS (United Kingdom)

    2014-11-01

    We improve on our previous treatments of the long-term evolution of protostellar disks by explicitly solving disk self-gravity in two dimensions. The current model is an extension of the one-dimensional layered accretion disk model of Bae et al. We find that gravitational instability (GI)-induced spiral density waves heat disks via compressional heating (i.e., PdV work), and can trigger accretion outbursts by activating the magnetorotational instability (MRI) in the magnetically inert disk dead zone. The GI-induced spiral waves propagate well inside of the gravitationally unstable region before they trigger outbursts at R ≲ 1 AU where GI cannot be sustained. This long-range propagation of waves cannot be reproduced with the previously used local α treatments for GI. In our standard model where zero dead-zone residual viscosity (α{sub rd}) is assumed, the GI-induced stress measured at the onset of outbursts is locally as large as 0.01 in terms of the generic α parameter. However, as suggested in our previous one-dimensional calculations, we confirm that the presence of a small but finite α{sub rd} triggers thermally driven bursts of accretion instead of the GI + MRI-driven outbursts that are observed when α{sub rd} = 0. The inclusion of non-zero residual viscosity in the dead zone decreases the importance of GI soon after mass feeding from the envelope cloud ceases. During the infall phase while the central protostar is still embedded, our models stay in a 'quiescent' accretion phase with M-dot {sub acc}∼10{sup −8}--10{sup −7} M{sub ⊙} yr{sup −1} over 60% of the time and spend less than 15% of the infall phase in accretion outbursts. While our models indicate that episodic mass accretion during protostellar evolution can qualitatively help explain the low accretion luminosities seen in most low-mass protostars, detailed tests of the mechanism will require model calculations for a range of protostellar masses with some constraint on the

  10. Accretion outbursts in self-gravitating protoplanetary disks

    International Nuclear Information System (INIS)

    Bae, Jaehan; Hartmann, Lee; Zhu, Zhaohuan; Nelson, Richard P.

    2014-01-01

    We improve on our previous treatments of the long-term evolution of protostellar disks by explicitly solving disk self-gravity in two dimensions. The current model is an extension of the one-dimensional layered accretion disk model of Bae et al. We find that gravitational instability (GI)-induced spiral density waves heat disks via compressional heating (i.e., PdV work), and can trigger accretion outbursts by activating the magnetorotational instability (MRI) in the magnetically inert disk dead zone. The GI-induced spiral waves propagate well inside of the gravitationally unstable region before they trigger outbursts at R ≲ 1 AU where GI cannot be sustained. This long-range propagation of waves cannot be reproduced with the previously used local α treatments for GI. In our standard model where zero dead-zone residual viscosity (α rd ) is assumed, the GI-induced stress measured at the onset of outbursts is locally as large as 0.01 in terms of the generic α parameter. However, as suggested in our previous one-dimensional calculations, we confirm that the presence of a small but finite α rd triggers thermally driven bursts of accretion instead of the GI + MRI-driven outbursts that are observed when α rd = 0. The inclusion of non-zero residual viscosity in the dead zone decreases the importance of GI soon after mass feeding from the envelope cloud ceases. During the infall phase while the central protostar is still embedded, our models stay in a 'quiescent' accretion phase with M-dot acc ∼10 −8 --10 −7 M ⊙ yr −1 over 60% of the time and spend less than 15% of the infall phase in accretion outbursts. While our models indicate that episodic mass accretion during protostellar evolution can qualitatively help explain the low accretion luminosities seen in most low-mass protostars, detailed tests of the mechanism will require model calculations for a range of protostellar masses with some constraint on the initial core angular momentum, which

  11. Minimum weight design of inhomogeneous rotating discs

    International Nuclear Information System (INIS)

    Jahed, Hamid; Farshi, Behrooz; Bidabadi, Jalal

    2005-01-01

    There are numerous applications for gas turbine discs in the aerospace industry such as in turbojet engines. These discs normally work under high temperatures while subjected to high angular velocities. Minimizing the weight of such items in aerospace applications results in benefits such as low dead weights and lower costs. High speed of rotation causes large centrifugal forces in a disc and simultaneous application of high temperatures reduces disc material strength. Thus, the latter effects tend to increase deformations of the disc under the applied loads. In order to obtain a reliable disc analysis and arrive at the corresponding correct stress distribution, solutions should consider changes in material properties due to the temperature field throughout the disc. To achieve this goal, an inhomogeneous disc model with variable thickness is considered. Using the variable material properties method, stresses are obtained for the disc under rotation and a steady temperature field. In this paper this is done by modelling the rotating disc as a series of rings of different but constant properties. The optimum disc profile is arrived at by sequentially proportioning the thicknesses of each ring to satisfy the stress requirements. This method vis-a-vis a mathematical programming procedure for optimization shows several advantages. Firstly, it is simple iterative proportioning in each design cycle not requiring involved mathematical operations. Secondly, due to its simplicity it alleviates the necessity of certain simplifications that are common in so-called rigorous mathematical procedures. The results obtained, compared to those published in the literature show agreement and superiority. A further advantage of the proposed method is the independence of the end results from the initially assumed point in the iterative design routine, unlike most methods published so far

  12. Spectroscopic Parameters of Lumbar Intervertebral Disc Material

    Science.gov (United States)

    Terbetas, G.; Kozlovskaja, A.; Varanius, D.; Graziene, V.; Vaitkus, J.; Vaitkuviene, A.

    2009-06-01

    There are numerous methods of investigating intervertebral disc. Visualization methods are widely used in clinical practice. Histological, imunohistochemical and biochemical methods are more used in scientific research. We propose that a new spectroscopic investigation would be useful in determining intervertebral disc material, especially when no histological specimens are available. Purpose: to determine spectroscopic parameters of intervertebral disc material; to determine emission spectra common for all intervertebral discs; to create a background for further spectroscopic investigation where no histological specimen will be available. Material and Methods: 20 patients, 68 frozen sections of 20 μm thickness from operatively removed intervertebral disc hernia were excited by Nd:YAG microlaser STA-01-TH third harmonic 355 nm light throw 0, 1 mm fiber. Spectrophotometer OceanOptics USB2000 was used for spectra collection. Mathematical analysis of spectra was performed by ORIGIN multiple Gaussian peaks analysis. Results: In each specimen of disc hernia were found distinct maximal spectral peaks of 4 types supporting the histological evaluation of mixture content of the hernia. Fluorescence in the spectral regions 370-700 nm was detected in the disc hernias. The main spectral component was at 494 nm and the contribution of the components with the peak wavelength values at 388 nm, 412 nm and 435±5 nm were varying in the different groups of samples. In comparison to average spectrum of all cases, there are 4 groups of different spectral signatures in the region 400-500 nm in the patient groups, supporting a clinical data on different clinical features of the patients. Discussion and Conclusion: besides the classical open discectomy, new minimally invasive techniques of treating intervertebral disc emerge (PLDD). Intervertebral disc in these techniques is assessed by needle, no histological specimen is taken. Spectroscopic investigation via fiber optics through the

  13. Biomaterials for intervertebral disc regeneration and repair.

    Science.gov (United States)

    Bowles, Robert D; Setton, Lori A

    2017-06-01

    The intervertebral disc contributes to motion, weight bearing, and flexibility of the spine, but is susceptible to damage and morphological changes that contribute to pathology with age and injury. Engineering strategies that rely upon synthetic materials or composite implants that do not interface with the biological components of the disc have not met with widespread use or desirable outcomes in the treatment of intervertebral disc pathology. Here we review bioengineering advances to treat disc disorders, using cell-supplemented materials, or acellular, biologically based materials, that provide opportunity for cell-material interactions and remodeling in the treatment of intervertebral disc disorders. While a field still in early development, bioengineering-based strategies employing novel biomaterials are emerging as promising alternatives for clinical treatment of intervertebral disc disorders. Copyright © 2017 Elsevier Ltd. All rights reserved.

  14. Development of rupture discs for the FBTR

    International Nuclear Information System (INIS)

    Chetal, S.C.; Raju, C.; Anandkumar, V.; Seetharaman, V.; Rajan, K.K.

    1984-01-01

    Rupture discs are required as a safety device for protecting the secondary sodium circuit and its components against high pressure surges due to accidental water steam leaks in sodium heated steam generator and the consequent sodium water reaction. For identical reasons, rupture discs are also required on the vessels used for decontamination of sodium components. Reverse buckling knife blade concept with austenitic stainless steel disc has been developed for the rupture disc assemblies required for Fast Breeder Test Reactor (FBTR). Hydroforming process without any die has been used for disc fabrication. One rupture disc assembly required for steam generator is undergoing sodium endurance test and has accumulated 4,500 hours. The present status of development work as demonstrated by room temperature experimental results as well as the scope for future work are discussed. (author)

  15. Radiation-driven Turbulent Accretion onto Massive Black Holes

    Energy Technology Data Exchange (ETDEWEB)

    Park, KwangHo; Wise, John H.; Bogdanović, Tamara, E-mail: kwangho.park@physics.gatech.edu [Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 (United States)

    2017-09-20

    Accretion of gas and interaction of matter and radiation are at the heart of many questions pertaining to black hole (BH) growth and coevolution of massive BHs and their host galaxies. To answer them, it is critical to quantify how the ionizing radiation that emanates from the innermost regions of the BH accretion flow couples to the surrounding medium and how it regulates the BH fueling. In this work, we use high-resolution three-dimensional (3D) radiation-hydrodynamic simulations with the code Enzo , equipped with adaptive ray-tracing module Moray , to investigate radiation-regulated BH accretion of cold gas. Our simulations reproduce findings from an earlier generation of 1D/2D simulations: the accretion-powered UV and X-ray radiation forms a highly ionized bubble, which leads to suppression of BH accretion rate characterized by quasi-periodic outbursts. A new feature revealed by the 3D simulations is the highly turbulent nature of the gas flow in vicinity of the ionization front. During quiescent periods between accretion outbursts, the ionized bubble shrinks in size and the gas density that precedes the ionization front increases. Consequently, the 3D simulations show oscillations in the accretion rate of only ∼2–3 orders of magnitude, significantly smaller than 1D/2D models. We calculate the energy budget of the gas flow and find that turbulence is the main contributor to the kinetic energy of the gas but corresponds to less than 10% of its thermal energy and thus does not contribute significantly to the pressure support of the gas.

  16. Effects of ice accretion on the aerodynamics of bridge cables

    DEFF Research Database (Denmark)

    Demartino, C.; Koss, Holger; Georgakis, Christos T.

    2015-01-01

    and stay cables. The aim of this paper is twofold; first, it was investigated the ice accretion process and the final shape of the ice accreted; then the aerodynamics of the ice accreted bridge cables was characterized, and related to the ice shape. Different climatic conditions, i.e. combinations...... of temperature, wind speed and yaw angle of accretion, were reproduced in a climatic wind tunnel, giving rise to different types of accretion. These were chosen such to generate the most common natural ice formations expected to produce bridge cable vibrations. A description of the geometric characteristics...

  17. Cooling of Accretion-Heated Neutron Stars

    Science.gov (United States)

    Wijnands, Rudy; Degenaar, Nathalie; Page, Dany

    2017-09-01

    We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.

  18. Reanalysis of the radii of the Benchmark eclipsing binary V578 Mon

    International Nuclear Information System (INIS)

    Garcia, E. V.; Stassun, Keivan G.; Torres, Guillermo

    2013-01-01

    V578 Mon is an eclipsing binary system in which both stars have masses above 10 M ☉ determined with an accuracy better than 3%. It is one of only five such massive eclipsing binaries known that also possess eccentric orbits and measured apsidal motions, thus making it an important benchmark for theoretical stellar evolution models. However, recently reported determinations of the radii of V578 Mon differ significantly from previously reported values. We reanalyze the published data for V578 Mon and trace the discrepancy to the use of an incorrect formulation for the stellar potentials in the most recent analysis. Here we report corrected radii for this important benchmark eclipsing binary.

  19. Queixas auditivas de disc jockeys da cidade de Recife Auditory complaints in disc jockeys in Recife

    Directory of Open Access Journals (Sweden)

    Eliza Maia de Britto Macedo

    2011-06-01

    Full Text Available OBJETIVO: investigar a ocorrência de queixas auditivas em disc jockeys da cidade de Recife/PE. MÉTODOS: foi realizada uma entrevista com 30 disc jockeys, com idade entre 19 e 28 anos, abordando informações ocupacionais, conhecimentos gerais sobre o ruído e queixas auditivas (diminuição da acuidade auditiva, desconforto a sons intensos, zumbido, sensação de ouvido abafado e otalgia. A análise foi realizada por meio de abordagem quantitativa, utilizando o teste estatístico t-student. RESULTADOS: dentre os dados mais relevantes, destacam-se: 46,7% dos disc jockeys apresentaram, espontaneamente, queixas auditivas, em especial, a diminuição da acuidade auditiva (relatada por todos os sujeitos; 14 disc jockeys (46,67% referiram desconforto a sons intensos e 13 (43,33% mencionaram zumbido. Todos afirmaram ter conhecimento sobre os riscos do ruído para a saúde auditiva, mas 76,7% não realizam qualquer medida preventiva de suas consequências. A perda auditiva foi referida pelos sujeitos como o principal risco da exposição a níveis intensos de pressão sonora. CONCLUSÃO: todos os disc jockeys apresentaram queixa de perda auditiva e, entre as demais queixas auditivas, destacaram-se o desconforto a sons intensos e o zumbido. Tendo em vista a irreversibilidade da perda auditiva induzida por elevados níveis de pressão sonora, os disc jockeys devem ser periodicamente avaliados a fim de que se confirme ou não a perda auditiva de que se queixaram e, caso ela exista, deve ser monitorada para que seja passível de intervenção pelo fonoaudiólogo. Desta forma, percebe-se a necessidade de atuação da Fonoaudiologia junto aos disc jockeys, uma vez que poder-se-á propiciar a otimização do exercício profissional com o mínimo de risco possível.PURPOSE: to investigate the occurrence of auditory complaints in disc jockeys from the city of Recife/PE. METHODS: an interview was carried through with 30 disc jockeys aged between 19 and 48 years

  20. MEASURING TINY MASS ACCRETION RATES ONTO YOUNG BROWN DWARFS

    International Nuclear Information System (INIS)

    Herczeg, Gregory J.; Cruz, Kelle L.; Hillenbrand, Lynne A.

    2009-01-01

    We present low-resolution Keck I/LRIS spectra spanning from 3200 to 9000 A of nine young brown dwarfs and three low-mass stars in the TW Hya Association and in Upper Sco. The optical spectral types of the brown dwarfs range from M5.5 to M8.75, though two have near-IR spectral types of early L dwarfs. We report new accretion rates derived from excess Balmer continuum emission for the low-mass stars TW Hya and Hen 3-600A and the brown dwarfs 2MASS J12073347-3932540, UScoCTIO 128, SSSPM J1102-3431, USco J160606.29-233513.3, DENIS-P J160603.9-205644, and Oph J162225-240515B, and upper limits on accretion for the low-mass star Hen 3-600B and the brown dwarfs UScoCTIO 112, Oph J162225-240515A, and USco J160723.82-221102.0. For the six brown dwarfs in our sample that are faintest at short wavelengths, the accretion luminosity or upper limit is measurable only when the image is binned over large wavelength intervals. This method extends our sensitivity to accretion rate down to ∼10 -13 M sun yr -1 for brown dwarfs. Since the ability to measure an accretion rate from excess Balmer continuum emission depends on the contrast between excess continuum emission and the underlying photosphere, for objects with earlier spectral types the upper limit on accretion rate is much higher. Absolute uncertainties in our accretion rate measurements of ∼3-5 include uncertainty in accretion models, brown dwarf masses, and distance. The accretion rate of 2 x 10 -12 M sun yr -1 onto 2MASS J12073347-3932540 is within 15% of two previous measurements, despite large changes in the Hα flux.

  1. The reports of thick discs' deaths are greatly exaggerated. Thick discs are NOT artefacts caused by diffuse scattered light

    Science.gov (United States)

    Comerón, S.; Salo, H.; Knapen, J. H.

    2018-02-01

    Recent studies have made the community aware of the importance of accounting for scattered light when examining low-surface-brightness galaxy features such as thick discs. In our past studies of the thick discs of edge-on galaxies in the Spitzer Survey of Stellar Structure in Galaxies - the S4G - we modelled the point spread function as a Gaussian. In this paper we re-examine our results using a revised point spread function model that accounts for extended wings out to more than 2\\farcm5. We study the 3.6 μm images of 141 edge-on galaxies from the S4G and its early-type galaxy extension. Thus, we more than double the samples examined in our past studies. We decompose the surface-brightness profiles of the galaxies perpendicular to their mid-planes assuming that discs are made of two stellar discs in hydrostatic equilibrium. We decompose the axial surface-brightness profiles of galaxies to model the central mass concentration - described by a Sérsic function - and the disc - described by a broken exponential disc seen edge-on. Our improved treatment fully confirms the ubiquitous occurrence of thick discs. The main difference between our current fits and those presented in our previous papers is that now the scattered light from the thin disc dominates the surface brightness at levels below μ 26 mag arcsec-2. We stress that those extended thin disc tails are not physical, but pure scattered light. This change, however, does not drastically affect any of our previously presented results: 1) Thick discs are nearly ubiquitous. They are not an artefact caused by scattered light as has been suggested elsewhere. 2) Thick discs have masses comparable to those of thin discs in low-mass galaxies - with circular velocities vc< 120 km s-1 - whereas they are typically less massive than the thin discs in high-mass galaxies. 3) Thick discs and central mass concentrations seem to have formed at the same epoch from a common material reservoir. 4) Approximately 50% of the up

  2. Accretion, primordial black holes and standard cosmology

    Indian Academy of Sciences (India)

    Primordial black holes evaporate due to Hawking radiation. We find that the evaporation times of primordial black holes increase when accretion of radiation is included. Thus, depending on accretion efficiency, more primordial black holes are existing today, which strengthens the conjecture that the primordial black holes ...

  3. Accretion Disks and Coronae in the X-Ray Flashlight

    Science.gov (United States)

    Degenaar, Nathalie; Ballantyne, David R.; Belloni, Tomaso; Chakraborty, Manoneeta; Chen, Yu-Peng; Ji, Long; Kretschmar, Peter; Kuulkers, Erik; Li, Jian; Maccarone, Thomas J.; Malzac, Julien; Zhang, Shu; Zhang, Shuang-Nan

    2018-02-01

    Plasma accreted onto the surface of a neutron star can ignite due to unstable thermonuclear burning and produce a bright flash of X-ray emission called a Type-I X-ray burst. Such events are very common; thousands have been observed to date from over a hundred accreting neutron stars. The intense, often Eddington-limited, radiation generated in these thermonuclear explosions can have a discernible effect on the surrounding accretion flow that consists of an accretion disk and a hot electron corona. Type-I X-ray bursts can therefore serve as direct, repeating probes of the internal dynamics of the accretion process. In this work we review and interpret the observational evidence for the impact that Type-I X-ray bursts have on accretion disks and coronae. We also provide an outlook of how to make further progress in this research field with prospective experiments and analysis techniques, and by exploiting the technical capabilities of the new and concept X-ray missions ASTROSAT, NICER, Insight-HXMT, eXTP, and STROBE-X.

  4. Accretion of a ghost condensate by black holes

    International Nuclear Information System (INIS)

    Frolov, Andrei V.

    2004-01-01

    The intent of this paper is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as a tenth of a solar mass per second for 10 MeV scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model

  5. The Correlation between Insertion Depth of Prodisc-C Artificial Disc and Postoperative Kyphotic Deformity: Clinical Importance of Insertion Depth of Artificial Disc.

    Science.gov (United States)

    Lee, Do-Youl; Kim, Se-Hoon; Suh, Jung-Keun; Cho, Tai-Hyoung; Chung, Yong-Gu

    2012-09-01

    This study was designed to investigate the correlation between insertion depth of artificial disc and postoperative kyphotic deformity after Prodisc-C total disc replacement surgery, and the range of artificial disc insertion depth which is effective in preventing postoperative whole cervical or segmental kyphotic deformity. A retrospective radiological analysis was performed in 50 patients who had undergone single level total disc replacement surgery. Records were reviewed to obtain demographic data. Preoperative and postoperative radiographs were assessed to determine C2-7 Cobb's angle and segmental angle and to investigate postoperative kyphotic deformity. A formula was introduced to calculate insertion depth of Prodisc-C artificial disc. Statistical analysis was performed to search the correlation between insertion depth of Prodisc-C artificial disc and postoperative kyphotic deformity, and to estimate insertion depth of Prodisc-C artificial disc to prevent postoperative kyphotic deformity. In this study no significant statistical correlation was observed between insertion depth of Prodisc-C artificial disc and postoperative kyphotic deformity regarding C2-7 Cobb's angle. Statistical correlation between insertion depth of Prodisc-C artificial disc and postoperative kyphotic deformity was observed regarding segmental angle (p<0.05). It failed to estimate proper insertion depth of Prodisc-C artificial disc effective in preventing postoperative kyphotic deformity. Postoperative segmental kyphotic deformity is associated with insertion depth of Prodisc-C artificial disc. Anterior located artificial disc leads to lordotic segmental angle and posterior located artificial disc leads to kyphotic segmental angle postoperatively. But C2-7 Cobb's angle is not affected by artificial disc location after the surgery.

  6. WIND-DRIVEN ACCRETION IN TRANSITIONAL PROTOSTELLAR DISKS

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Lile; Goodman, Jeremy J. [Princeton University Observatory, Princeton, NJ 08544 (United States)

    2017-01-20

    Transitional protostellar disks have inner cavities that are heavily depleted in dust and gas, yet most of them show signs of ongoing accretion, often at rates comparable to full disks. We show that recent constraints on the gas surface density in a few well-studied disk cavities suggest that the accretion speed is at least transsonic. We propose that this is the natural result of accretion driven by magnetized winds. Typical physical conditions of the gas inside these cavities are estimated for plausible X-ray and FUV radiation fields. The gas near the midplane is molecular and predominantly neutral, with a dimensionless ambipolar parameter in the right general range for wind solutions of the type developed by Königl, Wardle, and others. That is to say, the density of ions and electrons is sufficient for moderately good coupling to the magnetic field, but it is not so good that the magnetic flux needs to be dragged inward by the accreting neutrals.

  7. The three phases of galaxy formation

    Science.gov (United States)

    Clauwens, Bart; Schaye, Joop; Franx, Marijn; Bower, Richard G.

    2018-05-01

    We investigate the origin of the Hubble sequence by analysing the evolution of the kinematic morphologies of central galaxies in the EAGLE cosmological simulation. By separating each galaxy into disc and spheroidal stellar components and tracing their evolution along the merger tree, we find that the morphology of galaxies follows a common evolutionary trend. We distinguish three phases of galaxy formation. These phases are determined primarily by mass, rather than redshift. For M* ≲ 109.5M⊙ galaxies grow in a disorganised way, resulting in a morphology that is dominated by random stellar motions. This phase is dominated by in-situ star formation, partly triggered by mergers. In the mass range 109.5M⊙ ≲ M* ≲ 1010.5M⊙ galaxies evolve towards a disc-dominated morphology, driven by in-situ star formation. The central spheroid (i.e. the bulge) at z = 0 consists mostly of stars that formed in-situ, yet the formation of the bulge is to a large degree associated with mergers. Finally, at M* ≳ 1010.5M⊙ growth through in-situ star formation slows down considerably and galaxies transform towards a more spheroidal morphology. This transformation is driven more by the buildup of spheroids than by the destruction of discs. Spheroid formation in these galaxies happens mostly by accretion at large radii of stars formed ex-situ (i.e. the halo rather than the bulge).

  8. EARTH, MOON, SUN, AND CV ACCRETION DISKS

    International Nuclear Information System (INIS)

    Montgomery, M. M.

    2009-01-01

    Net tidal torque by the secondary on a misaligned accretion disk, like the net tidal torque by the Moon and the Sun on the equatorial bulge of the spinning and tilted Earth, is suggested by others to be a source to retrograde precession in non-magnetic, accreting cataclysmic variable (CV) dwarf novae (DN) systems that show negative superhumps in their light curves. We investigate this idea in this work. We generate a generic theoretical expression for retrograde precession in spinning disks that are misaligned with the orbital plane. Our generic theoretical expression matches that which describes the retrograde precession of Earths' equinoxes. By making appropriate assumptions, we reduce our generic theoretical expression to those generated by others, or to those used by others, to describe retrograde precession in protostellar, protoplanetary, X-ray binary, non-magnetic CV DN, quasar, and black hole systems. We find that spinning, tilted CV DN systems cannot be described by a precessing ring or by a precessing rigid disk. We find that differential rotation and effects on the disk by the accretion stream must be addressed. Our analysis indicates that the best description of a retrogradely precessing spinning, tilted, CV DN accretion disk is a differentially rotating, tilted disk with an attached rotating, tilted ring located near the innermost disk annuli. In agreement with the observations and numerical simulations by others, we find that our numerically simulated CV DN accretion disks retrogradely precess as a unit. Our final, reduced expression for retrograde precession agrees well with our numerical simulation results and with selective observational systems that seem to have main-sequence secondaries. Our results suggest that a major source to retrograde precession is tidal torques like that by the Moon and the Sun on the Earth. In addition, these tidal torques should be common to a variety of systems where one member is spinning and tilted, regardless if

  9. Sub-coulomb transfer method of a nucleon for measure orbital radii

    International Nuclear Information System (INIS)

    Aguilera R, E.F.; Murillo, G.; Ramirez, J.; Avila, O.

    1986-04-01

    The neutron transfer method is revised to measure neutron orbital radii and possible interest systems to apply it are determined. Its were carried out DWBA preliminary calculations for the system 209 Bi(d,t) 208 Bi. (Author)

  10. Doppler tomography in fusion plasmas and astrophysics

    DEFF Research Database (Denmark)

    Salewski, Mirko; Geiger, B.; Heidbrink, W. W.

    2015-01-01

    Doppler tomography is a well-known method in astrophysics to image the accretion flow, often in the shape of thin discs, in compact binary stars. As accretion discs rotate, all emitted line radiation is Doppler-shifted. In fast-ion Dα (FIDA) spectroscopy measurements in magnetically confined plasma......, the Dα-photons are likewise Doppler-shifted ultimately due to gyration of the fast ions. In either case, spectra of Doppler-shifted line emission are sensitive to the velocity distribution of the emitters. Astrophysical Doppler tomography has lead to images of accretion discs of binaries revealing bright...... and limits, analogies and differences in astrophysical and fusion plasma Doppler tomography and what can be learned by comparison of these applications....

  11. Mechanoreceptors in Diseased Cervical Intervertebral Disc and Vertigo.

    Science.gov (United States)

    Yang, Liang; Yang, Cheng; Pang, Xiaodong; Li, Duanming; Yang, Hong; Zhang, Xinwu; Yang, Yi; Peng, Baogan

    2017-04-15

    We collected the samples of cervical intervertebral discs from patients with vertigo to examine the distribution and types of mechanoreceptors in diseased cervical disc. The aim of this study was to determine whether mechanoreceptors are distributed more abundantly in cervical discs from patients with cervical spondylosis, and whether they are related to vertigo. Previous limited studies have found that normal cervical intervertebral discs are supplied with mechanoreceptors that have been considered responsible for proprioceptive functions. Several clinical studies have indicated that the patients with cervical spondylosis manifested significantly impaired postural control and subjective balance disturbance. We collected 77 samples of cervical discs from 62 cervical spondylosis patients without vertigo, 61 samples from 54 patients with vertigo, and 40 control samples from 8 cadaveric donors to investigate distribution of mechanoreceptors containing neurofilament (NF200) and S-100 protein immunoreactive nerve endings. The immunohistochemical investigation revealed that the most frequently encountered mechanoreceptors were the Ruffini corpuscles in all groups of cervical disc samples. They were obviously increased in the number and deeply ingrown into inner annulus fibrosus and even into nucleus pulposus in the diseased cervical discs from patients with vertigo in comparison with the discs from patients without vertigo and control discs. Only three Golgi endings were seen in the three samples from patients with vertigo. No Pacinian corpuscles were found in any samples of cervical discs. The diseased cervical discs from patients with vertigo had more abundant distribution of Ruffini corpuscles than other discs. A positive association between the increased number and ingrowth of Ruffini corpuscles in the diseased cervical disc and the incidence of vertigo in the patients with cervical spondylosis was found, which may indicate a key role of Ruffini corpuscles in the

  12. Transitional millisecond pulsars in the low-level accretion state

    Science.gov (United States)

    Jaodard, Amruta D.; Hessels, Jason W. T.; Archibald, Anne; Bogdanov, Slavko; Deller, Adam; Hernandez Santisteban, Juan; Patruno, Alessandro; D'Angelo, Caroline; Bassa, Cees; Amruta Jaodand

    2018-01-01

    In the canonical pulsar recycling scenario, a slowly spinning neutron star can be rejuvenated to rapid spin rates by the transfer of angular momentum and mass from a binary companion star. Over the last decade, the discovery of three transitional millisecond pulsars (tMSPs) has allowed us to study recycling in detail. These systems transition between accretion-powered (X-ray) and rotation-powered (radio) pulsar states within just a few days, raising questions such as: what triggers the state transition, when does the recycling process truly end, and what will the radio pulsar’s final spin rate be? Systematic multi-wavelength campaigns over the last decade have provided critical insights: multi-year-long, low-level accretion states showing coherent X-ray pulsations; extremely stable, bi-modal X-ray light curves; outflows probed by radio continuum emission; a surprising gamma-ray brightening during accretion, etc. In my thesis I am trying to bring these clues together to understand the low-level accretion process that recycles a pulsar. For example, recently we timed PSR J1023+0038 in the accretion state and found it to be spinning down ~26% faster compared to the non-accreting radio pulsar state. We are currently conducting simultaneous multi-wavelength campaigns (XMM, HST, Kepler and VLA) to understand the global variability of the accretion flow, as well as high-energy Fermi-LAT observations to probe the gamma-ray emission mechanism. I will highlight these recent developments, while also presenting a broad overview of tMSPs as exciting new laboratories to test low-level accretion onto magnetized neutron stars.

  13. Are galaxy discs optically thick?

    International Nuclear Information System (INIS)

    Disney, Michael; Davies, Jonathan; Phillipps, Steven

    1989-01-01

    We re-examine the classical optical evidence for the low optical depths traditionally assigned to spiral discs and argue that it is highly model-dependent and unconvincing. In particular, layered models with a physically thin but optically thick dust layer behave like optically thin discs. The opposite hypotheses, that such discs are optically thick is then examined in the light of modern evidence. We find it to be consistent with the near-infrared and IRAS observations, with the surface brightnesses, with the HI and CO column densities and with the Hα measurements. (author)

  14. Vascular complications of prosthetic inter-vertebral discs

    OpenAIRE

    Daly, Kevin J.; Ross, E. Raymond S.; Norris, Heather; McCollum, Charles N.

    2006-01-01

    Five consecutive cases of prosthetic inter-vertebral disc displacement with severe vascular complications on revisional surgery are described. The objective of this case report is to warn spinal surgeons that major vascular complications are likely with anterior displacement of inter-vertebral discs. We have not been able to find a previous report on vascular complications associated with anterior displacement of prosthetic inter-vertebral discs. In all five patients the prosthetic disc had e...

  15. Accretion disk emission from a BL Lacertae object

    International Nuclear Information System (INIS)

    Wandel, A.; Urry, C.M.

    1991-01-01

    It is suggested here that the UV and X-ray emission of BL Lac objects may originate in an accretion disk. Using detailed calculations of accretion disk spectra, the best-measured ultraviolet and soft X-ray spectra of the BL Lac object PKS 2155-304 are fitted, and the mass and accretion rate required is determined. The ultraviolet through soft X-ray continuum is well fitted by the spectrum of an accretion disk, but near-Eddington accretion rates are required to produce the soft X-ray excess. A hot disk or corona could Comptonize soft photons from the cool disk and produce the observed power-law spectrum in the 1-10 keV range. The dynamic time scale in the disk regions that contribute most of the observed ultraviolet and soft X-ray photons are consistent with the respective time scales for intensity variations observed in these two wave bands; the mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard X-ray variability. 37 refs

  16. TEMPERATURE STRUCTURE OF PROTOPLANETARY DISKS UNDERGOING LAYERED ACCRETION

    International Nuclear Information System (INIS)

    Lesniak, M. V.; Desch, S. J.

    2011-01-01

    We calculate the temperature structures of protoplanetary disks (PPDs) around T Tauri stars heated by both incident starlight and viscous dissipation. We present a new algorithm for calculating the temperatures in disks in hydrostatic and radiative equilibrium, based on Rybicki's method for iteratively calculating the vertical temperature structure within an annulus. At each iteration, the method solves for the temperature at all locations simultaneously, and converges rapidly even at high (>>10 4 ) optical depth. The method retains the full frequency dependence of the radiation field. We use this algorithm to study for the first time disks evolving via the magnetorotational instability. Because PPD midplanes are weakly ionized, this instability operates preferentially in their surface layers, and disks will undergo layered accretion. We find that the midplane temperatures T mid are strongly affected by the column density Σ a of the active layers, even for fixed mass accretion rate M-dot . Models assuming uniform accretion predict midplane temperatures in the terrestrial planet forming region several x 10 2 K higher than our layered accretion models do. For M-dot -7 M sun yr -1 and the column densities Σ a -2 associated with layered accretion, disk temperatures are indistinguishable from those of a passively heated disk. We find emergent spectra are insensitive to Σ a , making it difficult to observationally identify disks undergoing layered versus uniform accretion.

  17. Examination of turbine discs from nuclear power plants

    International Nuclear Information System (INIS)

    Czajkowski, C.J.; Weeks, J.R.

    1982-01-01

    Investigations were performed on a cracked turbine disc from the Cooper Nuclear Power Station, and on two failed turbine discs (governor and generator ends) from the Yankee-Rowe Nuclear Power Station. Cooper is a boiling water reactor (BWR) which went into commercial operation in July 1974, and Yankee-Rowe is a pressurized water reactor (PWR) which went into commercial operation in June 1961. Cracks were identified in the bore of the Cooper disc after 41,913 hours of operation, and the disc removed for repair. At Yankee-Rowe two discs failed after 100,000 hours of operation. Samples of the Cooper disc and both Yankee-Rowe disc (one from the governor and one from the generator end of the LP turbine) were sent to Brookhaven National Laboratory (BNL) for failure analysis

  18. Radiatively Driven Winds from Effective Boundary Layer around ...

    Indian Academy of Sciences (India)

    R. Narasimhan (Krishtel eMaging) 1461 1996 Oct 15 13:05:22

    Abstract. Matter accreting onto black holes suffers a standing or oscil- lating shock wave in much of the parameter space. The post-shock region is hot, puffed up and reprocesses soft photons from a Keplerian disc to produce the characteristic hard tail of the spectrum of accretion discs. The post-shock torus is also the base ...

  19. Journal of Astrophysics and Astronomy | Indian Academy of Sciences

    Indian Academy of Sciences (India)

    2016-01-27

    Jan 27, 2016 ... Matter accreting onto black holes suffers a standing or oscillating shock wave in much of the parameter space. The post-shock region is hot, puffed up and reprocesses soft photons from a Keplerian disc to produce the characteristic hard tail of the spectrum of accretion discs. The post-shock torus is also the ...

  20. SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS

    International Nuclear Information System (INIS)

    Reiners, A.; Basri, G.; Christensen, U. R.

    2009-01-01

    We have measured the surface magnetic flux on four accreting young brown dwarfs and one nonaccreting young very low mass (VLM) star utilizing high-resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1-2 kG had been proposed for one of the brown dwarfs, Two Micron All Sky Survey (2MASS) J1207334-393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilogauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3σ upper limit for the magnetic flux in 2MASS J1207334-393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of 5 or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It is interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently has an easy physical explanation.

  1. Cyclotron Lines in Accreting Neutron Star Spectra

    Science.gov (United States)

    Wilms, Jörn; Schönherr, Gabriele; Schmid, Julia; Dauser, Thomas; Kreykenbohm, Ingo

    2009-05-01

    Cyclotron lines are formed through transitions of electrons between discrete Landau levels in the accretion columns of accreting neutron stars with strong (1012 G) magnetic fields. We summarize recent results on the formation of the spectral continuum of such systems, describe recent advances in the modeling of the lines based on a modification of the commonly used Monte Carlo approach, and discuss new results on the dependence of the measured cyclotron line energy from the luminosity of transient neutron star systems. Finally, we show that Simbol-X will be ideally suited to build and improve the observational database of accreting and strongly magnetized neutron stars.

  2. Thin accretion disk around regular black hole

    Directory of Open Access Journals (Sweden)

    QIU Tianqi

    2014-08-01

    Full Text Available The Penrose′s cosmic censorship conjecture says that naked singularities do not exist in nature.So,it seems reasonable to further conjecture that not even a singularity exists in nature.In this paper,a regular black hole without singularity is studied in detail,especially on its thin accretion disk,energy flux,radiation temperature and accretion efficiency.It is found that the interaction of regular black hole is stronger than that of the Schwarzschild black hole. Furthermore,the thin accretion will be more efficiency to lost energy while the mass of black hole decreased. These particular properties may be used to distinguish between black holes.

  3. Blazar Variability from Turbulence in Jets Launched by Magnetically Arrested Accretion Flows

    International Nuclear Information System (INIS)

    Riordan, Michael O’; Pe’er, Asaf; McKinney, Jonathan C.

    2017-01-01

    Blazars show variability on timescales ranging from minutes to years, the former being comparable to and in some cases even shorter than the light-crossing time of the central black hole. The observed γ -ray light curves can be described by a power-law power density spectrum (PDS), with a similar index for both BL Lacs and flat-spectrum radio quasars. We show that this variability can be produced by turbulence in relativistic jets launched by magnetically arrested accretion flows (MADs). We perform radiative transport calculations on the turbulent, highly magnetized jet launching region of a MAD with a rapidly rotating supermassive black hole. The resulting synchrotron and synchrotron self-Compton emission, originating from close to the black hole horizon, is highly variable. This variability is characterized by PDS, which is remarkably similar to the observed power-law spectrum at frequencies less than a few per day. Furthermore, turbulence in the jet launching region naturally produces fluctuations in the plasma on scales much smaller than the horizon radius. We speculate that similar turbulent processes, operating in the jet at large radii (and therefore a high bulk Lorentz factor), are responsible for blazar variability over many decades in frequency, including on minute timescales.

  4. Blazar Variability from Turbulence in Jets Launched by Magnetically Arrested Accretion Flows

    Energy Technology Data Exchange (ETDEWEB)

    Riordan, Michael O’; Pe’er, Asaf [Physics Department, University College Cork, Cork (Ireland); McKinney, Jonathan C., E-mail: michael_oriordan@umail.ucc.ie [Department of Physics and Joint Space-Science Institute, University of Maryland, College Park, MD 20742 (United States)

    2017-07-10

    Blazars show variability on timescales ranging from minutes to years, the former being comparable to and in some cases even shorter than the light-crossing time of the central black hole. The observed γ -ray light curves can be described by a power-law power density spectrum (PDS), with a similar index for both BL Lacs and flat-spectrum radio quasars. We show that this variability can be produced by turbulence in relativistic jets launched by magnetically arrested accretion flows (MADs). We perform radiative transport calculations on the turbulent, highly magnetized jet launching region of a MAD with a rapidly rotating supermassive black hole. The resulting synchrotron and synchrotron self-Compton emission, originating from close to the black hole horizon, is highly variable. This variability is characterized by PDS, which is remarkably similar to the observed power-law spectrum at frequencies less than a few per day. Furthermore, turbulence in the jet launching region naturally produces fluctuations in the plasma on scales much smaller than the horizon radius. We speculate that similar turbulent processes, operating in the jet at large radii (and therefore a high bulk Lorentz factor), are responsible for blazar variability over many decades in frequency, including on minute timescales.

  5. Spins, moments and radii of Cd isotopes

    International Nuclear Information System (INIS)

    Hammen, Michael

    2013-01-01

    , the electric quadrupole moments, spins and changes in mean square charge radii are extracted. The obtained data reveal among other features an extremely linear behaviour of the quadrupole moments of the I=11/2 - isomeric states and a parabolic development in differences in mean square nuclear charge radii between ground and isomeric state. The development of charge radii between the shell closures is smooth, exposes a regular odd-even staggering and can be described and interpreted in the model of Zamick and Thalmi.

  6. Spins, moments and radii of Cd isotopes

    Energy Technology Data Exchange (ETDEWEB)

    Hammen, Michael

    2013-10-30

    recorded and the magnetic dipole moments, the electric quadrupole moments, spins and changes in mean square charge radii are extracted. The obtained data reveal among other features an extremely linear behaviour of the quadrupole moments of the I=11/2{sup -} isomeric states and a parabolic development in differences in mean square nuclear charge radii between ground and isomeric state. The development of charge radii between the shell closures is smooth, exposes a regular odd-even staggering and can be described and interpreted in the model of Zamick and Thalmi.

  7. Analysis of rabbit intervertebral disc physiology based on water metabolism. II. Changes in normal intervertebral discs under axial vibratory load

    International Nuclear Information System (INIS)

    Hirano, N.; Tsuji, H.; Ohshima, H.; Kitano, S.; Itoh, T.; Sano, A.

    1988-01-01

    Metabolic changes induced by axial vibratory load to the spine were investigated based on water metabolism in normal intervertebral discs of rabbits with or without pentobarbital anesthesia. Tritiated water concentration in the intervertebral discs of unanesthetized rabbits was reduced remarkably by axial vibration for 30 minutes using the vibration machine developed for this study. Repeated vibratory load for 18 and 42 hours duration showed the recovery of 3 H 2 O concentration of the intervertebral disc without anesthesia. Computer simulation suggested a reduction of blood flow surrounding the intervertebral disc following the vibration stress. However, no reduction of the 3 H 2 O concentration in the intervertebral disc was noted under anesthesia. Emotional stress cannot be excluded as a factor in water metabolism in the intervertebral disc

  8. INJURIES IN DISC GOLF - A DESCRIPTIVE CROSS-SECTIONAL STUDY

    DEFF Research Database (Denmark)

    Rahbek, Martin Amadeus; Nielsen, Rasmus Oestergaard

    2016-01-01

    BACKGROUND: Disc golf is rapidly increasing in popularity and more than two million people are estimated to regularly participate in disc golf activities. Despite this popularity, the epidemiology of injuries in disc golf remains under reported. PURPOSE: The purpose of the present study...... was to investigate the prevalence and anatomic distribution of injuries acquired through disc-golf participation in Danish disc golf players. METHODS: The study was a cross-sectional study conducted on Danish disc-golf players. In May 2015, invitations to complete a web-based questionnaire were spread online via...... social media, and around disc-golf courses in Denmark. The questionnaire included questions regarding disc-golf participation and the characteristics of injuries acquired through disc golf participation. The data was analyzed descriptively. RESULTS: An injury prevalence of 13.3% (95% CI: 6.7% to 19...

  9. Multiwavelength diagnostics of accretion in an X-ray selected sample of CTTSs

    Science.gov (United States)

    Curran, R. L.; Argiroffi, C.; Sacco, G. G.; Orlando, S.; Peres, G.; Reale, F.; Maggio, A.

    2011-02-01

    Context. High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in classical T Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies. Aims: We aim to test the hypothesis that the high density soft X-ray emission originates from accretion by analysing, in a homogeneous manner, optical accretion indicators for an X-ray selected sample of CTTSs. Methods: We analyse optical spectra of the X-ray selected sample of CTTSs and calculate the accretion rates based on measuring the Hα, Hβ, Hγ, He ii 4686 Å, He i 5016 Å, He i 5876 Å, O i 6300 Å, and He i 6678 Å equivalent widths. In addition, we also calculate the accretion rates based on the full width at 10% maximum of the Hα line. The different optical tracers of accretion are compared and discussed. The derived accretion rates are then compared to the accretion rates derived from the X-ray spectroscopy. Results: We find that, for each CTTS in our sample, the different optical tracers predict mass-accretion rates that agree within the errors, albeit with a spread of ≈ 1 order of magnitude. Typically, mass-accretion rates derived from Hα and He i 5876 Å are larger than those derived from Hβ, Hγ, and O i. In addition, the Hα full width at 10%, whilst a good indicator of accretion, may not accurately measure the mass-accretion rate. When the optical mass-accretion rates are compared to the X-ray derived mass-accretion rates, we find that: a) the latter are always lower (but by varying amounts); b) the latter range within a factor of ≈ 2 around 2 × 10-10 M⊙ yr-1, despite the former spanning a range of ≈ 3 orders of magnitude. We suggest that the systematic underestimate of the X-ray derived mass-accretion rates could depend on the density distribution inside the accretion streams, where the densest part of the stream is

  10. VARIABLE ACCRETION OUTBURSTS IN PROTOSTELLAR EVOLUTION

    International Nuclear Information System (INIS)

    Bae, Jaehan; Hartmann, Lee; Zhu, Zhaohuan; Gammie, Charles

    2013-01-01

    We extend the one-dimensional, two-zone models of long-term protostellar disk evolution with infall of Zhu et al. to consider the potential effects of a finite viscosity in regions where the ionization is too low for the magnetorotational instability (MRI) to operate (the d ead zone ) . We find that the presence of a small but finite dead zone viscosity, as suggested by simulations of stratified disks with MRI-active outer layers, can trigger inside-out bursts of accretion, starting at or near the inner edge of the disk, instead of the previously found outside-in bursts with zero dead zone viscosity, which originate at a few AU in radius. These inside-out bursts of accretion bear a qualitative resemblance to the outburst behavior of one FU Ori object, V1515 Cyg, in contrast to the outside-in burst models, which more closely resemble the accretion events in FU Ori and V1057 Cyg. Our results suggest that the type and frequency of outbursts are potentially a probe of transport efficiency in the dead zone. Simulations must treat the inner disk regions, R ∼< 0.5 AU, to show the detailed time evolution of accretion outbursts in general and to observe the inside-out bursts in particular.

  11. VARIABLE ACCRETION OUTBURSTS IN PROTOSTELLAR EVOLUTION

    Energy Technology Data Exchange (ETDEWEB)

    Bae, Jaehan; Hartmann, Lee [Department of Astronomy, University of Michigan, 500 Church St., Ann Arbor, MI 48105 (United States); Zhu, Zhaohuan [Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Peyton Hall, Princeton, NJ 08544 (United States); Gammie, Charles, E-mail: jaehbae@umich.edu, E-mail: lhartm@umich.edu, E-mail: zhuzh@astro.princeton.edu, E-mail: gammie@illinois.edu [Department of Astronomy, University of Illinois Urbana-Champaign, 1002 W. Green St., Urbana, IL 61801 (United States)

    2013-02-20

    We extend the one-dimensional, two-zone models of long-term protostellar disk evolution with infall of Zhu et al. to consider the potential effects of a finite viscosity in regions where the ionization is too low for the magnetorotational instability (MRI) to operate (the {sup d}ead zone{sup )}. We find that the presence of a small but finite dead zone viscosity, as suggested by simulations of stratified disks with MRI-active outer layers, can trigger inside-out bursts of accretion, starting at or near the inner edge of the disk, instead of the previously found outside-in bursts with zero dead zone viscosity, which originate at a few AU in radius. These inside-out bursts of accretion bear a qualitative resemblance to the outburst behavior of one FU Ori object, V1515 Cyg, in contrast to the outside-in burst models, which more closely resemble the accretion events in FU Ori and V1057 Cyg. Our results suggest that the type and frequency of outbursts are potentially a probe of transport efficiency in the dead zone. Simulations must treat the inner disk regions, R {approx}< 0.5 AU, to show the detailed time evolution of accretion outbursts in general and to observe the inside-out bursts in particular.

  12. m-Accretive extensions of a sectorial operator

    Energy Technology Data Exchange (ETDEWEB)

    Arlinskii, Yu M; Popov, A B [East-Ukrainian National University, Lugansk (Ukraine)

    2013-08-31

    A description of all the maximal accretive extensions and their resolvents is given for a densely defined closed sectorial operator in terms of abstract boundary conditions. These results are applied to parametrize all the m-accretive extensions of a symmetric operator in a planar model of one-centre point interaction. Bibliography: 40 titles.

  13. Accretion disks in active galactic nuclei

    International Nuclear Information System (INIS)

    Shields, G.A.

    1989-01-01

    Active galactic nuclei (AGN) have taunted astrophysicists for a quarter century. How do these objects produce huge luminosities---in some cases, far outshining our galaxy---from a region perhaps no larger than the solar system? Accretion onto supermassive black holes has been widely considered the best buy in theories of AGN. Much work has gone into accretion disk theory, searches for black holes in galactic nuclei, and observational tests. These efforts have not proved the disk model, but there is progress. Evidence for black holes in the nuclei of nearby galaxies is provided by observations of stellar velocities, and radiation from the disk's hot surface may be observed in the ultraviolet (UV) and neighboring spectral bands. In the review, the author describe some of the recent work on accretion disks in AGN, with an emphasis on points of contact between theory and observation

  14. Hydrodynamic forces on two moving discs

    Directory of Open Access Journals (Sweden)

    Burton D.A.

    2004-01-01

    Full Text Available We give a detailed presentation of a flexible method for constructing explicit expressions of irrotational and incompressible fluid flows around two rigid circular moving discs. We also discuss how such expressions can be used to compute the fluid-induced forces and torques on the discs in terms of Killing drives. Conformal mapping techniques are used to identify a meromorphic function on an annular region in C with a flow around two circular discs by a Mobius transformation. First order poles in the annular region correspond to vortices outside of the two discs. Inflows are incorporated by putting a second order pole at the point in the annulus that corresponds to infinity.

  15. Observational diagnostics of accretion on young stars and brown dwarfs

    Science.gov (United States)

    Stelzer, Beate; Argiroffi, Costanza

    I present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of the emission, new insight obtained from XMM-Newton and Chandra observations has unveiled contributions from accretion and outflow processes to the X-ray emission from young stars. Their smaller siblings, the brown dwarfs, have been shown to undergo a T Tauri phase on the basis of optical/IR observations of disks and measurements of accretion rates. Most re-cently, first evidence was found for X-rays produced by accretion in a young brown dwarf, complementing the suspected analogy between stars and substellar objects.

  16. Hydrodynamic simulations of accretion disks in cataclysmic variables

    International Nuclear Information System (INIS)

    Hirose, Masahito; Osaki, Yoji

    1990-01-01

    The tidal effects of secondary stars on accretion disks in cataclysmic variables are studied by two-dimensional hydrodynamical simulations. The time evolution of an accretion disk under a constant mass supply rate from the secondary is followed until it reaches a quasi-steady state. We have examined various cases of different mass ratios of binary systems. It is found that the accretion disk settles into a steady state of an elongated disk fixed in the rotating frame of the binary in a binary system with comparable masses of component stars. On the other hand, in the case of a low-mass secondary, the accretion disk develops a non-axisymmetric (eccentric) structure and finally settles into a periodically oscillating state in which a non-axisymmetric eccentric disk rotates in the opposite direction to the orbital motion of the binary in the rotating frame of the binary. The period of oscillation is a few percent longer than the orbital period of the binary, and it offers a natural explanation for the ''superhump'' periodicity of SU UMa stars. Our results thus confirm basically those of Whitehurst (1988, AAA 45.064.032) who discovered the tidal instability of an accretion disk in the case of a low-mass secondary. We then discuss the cause of the tidal instability. It is shown that the tidal instability of accretion disks is caused by a parametric resonance between particle orbits and an orbiting secondary star with a 1:3 period ratio. (author)

  17. Effect of Interbody Fusion on the Remaining Discs of the Lumbar Spine in Subjects with Disc Degeneration.

    Science.gov (United States)

    Ryu, Robert; Techy, Fernando; Varadarajan, Ravikumar; Amirouche, Farid

    2016-02-01

    To study effects (stress loads) of lumbar fusion on the remaining segments (adjacent or not) of the lumbar spine in the setting of degenerated adjacent discs. A lumbar spine finite element model was built and validated. The full model of the lumbar spine was a parametric finite element model of segments L 1-5 . Numerous hypothetical combinations of one-level lumbar spine fusion and one-level disc degeneration were created. These models were subjected to 10 Nm flexion and extension moments and the stresses on the endplates and consequently on the intervertebral lumbar discs measured. These values were compared to the stresses on healthy lumbar spine discs under the same load and fusion scenarios. Increased stress at endplates was observed only in the settings of L4-5 fusion and L3-4 disc degeneration (8% stress elevation at L2,3 in flexion or extension, and 25% elevation at L3,4 in flexion only). All other combinations showed less endplate stress than did the control model. For fusion at L3-4 and degeneration at L4-5 , the stresses in the endplates at the adjacent level inferior to the fused disc decreased for both loading disc height reductions. Stresses in flexion decreased after fusion by 29.5% and 25.8% for degeneration I and II, respectively. Results for extension were similar. For fusion at L2-3 and degeneration at L4-5 , stresses in the endplates decreased more markedly at the degenerated (30%), than at the fused level (14%) in the presence of 25% disc height reduction and 10 Nm flexion, whereas in extension stresses decreased more at the fused (24.3%) than the degenerated level (5.86%). For fusion at L3-4 and degeneration at L2-3 , there were no increases in endplate stress in any scenario. For fusion at L4-5 and degeneration at L3-4 , progression of degeneration from I to II had a significant effect only in flexion. A dramatic increase in stress was noted in the endplates of the degenerated disc (L3-4 ) in flexion for degeneration II. Stresses are greater

  18. Solar neutrinos and solar accretion of interstellar matter

    International Nuclear Information System (INIS)

    Newman, M.J.; Talbot, R.J. Jr.

    1976-01-01

    It is argued that if the Hoyle-Lyttleton mass accretion rate applies (Proc. Camb. Phil. Soc., Math. Phys. Sci. 35: 405 (1939)) the accretion of interstellar matter by the Sun is sufficient to enhance the surface heavy element abundances. This will also apply to other solar-type stars. The enhancement may be sufficient to allow the construction of consistent solar models with an interior heavy element abundance significantly lower than the observed surface abundance. This state of affairs lowers the predicted solar neutrino flux. It has been suggested that a similar enhancement of surface abundances might occur due to accretion of 'planetesimals' left over after formation of the solar system, and both processes may occur, thereby increasing the effect. The simple accretion model of Hoyle and Lyttleton is discussed mathematically. A crucial question to be answered by future research, however, is whether or not accretion on to the solar surface actually occurs. One of the most obvious obstacles is the outward flowing solar wind, and this is discussed. It appears that the outward flow can be reversed to an inward flow for certain interstellar cloud densities. (U.K.)

  19. Variation of diffusivity with the cation radii in molten salts of ...

    Indian Academy of Sciences (India)

    Abstract. A molecular dynamics study of the dependence of diffusivity of the cation on ionic radii in molten. AgI is reported. ... potential) to carry out molecular dynamics simulations on α-AgI. ..... There is no clustering of these ions. The anionic.

  20. Analogue Hawking radiation from astrophysical black-hole accretion

    International Nuclear Information System (INIS)

    Das, Tapas K

    2004-01-01

    We show that spherical accretion onto astrophysical black holes can be considered as a natural example of an analogue system. We provide, for the first time, an exact analytical scheme for calculating the analogue Hawking temperature and surface gravity for general relativistic accretion onto astrophysical black holes. Our calculation may bridge the gap between the theory of transonic astrophysical accretion and the theory of analogue Hawking radiation. We show that the domination of the analogue Hawking temperature over the actual Hawking temperature may be a real astrophysical phenomenon, though observational tests of this fact will at best be difficult and at worst might prove to be impossible. We also discuss the possibilities of the emergence of analogue white holes around astrophysical black holes. Our calculation is general enough to accommodate accreting black holes with any mass

  1. Migration of accreting giant planets

    Science.gov (United States)

    Robert, C.; Crida, A.; Lega, E.; Méheut, H.

    2017-09-01

    Giant planets forming in protoplanetary disks migrate relative to their host star. By repelling the gas in their vicinity, they form gaps in the disk's structure. If they are effectively locked in their gap, it follows that their migration rate is governed by the accretion of the disk itself onto the star, in a so-called type II fashion. Recent results showed however that a locking mechanism was still lacking, and was required to understand how giant planets may survive their disk. We propose that planetary accretion may play this part, and help reach this slow migration regime.

  2. Accretion and jets from stellar-mass to supermassive black holes

    NARCIS (Netherlands)

    Connors, R.M.T.

    2017-01-01

    Accretion and jets occur in many astrophysical systems across a multitude of size and mass scales, and environments. As such, the study of accretion and jet physics has for decades been, and still remains, a hot topic in astrophysics. Accretion onto black holes has particular significance for many

  3. Wind accretion and formation of disk structures in symbiotic binary systems

    Science.gov (United States)

    de Val-Borro, M.; Karovska, M.; Sasselov, D. D.; Stone, J. M.

    2015-05-01

    We investigate gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion. We study the mass accretion and formation of an accretion disk around the secondary caused by the strong wind from the primary late-type component using global 2D and 3D hydrodynamic numerical simulations. In particular, the dependence of the mass accretion rate on the mass loss rate, wind temperature and orbital parameters of the system is considered. For a typical slow and massive wind from an evolved star the mass transfer through a focused wind results in rapid infall onto the secondary. A stream flow is created between the stars with accretion rates of a 2--10% percent of the mass loss from the primary. This mechanism could be an important method for explaining periodic modulations in the accretion rates for a broad range of interacting binary systems and fueling of a large population of X-ray binary systems. We test the plausibility of these accretion flows indicated by the simulations by comparing with observations of the symbiotic variable system CH Cyg.

  4. The formation of stars by gravitational collapse rather than competitive accretion

    Science.gov (United States)

    Krumholz, Mark R.; McKee, Christopher F.; Klein, Richard I.

    2005-11-01

    There are two dominant models of how stars form. Under gravitational collapse, star-forming molecular clumps, of typically hundreds to thousands of solar masses (Msolar), fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems. In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (~0.5Msolar), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump. Competitive accretion models interpret brown dwarfs and free-floating planets as protostars ejected from star-forming clumps before they have accreted much mass; key predictions of this model are that such objects should lack disks, have high velocity dispersions, form more frequently in denser clumps, and that the mean stellar mass should vary within the Galaxy. Here we derive the rate of competitive accretion as a function of the star-forming environment, based partly on simulation, and determine in what types of environments competitive accretion can occur. We show that no observed star-forming region can undergo significant competitive accretion, and that the simulations that show competitive accretion do so because the assumed properties differ from those determined by observation. Our result shows that stars form by gravitational collapse, and explains why observations have failed to confirm predictions of the competitive accretion model.

  5. Thermal structure of the accreting earth

    International Nuclear Information System (INIS)

    Turcotte, D.L.; Pflugrath, J.C.

    1985-01-01

    The energy associated with the accretion of the earth and the segregation of the core is more than sufficient to melt the entire earth. In order to understand the thermal evolution of the early earth it is necessary to study the relevant heat transfer mechanisms. In this paper we postulate the existence of a global magma ocean and carry out calculations of the heat flux through it in order to determine its depth. In the solid mantle heat is transferred by the upward migration of magma. This magma supplies the magma ocean. The increase in the mantle liquidus with depth (pressure) is the dominant effect influencing heat transfer through the magma ocean. We find that a magma ocean with a depth of the order of 20 km would have existed as the earth accreted. We conclude that the core segregated and an atmosphere was formed during accretion

  6. Brown dwarf accretion: Nonconventional star formation over very long timescales

    Directory of Open Access Journals (Sweden)

    Ćirković Milan M.

    2005-01-01

    Full Text Available We investigate the process of accretion of interstellar gas by the Galactic population of brown dwarfs over very long timescales typical for physical eschatology. In particular, we use the classical Hoyle-Lyttleton-Bondi accretion model to investigate the rate at which brown dwarfs collect enough additional mass to become red dwarfs, accretion-induced changes in the mass function of the low- mass objects, and the corresponding accretion heating of brown dwarfs. In addition, we show how we can make the definition of the final mass function for stellar objects more precise.

  7. Mechanical design criteria for intervertebral disc tissue engineering.

    Science.gov (United States)

    Nerurkar, Nandan L; Elliott, Dawn M; Mauck, Robert L

    2010-04-19

    Due to the inability of current clinical practices to restore function to degenerated intervertebral discs, the arena of disc tissue engineering has received substantial attention in recent years. Despite tremendous growth and progress in this field, translation to clinical implementation has been hindered by a lack of well-defined functional benchmarks. Because successful replacement of the disc is contingent upon replication of some or all of its complex mechanical behaviors, it is critically important that disc mechanics be well characterized in order to establish discrete functional goals for tissue engineering. In this review, the key functional signatures of the intervertebral disc are discussed and used to propose a series of native tissue benchmarks to guide the development of engineered replacement tissues. These benchmarks include measures of mechanical function under tensile, compressive, and shear deformations for the disc and its substructures. In some cases, important functional measures are identified that have yet to be measured in the native tissue. Ultimately, native tissue benchmark values are compared to measurements that have been made on engineered disc tissues, identifying where functional equivalence was achieved, and where there remain opportunities for advancement. Several excellent reviews exist regarding disc composition and structure, as well as recent tissue engineering strategies; therefore this review will remain focused on the functional aspects of disc tissue engineering. Copyright 2009 Elsevier Ltd. All rights reserved.

  8. On the mechanism of self gravitating Rossby interfacial waves in proto-stellar accretion discs

    Science.gov (United States)

    Yellin-Bergovoy, Ron; Heifetz, Eyal; Umurhan, Orkan M.

    2016-05-01

    The dynamical response of edge waves under the influence of self-gravity is examined in an idealised two-dimensional model of a proto-stellar disc, characterised in steady state as a rotating vertically infinite cylinder of fluid with constant density except for a single density interface at some radius ?. The fluid in basic state is prescribed to rotate with a Keplerian profile ? modified by some additional azimuthal sheared flow. A linear analysis shows that there are two azimuthally propagating edge waves, kin to the familiar Rossby waves and surface gravity waves in terrestrial studies, which move opposite to one another with respect to the local basic state rotation rate at the interface. Instability only occurs if the radial pressure gradient is opposite to that of the density jump (unstably stratified) where self-gravity acts as a wave stabiliser irrespective of the stratification of the system. The propagation properties of the waves are discussed in detail in the language of vorticity edge waves. The roles of both Boussinesq and non-Boussinesq effects upon the stability and propagation of these waves with and without the inclusion of self-gravity are then quantified. The dynamics involved with self-gravity non-Boussinesq effect is shown to be a source of vorticity production where there is a jump in the basic state density In addition, self-gravity also alters the dynamics via the radial main pressure gradient, which is a Boussinesq effect. Further applications of these mechanical insights are presented in the conclusion including the ways in which multiple density jumps or gaps may or may not be stable.

  9. Basic properties of a stationary accretion disk surrounding a black hole

    International Nuclear Information System (INIS)

    Hoshi, Reiun

    1977-01-01

    The structure of a stationary accretion disk surrounding a black hole is studied by means of newly developed basic equations. The basic equations are derived under the assumption that the vertical distribution of disk matter is given by a polytrope. For a Keplerian accretion disk, basic equations reduce to a differential equation of the first order. We have found that solutions of an optically thick accretion disk converge to a limiting value, irrespective of the outer boundary condition. This gives the happy consequence that the inner structure of an optically thick accretion disk is determined irrespective of the outer boundary condition. On the contrary, an optically thin accretion disk shows bimodal behavior, that is, two physically distinct states exist depending on the outer boundary condition imposed at the outer edge of the accretion disk. (auth.)

  10. Magnetospheres of accreting compact objects in binary systems

    International Nuclear Information System (INIS)

    Aly, J.J.

    1985-09-01

    Bright pulsating X-ray sources (X-ray pulsars, AM Her stars,...) have been identified as strongly magnetized compact objects accreting matter from a binary companion. We give here a summary of some of the work which has been recently done to try to understand the interaction between the magnetic field of the compact object and the matter around. We examine in turn the models describing the interaction of the field with: i) a spherically symmetric accretion flow; ii) a thin keplerian accretion disk; iii) the companion itself. In all these cases, we pay particular attention to the following problems: i) how the external plasma interacting with the magnetosphere can get mixed with the field; ii) by which mechanism the magnetic field controls the mass-momentum-energy exchanges between the two stars. In conclusion, we compare the magnetosphere of an accreting compact object with that one of a planet [fr

  11. The barium-to-iron enrichment versus age relation of ancient disc stars

    Science.gov (United States)

    Fuhrmann, K.; Chini, R.; Kaderhandt, L.; Chen, Z.; Lachaume, R.

    2017-11-01

    We report an intrinsically precise relation of the barium-to-iron enrichment as a function of age for a local, volume-complete (N = 30) sample of ancient Population II (τ ≥ 12 Gyr) and intermediate-disc stars (τ ≃ 10 Gyr), which suggests a common, r-process-dominated nucleosynthesis site for both elements in the early stages of the Milky Way. Deviants from this empirical relation are to a large extent identified as formerly known or new blue straggler stars. We report in particular the striking case of the Population II star HD 159062, whose barium overabundance is difficult to explain without wind accretion of s-process material from a former asymptotic giant branch (AGB) primary that very likely survived as a white dwarf companion. The weak but significant barium enhancement that we measure for HR 3578 and 104 Tau also suggests that both may be accompanied by faint degenerate companions. If confirmed through precision astrometry or direct imaging observations, this would mean a very efficient method to uncover ancient stellar remnant companions around solar-type stars.

  12. Locally prepared antibiotic sensitivity discs: a substitute for imported ...

    African Journals Online (AJOL)

    Zones of inhibition were compared with those obtained from commercial antibiotic discs. Results obtained showed that discs prepared locally from antibiotic tablets, performed comparably with commercially obtained discs. There was no significant statistical difference between the two tested discs. We therefore recommend ...

  13. Imaging characteristics of noncontained migrating disc fragment and cyst

    International Nuclear Information System (INIS)

    Eerens, I.; Demaerel, P.; Haven, F.; Wilms, G.; Loon, J. van; Calenbergh, F. van

    2001-01-01

    The purpose of this article is to review less common presentations of degenerative disc disease on MR imaging. The images of eight patients were retrospectively analyzed. Six of them had transligamentous (or noncontained) disc herniations, the fragments of which were located in the posterior epidural space in three of them. One patient had a transdural disc fragment and one patient had a disc cyst. The cyst was located in the ventrolateral epidural space. On T2-weighted images, the migrated disc fragment returned a higher signal than the disc of origin in 6 of 7 patients. The disc cyst returned a signal similar to that of cerebrospinal fluid. The MR appearances of disc fragments can be puzzling, particularly if they are located in the posterior epidural space. It is important to recognize the abnormalities in order to differentiate them from less common lesions such as hematoma, abscess and neurinoma. (orig.)

  14. Imaging characteristics of noncontained migrating disc fragment and cyst

    Energy Technology Data Exchange (ETDEWEB)

    Eerens, I.; Demaerel, P.; Haven, F.; Wilms, G. [Dept. of Radiology, University Hospitals, Leuven (Belgium); Loon, J. van; Calenbergh, F. van [Dept. of Neurosurgery, University Hospitals, Leuven (Belgium)

    2001-05-01

    The purpose of this article is to review less common presentations of degenerative disc disease on MR imaging. The images of eight patients were retrospectively analyzed. Six of them had transligamentous (or noncontained) disc herniations, the fragments of which were located in the posterior epidural space in three of them. One patient had a transdural disc fragment and one patient had a disc cyst. The cyst was located in the ventrolateral epidural space. On T2-weighted images, the migrated disc fragment returned a higher signal than the disc of origin in 6 of 7 patients. The disc cyst returned a signal similar to that of cerebrospinal fluid. The MR appearances of disc fragments can be puzzling, particularly if they are located in the posterior epidural space. It is important to recognize the abnormalities in order to differentiate them from less common lesions such as hematoma, abscess and neurinoma. (orig.)

  15. Relationship of condylar position to disc position and morphology

    Energy Technology Data Exchange (ETDEWEB)

    Incesu, L.; Taskaya-Yilmaz, N. E-mail: nergizy@omu.edu.tr; Oeguetcen-Toller, M.; Uzun, E

    2004-09-01

    Introduction/objective: The purpose of this study was to assess whether condylar position, as depicted by magnetic resonance imaging, was an indicator of disc morphology and position. Methods and material: One hundred and twenty two TMJs of 61 patients with temporomandibular joint disorder were examined. Condylar position, disc deformity and degree of anterior disc displacement were evaluated by using magnetic resonance imaging. Results and discussion: Posterior condyle position was found to be the main feature of temporomandibular joints with slight and moderate anterior disc displacement. No statistical significance was found between the condylar position, and reducing and nonreducing disc positions. On the other hand, superior disc position was found to be statistically significant for centric condylar position. Conclusion: It was concluded that posterior condyle position could indicate anterior disc displacement whereas there was no relation between the position of condyle and the disc deformity.

  16. Gas-rich dwarfs and accretion phenomena in early-type galaxies

    Science.gov (United States)

    Silk, J.; Norman, C.

    1979-01-01

    An analysis is presented of the combined effects of cloud accretion and galactic winds and coronae. An accretion model is developed wherein gas-rich dwarf galaxies are accreted into galactic halos, which provides an adequate source of H I to account for observations of neutral gas in early-type galaxies. Accretion is found to fuel the wind, thereby regulating the accretion flow and yielding a time-dependent model for star formation, enrichment, and nuclear activity. The permissible parameter range for intergalactic gas clouds and galaxy groups is discussed, along with the frequency of gas-rich dwarfs and their large ratios of gas mass to luminosity. Also considered is the occurrence of gas stripping and the consequent formation of dwarf spheroidal systems that remain in the halo, and gas clouds that dissipate and suffer further infall. A cosmological implication of the model is that, because the characteristic time scale of a gas-rich dwarf galaxy to be accreted and lose its gas is comparable to a Hubble time, there may have been a far more extensive primordial distribution of such systems at earlier epochs.

  17. Thoracic spine disc-related abnormalities: longitudinal MR imaging assessment

    Energy Technology Data Exchange (ETDEWEB)

    Girard, Charles J.; Schweitzer, Mark E.; Morrison, William B.; Parellada, Joan A. [TJUH Radiology, Philadelphia, Pennsylvania (United States); Carrino, J.A. [Department of Radiology ASB-1, Harvard Medical School, Brigham and Women' s Hospital, L1, Room 002B, 75 Francis Street, MA 02115, Boston (United States)

    2004-04-01

    To describe and characterize the temporal changes in disc-related disorders of the thoracic spine using MR imaging. A retrospective longitudinal cohort study was carried out of 40 patients with two sequential thoracic spine MR images at variable intervals. The images were assessed for baseline presence of, new incidence of and changes in disc herniation, degenerative disc disease, endplate marrow signal alteration and Schmorl nodes. The range of follow-up was 4-149 weeks. Baseline presence was: disc herniation, 10% (49/480); degenerative disc disease, 14% (66/480); endplate marrow signal alteration, 2.3% (11/480); Schmorl nodes 9.6% (46/480). Most pre-existing lesions tended to remain unchanged. Herniations showed the most change, tending to improve in 27%. New incidence was: disc herniation, 1.5% (7/480), degenerative disc disease, 2% (10/480); endplate marrow signal alteration, 1.6% (8/480); Schmorl nodes, 2.1% (10/480). Disc degeneration was first visible at an 11-week interval and once established almost never changed over many weeks to months. Endplate signal alterations (Modic changes) were uncommon. Schmorl nodes show no change from baseline for up to 2 1/2 years. All findings predominated in the lower intervertebral levels from T6 to T10. The most prevalent thoracic spine disc-related findings are degeneration and herniation. Disc herniations predominate in the lower segments and are a dynamic phenomenon. Disc degeneration can be rapidly evolving but tends to remain unchanged after occurrence. Endplate marrow signal changes were an uncommon manifestation of thoracic disc disease. Schmorl nodes showed the least change over time. (orig.)

  18. Use NASA GES DISC Data in ArcGIS

    Science.gov (United States)

    Yang, Wenli; Pham, Long B.; Kempler, Steve

    2015-01-01

    This presentation describes GIS relevant data at NASA Goddard Earth Sciences Data and Information Services Center (GES DISC), GES DISC Services and Support for GIS Users, and use cases of GES DISC data in ArcGIS.

  19. Radiatively-suppressed spherical accretion under relativistic radiative transfer

    Science.gov (United States)

    Fukue, Jun

    2018-03-01

    We numerically examine radiatively-suppressed relativistic spherical accretion flows on to a central object with mass M under Newtonian gravity and special relativity. We simultaneously solve both the relativistic radiative transfer equation and the relativistic hydrodynamical equations for spherically symmetric flows under the double iteration process in the case of the intermediate optical depth. We find that the accretion flow is suppressed, compared with the freefall case in the nonrelativistic regime. For example, in the case of accretion on to a luminous core with accretion luminosity L*, the freefall velocity v normalized by the speed of light c under the radiative force in the nonrelativistic regime is β (\\hat{r}) = v/c = -√{(1-Γ _*)/(\\hat{r}+1-Γ _*)}, where Γ* (≡ L*/LE, LE being the Eddington luminosity) is the Eddington parameter and \\hat{r} (= r/rS, rS being the Schwarzschild radius) the normalized radius, whereas the infall speed at the central core is ˜0.7β(1), irrespective of the mass-accretion rate. This is due to the relativistic effect; the comoving flux is enhanced by the advective flux. We briefly examine and discuss an isothermal case, where the emission takes place in the entire space.

  20. Constraining jet physics in weakly accreting black holes

    Science.gov (United States)

    Markoff, Sera

    2007-04-01

    Outflowing jets are observed in a variety of astronomical objects such as accreting compact objects from X-ray binaries (XRBs) to active galactic nuclei (AGN), as well as at stellar birth and death. Yet we still do not know exactly what they are comprised of, why and how they form, or their exact relationship with the accretion flow. In this talk I will focus on jets in black hole systems, which provide the ideal test population for studying the relationship between inflow and outflow over an extreme range in mass and accretion rate. I will present several recent results from coordinated multi-wavelength studies of low-luminosity sources. These results not only support similar trends in weakly accreting black hole behavior across the mass scale, but also suggest that the same underlying physical model can explain their broadband spectra. I will discuss how comparisons between small- and large-scale systems are revealing new information about the regions nearest the black hole, providing clues about the creation of these weakest of jets. Furthermore, comparisons between our Galactic center nucleus Sgr A* and other sources at slightly higher accretion rates can illucidate the processes which drive central activity, and pave the way for new tests with upcoming instruments.

  1. Senescent intervertebral disc cells exhibit perturbed matrix homeostasis phenotype.

    Science.gov (United States)

    Ngo, Kevin; Patil, Prashanti; McGowan, Sara J; Niedernhofer, Laura J; Robbins, Paul D; Kang, James; Sowa, Gwendolyn; Vo, Nam

    2017-09-01

    Aging greatly increases the risk for intervertebral disc degeneration (IDD) as a result of proteoglycan loss due to reduced synthesis and enhanced degradation of the disc matrix proteoglycan (PG). How disc matrix PG homeostasis becomes perturbed with age is not known. The goal of this study is to determine whether cellular senescence is a source of this perturbation. We demonstrated that disc cellular senescence is dramatically increased in the DNA repair-deficient Ercc1 -/Δ mouse model of human progeria. In these accelerated aging mice, increased disc cellular senescence is closely associated with the rapid loss of disc PG. We also directly examine PG homeostasis in oxidative damage-induced senescent human cells using an in vitro cell culture model system. Senescence of human disc cells treated with hydrogen peroxide was confirmed by growth arrest, senescence-associated β-galactosidase activity, γH2AX foci, and acquisition of senescence-associated secretory phenotype. Senescent human disc cells also exhibited perturbed matrix PG homeostasis as evidenced by their decreased capacity to synthesize new matrix PG and enhanced degradation of aggrecan, a major matrix PG. of the disc. Our in vivo and in vitro findings altogether suggest that disc cellular senescence is an important driver of PG matrix homeostatic perturbation and PG loss. Published by Elsevier B.V.

  2. Photometric behaviour and the orbital period of the system HDE 245770=A 0535+26

    International Nuclear Information System (INIS)

    Gnedin, Yu.N.; Zajtseva, G.V.; Larionov, V.M.; Lyutyj, V.M.; Khozov, G.V.; Sheffer, E.K.

    1988-01-01

    The results of photometric observations (1981-1985) are given for the transient X-ray source A 0535+26, whose optical component is the Be star HDE 245770. An analysis of the photometric data enabled us to detect radiation of a disc of accreting matter that is gradually accumulated, precipitating afterwards on to the neutron star surface. The latter leads to an X-ray flare. Physical parameters of the accreting accumulating disc are determined. The characteristic timescale of formation of such a disc is likely to be ∼ 1000 days

  3. Highly Accreting Quasars at High Redshift

    Science.gov (United States)

    Martínez-Aldama, Mary L.; Del Olmo, Ascensión; Marziani, Paola; Sulentic, Jack W.; Negrete, C. Alenka; Dultzin, Deborah; Perea, Jaime; D'Onofrio, Mauro

    2017-12-01

    We present preliminary results of a spectroscopic analysis for a sample of type 1 highly accreting quasars (LLedd>0.2) at high redshift, z 2-3. The quasars were observed with the OSIRIS spectrograph on the GTC 10.4 m telescope located at the Observatorio del Roque de los Muchachos in La Palma. The highly accreting quasars were identified using the 4D Eigenvector 1 formalism, which is able to organize type 1 quasars over a broad range of redshift and luminosity. The kinematic and physical properties of the broad line region have been derived by fitting the profiles of strong UV emission lines such as AlIII, SiIII and CIII. The majority of our sources show strong blueshifts in the high-ionization lines and high Eddington ratios which are related with the productions of outflows. The importance of highly accreting quasars goes beyond a detailed understanding of their physics: their extreme Eddington ratio makes them candidates standard candles for cosmological studies.

  4. Magnetohydrodynamic Simulations of Black Hole Accretion

    Science.gov (United States)

    Avara, Mark J.

    Black holes embody one of the few, simple, solutions to the Einstein field equations that describe our modern understanding of gravitation. In isolation they are small, dark, and elusive. However, when a gas cloud or star wanders too close, they light up our universe in a way no other cosmic object can. The processes of magnetohydrodynamics which describe the accretion inflow and outflows of plasma around black holes are highly coupled and nonlinear and so require numerical experiments for elucidation. These processes are at the heart of astrophysics since black holes, once they somehow reach super-massive status, influence the evolution of the largest structures in the universe. It has been my goal, with the body of work comprising this thesis, to explore the ways in which the influence of black holes on their surroundings differs from the predictions of standard accretion models. I have especially focused on how magnetization of the greater black hole environment can impact accretion systems.

  5. Intervertebral Disc Characteristic on Progressive Neurological Deficit

    Directory of Open Access Journals (Sweden)

    Farid Yudoyono

    2017-09-01

    Full Text Available Objective: To examine the intervertebral disc characteristic on magnetic resonance imaging (MRI in lumbar herniated disc (LHD patients with progressive neurological deficit. Methods: Patients were collected retrospectively from Dr. Hasan Sadikin General Hospital Database from 2011–2013 with LHD, had neurological deficit such as radiculopathy and cauda equine syndrome for less than four weeks with a positive sign confirmed by neurological examination and confirmatory with MRI examination. Results: A total of 14 patients with lumbar herniated disc disease (10 males, 4 females suffered from progressive neurological deficit with an average age of (52.07±10.9 years old. Early disc height was 9.38±0.5 mm and progressive neurological deficit state disc height was 4.03±0.53 mm, which were significantly different statisticaly (p<0.01. Symptoms of radiculopathy were seen in 11 patients and cauda equine syndrome in three patients. Modic changes grade 1 was found in five patients, grade 2 in eight patients,grade 3 in one patient, Pfirmman grade 2 in eleven patients and grade 3 in three patients. Thecal sac compression 1/3 compression was seen in four patients and 2/3 compression in ten patients. Conclusions: Neurosurgeon should raise concerns on the characteristic changes of intervertebral disc in magnetic resonance imaging examination to avoid further neural injury in lumbar herniated disc patients.

  6. FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

    International Nuclear Information System (INIS)

    Kirk, Helen; Myers, Philip C.; Bourke, Tyler L.; Gutermuth, Robert A.; Wilson, Grant W.; Hedden, Abigail

    2013-01-01

    One puzzle in understanding how stars form in clusters is the source of mass—is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of ∼30 M ☉ Myr –1 (inferred from the N 2 H + velocity gradient along the filament), and radially contracting onto the filament at ∼130 M ☉ Myr –1 (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

  7. Theories of magnetospheres around accreting compact objects

    International Nuclear Information System (INIS)

    Vasyliunas, V.M.

    1979-01-01

    A wide class of galactic X-ray sources are believed to be binary systems where mass is flowing from a normal star to a companion that is a compact object, such as a neutron star. The strong magnetic fields of the compact object create a magnetosphere around it. We review the theoretical models developed to describe the properties of magnetospheres in such accreting binary systems. The size of the magnetosphere can be estimated from pressure balance arguments and is found to be small compared to the over-all size of the accretion region but large compared object if the latter is a neutron star. In the early models the magnetosphere was assumed to have open funnels in the polar regions, through which accreting plasma could pour in. Later, magnetically closed models were developed, with plasma entry made possible by instabilities at the magnetosphere boundary. The theory of plasma flow inside the magnetosphere has been formulated in analogy to a stellar wind with reversed flow; a complicating factor is the instability of the Alfven critical point for inflow. In the case of accretion via a well-defined disk, new problems if magnetospheric structure appear, in particular the question to what extent and by what process the magnetic fields from the compact object can penetrate into the acretion disk. Since the X-ray emission is powered by the gravitational energy released in the accretion process, mass transfer into the magnetosphere is of fundamental importance; the various proposed mechanisms are critically examined. (orig.)

  8. Hot accreting white dwarfs in the quasi-static approximation

    International Nuclear Information System (INIS)

    Iben, I. Jr.

    1982-01-01

    Properties of white dwarfs which are accreting hydrogen-rich matter at rates in the range 1.5 x 10 -9 to 2.5 x 10 -7 M/sub sun/ yr -1 are investigated in several approximations. Steady-burning models, in which matter is processed through nuclear-burning shells as rapidly as it is accreted, provide a framework for understanding the properties of models in which thermal pulses induced by hydrogen burning and helium burning are allowed to occur. In these latter models, the underlying carbon-oxygen core is chosen to be in a cycle-averaged steady state with regard to compressional heating and neutrino losses. Several of these models are evolved in the quasi-static approximation. Combining results obtained in the steady-burning approximation with those obtained in the quasi-static approximation, expressions are obtained for estimating, as functions of accretion rate and white dwarf mass, the thermal pulse recurrence period and the duration of hydrogen-burning phases. The time spent by an accreting model burning hydrogen as a large star of giant dimensions versus time spent burning hydrogen as a hot dwarf is also estimated as a function of model mass and accretion rate. Finally, suggestions for detecting observational counterparts of the theoretical models and suggestions for further theoretical investigations are offered. Subject headings: stars: accretion: stars: interiors: stars: novae: stars: symbiotic: stars: white dwarfs

  9. Only marginal alignment of disc galaxies

    Science.gov (United States)

    Andrae, René; Jahnke, Knud

    2011-12-01

    Testing theories of angular-momentum acquisition of rotationally supported disc galaxies is the key to understanding the formation of this type of galaxies. The tidal-torque theory aims to explain this acquisition process in a cosmological framework and predicts positive autocorrelations of angular-momentum orientation and spiral-arm handedness, i.e. alignment of disc galaxies, on short distance scales of 1 Mpc h-1. This disc alignment can also cause systematic effects in weak-lensing measurements. Previous observations claimed discovering these correlations but are overly optimistic in the reported level of statistical significance of the detections. Errors in redshift, ellipticity and morphological classifications were not taken into account, although they have a significant impact. We explain how to rigorously propagate all the important errors through the estimation process. Analysing disc galaxies in the Sloan Digital Sky Survey (SDSS) data base, we find that positive autocorrelations of spiral-arm handedness and angular-momentum orientations on distance scales of 1 Mpc h-1 are plausible but not statistically significant. Current data appear not good enough to constrain parameters of theory. This result agrees with a simple hypothesis test in the Local Group, where we also find no evidence for disc alignment. Moreover, we demonstrate that ellipticity estimates based on second moments are strongly biased by galactic bulges even for Scd galaxies, thereby corrupting correlation estimates and overestimating the impact of disc alignment on weak-lensing studies. Finally, we discuss the potential of future sky surveys. We argue that photometric redshifts have too large errors, i.e. PanSTARRS and LSST cannot be used. Conversely, the EUCLID project will not cover the relevant redshift regime. We also discuss the potentials and problems of front-edge classifications of galaxy discs in order to improve the autocorrelation estimates of angular-momentum orientation.

  10. CO{sub 2} ICE TOWARD LOW-LUMINOSITY EMBEDDED PROTOSTARS: EVIDENCE FOR EPISODIC MASS ACCRETION VIA CHEMICAL HISTORY

    Energy Technology Data Exchange (ETDEWEB)

    Kim, Hyo Jeong; Evans, Neal J. II [Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400 Austin, TX 78712-1205 (United States); Dunham, Michael M. [Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520 (United States); Lee, Jeong-Eun [Department of Astronomy and Space Science, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701 (Korea, Republic of); Pontoppidan, Klaus M., E-mail: hyojeong@astro.as.utexas.edu [Space Telescope Science Institute, Baltimore, MD 21218 (United States)

    2012-10-10

    We present Spitzer IRS spectroscopy of CO{sub 2} ice bending mode spectra at 15.2 {mu}m toward 19 young stellar objects (YSOs) with luminosity lower than 1 L{sub Sun} (3 with luminosity lower than 0.1 L{sub Sun }). Ice on dust grain surfaces can encode the history of heating because pure CO{sub 2} ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1L{sub Sun} do not provide the conditions needed to produce pure CO{sub 2} ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO{sub 2} ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO{sub 2} component, traced by the presence of a characteristic band splitting in the 15.2 {mu}m bending mode. About half of the sources (9 out of 19) in the low-luminosity sample have evidence for pure CO{sub 2} ice, and 6 of these have significant double-peaked features, which are very strong evidence of pure CO{sub 2} ice. The presence of the pure CO{sub 2} ice component indicates that the dust temperature, and hence luminosity of the central star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO{sub 2} ice is converted to pure CO{sub 2} ice during each high-luminosity phase, explains the presence of pure CO{sub 2} ice, the total amount of CO{sub 2} ice, and the observed residual C{sup 18}O gas.

  11. Theory of quasi-spherical accretion in X-ray pulsars

    Science.gov (United States)

    Shakura, N.; Postnov, K.; Kochetkova, A.; Hjalmarsdotter, L.

    2012-02-01

    A theoretical model for quasi-spherical subsonic accretion on to slowly rotating magnetized neutron stars is constructed. In this model, the accreting matter subsonically settles down on to the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere. The settling regime of accretion can be realized for moderate accretion rates ? g s-1. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of the spin-up/spin-down rates (the angular rotation frequency derivative ?, and ? near the torque reversal) of X-ray pulsars with known orbital periods, it is possible to determine the main dimensionless parameters of the model, as well as to estimate the magnetic field of the neutron star. We illustrate the model by determining these parameters for three wind-fed X-ray pulsars GX 301-2, Vela X-1 and GX 1+4. The model explains both the spin-up/spin-down of the pulsar frequency on large time-scales and the irregular short-term frequency fluctuations, which can correlate or anticorrelate with the X-ray flux fluctuations in different systems. It is shown that in real pulsars an almost iso-angular-momentum rotation law with ω˜ 1/R2, due to strongly anisotropic radial turbulent motions sustained by large-scale convection, is preferred.

  12. Formation of primordial supermassive stars by rapid mass accretion

    Energy Technology Data Exchange (ETDEWEB)

    Hosokawa, Takashi; Yoshida, Naoki [Department of Physics and Research Center for the Early Universe, The University of Tokyo, Tokyo 113-0033 (Japan); Yorke, Harold W. [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Inayoshi, Kohei; Omukai, Kazuyuki, E-mail: takashi.hosokawa@phys.s.u-tokyo.ac.jp, E-mail: hosokwtk@gmail.com [Department of Physics, Kyoto University, Kyoto 606-8502 (Japan)

    2013-12-01

    Supermassive stars (SMSs) forming via very rapid mass accretion ( M-dot {sub ∗}≳0.1 M{sub ⊙} yr{sup −1}) could be precursors of supermassive black holes observed beyond a redshift of about six. Extending our previous work, here we study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10{sup 4–5} M {sub ☉}. Our stellar evolution calculations show that a star becomes supermassive while passing through the 'supergiant protostar' stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass until ≅ 100 AU for M {sub *} ≳ 10{sup 4} M {sub ☉}, after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 10{sup 4} K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M {sub *} ≳ 10{sup 5} M {sub ☉} can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10{sup 5} M {sub ☉}. An extremely massive black hole should form after the collapse of the inner core.

  13. MN Lup: X-RAYS FROM A WEAKLY ACCRETING T TAURI STAR

    International Nuclear Information System (INIS)

    Günther, H. M.; Wolk, S. J.; Wolter, U.; Robrade, J.

    2013-01-01

    Young T Tauri stars (TTS) are surrounded by an accretion disk, which over time disperses due to photoevaporation, accretion, and possibly planet formation. The accretion shock on the central star produces an UV/optical veiling continuum, line emission, and X-ray signatures. As the accretion rate decreases, the impact on the central star must change. In this article we study MN Lup, a young star where no indications of a disk are seen in IR observations. We present XMM-Newton and VLT/UVES observations, some of them taken simultaneously. The X-ray data show that MN Lup is an active star with L X /L bol close to the saturation limit. However, we find high densities (n e > 3 × 10 10 cm –3 ) in the X-ray grating spectrum. This can be well fitted using an accretion shock model with an accretion rate of 2 × 10 –11 M ☉ yr –1 . Despite the simple Hα line profile which has a broad component, but no absorption signatures as typically seen on accreting TTS, we find rotational modulation in Ca II K and in photospheric absorption lines. These line profile modulations do not clearly indicate the presence of a localized hot accretion spot on the star. In the Hα line we see a prominence in absorption about 2R * above the stellar surface—the first of its kind on a TTS. MN Lup is also the only TTS where accretion is seen, but no dust disk is detected that could fuel it. We suggest that MN Lup presents a unique and short-lived state in the disk evolution. It may have lost its dust disk only recently and is now accreting the remaining gas at a very low rate.

  14. Evaluation of bone and disc configuration in TMJ internal derangement

    International Nuclear Information System (INIS)

    Park, Cheol Woo; Hwang, Eui Hwan; Lee, Sang Rae

    2001-01-01

    To investigate bone and disc configuration on MR images in internal derangement related to age. MR images of 150 TMJs in 107 patients were analyzed to determine the morphologic changes. Two groups were distinguished to be correlated with age. Group 1 consisted of TMJs that were diagnosed as having anterior disc displacement with reduction (ADDwR), and Group 2 consisted of TMJs that were diagnosed as having anterior disc displacement without reduction (ADDwR). We assessed the configuration of the articular disc, degree of anterior disc displacement, and osseous changes of TMJs. The third decade (83 of 150 joints) was most frequent in this study. In the ADDwR group biconcave disc was most frequent at all ages except fifth decade, but in the ADDwoR group deformed discs was most frequent at third and forth decades. In the ADDwR group slightly displaced discs was most frequent at all ages, but in the ADDwoR group severely displaced discs was most frequent at second decade, and the degree of disc displacement was increased with aging over 30 years of age. TM joints showed osseous changes in 17% of the ADDwR group, and in 30% of the ADDwoR group. MR findings of osseous changes of the TMJ were not found to be significantly correlated with age. The prevalence of deformation of disc, displacement of disc, and osseous changes of TMJ was higher in the ADDwoR group than in the ADDwR group. MR findings of disc configuration and degree of disc displacement were found to be correlated with age

  15. A computational study of intervertebral disc degeneration in relation to changes in regional tissue composition and disc nutrition

    OpenAIRE

    Ruiz Wills, Carlos

    2015-01-01

    Up to 85% of the world population suffers from low back pain, a clinical condition often related to the intervertebral disc (IVD) degeneration (DD). Altered disc cell nutrition affects cell viability and can generate catabolic cascades that degrade the extracellular matrix (ECM). Also, a major degenerative biochemical change in the disc is the proteoglycan (PG) loss, which affects the osmotic pressure and hydration that is critical for cell nutrition. However, the relationship between biochem...

  16. Schrödinger evolution of self-gravitating discs

    Science.gov (United States)

    Batygin, Konstantin

    2018-04-01

    An understanding of the long-term evolution of self-gravitating discs ranks among the classic outstanding problems of astrophysics. In this work, we show that the secular inclination dynamics of a geometrically thin quasi-Keplerian disc, with a surface density profile that scales as the inverse square-root of the orbital radius, are described by the time-dependent Schrödinger equation. Within the context of this formalism, nodal bending waves correspond to the eigenmodes of a quasi-particle's wavefunction, confined in an infinite square well with boundaries given by the radial extent of the disc. We further show that external secular perturbations upon self-gravitating discs exhibit a mathematical similarity to quantum scattering theory. Employing this framework, we derive an analytic criterion for the gravitational rigidity of a nearly-Keplerian disc under external perturbations. Applications of the theory to circumstellar discs and Galactic nuclei are discussed.

  17. Gas Modelling in the Disc of HD 163296

    Science.gov (United States)

    Tilling, I.; Woitke, P.; Meeus, G.; Mora, A.; Montesinos, B.; Riviere-Marichalar, P.; Eiroa, C.; Thi, W. -F.; Isella, A.; Roberge, A.; hide

    2011-01-01

    We present detailed model fits to observations of the disc around the Herbig Ae star HD 163296. This well-studied object has an age of approx. 4Myr, with evidence of a circumstellar disc extending out to approx. 540AU. We use the radiation thermo-chemical disc code ProDiMo to model the gas and dust in the circumstellar disc of HD 163296, and attempt to determine the disc properties by fitting to observational line and continuum data. These include new Herschel/PACS observations obtained as part of the open-time key program GASPS (Gas in Protoplanetary Systems), consisting of a detection of the [Oi] 63 m line and upper limits for several other far infrared lines. We complement this with continuum data and ground-based observations of the CO-12 3-2, 2-1 and CO-13 J=1-0 line transitions, as well as the H2 S(1) transition. We explore the effects of stellar ultraviolet variability and dust settling on the line emission, and on the derived disc properties. Our fitting efforts lead to derived gas/dust ratios in the range 9-100, depending on the assumptions made. We note that the line fluxes are sensitive in general to the degree of dust settling in the disc, with an increase in line flux for settled models. This is most pronounced in lines which are formed in the warm gas in the inner disc, but the low excitation molecular lines are also affected. This has serious implications for attempts to derive the disc gas mass from line observations. We derive fractional PAH abundances between 0.007 and 0.04 relative to ISM levels. Using a stellar and UV excess input spectrum based on a detailed analysis of observations, we find that the all observations are consistent with the previously assumed disc geometry

  18. The axial dipole moment of two intersecting spheres of equal radii

    DEFF Research Database (Denmark)

    McAllister, Iain Wilson

    1988-01-01

    The use of a finite number of image charges to solve electrostatic problems associated with two conducting spheres intersecting at an angle of pi/n (n an integer) has been known for over a century. If, however, only spheres of equal radii are considered, it is possible to extend the permissible...

  19. Investigation of the Effects of Expectation Values for Radii on the ...

    Indian Academy of Sciences (India)

    mation (NCA) wave functions to calculate expectation values of radii. The transition probability ... 1. Introduction. The optical properties of carbon, nitrogen and oxygen atoms are important in both atmospheric and ...... Zheng, N. W. 1988a, A new outline of atomic theory (Jiang Su Education Press). Zheng, N. W. 1988b, Chin.

  20. Nuclear charge radii from X-ray transitions in muonic C, O and N

    CERN Document Server

    Dubler, T; Schellenberg, L; Schneuwly, H; Vuilleumier, J L; Walter, H K

    1973-01-01

    Energies of muonic X-rays permit an almost model independent determination of nuclear root mean square (rms) radii for light and medium nuclei, which can be compared to those obtained from elastic electron scattering experiments at low momentum transfer. In the present experiment the X-ray energies of the K series of C, N and O up to the 6p-1s transition are determined with an accuracy of +or-15 eV from which rms radii are deduced comparable in precision to the electron scattering data. Muons from the CERN muon channel were stopped in a target, which contained 70 gr dimethylglyoxim (C/sub 4/H /sub 8/N/sub 2/O/sub 2/) and 18 gr rhodium powder. (0 refs).

  1. LUNAR ACCRETION FROM A ROCHE-INTERIOR FLUID DISK

    Energy Technology Data Exchange (ETDEWEB)

    Salmon, Julien; Canup, Robin M., E-mail: julien@boulder.swri.edu, E-mail: robin@boulder.swri.edu [Department of Space Studies, Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302 (United States)

    2012-11-20

    We use a hybrid numerical approach to simulate the formation of the Moon from an impact-generated disk, consisting of a fluid model for the disk inside the Roche limit and an N-body code to describe accretion outside the Roche limit. As the inner disk spreads due to a thermally regulated viscosity, material is delivered across the Roche limit and accretes into moonlets that are added to the N-body simulation. Contrary to an accretion timescale of a few months obtained with prior pure N-body codes, here the final stage of the Moon's growth is controlled by the slow spreading of the inner disk, resulting in a total lunar accretion timescale of {approx}10{sup 2} years. It has been proposed that the inner disk may compositionally equilibrate with the Earth through diffusive mixing, which offers a potential explanation for the identical oxygen isotope compositions of the Earth and Moon. However, the mass fraction of the final Moon that is derived from the inner disk is limited by resonant torques between the disk and exterior growing moons. For initial disks containing <2.5 lunar masses (M{sub Last-Quarter-Moon }), we find that a final Moon with mass > 0.8 M{sub Last-Quarter-Moon} contains {<=}60% material derived from the inner disk, with this material preferentially delivered to the Moon at the end of its accretion.

  2. Magnetic resonance imaging of intervertebral disc degeneration

    International Nuclear Information System (INIS)

    Maeda, Hiroshi; Noguchi, Masao; Kira, Hideaki; Fujiki, Hiroshi; Shimokawa, Isao; Hinoue, Kaichi.

    1993-01-01

    The aim of this study was to correlate the degree of lumbar intervertebral disc degeneration with findings of magnetic resonance imaging (MRI). Seventeen autopsied (from 7 patients) and 21 surgical (from 20 patients) intervertebral discs were used as specimens for histopathological examination. In addition, 21 intervertebral discs were examined on T2-weighted images. Histopathological findings from both autopsied and surgical specimens were well correlated with MRI findings. In particular, T2-weighted images reflected increased collagen fibers and rupture within the fibrous ring accurately. However, when severely degenerated intervertebral discs and hernia protruding the posterior longitudinal ligament existed, histological findings were not concordant well with T2-weighted images. Morphological appearances of autopsy specimens, divided into four on T2-weighted images, were well consistent with histological degeneration. This morphological classification, as shown on T2-weighted images, could also be used in the evaluation of intervertebral disc degeneration. (N.K.)

  3. Magnetic resonance imaging of intervertebral disc degeneration

    Energy Technology Data Exchange (ETDEWEB)

    Maeda, Hiroshi; Noguchi, Masao (Kitakyushu City Yahata Hospital, Fukuoka (Japan)); Kira, Hideaki; Fujiki, Hiroshi; Shimokawa, Isao; Hinoue, Kaichi

    1993-02-01

    The aim of this study was to correlate the degree of lumbar intervertebral disc degeneration with findings of magnetic resonance imaging (MRI). Seventeen autopsied (from 7 patients) and 21 surgical (from 20 patients) intervertebral discs were used as specimens for histopathological examination. In addition, 21 intervertebral discs were examined on T2-weighted images. Histopathological findings from both autopsied and surgical specimens were well correlated with MRI findings. In particular, T2-weighted images reflected increased collagen fibers and rupture within the fibrous ring accurately. However, when severely degenerated intervertebral discs and hernia protruding the posterior longitudinal ligament existed, histological findings were not concordant well with T2-weighted images. Morphological appearances of autopsy specimens, divided into four on T2-weighted images, were well consistent with histological degeneration. This morphological classification, as shown on T2-weighted images, could also be used in the evaluation of intervertebral disc degeneration. (N.K.).

  4. Broad-Band Variability in Accreting Compact Objects

    Directory of Open Access Journals (Sweden)

    S. Scaringi

    2015-02-01

    Full Text Available Cataclysmic variable stars are in many ways similar to X-ray binaries. Both types of systems possess an accretion disk, which in most cases can reach the surface (or event horizon of the central compact object. The main difference is that the embedded gravitational potential well in X-ray binaries is much deeper than those found in cataclysmic variables. As a result, X-ray binaries emit most of their radiation at X-ray wavelengths, as opposed to cataclysmic variables which emit mostly at optical/ultraviolet wavelengths. Both types of systems display aperiodic broad-band variability which can be associated to the accretion disk. Here, the properties of the observed X-ray variability in XRBs are compared to those observed at optical wavelengths in CVs. In most cases the variability properties of both types of systems are qualitatively similar once the relevant timescales associated with the inner accretion disk regions have been taken into account. The similarities include the observed power spectral density shapes, the rms-flux relation as well as Fourier-dependant time lags. Here a brief overview on these similarities is given, placing them in the context of the fluctuating accretion disk model which seeks to reproduce the observed variability.

  5. ACCRETION AND MAGNETIC RECONNECTION IN THE CLASSICAL T TAURI BINARY DQ TAU

    International Nuclear Information System (INIS)

    Tofflemire, Benjamin M.; Mathieu, Robert D.; Ardila, David R.; Akeson, Rachel L.; Ciardi, David R.; Johns-Krull, Christopher; Herczeg, Gregory J.; Quijano-Vodniza, Alberto

    2017-01-01

    The theory of binary star formation predicts that close binaries ( a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory.

  6. ACCRETION AND MAGNETIC RECONNECTION IN THE CLASSICAL T TAURI BINARY DQ TAU

    Energy Technology Data Exchange (ETDEWEB)

    Tofflemire, Benjamin M.; Mathieu, Robert D. [Department of Astronomy, University of Wisconsin–Madison, 475 North Charter Street, Madison, WI 53706 (United States); Ardila, David R. [The Aerospace Corporation, M2-266, El Segundo, CA 90245 (United States); Akeson, Rachel L.; Ciardi, David R. [NASA Exoplanet Science Institute, IPAC/Caltech, Pasadena, CA 91125 (United States); Johns-Krull, Christopher [Department of Physics and Astronomy, Rice University, Houston, TX 77005 (United States); Herczeg, Gregory J. [The Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871 (China); Quijano-Vodniza, Alberto [University of Nariño Observatory, Pasto, Nariño (Colombia)

    2017-01-20

    The theory of binary star formation predicts that close binaries ( a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory.

  7. FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

    Energy Technology Data Exchange (ETDEWEB)

    Kirk, Helen; Myers, Philip C.; Bourke, Tyler L. [Radio and Geoastronomy Division, Harvard Smithsonian Center for Astrophysics, MS-42, Cambridge, MA, 02138 (United States); Gutermuth, Robert A.; Wilson, Grant W. [Department of Astronomy, University of Massachusetts Amherst, Amherst, MA 01003 (United States); Hedden, Abigail, E-mail: kirkh@mcmaster.ca [Army Research Labs, Adelphi, MD 20783 (United States)

    2013-04-01

    One puzzle in understanding how stars form in clusters is the source of mass-is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of {approx}30 M{sub Sun} Myr{sup -1} (inferred from the N{sub 2}H{sup +} velocity gradient along the filament), and radially contracting onto the filament at {approx}130 M{sub Sun} Myr{sup -1} (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

  8. Modeling X-ray Absorbers in AGNs with MHD-Driven Accretion-Disk Winds

    Science.gov (United States)

    Fukumura, Keigo; Kazanas, D.; Shrader, C. R.; Tombesi, F.; Contopoulos, J.; Behar, E.

    2013-04-01

    We have proposed a systematic view of the observed X-ray absorbers, namely warm absorbers (WAs) in soft X-ray and highly-ionized ultra-fast outflows (UFOs), in the context of magnetically-driven accretion-disk wind models. While potentially complicated by variability and thermal instability in these energetic outflows, in this simplistic model we have calculated 2D kinematic field as well as density and ionization structure of the wind with density profile of 1/r corresponding to a constant column distribution per decade of ionization parameter. In particular we show semi-analytically that the inner layer of the disk-wind manifests itself as the strongly-ionized fast outflows while the outer layer is identified as the moderately-ionized absorbers. The computed characteristics of these two apparently distinct absorbers are consistent with X-ray data (i.e. a factor of ~100 difference in column and ionization parameters as well as low wind velocity vs. near-relativistic flow). With the predicted contour curves for these wind parameters one can constrain allowed regions for the presence of WAs and UFOs.The model further implies that the UFO's gas pressure is comparable to that of the observed radio jet in 3C111 suggesting that the magnetized disk-wind with density profile of 1/r is a viable agent to help sustain such a self-collimated jet at small radii.

  9. The comparative study of lumbar disc disruption with MRI and CT discography

    International Nuclear Information System (INIS)

    Chen Xingcan; Liu Naifang; Li Xiaohong; Xu Wengen; Zou Qing; Yang Yonghong

    2005-01-01

    Objective: To compare MRI with CT discography (CTD) for diagnostic assessment of lumbar disc disruption. Methods: Paired comparative examination in 16 patients with chronic lower back pain without radicular pain and no disc herniation was conducted using CT or MRI. The standard of CTD classification and positive disc was formulated and the correlation between the induced lower back pain and dosage used in CTD was observed. Results: For a total of 21 discs in the 16 patients, CTD showed the disc as type 2 in 12 discs and type 5 in 1 disc with 13 positive discs, while MRI only showed the high-intensity zone of posterior annulus in 6 discs as the indirect sign of disc disruption and disc degeneration in 7 discs. Conclusion: CTD was the only method for showing the direct sign of disc disruption. The induced lower back pain was related with the type of disc disruption. MRI can show some of the indirect signs of disc disruption and CTD can show the direct sign of disc disruption. (authors)

  10. Hot accretion disks with electron-positron pairs

    International Nuclear Information System (INIS)

    White, T.R.; Lightman, A.P.

    1989-01-01

    The hot thermal accretion disks of the 1970s are studied and consideration is given to the effects of electron-positron pairs, which were originally neglected. It is found that disks cooled by internally produced photons have a critical accretion rate above which equilibrium is not possible in a radial annulus centered around r = 10 GM/c-squared, where M is the mass of the central object. This confirms and extends previous work by Kusunose and Takahara. Above the critical rate, pairs are created more rapidly than they can be destroyed. Below the critical rate, there are two solutions to the disk structure, one with a high pair density and one with a low pair density. Depending on the strength of the viscosity, the critical accretion rate corresponds to a critical luminosity of about 3-10 percent of the Eddington limit. 32 refs

  11. Exoplanet recycling in massive white-dwarf debris discs

    Science.gov (United States)

    van Lieshout, R.; Kral, Q.; Charnoz, S.; Wyatt, M. C.; Shannon, A.

    2018-05-01

    Several tens of white dwarfs are known to host circumstellar discs of dusty debris, thought to arise from the tidal disruption of rocky bodies originating in the star's remnant planetary system. This paper investigates the evolution of such discs if they are very massive, as may be the case if their progenitor was a terrestrial planet, moon, or dwarf planet. Assuming the discs are physically thin and flat, like Saturn's rings, their evolution is governed by Poynting-Robertson drag or viscous spreading, where the disc's effective viscosity is due to self-gravity wakes. For discs with masses ≳ 1026 g, located in the outer parts of the tidal disruption zone, viscous spreading dominates the evolution, and mass is transported both in- and outwards. When outwards-spreading material flows beyond the Roche limit, it coagulates into new (minor) planets in a process analogous to the ongoing formation of moonlets at the outer edge of Saturn's rings. The newly formed bodies migrate outwards by exchanging angular momentum with the disc and coalesce into larger objects through mutual collisions. Eventually, the disc's Roche-limit overflow recycles tens of percent of the original disc mass; most ends up in a single large body near 2:1 mean-motion resonance with the disc's outer edge. Hence, the recycling of a tidally disrupted super-Earth, for example, could yield an Earth-mass planet on a ˜10-h orbit, located in the habitable zone for 2-to-10-Gyr-old white dwarfs. The recycling process also creates a population of smaller bodies just outside the Roche limit, which may explain the minor planets recently postulated to orbit WD 1145+017.

  12. Undergoing spherically symmetric steady-state accretion stability of white dwarfs

    Energy Technology Data Exchange (ETDEWEB)

    Sienkiewicz, R [Polska Akademia Nauk, Warsaw. N. Copernicus Astronomical Center

    1980-01-01

    Thermal and vibrational stabilities of accreting white dwarfs with steady-state nuclear burning were considered, assuming spherically symmetric accretion of the hydrogen-rich matter and using linear stability analysis. Almost all models with masses 0.2 M(sun) - 1.39 M(sun) were found to be unstable in some way. The type of instability expected to dominate is given as a function of the accretion rate. For most accretion rates it is the thermal instability. Oscillation periods are given for the models in which the vibrational instability is the most violent one. These periods are of the order of seconds or minutes. We expect that our stability analysis may suggest the cause of the variabilities of the hot components of some symbiotic stars, for a wide range of the accretion rates. In this case our models may serve as the initial conditions for evolutionary computations. The results predict that short-period oscillations should be observed in some hot nuclei of planetary nebulae.

  13. Accreting planets as dust dams in 'transition' disks

    International Nuclear Information System (INIS)

    Owen, James E.

    2014-01-01

    We investigate under what circumstances an embedded planet in a protoplanetary disk may sculpt the dust distribution such that it observationally presents as a 'transition' disk. We concern ourselves with 'transition' disks that have large holes (≳ 10 AU) and high accretion rates (∼10 –9 -10 –8 M ☉ yr –1 ), particularly, those disks which photoevaporative models struggle to explain. Adopting the observed accretion rates in 'transition' disks, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small (s ≲ 1 μm) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disk with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of a 'transition' disk (NIR dip in the spectral energy distribution (SED), millimeter cavity, and high accretion rate). At suitably high planet masses (≳ 3-4 M J ), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disk that is optically thin to infrared radiation. The planet-disk system will present as a 'transition' disk with a dip in the SED only when the planet mass and planetary accretion rate are high enough. At other times, it will present as a disk with a primordial SED, but with a cavity in the millimeter, as observed in a handful of protoplanetary disks.

  14. Intermediate polars as low-field magnetic cataclysmic variables

    International Nuclear Information System (INIS)

    Wickramasinghe, D.T.; Kinwah Wu; Ferrario, Lilia

    1991-01-01

    We present the first detailed calculations of the polarization properties of extended accretion shocks on the surface of a magnetic white dwarf where allowance is made both for field spread and for the change in shock height as a function of specific accretion rate. These results are used to show conclusively that the null detection of circular polarization in most IPs imply fields of less than 5 MG. We suggest that the X-ray properties of MCVs depends critically on the fractional area of the white-dwarf surface over which accretion occurs, and on the type of accretion (smooth or clumpy). We argue that in the known IPs, accretion occurs via a disc. The accretion flow is smooth and a strong shock forms making them a powerful source of hard X-rays. We propose that there is a new class of MCV distinct from the IPs, where the white dwarf is asynchronous and accretes without a disc in which the accretion is clumpy and the radiation is mainly in the EUV region. (author)

  15. An Accretion Model for Anomalous X-Ray Pulsars

    Science.gov (United States)

    Chatterjee, Pinaki; Hernquist, Lars; Narayan, Ramesh

    2000-05-01

    We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star toward a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength, and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to the situation in models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.

  16. Disc degeneration: current surgical options

    Directory of Open Access Journals (Sweden)

    C Schizas

    2010-10-01

    Full Text Available Chronic low back pain attributed to lumbar disc degeneration poses a serious challenge to physicians. Surgery may be indicated in selected cases following failure of appropriate conservative treatment. For decades, the only surgical option has been spinal fusion, but its results have been inconsistent. Some prospective trials show superiority over usual conservative measures while others fail to demonstrate its advantages. In an effort to improve results of fusion and to decrease the incidence of adjacent segment degeneration, total disc replacement techniques have been introduced and studied extensively. Short-term results have shown superiority over some fusion techniques. Mid-term results however tend to show that this approach yields results equivalent to those of spinal fusion. Nucleus replacement has gained some popularity initially, but evidence on its efficacy is scarce. Dynamic stabilisation, a technique involving less rigid implants than in spinal fusion and performed without the need for bone grafting, represents another surgical option. Evidence again is lacking on its superiority over other surgical strategies and conservative measures. Insertion of interspinous devices posteriorly, aiming at redistributing loads and relieving pain, has been used as an adjunct to disc removal surgery for disc herniation. To date however, there is no clear evidence on their efficacy. Minimally invasive intradiscal thermocoagulation techniques have also been tried, but evidence of their effectiveness is questioned. Surgery using novel biological solutions may be the future of discogenic pain treatment. Collaboration between clinicians and basic scientists in this multidisciplinary field will undoubtedly shape the future of treating symptomatic disc degeneration.

  17. Highly Accreting Quasars at High Redshift

    Directory of Open Access Journals (Sweden)

    Mary L. Martínez-Aldama

    2018-01-01

    Full Text Available We present preliminary results of a spectroscopic analysis for a sample of type 1 highly accreting quasars (L/LEdd ~ 1.0 at high redshift, z ~2–3. The quasars were observed with the OSIRIS spectrograph on the GTC 10.4 m telescope located at the Observatorio del Roque de los Muchachos in La Palma. The highly accreting quasars were identified using the 4D Eigenvector 1 formalism, which is able to organize type 1 quasars over a broad range of redshift and luminosity. The kinematic and physical properties of the broad line region have been derived by fitting the profiles of strong UV emission lines such as Aliiiλ1860, Siiii]λ1892 and Ciii]λ1909. The majority of our sources show strong blueshifts in the high-ionization lines and high Eddington ratios which are related with the productions of outflows. The importance of highly accreting quasars goes beyond a detailed understanding of their physics: their extreme Eddington ratio makes them candidates standard candles for cosmological studies.

  18. SIMULTANEOUS DISC HERNIATION IN PATIENTS WITH MULTIPLE SCLEROSIS

    Directory of Open Access Journals (Sweden)

    Kalina V. Drenska

    2013-04-01

    Full Text Available Background: Multiple sclerosis (MS is a chronic autoimmune, inflammatory demyelinating disease of the central nervous system. Commonly, MS patients present with accompanying degenerative vertebral disc diseases. Simultaneous disc herniations situated in the cervical or lumbosacral spine can mimic the clinical symptoms of MS and worsen patients’ quality of life.Objective: to investigate the incidence rate and clinical impact of accompanying disc herniations in patients with MS.Material and methods: Our study covered 330 patients (220 females and 110 males, mean age 40.5±12.4 years with clinically definite MS, according to McDonald’s criteria. Comprehensive neurological examinations, EDSS (Expanded Disability Status Scale assessments, and MRI neuroimaging were carried out. Statistical data processing was performed by using the method of variation analysis.Results: Relapsing-remitting MS (RRMS was diagnosed in 280 patients while 50 patients presented with secondary progressive MS (SPMS. Disc herniation was found in 64 (19.4% of our patients. Cervical disc pathology was detected in 38 patients (11.5% of the cases and lumbosacral - in 26 (7.9% of the cases. EDSS scores ranged from 2.5 to 5.5. EDSS evaluation showed statistically significantly worse scores in MS patients with disc herniation comorbidity (p<0, 05.Conclusion: Our own data confirm the assumption that MS patients often present with accompanying degenerative disc pathology. We suggest that comorbidity of disc herniation and MS exert an additional unfavorable effect on patient’s disability and individual quality of life.

  19. CT in diagnosis of recurrent vertebral disc hernias after preceding lumbar disc prolapse surgery

    International Nuclear Information System (INIS)

    Burval, S.; Nekula, J.; Vaverka, M.; Veliskova, J.

    1992-01-01

    20 patients with recurrent symptoms following operations for disc prolapse and resistant to treatment were studied by CT, using plain and enhanced images. The results have been analysed. In 10 patients a recurrence of disc prolapse was diagnosed, and this was confirmed surgically in 8 cases. In 2 patients there was epidural scarring. The findings indicate that differential diagnosis between scarring and recurring prolapse can by accurately made by this technique. (orig.) [de

  20. Accretion onto a noncommutative-inspired Schwarzschild black hole

    Science.gov (United States)

    Gangopadhyay, Sunandan; Paik, Biplab; Mandal, Rituparna

    2018-05-01

    In this paper, we investigate the problem of ordinary baryonic matter accretion onto the noncommutative (NC) geometry-inspired Schwarzschild black hole. The fundamental equations governing the spherically symmetric steady state matter accretion are deduced. These equations are seen to be modified due to the presence of noncommutativity. The matter accretion rate is computed and is found to increase rapidly with the increase in strength of the NC parameter. The sonic radius reduces while the sound speed at the sonic point increases with the increase in the strength of noncommutativity. The profile of the thermal environment is finally investigated below the sonic radius and at the event horizon and is found to be affected by noncommutativity.