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Sample records for two-pole accretion polar

  1. Massive thin accretion discs. Pt. 2; Polarization

    Energy Technology Data Exchange (ETDEWEB)

    Laor, A.; Netzer, H. (Tel Aviv Univ. (Israel)); Piran, T. (Hebrew Univ., Jerusalem (Israel). Racah Inst. of Physics)

    1990-02-15

    Thin accretion discs around massive black holes are believed to produce much of the observed optical-UV emission from AGN. Classical calculations predict that this radiation is highly polarized at large inclination angles, in contrast to observations of quasars and Seyfert galaxies. We have calculated the spectrum and polarization of such discs using an improved radiative transfer method with all the relevant opacity sources, and a full general relativistic treatment of the radiation propagation. (author).

  2. Do observations reveal accretion discs in intermediate polars

    Energy Technology Data Exchange (ETDEWEB)

    Hellier, C. (University Coll., London (UK). Mullard Space Science Lab.)

    1991-08-15

    It has been proposed that intermediate polars do not accrete through discs and that they may not possess discs. Observations of eclipses and emission lines provide strong evidence that discs are present in many intermediate polars, although it is less clear whether the accretion flows through these discs. An analysis of the EXOSAT database shows that many systems have orbital and beat period modulations which are small compared to the spin-pulses, suggesting disc accretion. There are, though, exceptions, notably TX Col where dominant orbital and beat period modulations indicate discless accretion. (author).

  3. Twisted accretion curtains in the intermediate polar FO Aquarii

    CERN Document Server

    Evans, P A; Cropper, G R M; Hellier, Coel; Cropper, Gavin Ramsay & Mark

    2004-01-01

    We report on a ~37-ks XMM-Newton observation of the intermediate polar FO Aquarii, presenting X-ray and UV data from the EPIC and OM cameras. We find that the system has changed from its previously reported state of disc-overflow accretion to one of purely disc-fed accretion. We detect the previously reported `notch' feature in the X-ray spin pulse, and explain it as a partial occultation of the upper accretion pole. Maximum flux of the quasi-sinusoidal UV pulse coincides with the notch, in keeping with this idea. However, an absorption dip owing to the outer accretion curtains occurs 0.27 later than the expected phase, which implies that the accretion curtains are twisted, trailing the magnetic poles. This result is the opposite of that reported in PQ Gem, where accreting field lines were found to lead the pole. We discuss how such twists relate to the accretion torques and thus the observed period changes of the white dwarfs, but find no simple connection.

  4. Anchoring Polar Magnetic Field in a Stationary Thick Accretion Disk

    Science.gov (United States)

    Samadi, Maryam; Abbassi, Shahram

    2017-08-01

    We investigate the properties of a hot accretion flow bathed in a poloidal magnetic field. We consider an axisymmetric viscous-resistive flow in the steady-state configuration. We assume that the dominant mechanism of energy dissipation is due to turbulence viscosity and magnetic diffusivity. A certain fraction of that energy can be advected toward the central compact object. We employ the self-similar method in the radial direction to find a system of ODEs with just one varible, θ in the spherical coordinates. For the existence and maintenance of a purely poloidal magnetic field in a rotating thick disk, we find that the necessary condition is a constant value of angular velocity along a magnetic field line. We obtain an analytical solution for the poloidal magnetic flux. We explore possible changes in the vertical structure of the disk under the influences of symmetric and asymmetric magnetic fields. Our results reveal that a polar magnetic field with even symmetry about the equatorial plane makes the disk vertically thin. Moreover, the accretion rate decreases when we consider a strong magnetic field. Finally, we notice that hot magnetized accretion flows can be fully advected even in a slim shape.

  5. Probing the Accretion Geometry of Black Holes with X-Ray Polarization

    Science.gov (United States)

    Schnitman, Jeremy D.

    2011-01-01

    In the coming years, new space missions will be able to measure X-ray polarization at levels of 1% or better in the approx.1-10 keV energy band. In particular, X-ray polarization is an ideal tool for determining the nature of black hole (BH) accretion disks surrounded by hot coronae. Using a Monte Carlo radiation transport code in full general relativity, we calculate the spectra and polarization features of these BH systems. At low energies, the signal is dominated by the thermal flux coming directly from the optically thick disk. At higher energies, the thermal seed photons have been inverse-Compton scattered by the corona, often reflecting back off the disk before reaching the observer, giving a distinctive polarization signature. By measuring the degree and angle of this X-ray polarization, we can infer the BH inclination, the emission geometry of the accretion flow, and also determine the spin of the black hole.

  6. A Numerical Model for Accretion in Intermediate Polars with Dipolar Magnetic Fields

    CERN Document Server

    Isakova, P B; Bisikalo, D V

    2016-01-01

    A three-dimensional numerical model for an accretion process investigation in the magnetosphere of a white dwarf in magnetic cataclysmic variables is developed. The model assumes that the white dwarf has a dipole magnetic field with its symmetry axis inclined to the rotation axis. The model is based on the equations of modified MHD, that describe the mean flow parameters in the wave MHD turbulence. Diffusion of the magnetic field and radiative heating and cooling are taken into account. The suitability of the model is confirmed by modeling the accretion in a typical intermediate polar. The computations show that a magnetosphere forms around the accretor, with the accretion occurring via columns. The accretion columns have a curtain-like shape, and arc-shaped zones of energy release form on the surface of the white dwarf in the magnetic poles area as a result of the matter infall.

  7. Turbulence-driven Polar Winds from T Tauri Stars Energized by Magnetospheric Accretion

    CERN Document Server

    Cranmer, Steven R

    2008-01-01

    Pre-main-sequence stars are observed to be surrounded by both accretion flows and some kind of wind or jet-like outflow. Recent work by Matt and Pudritz has suggested that if classical T Tauri stars exhibit stellar winds with mass loss rates about 0.1 times their accretion rates, the wind can carry away enough angular momentum to keep the stars from being spun up unrealistically by accretion. This paper presents a preliminary set of theoretical models of accretion-driven winds from the polar regions of T Tauri stars. These models are based on recently published self-consistent simulations of the Sun's coronal heating and wind acceleration. In addition to the convection-driven MHD turbulence (which dominates in the solar case), we add another source of wave energy at the photosphere that is driven by the impact of plasma in neighboring flux tubes undergoing magnetospheric accretion. This added energy, determined quantitatively from the far-field theory of MHD wave generation, is sufficient to produce T Tauri-l...

  8. Nova V1425 Aquilae 1995 The Early Appearance of Accretion Processes in An Intermediate Polar Candidate

    CERN Document Server

    Retter, A; Kovo-Kariti, O

    1997-01-01

    Continuous CCD photometry of Nova Aquilae 1995 was performed through the standard B,V,R and I filters during three nights in 1995 and with the I filter during 18 nights in 1996. The power spectrum of the 1996 data reveals three periodicities in the light curve: 0.2558 d, 0.06005 d and 0.079 d, with peak-to-peak amplitudes of about 0.012, 0.014 and 0.007 mag. respectively. The two shorter periods are absent from the power spectrum of the 1995 light curve, while the long one is probably already present in the light curve of that year. We propose that V1425 Aql should be classified as an Intermediate - Polar CV. Accordingly the three periods are interpreted as the orbital period of the underlying binary system, the spin period of the magnetic white dwarf and the beat period between them. Our results suggest that no later than 15 months after the outburst of the nova, accretion processes are taking place in this stellar system. Matter is being transferred from the cool component, most likely through an accretion ...

  9. Two-pole structure of the D0* (2400)

    Science.gov (United States)

    Albaladejo, Miguel; Fernandez-Soler, Pedro; Guo, Feng-Kun; Nieves, Juan

    2017-04-01

    The so far only known charmed non-strange scalar meson is dubbed as D0* (2400) in the Review of Particle Physics. We show, within the framework of unitarized chiral perturbation theory, that there are in fact two (I = 1 / 2 ,JP =0+) poles in the region of the D0* (2400) in the coupled-channel Dπ, Dη and Ds K bar scattering amplitudes. With all the parameters previously fixed, we predict the energy levels for the coupled-channel system in a finite volume, and find that they agree remarkably well with recent lattice QCD calculations. This successful description of the lattice data is regarded as a strong evidence for the two-pole structure of the D0* (2400). With the physical quark masses, the poles are located at (2105-8+6 - i102-12+10) MeV and (2451-26+36 - i134-8+7) MeV, with the largest couplings to the Dπ and Ds K bar channels, respectively. Since the higher pole is close to the Ds K bar threshold, we expect it to show up as a threshold enhancement in the Ds K bar invariant mass distribution. This could be checked by high-statistic data in future experiments. We also show that the lower pole belongs to the same SU(3) multiplet as the Ds0 * (2317) state. Predictions for partners in the bottom sector are also given.

  10. Magnetized accretion

    Science.gov (United States)

    Heyvaerts, J.

    This lecture reviews in simple terms the general subject of large scale magnetic field coupling to plasma flows in the vicinity of accreting compact stars. The relevant astrophysical phenomenology is summarized. Disk interaction with the magnetosphere of accreting stars is first discussed, in particular the structure of the magnetopause, its stability and plasma ejection in so-called propeller systems. The physics of accretion/ejection is then considered. Acceleration and focusing mechanisms of jets from accretion disks around compact stars or black holes and the question of the self-consistency of accretion and ejection are described. By contrast, small scale MHD turbulence in disks is not discussed, neither are accretion columns near the polar caps of neutron stars or white dwarfs. The reader is only assumed to have some basic knowledge of astrophysics and of fluid mechanics and electromagnetism.

  11. First science with SALT: peering at the accreting polar caps of the eclipsing polar SDSS J015543.40+002807.2

    CERN Document Server

    O'Donoghue, D; Balona, L A; Bester, D; Botha, L; Brink, J; Carter, D B; Charles, P A; Christians, A; Ebrahim, F; Emmerich, R; Esterhuyse, W; Evans, G P; Fourie, C; Fourie, P; Gajjar, H; Gordon, M; Gumede, C; De Kock, M; Koeslag, A; Koorts, W P; Kriel, H; Marang, F; Meiring, J G; Menzies, J W; Menzies, P; Metcalfe, D; Meyer, B; Nel, L; O'Connor, J; Osman, F; Plessis, C; Rall, H; Riddick, A; Romero-Colmenero, E; Potter, S B; Sass, C; Schalekamp, H; Sessions, N; Siyengo, S; Sopela, V; Steyn, H; Stoffels, J; Stoltz, J; Swart, G; Swat, A; Swiegers, J; Tiheli, T; Väisänen, P; Whittaker, W; Van Wyk, F

    2006-01-01

    We describe briefly the properties of the recently completed Southern African Large Telescope (SALT), along with its first light imager SALTICAM. Using this instrument, we present 4.3 hr of high speed unfiltered photometric observations of the eclipsing polar SDSSJ015543.40+002807.2 with time resolution as short as 112 ms, the highest quality observations of this kind of any polar to date. The system was observed during its high luminosity state. Two accreting poles are clearly seen in the eclipse light curve. The binary system parameters have been constrained: the white dwarf mass is at the low end of the range expected for cataclysmic variables. Correlations between the positions of the accretion regions on or near the surface of the white dwarf and the binary system parameters were established. The sizes of the accretion regions and their relative movement from eclipse to eclipse were estimated: they are typically 4-7 deg depending on the mass of the white dwarf. The potential of these observations will on...

  12. Long orbital period pre-polars containing an early K-type donor stars. Bottleneck accretion mechanism in action

    CERN Document Server

    Tovmassian, G; Zharikov, S; Reichart, D E; Haislip, J B; Ivarsen, K M; LaCluyze, A P; Moore, J P; Miroshnichenko, A S

    2016-01-01

    We studied two objects identified as a Cataclysmic Variables (CVs) with periods exceeding the natural boundary for Roche lobe filling ZAMS secondary stars. We present observational results for V1082 Sgr with 20.82 h orbital period, an object that shows low luminosity state, when its flux is totally dominated by a chromospherically active K- star with no signs of ongoing accretion. Frequent accretion shut-offs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 h binary system. They both have early K-type stars as a donor star. We argue, that similar to the shorter period pre-polars containing M-dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar ...

  13. Asynchronous Polar BY Cam: the Spin-orbital Synchronization and Variation of Accretion Geometry on the 8-year Time Scale

    Science.gov (United States)

    Pavlenko, E.; Andreev, M.; Babina, Y.; Malanushenko, V.

    2013-01-01

    Attempts to estimate the time of a spin-orbital synchronization for asynchronous polar BY Cam have been made many times since 1994 year. However the estimates obtained did not coincide in a wide region, varying from 150 years (Piirola et al. 1994) up to >3500 years (Kafka et al. 2005). We have undertaken photometric observations of BY Cam over 8.1 years (2004 - 2012) and collected an array of data covering 998 hours during 178 nights. Analyzing the data, we have obtained the most reliable estimate of the spin-orbital time synchronization, Ts = 250±20 years, which agrees very well with both Ts = 150- 290 yrs for asynchronous polar V1500 Cyg (Pavlenko & Pelt 1991); (Stockman et al. 1988) and the theoretically predicted Ts < 1000 yr for asynchronous polars as a whole (Andronov 1987). We also found that the accretion stream switches between two dipole magnetic poles and the equatorial magnetic poles during a synodic ˜ 15-d cycle; the number of switching and their phases can be kept during neighbor cycles but varies on a scale of years. Probably this may depend on the phase of the long-term ˜ 1500 day periodicity (Andreev et al. 2012).

  14. XMM-Newton and optical observations of the eclipsing polar CSS081231:071126+440405

    CERN Document Server

    Worpel, H

    2015-01-01

    Aims: We aim to study the temporal and spectral behaviour of the eclipsing polar CSS081231:071126+440405 from the infrared to the X-ray regime. Methods: We obtained phase-resolved XMM-Newton X-ray observations on two occasions in 2012 and 2013 in different states of accretion. In 2013 the XMM-Newton X-ray and UV data were complemented by optical photometric and spectroscopic observations. Results: CSS081231 displays two-pole accretion in the high state. The magnetic fields of the two poles are 36 and 69 MG, indicating a non-dipolar field geometry. The X-ray spectrum of the main accreting pole with the lower field comprises a hot thermal component from the cooling accretion plasma, $kT_{plas}$ of a few tens of keV, and a blackbody-like component from the accretion area with $kT_{rm bb} \\sim$ 50-100\\,eV. The high-field pole which was located opposite to the mass-donating star accretes at a low rate and has a plasma temperature of about 4\\,keV. At both occasions the X-ray eclipse midpoint precedes the optical ec...

  15. Accretion of Interplanetary Dust: A New Record from He-3 In Polar Ice Cores

    Science.gov (United States)

    Brook, Edward

    2002-01-01

    This grant funded measurements of extraterrestrial He-3 in particles extracted from polar ice samples. The overall objective was to develop measurements of He-3 as tracers of the flux of interplanetary dust particles (IDP's) to the earth. To our knowledge these are the first such measurements, apart from our earlier work. The project also funded an EPO activity - a climate and global change workshop for high school science teachers.

  16. XMM-Newton observation of the long-period polar V1309 Ori: The case for pure blobby accretion

    CERN Document Server

    Schwarz, R; Beuermann, K; Burwitz, V

    2005-01-01

    Using XMM-Newton we have obtained the first X-ray observation covering a complete orbit of the longest period polar, V1309 Ori. The X-ray light curve is dominated by a short, bright phase interval with EPIC pn count rates reaching up to 15 cts/sec per 30 sec resolution bin. The bright phase emission is well described by a single blackbody component with kT_bb = (45 +- 3) eV. The absence of a bremsstrahlung component at photon energies above 1 keV yields a flux ratio F_bb/F_br > 6700. This represents the most extreme case of a soft X-ray excess yet observed in an AM Herculis star. The bright, soft X-ray emission is subdivided into a series of individual flare events supporting the hypothesis that the soft X-ray excess in V1309 is caused by accretion of dense blobs. In addition to the bright phase emission, a faint, hard X-ray component is visible throughout the binary orbit with an almost constant count rate of 0.01 cts/sec. Spectral modelling indicates that this emission originates from a complex multi-temper...

  17. Magnet losses in inverter-fed two-pole PM machines

    DEFF Research Database (Denmark)

    Gonzalez, Adolfo Garcia; Millinger, J.; Soulard, J.

    2016-01-01

    This article deals with the estimation of magnet losses in a permanent-magnet motor inserted in a nut-runner. This type of machine has interesting features such as being two-pole, slot-less and running at a high speed (30000 rpm). Two analytical models were chosen from the literature. A numerical...... estimation of the losses with 2D Finite Element Method was carried out. A detailed investigation of the effect of simulation settings (e.g., mesh size, time-step, remanence flux density in the magnet, superposition of the losses, etc.) was performed. Finally, calculation of losses with 3D-FEM were also run...

  18. On the area of accretion curtains from fast aperiodic time variability of the intermediate polar EX Hya

    CERN Document Server

    Semena, Andrey N; Buckley, David A H; Kotze, Marissa M; Khabibullin, Ildar I; Breytenbach, Hannes; Gulbis, Amanda A S; Coppejans, Rocco; Potter, Stephen B

    2014-01-01

    We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya. It was previously shown that one may expect the rapid flux variability of mCVs to be smeared out at timescales shorter than the cooling time of hot plasma in the post shock region of the accretion curtain near the WD surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the post-shock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightest mCV EX Hya with the help of optical telescopes, namely SALT and the SAAO 1.9m telescope. We place upper limits on the plasma cooling timescale $\\tau<$0.3 sec, on the fractional area of the accretion curtain footprint $f<1.6\\times10^{-4}$, and a lower limit on the specific mass accretion rate $\\dot{M}/A \\gtrsim $3 g/sec/cm$^{-2}$. We show that measurements of accretion column footprints v...

  19. YY Draconis and V709 Cassiopeiae two intermediate polars with weak magnetic fields

    CERN Document Server

    Norton, A J; Allan, A R; Hellier, C; Allan, Alasdair; Hellier, Coel

    1999-01-01

    We present data from long ROSAT HRI observations of the intermediate polars YY Dra and V709 Cas which show that V709 Cas, like YY Dra, exhibits a double-peaked X-ray pulse profile. Neither system shows evidence for X-ray beat period or orbital modulation, so both must be disc-fed accretors seen at low inclination angles. We argue that the short spin periods of the white dwarfs in these objects indicate that they have weak magnetic fields, so the radius at which material is captured by the field lines is relatively small. Consequently the footprints of the disc-fed accretion curtains on the white dwarf surface are large. The optical depths to X-ray emission within the accretion curtains are therefore lowest in the direction along the magnetic field lines, and highest in the direction parallel to the white dwarf surface, such that the emission from the two poles conspires to produce double-peaked X-ray pulse profiles. We emphasise that such a pulse profile is not a unique indicator of two-pole accretion however...

  20. Two-pole structure of the $D^\\ast_0(2400)$

    CERN Document Server

    Albaladejo, Miguel; Guo, Feng-Kun; Nieves, Juan

    2016-01-01

    The so far only known charmed non-strange scalar meson is dubbed as $D_0^*(2400)$ in the Review of Particle Physics. We show, within the framework of unitarized chiral perturbation theory, that there are in fact two $(I=1/2, J^P=0^+)$ poles in the region of the $D_0^*(2400)$ in the coupled-channel $D\\pi$, $D\\eta$ and $D_s\\bar K$ scattering amplitudes. With all the parameters previously fixed, we predict the energy levels for the coupled-channel system in a finite volume, and find that they agree remarkably well with recent lattice QCD calculations. This successful description of the lattice data is regarded as a strong evidence for the two-pole structure of the $D_0^*(2400)$. With the physical quark masses, the poles are located at $\\left(2105^{+6}_{-8}-i\\,102^{+10}_{-12}\\right)$~MeV and $\\left(2451^{+36}_{-26}-i\\,134^{+7}_{-8}\\right)$~MeV, with the largest couplings to the $D\\pi$ and $D_s\\bar K$ channels, respectively. Since the higher pole is close to the $D_s\\bar K$ threshold, we expect it to show up as a ...

  1. Proposal for an optimum water management method using two-pole simultaneous measurement

    Energy Technology Data Exchange (ETDEWEB)

    Yamauchi, M.; Sugiura, K.; Yamauchi, T. [Osaka Prefectural College of Technology, Neyagawa, Osaka 5728572 (Japan); Taniguchi, T. [Eneos Celltech Co., Ltd., Sakata, Oizumi, Ora-gun, Gunma 3700596 (Japan); Itoh, Y. [Sanyo Electric Co., Ltd., Hirakata, Osaka 5738534 (Japan)

    2009-08-01

    Most designers of Polymer Electrolyte Fuel Cells (PEFCs) supply the PEFC with humidified gas to prevent its membrane from drying. Because the steam generated by the electrochemical reaction is added to a humidified supply gas, the steam partial pressure in the cathode channel forces a supersaturated state. Therefore, the PEFC has water management issues, such as flooding and plugging. Many researchers have studied these issues in the cathode side using a visualization technique, and have introduced water repellency processing to the gas channel and GDL (gas diffusion layer) as a solution. However, the flooding/plugging phenomena in the cathode do not occur alone, and are influenced by the flooding/plugging phenomena in the anode channel through the membrane. Moreover, the water transport phenomenon through the membrane is affected by the locations of the flooding/plugging phenomena in each gas channel. Therefore, we aim to examine the water transport phenomenon through the membrane by the two-pole simultaneous image measurement, and to propose an optimum water management method. This work shows that the flooding/plugging phenomena on the anode side are clearly related to water transportation from the cathode side through the membrane. (author)

  2. Photon Bubbles in Accretion Discs

    OpenAIRE

    Gammie, Charles F.

    1998-01-01

    We show that radiation dominated accretion discs are likely to suffer from a ``photon bubble'' instability similar to that described by Arons in the context of accretion onto neutron star polar caps. The instability requires a magnetic field for its existence. In an asymptotic regime appropriate to accretion discs, we find that the overstable modes obey the remarkably simple dispersion relation \\omega^2 = -i g k F(B,k). Here g is the vertical gravitational acceleration, B the magnetic field, ...

  3. Accretion Discs in Blazars

    OpenAIRE

    Jolley, E. J. D.; Kuncic, Z.; Bicknell, G. V.; Wagner, S.(Max-Planck-Institut für Kernphysik, 69117, Heidelberg, Germany)

    2009-01-01

    The characteristic properties of blazars (rapid variability, strong polarization, high brightness) are widely attributed to a powerful relativistic jet oriented close to our line of sight. Despite the spectral energy distributions (SEDs) being strongly jet-dominated, a "big blue bump" has been recently detected in sources known as flat spectrum radio quasars (FSRQs). These new data provide a unique opportunity to observationally test coupled jet-disc accretion models in these extreme sources....

  4. Finite element analysis of relative shaft vibrations of two-pole induction motors with static rotor eccentricity

    Energy Technology Data Exchange (ETDEWEB)

    Werner, Ulrich [Siemens AG, Nuernberg (Germany). Industry, Drive Technologies, Large Drives, Industry Development

    2010-03-15

    The paper shows a computational methodology for calculating the relative shaft vibrations in the sleeve bearings of two-pole induction machines regarding excitation due to an electromagnetic force, which is caused by static rotor eccentricity. For a worst case calculation concerning the height of exciting magnetic force electromagnetic field damping effects and magnetic resistance concerning the homopolar flux are neglected. The calculated magnetic force, acting on the rotor core with double supply frequency in direction of the smallest air gap, is implemented into a finite element rotor dynamic model. With this model the influence of the rotor speed as well as influence of the direction of the magnetic force on the relative shaft displacements can be analyzed. Therefore the paper shows a computational methodology to check, whether the rotor-bearing design is sensitive for electromagnetic excitations due to static rotor eccentricity and prepares therefore the possibility to introduce improvements during the design phase of the induction motor. (orig.)

  5. A tale of two poles: Toward understanding the presence, distribution, and origin of volatiles at the polar regions of the Moon and Mercury

    Science.gov (United States)

    Lawrence, David J.

    2017-01-01

    The Moon and Mercury both have permanently shaded regions (PSRs) at their poles, which are locations that do not see the Sun for geologically long periods of time. The PSRs of the Moon and Mercury have very cold temperatures (Mercury's poles are significantly larger than at the Moon's poles. This paper documents the study of PSR volatiles at the Moon and Mercury that has taken place over the past 60 years. Starting with speculative ideas in the 1950s and 1960s, the field of PSR volatiles has emerged into a thriving subfield of planetary science that has significant implications for scientific studies of the solar system, as well as future human exploration of the solar system. While much has been learned about PSRs and PSR volatiles, many foundational aspects of PSRs are still not understood. One of the most important unanswered questions is why the PSR volatile concentrations at the Moon and Mercury are so different. After describing the initial predictions and measurements of PSR volatiles, this paper documents a variety of PSR measurements, summarizes the current understanding of PSR volatiles, and then suggests what new measurements and studies are needed to answer many of the remaining open questions about PSR volatiles.

  6. Effects of slotting and unipolar flux on magnetic pull in a two-pole induction motor with an extra four-pole stator winding

    Energy Technology Data Exchange (ETDEWEB)

    Sinervo, A.

    2013-06-01

    This thesis is about the radial magnetic forces between the rotor and stator in twopole induction machines. The magnetic forces arise from rotor eccentricity. The asymmetric air-gap makes the flux density on one side of the rotor stronger than on the opposite side. This produces magnetic pull. The magnetic flux density distribution in the air-gap can be expressed with spatial harmonics, i.e. flux densities with different pole-pair numbers. In two-pole machines, the main part of the magnetic force is produced by the interaction of two- and fourpole flux unless the four-pole flux is damped by parallel paths in the stator winding or an extra four-pole stator winding. The rest of the force comes from the interaction of two-pole and unipolar flux and from the higher harmonics of the air-gap flux of which the slot harmonics are a major part. The force caused by the higher harmonics and the unipolar flux is studied in the case where a four-pole stator winding is used to reduce the four-pole flux. The higher harmonics are found to produce, in addition to the traditional unbalanced magnetic pull, a force similar to the effect of the unipolar flux and the two can be distinguished only by measuring the unipolar flux. In measurements at various operation points, the higher harmonics are found to produce much more force than the unipolar flux and two-pole flux but the unipolar flux is still significant. The four-pole winding also is used to actively control the four-pole flux and the magnetic forces. Designing the controller requires a low order model of the system. Such a model is derived and the effect of the slot harmonics and the unipolar flux are included in the model. Different measurements techniques and methods are presented to identify and validate the control model. The operation point dependence of the system dynamics is studied via measurements. All results are obtained from a 30 kW test motor. The rotor of the test machine has a long flexible shaft on external

  7. Design and fabrication of a nanofibrous polycaprolactone tubular nerve guide for peripheral nerve tissue engineering using a two-pole electrospinning system.

    Science.gov (United States)

    Panahi-Joo, Y; Karkhaneh, A; Nourinia, A; Abd-Emami, B; Negahdari, B; Renaud, P; Bonakdar, S

    2016-04-12

    Nerve guidance conduits are considered to be the new generation of scaffolds designed for nerve disorders. A tubular construct with a highly aligned fibrous structure, mimicking the endoneurium layer surrounding inner axons of a nerve fascicle, is a suitable candidate for a nerve guide. In this paper a new approach for the fabrication of 3D tubular nerve guides is introduced using simulation of a two-pole electrospinning system and describing its mechanism. The structure of this scaffold is then optimized using the Taguchi statistical method and after morphological studies by scanning electron microscopy, the crystallinity, tensile strength and protein adsorption of these highly aligned fibres are investigated, comparing them with semi-aligned and random fibres produced via conventional mandrel electrospinning. Cell attachment, proliferation and migration of PC12 neuronal like cells are studied on highly aligned, semi aligned and random structures, and morphological change and elongation are observed in PC12 cells. The results of these studies suggest that conduits fabricated using two-pole electrospinning are a suitable and promising scaffold for peripheral and even spinal nerve regeneration. This nerve guide has a great potential for further advanced modifications and regeneration in higher levels.

  8. Formation of polar ring galaxies

    CERN Document Server

    Bournaud, F

    2003-01-01

    Polar ring galaxies are peculiar systems in which a gas rich, nearly polar ring surrounds an early-type or elliptical host galaxy. Two formation scenarios for these objects have been proposed: they are thought to form either in major galaxy mergers or by tidal accretion of the polar material from a gas rich donor galaxy. Both scenarios are studied through N-body simulations including gas dynamics and star formation. Constraints on physical parameters are drawn out, in order to determine which scenario is the most likely to occur. Polar ring galaxies from each scenario are compared with observations and we discuss whether the accretion scenario and the merging scenario account for observational properties of polar ring galaxies. The conclusion of this study is that the accretion scenario is both the most likely and the most supported by observations. Even if the merging scenario is rather robust, most polar ring galaxies are shown to be the result of tidal gas accretion events.

  9. Integrins and epithelial cell polarity.

    Science.gov (United States)

    Lee, Jessica L; Streuli, Charles H

    2014-08-01

    Cell polarity is characterised by differences in structure, composition and function between at least two poles of a cell. In epithelial cells, these spatial differences allow for the formation of defined apical and basal membranes. It has been increasingly recognised that cell-matrix interactions and integrins play an essential role in creating epithelial cell polarity, although key gaps in our knowledge remain. This Commentary will discuss the mounting evidence for the role of integrins in polarising epithelial cells. We build a model in which both inside-out signals to polarise basement membrane assembly at the basal surface, and outside-in signals to control microtubule apical-basal orientation and vesicular trafficking are required for establishing and maintaining the orientation of epithelial cell polarity. Finally, we discuss the relevance of the basal integrin polarity axis to cancer. This article is part of a Minifocus on Establishing polarity.

  10. Black hole accretion discs

    CERN Document Server

    Lasota, Jean-Pierre

    2015-01-01

    This is an introduction to models of accretion discs around black holes. After a presentation of the non-relativistic equations describing the structure and evolution of geometrically thin accretion discs we discuss their steady-state solutions and compare them to observation. Next we describe in detail the thermal-viscous disc instability model and its application to dwarf novae for which it was designed and its X-ray irradiated-disc version which explains the soft X--ray transients, i.e. outbursting black-hole low-mass X-ray binaries. We then turn to the role of advection in accretion flow onto black holes illustrating its action and importance with a toy model describing both ADAFs and slim discs. We conclude with a presentation of the general-relativistic formalism describing accretion discs in the Kerr space-time.

  11. Observations of Accreting Pulsars

    Science.gov (United States)

    Bildsten, Lars; Chakrabarty, Deepto; Chiu, John; Finger, Mark H.; Koh, Danny T.; Nelson, Robert W.; Prince, Thomas A.; Rubin, Bradley C.; Scott, D. Matthew; Stollberg, Mark; Vaughan, Brian A.; Wilson, Colleen A.; Wilson, Robert B.

    1997-01-01

    We summarize 5 years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered five new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A0535+26, GRO J2058+42, 4U 1145-619, and A1118-616), and also measured the accretion torque history during outbursts of six of those transients whose orbital param- eters were also known. We have also continuously measured the pulsed flux and spin frequency for eiaht persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic neutron stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long timescales, which blurs the con- ventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars but are uncorrelated, or even anti- correlated, in persistent sources. We describe daily folded pulse profiles, frequency, and flux measurements that are available through the Compton Observatory Science Support Center at NASA/Goddard Space Flight Center.

  12. 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...

  13. Black hole accretion discs

    OpenAIRE

    Lasota, Jean-Pierre

    2015-01-01

    This is an introduction to models of accretion discs around black holes. After a presentation of the non-relativistic equations describing the structure and evolution of geometrically thin accretion discs we discuss their steady-state solutions and compare them to observation. Next we describe in detail the thermal-viscous disc instability model and its application to dwarf novae for which it was designed and its X-ray irradiated-disc version which explains the soft X--ray transients, i.e. ou...

  14. Dynamos in accretion discs

    OpenAIRE

    Brandenburg, A.; von Rekowski, B.

    2007-01-01

    It is argued that accretion discs in young stellar objects may have hot coronae that are heated by magnetic reconnection. This is a consequence of the magneto-rotational instability driving turbulence in the disc. Magnetic reconnection away from the midplane leads to heating of the corona which, in turn, contributes to driving disc winds.

  15. Viscosity in accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Katz, J.I.

    1980-01-01

    Both HerX-1 and SS433 may contain accretion disks slaved to a precessing companion star. If so, it is possible to bound the effective viscosity in these disks. The results, in terms of the disk parameter alpha, are lower bounds of 0.01 for HerX-1 and of 0.1 for SS433.

  16. Accretion discs trapped near corotation

    NARCIS (Netherlands)

    D'Angelo, C.R.; Spruit, H.C.

    2012-01-01

    We show that discs accreting on to the magnetosphere of a rotating star can end up in a trapped state, in which the inner edge of the disc stays near the corotation radius, even at low and varying accretion rates. The accretion in these trapped states can be steady or cyclic; we explore these states

  17. Massive star formation by accretion. I. Disc accretion

    Science.gov (United States)

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

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  18. Accretion of the Earth.

    Science.gov (United States)

    Canup, Robin M

    2008-11-28

    The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 45 degrees and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon.

  19. 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.

  20. Polar ring galaxies formation and properties

    CERN Document Server

    Bournaud, F; Bournaud, Frederic; Combes, Francoise

    2003-01-01

    Formation scenarios for polar ring galaxies are studied through N-body simulatio ns that are compared with existing observations. It is shown that polar rings ar e likely to be formed by tidal accretion of the polar material from a gas rich d onor galaxy. The distribution of dark matter in polar ring galaxies is studied: dark halos seem to be flattened towards the polar rings.

  1. Rayleigh-Taylor-Unstable Accretion and Variability of Magnetized Stars: Global Three-Dimensional Simulations

    CERN Document Server

    Kulkarni, Akshay K

    2008-01-01

    We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, $\\Theta\\lesssim30^\\circ$, between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accre...

  2. Accretion to Magnetized Stars through the Rayleigh-Taylor Instability: Global Three-Dimensional Simulations

    CERN Document Server

    Kulkarni, Akshay K

    2008-01-01

    We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It manifests itself in the form of tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, $\\Theta\\lesssim30^\\circ$, between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermitten...

  3. Massive star formation by accretion I. Disc accretion

    CERN Document Server

    Haemmerlé, Lionel; Meynet, Georges; Maeder, André; Charbonnel, Corinne

    2016-01-01

    Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the HR diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the Hertzsprung-Russell (HR) diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the acc...

  4. Magnetic field evolution of accreting neutron stars

    CERN Document Server

    Istomin, Ya N

    2016-01-01

    The flow of a matter, accreting onto a magnetized neutron star, is accompanied by an electric current. The closing of the electric current occurs in the crust of a neutron stars in the polar region across the magnetic field. But the conductivity of the crust along the magnetic field greatly exceeds the conductivity across the field, so the current penetrates deep into the crust down up to the super conducting core. The magnetic field, generated by the accretion current, increases greatly with the depth of penetration due to the Hall conductivity of the crust is also much larger than the transverse conductivity. As a result, the current begins to flow mainly in the toroidal direction, creating a strong longitudinal magnetic field, far exceeding an initial dipole field. This field exists only in the narrow polar tube of $r$ width, narrowing with the depth, i.e. with increasing of the crust density $\\rho$, $r\\propto \\rho^{-1/4}$. Accordingly, the magnetic field $B$ in the tube increases with the depth, $B\\propto...

  5. Analyzing the Spectra of Accreting X-Ray Pulsars

    Science.gov (United States)

    Wolff, Michael

    This proposal seeks funding for the analysis of accretion-powered X-ray pulsar spectra from NASA/ HEASARC archived X-ray data. Spectral modeling of accreting X-ray pulsars can tell us a great deal about the physical conditions in and near high mass X-ray binary systems. Such systems have accretion flows where plasma is initially channeled from an accretion disk by the strong neutron star magnetic field, eventually falling onto the magnetic polar cap of the neutron star compact object. Many of these accreting X-ray pulsars have X-ray spectra that consist of broad power-law continua with superposed cyclotron resonant scattering features indicating magnetic field strengths above 10^12 G. The energies of these cyclotron line features have recently been shown to vary with X-ray luminosity in a number of sources such as Her X-1 and V 0332+53, a phenomenon not well understood. Another recent development is the relatively new analytic model for the spectral continuum formation in accretion-powered pulsar systems developed by Becker & Wolff. In their formalism the accretion flows are assumed to go through radiation- dominated radiative shocks and settle onto the neutron star surface. The radiation field consists of strongly Comptonized bremsstrahlung emission from the entire plasma, Comptonized cyclotron emission from the de-excitations of Landau-excited electrons in the neutron star magnetic field, and Comptonized black-body emission from a thermal mound near the neutron star surface. We seek to develop the data analysis tools to apply this model framework to the X-ray data from a wide set of sources to make progress characterizing the basic accretion properties (e.g., magnetic field strength, plasma temperatures, polar cap size, accretion rate per unit area, dominance of bulk vs. thermal Comptonization) as well as understanding the variations of the cyclotron line energies with X-ray luminosity. The three major goals of our proposed work are as follows: In the first year

  6. Constraints on Slim Accretion Discs

    Institute of Scientific and Technical Information of China (English)

    CAI Zhen-Yi; GU Wei-Min; LU Ju-Fu

    2008-01-01

    @@ We show that when the gravitational force in the vertical direction is correctly calculated, the well-known Sshaped sequence of thermal equilibrium solutions can be constructed only for small radii of black hole accretion flows, such that slim accretion discs can possibly exist only in the inner regions of these flows.

  7. Protostellar accretion traced with chemistry

    DEFF Research Database (Denmark)

    Frimann, Søren; Jørgensen, Jes K.; Padoan, Paolo;

    2016-01-01

    Context. Understanding how protostars accrete their mass is a centralquestion of star formation. One aspect of this is trying to understandwhether the time evolution of accretion rates in deeply embedded objectsis best characterised by a smooth decline from early to late stages orby intermittent ...

  8. 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

  9. Accretion by the Galaxy

    Directory of Open Access Journals (Sweden)

    Binney J.

    2012-02-01

    Full Text Available Cosmology requires at least half of the baryons in the Universe to be in the intergalactic medium, much of which is believed to form hot coronae around galaxies. Star-forming galaxies must be accreting from their coronae. Hi observations of external galaxies show that they have Hi halos associated with star formation. These halos are naturally modelled as ensembles of clouds driven up by supernova bubbles. These models can fit the data successfully only if clouds exchange mass and momentum with the corona. As a cloud orbits, it is ablated and forms a turbulent wake where cold high-metallicity gas mixes with hot coronal gas causing the prompt cooling of the latter. As a consequence the total mass of Hi increases. This model has recently been used to model the Leiden-Argentina-Bonn survey of Galactic Hi. The values of the model’s parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a remarkable degree of consistency, despite the very different natures of the two external galaxies and the dramatic difference in the nature of the data for our Galaxy and the external galaxies. The parameter values are also consistent with hydrodynamical simulations of the ablation of individual clouds. The model predicts that a galaxy that loses its cool-gas disc for instance through a major merger cannot reform it from its corona; it can return to steady star formation only if it can capture a large body of cool gas, for example by accreting a gas-rich dwarf. Thus the model explains how major mergers can make galaxies “red and dead.”

  10. Dynamics of core accretion

    Science.gov (United States)

    Nelson, Andrew F.; Ruffert, Maximilian

    2013-02-01

    We perform three-dimensional hydrodynamic simulations of gas flowing around a planetary core of mass Mpl = 10M⊕ embedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behaviour following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate-temperature background disc with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the `Piecewise Parabolic Method' with as many as six fixed nested grids, providing spatial resolution on the finest grid comparable to the present-day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static envelope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disc. It is sensitive to the thermodynamic treatment of the gas, as modelled by prescribed equations of state (either `locally isothermal' or `locally isentropic') and the temperature of the background disc material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influences the magnitude and character of hydrodynamic feedback of the small-scale flow on the background, and we conclude that accurate modelling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling however, as defined by locally isothermal or

  11. Accretion discs trapped near corotation

    OpenAIRE

    D'Angelo, C.R.; Spruit, H.C.

    2012-01-01

    We show that discs accreting on to the magnetosphere of a rotating star can end up in a trapped state, in which the inner edge of the disc stays near the corotation radius, even at low and varying accretion rates. The accretion in these trapped states can be steady or cyclic; we explore these states over a wide range of parameter space. We find two distinct regions of instability: one related to the buildup and release of mass in the disc outside corotation, and the other to mass storage with...

  12. Cooling of young stars growing by disk accretion

    CERN Document Server

    Rafikov, Roman

    2007-01-01

    In the initial formation stages young stars must acquire a significant fraction of their mass by accretion from a circumstellar disk that forms in the center of a collapsing protostellar cloud. Throughout this period mass accretion rates through the disk can reach 10^{-6}-10^{-5} M_Sun/yr leading to substantial energy release in the vicinity of stellar surface. We study the impact of irradiation of the stellar surface produced by the hot inner disk on properties of accreting fully convective low-mass stars, and also look at objects such as young brown dwarfs and giant planets. At high accretion rates irradiation raises the surface temperature of the equatorial region above the photospheric temperature T_0 that a star would have in the absence of accretion. The high-latitude (polar) parts of the stellar surface, where disk irradiation is weak, preserve their temperature at the level of T_0. In strongly irradiated regions an almost isothermal outer radiative zone forms on top of the fully convective interior, l...

  13. MRI-driven Accretion onto Magnetized stars: Axisymmetric MHD Simulations

    CERN Document Server

    Romanova, Marina M; Koldoba, Alexander V; Lovelace, Richard V E

    2011-01-01

    We present the first results of a global axisymmetric simulation of accretion onto rotating magnetized stars from a turbulent, MRI-driven disk. The angular momentum is transported outward by the magnetic stress of the turbulent flow with a rate corresponding to a Shakura-Sunyaev viscosity parameter alpha\\approx 0.01-0.04. The result of the disk-magnetosphere interaction depends on the orientation of the poloidal field in the disk relative to that of the star at the disk-magnetosphere boundary. If fields have the same polarity, then the magnetic flux is accumulated at the boundary and blocks the accretion which leads to the accumulation of matter at the boundary. Subsequently, this matter accretes to the star in outburst before accumulating again. Hence, the cycling, `bursty' accretion is observed. If the disc and stellar fields have opposite polarity, then the field reconnection enhances the penetration of the disk matter towards the deeper field lines of the magnetosphere. However, the magnetic stress at the...

  14. He-Accreting WDs: accretion regimes and final outcomes

    CERN Document Server

    Piersanti, L; Yungelson, L R

    2014-01-01

    The behaviour of carbon-oxygen white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyze the thermal response of the accreting WD to mass deposition at different time scales. The analysis has been performed for initial WDs masses and accretion rates in the range (0.60 - 1.02) Msun and 1.e-9 - 1.e-5 Msun/yr, respectively. Thermal regimes in the parameters space M_{WD} - dot{M}_{He}, leading to formation of red-giant-like structure, steady burning of He, mild, strong and dynamical flashes have been identified and the transition between those regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated in order to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building-up of a He-rich layer via H-burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomi...

  15. Dynamics of continental accretion.

    Science.gov (United States)

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

  16. The Magnetospheres of (Accreting Neutron Stars

    Directory of Open Access Journals (Sweden)

    Wilms J.

    2014-01-01

    Full Text Available I give an overview of the most important observational tools to study the magnetospheres of accreting neutron stars, with a focus on accreting neutron stars in high mass X-ray binary systems. Topics covered are the different types of accretion onto neutron stars and the structure of the accretion column, and how models for these can be tested with observations.

  17. Eclipse mapping of accretion discs

    OpenAIRE

    Baptista, Raymundo

    2000-01-01

    The eclipse mapping method is an inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs. In this review I present the basics of the method and discuss its different implementations. I summarize the most important results obtained to date and discuss how they have helped to improve our understanding of accretion physics, from testing the theoretical radial brightness temperature dist...

  18. Self-gravitating accretion discs

    OpenAIRE

    Lodato, G.

    2008-01-01

    I review recent progresses in the dynamics and the evolution of self-gravitating accretion discs. Accretion discs are a fundamental component of several astrophysical systems on very diverse scales, and can be found around supermassive black holes in Active Galactic Nuclei (AGN), and also in our Galaxy around stellar mass compact objects and around young stars. Notwithstanding the specific differences arising from such diversity in physical extent, all these systems share a common feature whe...

  19. The Rate of Gas Accretion onto Black Holes Drives Jet Velocity

    CERN Document Server

    King, Ashley L; Bietenholz, Michael; Gultekin, Kayhan; Reynolds, Mark; Mioduszewski, Amy; Rupen, Michael; Bartel, Norbert

    2014-01-01

    Accreting black holes are observed to launch relativistic, collimated jets of matter and radiation. In some sources, discrete ejections have been detected with highly relativistic velocities. These particular sources typically have very high mass accretion rates, while sources lower knot velocities are predominantly associated with black holes with relatively low mass accretion rates. We quantify this behavior by examining knot velocity with respect to X-ray luminosity, a proxy for mass accretion rate onto the black hole. We find a positive correlation between the mass-scaled X-ray luminosity and jet knot velocity. In addition, we find evidence that the jet velocity is also a function of polar angle, supporting the "spine-sheath" model of jet production. Our results reveal a fundamental aspect of how accretion shapes mechanical feedback from black holes into their host environments.

  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. How do accretion discs break?

    Science.gov (United States)

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  2. Twisted accretion discs: Pt. 5; Viscous evolution

    Energy Technology Data Exchange (ETDEWEB)

    Kumar, S. (Max-Planck-Institut fuer Physik und Astrophysik, Garching (Germany, F.R.). Inst. fuer Astrophysik)

    1990-08-15

    The time-dependence of accretion discs with orbits tilted out of the symmetry plane is studied. The effects of mass inflow modulation, and tilt variation at the disc outer edge, are examined for both circumbinary discs and for discs around compact objects. The appendices extend the numerical work to some analytic examples of tilt diffusion and external forcing effects. It is also shown that the disc must not be treated as a rigid tilted object if global angular momentum is to be conserved. These results are relevant to the problem of long-term periodicities of the light curves in Her X1 and {epsilon} Aur, the S-type symmetry of radio jets, warped gas discs in galaxies and polar rings in Neptune. Twisted discs may also arise in star-forming regions. (author).

  3. Terrestrial Planets Accreted Dry

    Science.gov (United States)

    Albarede, F.; Blichert-Toft, J.

    2007-12-01

    Plate tectonics shaped the Earth, whereas the Moon is a dry and inactive desert. Mars probably came to rest within the first billion years of its history, and Venus, although internally very active, has a dry inferno for its surface. The strong gravity field of a large planet allows for an enormous amount of gravitational energy to be released, causing the outer part of the planetary body to melt (magma ocean), helps retain water on the planet, and increases the pressure gradient. The weak gravity field and anhydrous conditions prevailing on the Moon stabilized, on top of its magma ocean, a thick buoyant plagioclase lithosphere, which insulated the molten interior. On Earth, the buoyant hydrous phases (serpentines) produced by reactions between the terrestrial magma ocean and the wet impactors received from the outer Solar System isolated the magma and kept it molten for some few tens of million years. The elemental distributions and the range of condensation temperatures show that the planets from the inner Solar System accreted dry. The interior of planets that lost up to 95% of their K cannot contain much water. Foundering of their wet surface material softened the terrestrial mantle and set the scene for the onset of plate tectonics. This very same process may have removed all the water from the surface of Venus 500 My ago and added enough water to its mantle to make its internal dynamics very strong and keep the surface very young. Because of a radius smaller than that of the Earth, not enough water could be drawn into the Martian mantle before it was lost to space and Martian plate tectonics never began. The radius of a planet therefore is the key parameter controlling most of its evolutional features.

  4. YY Sex: a Polar Candidate

    Science.gov (United States)

    Gabdeev, M. M.; Shimanskiy, V. V.; Borisov, N. V.; Tazieva, Z. R.

    2017-06-01

    We present spectroscopic investigations of a cataclysmic variable star, YY Sex. There are some uncertainties in the classification of this object. We calculate Doppler maps for Hβ and HeII λ4686Å and show that there is no sign of disk accretion in YY Sex. Consequently, we conclude that YY Sex is a polar.

  5. AGN accretion disks as spatially resolved by polarimetry

    CERN Document Server

    Kishimoto, Makoto; Blaes, Omer; Lawrence, Andy; Boisson, Catherine; Albrecht, Marcus; Leipski, Christian

    2008-01-01

    A crucial difficulty in understanding the nature of the putative accretion disk in AGNs is that some of its key intrinsic spectral signatures cannot be observed directly. The strong emissions from the broad-line region (BLR) and the obscuring torus, which are generally yet to be spatially resolved, essentially 'bury' such signatures. Here we argue that we can actually isolate the disk emission spectrum by using optical and near-infrared polarization of quasars and uncover the important spectral signatures. In these quasars, the polarization is considered to originate from electron scattering interior to the BLR, so that the polarized flux shows the disk spectrum with all the emissions from the BLR and torus eliminated. The polarized flux observations have now revealed a Balmer edge feature in absorption and a blue near-infrared spectral shape consistent with a specific and robust theoretical prediction. These results critically verify the long-standing picture of an optically thick and locally heated disk in ...

  6. Self consistent modeling of accretion columns in accretion powered pulsars

    Science.gov (United States)

    Falkner, Sebastian; Schwarm, Fritz-Walter; Wolff, Michael Thomas; Becker, Peter A.; Wilms, Joern

    2016-04-01

    We combine three physical models to self-consistently derive the observed flux and pulse profiles of neutron stars' accretion columns. From the thermal and bulk Comptonization model by Becker & Wolff (2006) we obtain seed photon continua produced in the dense inner regions of the accretion column. In a thin outer layer these seed continua are imprinted with cyclotron resonant scattering features calculated using Monte Carlo simulations. The observed phase and energy dependent flux corresponding to these emission profiles is then calculated, taking relativistic light bending into account. We present simulated pulse profiles and the predicted dependency of the observable X-ray spectrum as a function of pulse phase.

  7. Externally Fed Accretion onto Protostars

    CERN Document Server

    Dalba, Paul A

    2012-01-01

    The asymmetric molecular emission lines from dense cores reveal slow, inward motion in the clouds' outer regions. This motion is present both before and after the formation of a central star. Motivated by these observations, we revisit the classic problem of steady, spherical accretion of gas onto a gravitating point mass, but now include self-gravity of the gas and impose a finite, subsonic velocity as the outer boundary condition. We find that the accretion rate onto the protostar is lower than values obtained for isolated, collapsing clouds, by a factor that is the Mach number of the outer flow. Moreover, the region of infall surrounding the protostar spreads out more slowly, at a speed close to the subsonic, incoming velocity. Our calculation, while highly idealized, provides insight into two longstanding problems -- the surprisingly low accretion luminosities of even the most deeply embedded stellar sources, and the failure so far to detect spatially extended, supersonic infall within their parent dense ...

  8. Atmospheric Ice Accretion Measurement Techniques

    Directory of Open Access Journals (Sweden)

    M Virk

    2016-09-01

    Full Text Available Atmospheric icing on structures has proven to be an area of concern in many cold climate geographical regions like arctic and alpine. Difficulties encountered by the communication, construction and power industries in these areas are the subject of intense investigations for researchers from decades. Three main methods of investigation are generally employed by researchers to study atmospheric ice accretion on structures: a continuous field measurements, b lab based simulations using icing wind tunnel & c numerical modelling. This paper presents a brief review study of various techniques to understand and measure the atmospheric ice accretion on structures, anti/de icing techniques and important parameters for numerical modelling of atmospheric ice accretion.

  9. Eclipse mapping of accretion discs

    CERN Document Server

    Baptista, R

    2000-01-01

    The eclipse mapping method is an inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs. In this review I present the basics of the method and discuss its different implementations. I summarize the most important results obtained to date and discuss how they have helped to improve our understanding of accretion physics, from testing the theoretical radial brightness temperature distribution and measuring mass accretion rates to showing the evolution of the structure of a dwarf novae disc through its outburst cycle, from isolating the spectrum of a disc wind to revealing the geometry of disc spiral shocks. I end with an outline of the future prospects.

  10. Black hole accretion disc impacts

    CERN Document Server

    Pihajoki, Pauli

    2015-01-01

    We present an analytic model for computing the luminosity and spectral evolution of flares caused by a supermassive black hole impacting the accretion disc of another supermassive black hole. Our model includes photon diffusion, emission from optically thin regions and relativistic corrections to the observed spectrum and time-scales. We test the observability of the impact scenario with a simulated population of quasars hosting supermassive black hole binaries. The results indicate that for a moderate binary mass ratio of 0.3, and impact distances of 100 primary Schwarzschild radii, the accretion disc impacts can be expected to equal or exceed the host quasar in brightness at observed wavelength {\\lambda} = 510 nm up to z = 0.6. We conclude that accretion disc impacts may function as an independent probe for supermassive black hole binaries. We release the code used for computing the model light curves to the community.

  11. Optical polarimetry of PKS 2155 - 304 and constraints on accretion disk models for BL Lacertae objects

    Science.gov (United States)

    Smith, Paul S.; Sitko, Michael L.

    1991-01-01

    Optical broad-band polarimetry and photometry of the BL Lacertae object PKS 2155 - 304 during late 1990 are presented. Variability in both flux and linear polarization was moderate during this period. The optical polarization ranged from 2 to 7 percent while photometric variations were within 0.2 mag (V = 13.2-13.4). Accurate multicolor measurements were made to study any wavelength dependence of the polarization with an eye to using these data to test the model of Wandel and Urry (1991) which proposes that the UV to soft X-ray continuum of this object arises from an accretion disk. Wavelength-dependent polarization (WDP) is observed in PKS 2155 - 304. However, the polarization is always seen to decrease with wavelength when WDP is observed. This is opposite to the expected sense of WDP if the UV continuum is dominated by an accretion disk.

  12. Dissecting accretion and outflows in accreting white dwarf binaries

    CERN Document Server

    de Martino, D; Balman, S; Bernardini, F; Bianchini, A; Bode, M; Bonnet-Bidaud, J -M; Falanga, M; Greiner, J; Groot, P; Hernanz, M; Israel, G; Jose, J; Motch, C; Mouchet, M; Norton, A J; Nucita, A; Orio, M; Osborne, J; Ramsay, G; Rodriguez-Gil, P; Scaringi, S; Schwope, A; Traulsen, I; Tamburini, F

    2015-01-01

    This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of accreting white dwarfs. For a summary, we refer to the paper.

  13. Accretion physics: It's not U, it's B

    Science.gov (United States)

    Miller, Jon

    2017-03-01

    Black holes grow by accreting mass, but the process is messy and redistributes gas and energy into their environments. New evidence shows that magnetic processes mediate both the accretion and ejection of matter.

  14. Slim Discs with Varying Accretion Rates

    Institute of Scientific and Technical Information of China (English)

    JIAO Cheng-Liang; LU Ju-Fu

    2009-01-01

    @@ It was revealed in our previous studies that there exists a maximal possible accretion rate for slim discs with constant accretion rates because the correctly calculated vertical gravitational force can only gather some limited amount of accreted matter. Here we show that when the accretion rate is not constant and instead decreases with decreasing radius because of outflows, such that the amount of accreted matter is adjusted to be within the allowed limit, global slim disc solutions can be constructed even for the case that accretion rates at large radii apparently exceed the maximal possible value. This result further demonstrates that outflows seem to be unavoidable for accretion flows with large accretion rates at large radii.

  15. Perturbation growth in accreting filaments

    Science.gov (United States)

    Clarke, S. D.; Whitworth, A. P.; Hubber, D. A.

    2016-05-01

    We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long filaments as they form and grow by accretion. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length-scale which is roughly four times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multiwavelength density power spectrum, there exists a clear preferred core separation equal to the largest peak in the dispersion relation. Our results allow one to estimate a minimum age for a filament which is breaking up into regularly spaced fragments, as well as an average accretion rate. We apply the model to observations of filaments in Taurus by Tafalla & Hacar and find accretion rates consistent with those estimated by Palmeirim et al.

  16. Outflow from Hot Accretion Flows

    CERN Document Server

    Yuan, Feng; Wu, Maochun

    2012-01-01

    Numerical simulations of hot accretion flows have shown that the mass accretion rate decreases with decreasing radius. Two models have been proposed to explain this result. In the adiabatic inflow-outflow solution (ADIOS), it is thought to be due to the loss of gas in outflows. In the convection-dominated accretion flow (CDAF) model, it is explained as because that the gas is locked in convective eddies. In this paper we use hydrodynamical (HD) and magnetohydrodynamical (MHD) simulations to investigate which one is physical. We calculate and compare various properties of inflow (gas with an inward velocity) and outflow (gas with an outward velocity). Systematic and significant differences are found. For example, for HD flows, the temperature of outflow is higher than inflow; while for MHD flows, the specific angular momentum of outflow is much higher than inflow. We have also analyzed the convective stability of MHD accretion flow and found that they are stable. These results suggest that systematic inward an...

  17. The Black Hole Accretion Code

    CERN Document Server

    Porth, Oliver; Mizuno, Yosuke; Younsi, Ziri; Rezzolla, Luciano; Moscibrodzka, Monika; Falcke, Heino; Kramer, Michael

    2016-01-01

    We present the black hole accretion code (BHAC), a new multidimensional general-relativistic magnetohydrodynamics module for the MPI-AMRVAC framework. BHAC has been designed to solve the equations of ideal general-relativistic magnetohydrodynamics in arbitrary spacetimes and exploits adaptive mesh refinement techniques with an efficient block-based approach. Several spacetimes have already been implemented and tested. We demonstrate the validity of BHAC by means of various one-, two-, and three-dimensional test problems, as well as through a close comparison with the HARM3D code in the case of a torus accreting onto a black hole. The convergence of a turbulent accretion scenario is investigated with several diagnostics and we find accretion rates and horizon-penetrating fluxes to be convergent to within a few percent when the problem is run in three dimensions. Our analysis also involves the study of the corresponding thermal synchrotron emission, which is performed by means of a new general-relativistic radi...

  18. 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.)

  19. New mechanism of radiation polarization in Seyfert-1 AGNs

    CERN Document Server

    Silant'ev, N A; Piotrovich, M Yu; Natsvlishvili, T M; Buliga, S D

    2016-01-01

    In most of Seyfert-1 active galactic nucei (AGN) the optical linear continuum polarization degree is usually small (less than 1%) and the polarization position angle is nearly parallel to the AGN radio-axis. However, there are many types-1 AGNs with unexplained intermediate values for both positional angles and polarization degrees. Our explanation of polarization degree and positional angle of Seyfert-1 AGNs focuses on the reflection of non-polarized radiation from sub-parsec jets in optically thick accretion discs. The presence of a magnetic field surrounding the scattering media will induce Faraday rotation of the polarization plane that may explain the intermediate values of positional angles if there is a magnetic field component normal to the accretion disc. The Faraday rotation depolarization effect in disc diminishes the competition between polarization of the reflected radiation with the parallel component of polarization and the perpendicular polarization from internal radiation of disc (the Milne p...

  20. Accretion of Jupiter-mass planets in the limit of vanishing viscosity

    Energy Technology Data Exchange (ETDEWEB)

    Szulágyi, J.; Morbidelli, A.; Crida, A. [University of Nice-Sophia Antipolis, CNRS, Observatoire de la Côte d' Azur, Laboratoire Lagrange, F-06304, Nice (France); Masset, F., E-mail: jszulagyi@oca.eu [Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, P.O. Box 48-3, 62251 Cuernavaca, Morelos (Mexico)

    2014-02-20

    In the core-accretion model, the nominal runaway gas-accretion phase brings most planets to multiple Jupiter masses. However, known giant planets are predominantly Jupiter mass bodies. Obtaining longer timescales for gas accretion may require using realistic equations of states, or accounting for the dynamics of the circumplanetary disk (CPD) in the low-viscosity regime, or both. Here we explore the second way by using global, three-dimensional isothermal hydrodynamical simulations with eight levels of nested grids around the planet. In our simulations, the vertical inflow from the circumstellar disk (CSD) to the CPD determines the shape of the CPD and its accretion rate. Even without a prescribed viscosity, Jupiter's mass-doubling time is ∼10{sup 4} yr, assuming the planet at 5.2 AU and a Minimum Mass Solar Nebula. However, we show that this high accretion rate is due to resolution-dependent numerical viscosity. Furthermore, we consider the scenario of a layered CSD, viscous only in its surface layer, and an inviscid CPD. We identify two planet-accretion mechanisms that are independent of the viscosity in the CPD: (1) the polar inflow—defined as a part of the vertical inflow with a centrifugal radius smaller than two Jupiter radii and (2) the torque exerted by the star on the CPD. In the limit of zero effective viscosity, these two mechanisms would produce an accretion rate 40 times smaller than in the simulation.

  1. Physical Environment of Accreting Neutron Stars

    Directory of Open Access Journals (Sweden)

    J. Wang

    2016-01-01

    Full Text Available Neutron stars (NSs powered by accretion, which are known as accretion-powered NSs, always are located in binary systems and manifest themselves as X-ray sources. Physical processes taking place during the accretion of material from their companions form a challenging and appealing topic, because of the strong magnetic field of NSs. In this paper, we review the physical process of accretion onto magnetized NS in X-ray binary systems. We, firstly, give an introduction to accretion-powered NSs and review the accretion mechanism in X-ray binaries. This review is mostly focused on accretion-induced evolution of NSs, which includes scenario of NSs both in high-mass binaries and in low-mass systems.

  2. Perturbation growth in accreting filaments

    CERN Document Server

    Clarke, Seamus D; Hubber, David A

    2016-01-01

    We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long, initially sub-critical but accreting filaments. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length scale which is roughly 4 times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multi-wavelength density power spectrum there exists a clear preferred core separation equal to the largest peak in the dispe...

  3. Poynting Jets from Accretion Disks

    CERN Document Server

    Lovelace, R V E; Ustyugova, G V; Romanova, M M; Colgate, S A

    2002-01-01

    The powerful narrow jets observed to emanate from many compact accreting objects may arise from the twisting of a magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, {\\it hydromagnetic outflows}, which have a significant mass flux and have energy and angular momentum carried by both the matter and the electromagnetic field and, Poynting outflows, where the mass flux is negligible and energy and angular momentum are carried predominantly by the electromagnetic field. Here we consider a Keplerian disk initially threaded by a dipole-like magnetic field and we present solutions of the force-free Grad-Shafranov equation for the coronal plasma. We find solutions with Poynting jets where there is a continuous outflow of energy and toroidal magnetic flux from the disk into the external space. This behavior contradicts the commonly accepted ``theorem'' of Solar plasma physics that the motio...

  4. Ringed accretion disks: equilibrium configurations

    CERN Document Server

    Pugliese, D

    2015-01-01

    We investigate a model of ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the General Relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can be then determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We pr...

  5. Dynamics of warped accretion discs

    OpenAIRE

    Tremaine, Scott; Davis, Shane W.

    2013-01-01

    Accretion discs are present around both stellar-mass black holes in X-ray binaries and supermassive black holes in active galactic nuclei. A wide variety of circumstantial evidence implies that many of these discs are warped. The standard Bardeen--Petterson model attributes the shape of the warp to the competition between Lense--Thirring torque from the central black hole and viscous angular-momentum transport within the disc. We show that this description is incomplete, and that torques from...

  6. Counter-Rotating Accretion Discs

    CERN Document Server

    Dyda, Sergei; Ustyugova, Galina V; Romanova, Marina M; Koldoba, Alexander V

    2014-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud onto the surface of an existing co-rotating disc or from the counter-rotating gas moving radially inward to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc center. We discuss high-resolution axisymmetric hydrodynamic simulations of a viscous counter-rotating disc for cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic $\\alpha-$viscosity including all terms in the viscous stress tensor. For the vertically separated components a shear layer forms between them. The middle of this layer free-falls to the disk center. The accretion rates are increased by factors $\\sim 10^2-10^4$ over that of a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dep...

  7. Magnetic flux stabilizing thin accretion disks

    CERN Document Server

    Sadowski, Aleksander

    2016-01-01

    We calculate the minimal amount of large-scale poloidal magnetic field that has to thread the inner, radiation-over-gas pressure dominated region of a thin disk for its thermal stability. Such a net field amplifies the magnetization of the saturated turbulent state and makes it locally stable. For a $10 M_\\odot$ black hole the minimal magnetic flux is $10^{24}(\\dot M/\\dot M_{\\rm Edd})^{20/21}\\,\\rm G\\cdot cm^{2}$. This amount is compared with the amount of uniform magnetic flux that can be provided by the companion star -- estimated to be in the range $10^{22}-10^{24}\\,\\rm G\\cdot cm^2$. If accretion rate is large enough, the companion is not able to provide the required amount and such a system, if still sub-Eddington, must be thermally unstable. The peculiar variability of GRS 1915+105, an X-ray binary with the exceptionally high BH mass and near-Eddington luminosity, may result from the shortage of large scale poloidal field of uniform polarity.

  8. Magnetic flux stabilizing thin accretion discs

    Science.gov (United States)

    Sądowski, Aleksander

    2016-10-01

    We calculate the minimal amount of large-scale poloidal magnetic field that has to thread the inner, radiation-over-gas pressure dominated region of a thin disc for its thermal stability. Such a net field amplifies the magnetization of the saturated turbulent state and makes it locally stable. For a 10 M⊙ black hole the minimal magnetic flux is 10^{24}(dot{M}/dot{M}_Edd)^{20/21} G cm2. This amount is compared with the amount of uniform magnetic flux that can be provided by the companion star - estimated to be in the range 1022-1024 G cm2. If accretion rate is large enough, the companion is not able to provide the required amount and such a system, if still sub-Eddington, must be thermally unstable. The peculiar variability of GRS 1915+105, an X-ray binary with the exceptionally high BH mass and near-Eddington luminosity, may result from the shortage of large-scale poloidal field of uniform polarity.

  9. Observational evidence for matter propagation in accretion flows

    CERN Document Server

    Revnivtsev, M; Kniazev, A; Burenin, R; Buckley, D A H; Churazov, E

    2010-01-01

    We study simultaneous X-ray and optical observations of three intermediate polars EX Hya, V1223 Sgr and TV Col with the aim to understand the propagation of matter in their accretion flows. We show that in all cases the power spectra of flux variability of binary systems in X-rays and in optical band are similar to each other and the majority of X-ray and optical fluxes are correlated with time lag <1 sec. These findings support the idea that optical emission of accretion disks, in these binary systems,largely originates as reprocessing of X-ray luminosity of their white dwarfs. In the best obtained dataset of EX Hya we see that the optical lightcurve unambiguously contains some component, which leads the X-ray emission by ~7 sec. We interpret this in the framework of the model of propagating fluctuations and thus deduce the time of travel of the matter from the innermost part of the truncated accretion disk to the white dwarf surface. This value agrees very well with the time expected for matter threaded ...

  10. Black hole feedback from thick accretion discs

    CERN Document Server

    Sadowski, Aleksander; Abramowicz, Marek A; Narayan, Ramesh

    2015-01-01

    We study energy flows in geometrically thick accretion discs, both optically thick and thin, using general relativistic, three-dimensional simulations of black hole accretion flows. We find that for non-rotating black holes the efficiency of the total feedback from thick accretion discs is $3\\%$ - roughly half of the thin disc efficiency. This amount of energy is ultimately distributed between outflow and radiation, the latter scaling weakly with the accretion rate for super-critical accretion rates, and returned to the interstellar medium. Accretion on to rotating black holes is more efficient because of the additional extraction of rotational energy. However, the jet component is collimated and likely to interact only weakly with the environment, whereas the outflow and radiation components cover a wide solid angle.

  11. Interpreting MAD within multiple accretion regimes

    CERN Document Server

    Mocz, Philip

    2014-01-01

    General relativistic magnetohydrodynamic (GRMHD) simulations of accreting black holes in the radiatively inefficient regime show that systems with sufficient magnetic poloidal flux become magnetically arrested disc (MAD) systems, with a well-defined relationship between the magnetic flux and the mass accretion rate. Recently, Zamaninasab (2014) report that the jet magnetic flux and accretion disc luminosity are tightly correlated over 7 orders of magnitude for a sample of 76 radio-loud active galaxies, concluding that the data are explained by the MAD mode of accretion. Their analysis assumes radiatively efficient accretion, and their sample consists primarily of radiatively efficient sources, while GRMHD simulations of MAD thus far have been carried out in the radiatively inefficient regime. We propose a model to interpret MAD systems in the context of multiple accretion regimes, and apply it to the sample in Zamaninasab (2014), along with additional radiatively inefficient sources from archival data. We sho...

  12. 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.

  13. Theory of Disk Accretion onto Magnetic Stars

    CERN Document Server

    Lai, Dong

    2014-01-01

    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 stars and protostellar systems, as well as possible connections to protoplanetary disks and exoplanets.

  14. Lithium synthesis in microquasar accretion.

    Science.gov (United States)

    Iocco, Fabio; Pato, Miguel

    2012-07-13

    We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes 6Li and 7Li can be produced, the exact figures varying with the characteristics of the torus and reaching as much as 10(-2) M⊙ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis.

  15. Protostellar accretion traced with chemistry

    DEFF Research Database (Denmark)

    Frimann, Søren; Jørgensen, Jes Kristian; Dunham, Michael M.

    2017-01-01

    . Our aim is to characterise protostellar accretion histories towards individual sources by utilising sublimation and freeze-out chemistry of CO. Methods. A sample of 24 embedded protostars are observed with the Submillimeter Array (SMA) in context of the large program "Mass Assembly of Stellar Systems...... and their Evolution with the SMA" (MASSES). The size of the C18O-emitting region, where CO has sublimated into the gas-phase, is measured towards each source and compared to the expected size of the region given the current luminosity. The SMA observations also include 1.3 mm continuum data, which are used...

  16. Counter-Rotating Accretion Discs

    OpenAIRE

    Dyda, Sergei; Lovelace, Richard V. E.; Ustyugova, Galina V.; Romanova, Marina M.; Koldoba, Alexander V.

    2014-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud onto the surface of an existing co-rotating disc or from the counter-rotating gas moving radially inward to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc center. We discuss high-resolution axisymmetric hydrodynamic simulations of a viscous counter-rotating disc for cases where the tw...

  17. Advection of magnetic flux by accretion disks around neutron stars

    Science.gov (United States)

    Flores-Tulian, S.; Reisenegger, A.

    The aim of our research is to address why millisecond pulsars have relatively weak surface magnetic fields, of about 10^8 G, with a narrow spread. We propose that the accretion of plasma from the companion star fully screens the original neutron star field, but the accretion disk carries additional magnetic flux from the companion star, or itself can generate field by means of dynamo processes. For a strongly magnetized star, the field prevents the disk from approaching the star. The accretion is along the field lines and deposits the matter on the polar cap. Then, the accreted plasma flows, dragging with itself the magnetic field lines, from the pole to the equator (Payne & Melatos 2004). In a following stage, when the star becomes non-magnetic, because the field has been buried, the disk touches the star. We suggest that some effective mechanism of magnetic flux transport such as that proposed by Spruit & Uzdensky 2005 (or Bisnovatyi-Kogan & Lovelace 2007), operates and necessarily leads to a "strongly magnetized disk''. It becomes laminar because the magneto-rotational instability saturates (it is considered to be responsible for turbulence in the disk), and the magnetic difussivity is negligible. Then, the loss of angular momentum allowing the accretion is only caused by the magneto-centrifugal disk-wind (Blandford & Payne 1982). Meanwhile, the wind-driven transport of the magnetic flux by the disk re-magnetizes the star. This process continues until the Lorentz force due to the star's magnetic field forbids any further accretion of matter and magnetic flux, in the Ideal Magneto-Hydro-Dynamics approach. Additional of material can fall onto the star (but at lower rate) if some instability process sets in, allowing the diffusion of mass through the magnetic field lines (e.g the Interchange Instability, Spruit & Taam 1990). All these processes might lead to an asymptotic magnetic field of 10^8 G,as is inferred from observations. We are developing a self

  18. Polar Ring Galaxies and Warps

    Science.gov (United States)

    Combes, F.

    Polar ring galaxies, where matter is in equilibrium in perpendicular orbits around spiral galaxies, are ideal objects to probe the 3D shapes of dark matter halos. The conditions to constrain the halos are that the perpendicular system does not strongly perturb the host galaxy, or that it is possible to derive back its initial shape, knowing the formation scenario of the polar ring. The formation mechanisms are reviewed: mergers, tidal accretion, or gas accretion from cosmic filaments. The Tully-Fisher diagram for polar rings reveals that the velocity in the polar plane is higher than in the host plane, which can only be explained if the dark matter is oblate and flattened along the polar plane. Only a few individual systems have been studied in details, and 3D shapes of their haloes determined by several methods. The high frequency of warps could be explained by spontaneous bending instability, if the disks are sufficiently self-gravitating, which can put constraints on the dark matter flattening.

  19. Polar ring galaxies and warps

    CERN Document Server

    Combes, F

    2005-01-01

    Polar ring galaxies, where matter is in equilibrium in perpendicular orbits around spiral galaxies, are ideal objects to probe the 3D shapes of dark matter halos. The conditions to constrain the halos are that the perpendicular system does not strongly perturb the host galaxy, or that it is possible to derive back its initial shape, knowing the formation scenario of the polar ring. The formation mechanisms are reviewed: mergers, tidal accretion, or gas accretion from cosmic filaments. The Tully-Fisher diagram for polar rings reveals that the velocity in the polar plane is higher than in the host plane, which can only be explained if the dark matter is oblate and flattened along the polar plane. Only a few individual systems have been studied in details, and 3D shapes of their haloes determined by several methods. The high frequency of warps could be explained by spontaneous bending instability, if the disks are sufficiently self-gravitating, which can put constraints on the dark matter flattening.

  20. Full 3-D MHD calculations of accretion flow Structure in magnetic cataclysmic variable stars with strong and complex magnetic fields

    CERN Document Server

    Zhilkin, A G; Mason, P A; 10.1134/S1063772912040087

    2012-01-01

    We performed 3D MHD calculations of stream accretion in cataclysmic variable stars for which the white dwarf primary star possesses a strong and complex magnetic field. These calculations are motivated by observations of polars; cataclysmic variables containing white dwarfs with magnetic fields sufficiently strong to prevent the formation of an accretion disk. So an accretion stream flows from the L1 point and impacts directly onto one or more spots on the surface of the white dwarf. Observations indicate that the white dwarf, in some binaries, possesses a complex (non-dipolar) magnetic field. We perform simulations of 10 polars or equivalently one asynchronous polar at 10 different beat phases. Our models have an aligned dipole plus quadrupole magnetic field centered on the white dwarf primary. We find that for a sufficiently strong quadrupole component an accretion spot occurs near the magnetic equator for slightly less than half of our simulations while a polar accretion zone is active for most of the rest...

  1. RINGED ACCRETION DISKS: EQUILIBRIUM CONFIGURATIONS

    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)

    2015-12-15

    We investigate a model of a ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the general relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can then be determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We present also a perturbation analysis based on evolution of the oscillating components of the ringed disk. The dynamics of the unstable phases of the ringed disk evolution seems to be promising in relation to high-energy phenomena demonstrated in active galactic nuclei.

  2. Plasma physics of accreting neutron stars

    Science.gov (United States)

    Ghosh, Pranab; Lamb, Frederick K.

    1991-01-01

    Plasma concepts and phenomena that are needed to understand X- and gamma-ray sources are discussed. The capture of material from the wind or from the atmosphere or envelope of a binary companion star is described and the resulting types of accretion flows discussed. The reasons for the formation of a magnetosphere around the neutron star are explained. The qualitative features of the magnetospheres of accreting neutron stars are then described and compared with the qualitative features of the geomagnetosphere. The conditions for stable flow and for angular and linear momentum conservation are explained in the context of accretion by magnetic neutron stars and applied to obtain rough estimates of the scale of the magnetosphere. Accretion from Keplerian disks is then considered in some detail. The radial structure of geometrically thin disk flows, the interaction of disk flows with the neutron star magnetosphere, and models of steady accretion from Keplerian disks are described. Accretion torques and the resulting changes in the spin frequencies of rotating neutron stars are considered. The predicted behavior is then compared with observations of accretion-powered pulsars. Magnetospheric processes that may accelerate particles to very high energies, producing GeV and, perhaps, TeV gamma-rays are discussed. Finally, the mechanisms that decelerate and eventually stop accreting plasma at the surfaces of strongly magnetic neutron stars are described.

  3. 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).

  4. Accretion, primordial black holes and standard cosmology

    Indian Academy of Sciences (India)

    B Nayak; P Singh

    2011-01-01

    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 are the proper candidates for dark matter.

  5. State Transitions of Black Hole Accretion Flows

    Institute of Scientific and Technical Information of China (English)

    卢炬甫; 潘刘彬

    2001-01-01

    We show that the thermal instability-triggered transition from the state of the Shakura-Sunyaev disc to the state of the advection-dominated accretion flow is possible for black hole accretion flows composed of two-temperature plasma with bremsstrahlung and synchrotron radiation and Comptonization.

  6. Pycnonuclear burning and accreting neutron stars

    CERN Document Server

    Yakovlev, D G

    2002-01-01

    We outline the phenomenon of deep crustal heating in transiently accreting neutron stars. It is produced by nuclear transformations (mostly, by pycnonuclear reactions) in accreted matter while this matter sinks to densities rho > 10^{10} g/cc under the weight of freshly accreted material. We consider then thermal states of transiently accreting neutron stars (with mean mass accretion rates \\dot{M}=(10^{-14}-10^{-9}) M_\\odot/yr) determined by deep crustal heating. In a simplified fashion we study how the thermal flux emergent from such stars depends on the properties of superdense matter in stellar interiors. We analyze the most important regulators of the thermal flux: strong superfluidity in the cores of low-mass stars and fast neutrino emission (in nucleon, pion-condensed, kaon-condensed, or quark phases of dense matter) in the cores of high-mass stars. We compare the results with observations of soft X-ray transients in quiescent states.

  7. Accretion and Outflow Activity in Brown Dwarfs

    CERN Document Server

    Riaz, B

    2013-01-01

    An investigation of the magnetospheric accretion and outflow signatures in sub-stellar objects is a natural extension of similar studies conducted on classical T Tauri stars (CTTS), and helps understand if brown dwarfs (BDs) follow the same formation mechanism as stars. Over the past decade, evidence for accretion in very low-mass stars (VLMs) and BDs has been accumulated using various techniques, which indicates that the overall accretion characteristics are continuous across the sub-stellar boundary. Outflow activity in VLMs and BDs has been confirmed based on spectro-astrometry of forbidden emission lines observed in the optical, and in millimetre continuum images of CO J=2-1 emission. This review summarizes the past and current state of observational work on accretion and outflow activity in VLMs and BDs, particularly with the advent of new instruments such as VLT/X-Shooter which has allowed the study of several accretion and outflow indicators over a wider wavelength range.

  8. Spiral shocks and accretion in discs

    Energy Technology Data Exchange (ETDEWEB)

    Spruit, H.C.; Matsuda, T.; Inoue, M.; Sawada, K.

    1987-12-01

    Recent numerical and analytical results on disc-like accretion with shock waves as the only dissipation mechanism are compared. The global properties of the process are similar to those of the viscous (..cap alpha..) disc model, but precise values of the effective ..cap alpha.. value as a function of the accretion rate can be calculated. At low values of the ratio of specific heats (..gamma.. < 1.45) accretion is possible without radiative losses. Such adiabatic accretion can occur in practice at high accretion rates on to low mass objects and may be important in the formation of planets. Following previous authors, it is pointed out that non-axisymmetric perturbations in the outer parts of a disc increase in amplitude as they propagate in and cause spiral shocks more easily in a disc than perturbations originating in the inner parts.

  9. Late Accretion and the Late Veneer

    CERN Document Server

    Morbidelli, Alessandro

    2014-01-01

    The concept of Late Veneer has been introduced by the geochemical community to explain the abundance of highly siderophile elements in the Earth's mantle and their chondritic proportions relative to each other. However, in the complex scenario of Earth accretion, involving both planetesimal bombardment and giant impacts from chondritic and differentiated projectiles, it is not obvious what the "Late Veneer" actually corresponds to. In fact, the process of differentiation of the Earth was probably intermittent and there was presumably no well-defined transition between an earlier phase where all metal sunk into the core and a later phase in which the core was a closed entity separated from the mantle. In addition, the modellers of Earth accretion have introduced the concept of "Late Accretion", which refers to the material accreted by our planet after the Moon-forming event. Characterising Late Veneer, Late Accretion and the relationship between the two is the major goal of this chapter.

  10. Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AA Tau

    Science.gov (United States)

    Donati, J.-F.; Skelly, M. B.; Bouvier, J.; Gregory, S. G.; Grankin, K. N.; Jardine, M. M.; Hussain, G. A. J.; Ménard, F.; Dougados, C.; Unruh, Y.; Mohanty, S.; Aurière, M.; Morin, J.; Farès, R.; MAPP Collaboration

    2010-12-01

    From observations collected with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope (CFHT) and with the NARVAL spectropolarimeter at the Télescope Bernard Lyot (TBL), we report the detection of Zeeman signatures on the prototypical classical T Tauri star AA Tau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AA Tau. We find that AA Tau hosts a 2-3 kG magnetic dipole tilted at ≃20° to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AA Tau host large cool spots at photospheric level and accretion regions at chromospheric level. The accretion rate at the surface of AA Tau at the time of our observations (estimated from the emission in the He I D3 line mainly) is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2 in average (in logarithmic scale and in M⊙ yr-1); this is an order of magnitude smaller than the disc accretion rate at which the magnetic truncation radius (below which the disc is disrupted by the stellar magnetic field) matches the corotation radius (where the Keplerian period equals the stellar rotation period) - a necessary condition for accretion to occur. It suggests that AA Tau is largely in the propeller regime, with most of the accreting material in the inner disc regions being expelled outwards and only a small fraction accreted towards the surface of the star. The strong variability in the observed surface mass accretion rate and the systematic time-lag of optical occultations (by the warped accretion disc) with respect to magnetic and accretion-powered emission maxima also support this conclusion. Our results imply that AA Tau is being actively spun-down by the star/disc magnetic coupling and appears as an ideal laboratory for studying angular momentum losses of forming suns

  11. Superhumps, resonances and accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Whitehurst, R.; King, A. (Leicester Univ. (UK). Dept. of Astronomy)

    1991-03-01

    The structure of accretion discs within binary systems is shown to be influenced by the excitation of resonances within the disc. Of particular importance is that near the 3:1 commensurability with the stars' orbit. This can be used to explain the superhump phenomenon of SU Ursae Majoris dwarf novae in superoutburst. This resonance can only appear for mass ratios which satisfy M{sub 2}/M{sub 1} < {approx equal} 0.25-0.33: for larger mass ratios the available resonances are weaker and of the wrong form to produce the superhump phenomenon. The mass-transfer stream is shown to be an important contributor to the growth rate of the resonance. (author).

  12. Protoplanetary Accretion by Collisional Fusion

    CERN Document Server

    Wettlaufer, J S

    2009-01-01

    The formation of a solar system is believed to have followed a multi-stage process around a protostar. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag; there is a ``bottleneck'' at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Thus, successful planetary accretion requires rapid planetesimal growth to km scale. A commonly accepted picture is that for collisional velocities $V_c$ above a certain threshold collisional velocity, ${V_{th}} \\sim$ 0.1-10 cm s$^{-1}$, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all collisions the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt explicitly with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter--p...

  13. Heating of protostellar accretion disks

    Science.gov (United States)

    de Campos, R. R.; Jatenco-Pereira, V.

    2017-07-01

    The magneto-rotational instability (MRI) is believed to be the mechanism responsible for a magneto-hydrodynamic turbulence that could lead to the accretion observed in protoplanetary disks. The need of a minimum amount of ionization in protostellar accretion disks is necessary for the MRI to take place. There are in the literature several studies that include the damping of Alfvén waves as an additional heating source besides the viscous heating mechanism in a geometrically thin and optically thick disk. The damping of the waves transfers energy to the disk increasing the temperature and consequently its ionization fraction, making possible the presence of the MRI in a large part of the disk. We analyzed the contribution of non-ideal effects such as Ohmic and ambipolar diffusion for the disk heating and compare these heating rates with those obtained by damping of Alfvén waves. In order to study these non-ideal effects, we have estimated the radiation emission of each effect through the energy conservation equation, and associated each emission with a black body radiation, which enabled us to assign a temperature contribution of each effect. Using the ATHENA code we were able to simulate the disk at different radial distances, and estimate the electric current density needed to calculate the radiation emission associated with each effect. Once we have those data, we were able to compare the results with other heating sources, like viscosity and Alfvén waves damping, and we concluded that the Ohmic and ambipolar diffusions do not heat the disk in any significant way.

  14. Jittering-jets explosion triggered by the standing accretion shock instability

    CERN Document Server

    Papish, Oded; Soker, Noam

    2015-01-01

    We show that the standing accretion shock instability (SASI) that has been used to ease the shock revival in core collapse supernovae (CCSNe) neutrino-driven explosion models, might play a much more decisive role in supplying the stochastic angular momentum required to trigger an explosion with jittering jets. To play a minor role in neutrino-based explosion models, the kinetic energy of the gas inside the stalled shock associated with the transverse (non-radial) motion should be about more than ten percent of the energy of the accreted gas. We find that this implies a stochastic angular momentum that can reach about five percent of the Keplerian specific angular momentum around the newly born neutron star. Such an accretion flow leaves an open conical region along the poles with an average opening angle of about 5 degrees. The outflow from the open polar region powers an explosion according to the jittering-jets model.

  15. Pulsed Accretion in a Variable Protostar

    CERN Document Server

    Muzerolle, James; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-01

    Periodic increases in luminosity arising from variable accretion rates have been predicted for some close pre-main sequence binary stars as they grow from circumbinary disks. The phenomenon is known as "pulsed accretion" and can affect the orbital evolution and mass distribution of young binaries, as well as the potential for planet formation in the circumbinary environment. Accretion variability is a common feature of young stars, with a large range of amplitudes and timescales as measured from multi-epoch observations at optical and infrared wavelengths. Periodic variations consistent with pulsed accretion have been seen in only a few young binaries via optical accretion tracers, albeit intermittently with accretion luminosity variations ranging from 0-50 percent from orbit to orbit. Here we report on a young protostar (age ~10^5 yr) that exhibits periodic variability in which the infrared luminosity increases by a factor of 10 in roughly one week every 25.34 days. We attribute this to pulsed accretion asso...

  16. X-ray Polarization in Relativistic Jets

    CERN Document Server

    McNamara, Aimee L; Wu, Kinwah

    2009-01-01

    We investigate the polarization properties of Comptonized X-rays from relativistic jets in Active Galactic Nuclei (AGN) using Monte Carlo simulations. We consider three scenarios commonly proposed for the observed X-ray emission in AGN: Compton scattering of blackbody photons emitted from an accretion disk; scattering of cosmic microwave background (CMB) photons; and self-Comptonization of intrinsically polarized synchrotron photons emitted by jet electrons. Our simulations show that for Comptonization of disk and CMB photons, the degree of polarization of the scattered photons increases with the viewing inclination angle with respect to the jet axis. In both cases the maximum linear polarization is approximately 20%. In the case of synchrotron self-Comptonization (SSC), we find that the resulting X-ray polarization depends strongly on the seed synchrotron photon injection site, with typical fractional polarizations of approximately P = 10 - 20% when synchrotron emission is localized near the jet base, while ...

  17. Long-term Multiwavelength Observations of Polars

    Science.gov (United States)

    Santana, Joshua; Mason, Paul A.

    2016-06-01

    Polars are cataclysmic variables with the highest magnetic field strengths (10-250 MG). Matter is accreted after being funneled by the strong magnetic field of the white dwarf. We perform a meta-study of multi-wavelength data of polars. Many polars have been observed in surveys, such as SDSS, 2MASS, ROSAT, just to name a few. Some polars have now been detected by the JVLA, part of an expanding class of radio CVs. A large subset of polars have long-term optical light curves from CRTS and AAVSO. We suggest that the long term light curves of polars display a variety of signature behaviors and may be grouped accordingly. Additional characteristics such a binary period, magnetic field strengths, X-ray properties, and distance estimates are examined in context with long-term observations.

  18. Accretion in Saturn's F Ring

    Science.gov (United States)

    Meinke, B. K.; Esposito, L. W.; Stewart, G.

    2012-12-01

    Saturn's F ring is the solar system's principal natural laboratory for direct observation of accretion and disruption processes. The ring resides in the Roche zone, where tidal disruption competes with self-gravity, which allows us to observe the lifecycle of moonlets. Just as nearby moons create structure at the B ring edge (Esposito et al. 2012) and the Keeler gap (Murray 2007), the F ring "shepherding" moons Prometheus and Pandora stir up ring material and create observably changing structures on timescales of days to decades. In fact, Beurle et al (2010) show that Prometheus makes it possible for "distended, yet gravitationally coherent clumps" to form in the F ring, and Barbara and Esposito (2002) predicted a population of ~1 km bodies in the ring. In addition to the observations over the last three decades, the Cassini Ultraviolet Imaging Spectrograph (UVIS) has detected 27 statistically significant features in 101 occultations by Saturn's F ring since July 2004. Seventeen of those 27 features are associated with clumps of ring material. Two features are opaque in occultation, which makes them candidates for solid objects, which we refer to as Moonlets. The 15 other features partially block stellar signal for 22 m to just over 3.7 km along the radial expanse of the occultation. Upon visual inspection of the occultation profile, these features resemble Icicles, thus we will refer to them as such here. The density enhancements responsible for such signal attenuations are likely due to transient clumping of material, evidence that aggregations of material are ubiquitous in the F ring. Our lengthy observing campaign reveals that Icicles are likely transient clumps, while Moonlets are possible solid objects. Optical depth is an indicator of clumping because more-densely aggregated material blocks more light; therefore, it is natural to imagine moonlets as later evolutionary stage of icicle, when looser clumps of material compact to form a feature that appears

  19. Radiation from optically thin accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Tylenda, R. (Polska Akademia Nauk, Torun. Pracownia Astrofizyki)

    1981-01-01

    Accretion discs in cataclysmic variables with low rates of mass transfer, M < or approx. 10/sup 16/g s/sup -1/, have outer regions optically thin in continuum. A simple approach that allows one to calculate the radiation spectra from such discs is presented. A great number of disc models has been obtained in order to study the influence of various parameters (accretion rate, outer radius of the disc, inclination angle, mass of the accreting degenerate dwarf, viscosity parameter) of discs on the outgoing continuous spectra, emission lines and the UBV colours.

  20. Increased Understanding of Accretion in Massive YSOs

    Science.gov (United States)

    De Wit, Willem-Jan; Caratti, A.; Kraus, S.

    2017-06-01

    That massive stars up to 20Msol form by disk accretion is by now reasonably well established. We will present the latest observational results for the formation of single and multiple massive YSOs. By means of optical interferometry using the newly commissioned instrument Gravity at the VLT-I, we show the discovery of a young, embedded, 170AU-wide binary and is the most massive and most compact accreting young binary to date. We will also present the results of a multi-site multi-epoch follow-up campaign of the first well studied accretion outburst in a massive YSO.

  1. Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AATau

    CERN Document Server

    Donati, JF; Bouvier, J; Gregory, SG; Grankin, KN; Jardine, MM; Hussain, GAJ; Menard, F; Dougados, C; Unruh, Y; Mohanty, S; Auriere, M; Morin, J; Fares, R

    2010-01-01

    From observations collected with the ESPaDOnS & NARVAL spectropolarimeters at CFHT and TBL, we report the detection of Zeeman signatures on the prototypical classical TTauri star AATau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AATau. We find that AATau hosts a 2-3kG magnetic dipole tilted at ~20deg to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AATau host large cool spots at photospheric level and accretion regions at chromospheric level. The logarithmic accretion rate at the surface of AATau at the time of our observations is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2 in average (in Msun/yr); this is an order of magnitude smaller than the disc accretion rate at which the magnetic truncation radius (below which the disc is disrupted by the s...

  2. The beaming of subhalo accretion

    Science.gov (United States)

    Libeskind, Noam I.

    2016-10-01

    We examine the infall pattern of subhaloes onto hosts in the context of the large-scale structure. We find that the infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (~ 10 Mpc) scale - a scale accessible by current surveys of peculiar velocities - it follows that findings presented here open a new window into the relation between the observed large scale structure unveiled by current surveys of peculiar velocities and the preferential infall direction of the Local Group. This may shed light on the unexpected alignments of dwarf galaxies seen in the Local Group.

  3. Probing the parsec-scale accretion flow of 3C 84 with millimeter wavelength polarimetry

    Energy Technology Data Exchange (ETDEWEB)

    Plambeck, R. L. [Radio Astronomy Laboratory, University of California, Berkeley, CA 94720 (United States); Bower, G. C.; Rao, Ramprasad [Academia Sinica Institute for Astronomy and Astrophysics (ASIAA), 645 N. Aohoku Pl., Hilo, HI 96720 (United States); Marrone, D. P. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Jorstad, S. G.; Marscher, A. P. [Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States); Doeleman, S. S.; Fish, V. L. [MIT Haystack Observatory, Route 40, Westford, MA 01886 (United States); Johnson, M. D., E-mail: plambeck@berkeley.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

    2014-12-10

    We report the discovery of Faraday rotation toward radio source 3C 84, the active galactic nucleus in NGC 1275 at the core of the Perseus Cluster. The rotation measure (RM), determined from polarization observations at wavelengths of 1.3 and 0.9 mm, is (8.7 ± 2.3)× 10{sup 5} rad m{sup –2}, among the largest ever measured. The RM remained relatively constant over a 2 yr period even as the intrinsic polarization position angle wrapped through a span of 300°. The Faraday rotation is likely to originate either in the boundary layer of the radio jet from the nucleus or in the accretion flow onto the central black hole. The accretion flow probably is disk-like rather than spherical on scales of less than a parsec, otherwise the RM would be even larger.

  4. Accretion disks around a mass with quadrupole

    CERN Document Server

    Abishev, Medeu; Quevedo, Hernando; Toktarbay, Saken

    2015-01-01

    We consider the stability properties of test particles moving along circular orbits around a mass with quadrupole. We show that the quadrupole modifies drastically the properties of an accretion disk made of such test particles.

  5. Foundations of Black Hole Accretion Disk Theory

    National Research Council Canada - National Science Library

    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...

  6. Radiatively inefficient MHD accretion-ejection structures

    CERN Document Server

    Casse, F; Casse, Fabien; Keppens, Rony

    2004-01-01

    We present magnetohydrodynamic simulations of a resistive accretion disk continuously launching transmagnetosonic, collimated jets. We time-evolve the full set of magnetohydrodynamic equations, but neglect radiative losses in the energetics (radiatively inefficient). Our calculations demonstrate that a jet is self-consistently produced by the interaction of an accretion disk with an open, initially bent large-scale magnetic field. A constant fraction of heated disk material is launched in the inner equipartition disk regions, leading to the formation of a hot corona and a bright collimated, super-fastmagnetosonic jet. We illustrate the complete dynamics of the ``hot'' near steady-state outflow (where thermal pressure $\\simeq$ magnetic pressure) by showing force balance, energy budget and current circuits. The evolution to this near stationary state is analyzed in terms of the temporal variation of energy fluxes controlling the energetics of the accretion disk. We find that unlike advection-dominated accretion...

  7. Accretion disks around a mass with quadrupole

    Science.gov (United States)

    Abishev, M.; Boshkayev, K.; Quevedo, H.; Toktarbay, S.

    We consider the stability properties of circular orbits of test particles moving around a mass with quadrupole. We show that the quadrupole modifies drastically the properties of an accretion disk made of such test particles.

  8. Coronal Neutrino Emission in Hypercritical Accretion Flows

    CERN Document Server

    Kawabata, R; Kawanaka, N

    2007-01-01

    Hypercritical accretion flows onto stellar mass black holes (BHs) are commonly considered as a promising model of central engines of gamma-ray bursts (GRBs). In this model a certain fraction of gravitational binding energy of accreting matter is deposited to the energy of relativistic jets via neutrino annihilation and/or magnetic fields. However, some recent studies have indicated that the energy deposition rate by neutrino annihilation is somewhat smaller than that needed to power a GRB. To overcome this difficulty, Ramirez-Ruiz & Socrates (2005) proposed that high energy neutrinos from hot corona above the accretion disk might enhance the efficiency of energy deposition. We elucidate the disk corona model in the context of hypercritical accretion flows. From the energy balance in the disk and the corona, we can calculate the disk and coronal temperature, Td and Tc, and neutrino spectra, taking into account the neutrino cooling processes by neutrino-electron scatterings and neutrino pair productions. Th...

  9. Some Interesting Behaviour of Accreting Particles in the Gap Region of Black Hole Accretion Discs

    Institute of Scientific and Technical Information of China (English)

    WANG Ding-Xiong; XIAO Kan; LEI Wei-Hua

    2001-01-01

    Some interesting behaviour of accreting particles in the gap region between the horizon of the Kerr black hole and the inner edge of the surrounding disc is investigated. The following results are obtained. (i) Spacetime coincidence of the maximum of angular velocity of accreting particles and that of the black hole horizon is extended to the more general disc-accretion. (ii) The possibility is discussed of negative energy of accreting particles in prograde orbit inside the ergosphere of the Kerr black hole, which is surrounded by strong enough magnetic field.

  10. Gas dynamics for accretion disk simulations

    Science.gov (United States)

    Whitehurst, R.

    1994-01-01

    The behavior of accretion disks can largely be understood in terms of the basic physical processes of mass, energy, and momentum conservation. Despite this, detailed modeling of these systems using modern computational techniques is challenging and controversial. Disturbing differences exist between methods used widely in astrophysics, namely Eulerian finite-difference techniques and particle codes such as SPH. Therefore neither technique is fully satisfactory for accretion disk simulations. This paper describes a new fully Lagrangian method designed to resolve these difficulties.

  11. Lyman edges in AGN accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Czerny, B. (Copernicus Astronomical Center, Warsaw (Poland)); Pojmanski, G. (Warsaw Univ. (Poland). Obserwatorium Astronomiczne)

    1990-07-01

    We show that the basic difference in the two principal approaches to predictions of the Lyman edge in an accretion disc lies in the implicit assumption about the density of the radiating gas. Independent from the details, models predict a broad range of the edge sizes, both in absorption and in emission. Observed spectra do not exhibit any strong feature at 912 A but may still be consistent with an accretion disc mechanism if more advanced theory is developed. (author).

  12. Energy-dependent variability from accretion flows

    OpenAIRE

    Zdziarski, Andrzej A.

    2005-01-01

    We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependencies of the level of local variability (as expected from propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As example local emission, we consider blackb...

  13. ASYMMETRIC ACCRETION FLOWS WITHIN A COMMON ENVELOPE

    Energy Technology Data Exchange (ETDEWEB)

    MacLeod, Morgan; Ramirez-Ruiz, Enrico [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)

    2015-04-10

    This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). Flow in the vicinity of the embedded object is gravitationally focused, leading to drag and potentially to gas accretion. This process has been studied numerically and analytically in the context of Hoyle–Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of three-dimensional HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by one to two orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically derived rates of drag and accretion as a function of the density gradient. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.

  14. Magnetically controlled accretion onto a black hole

    CERN Document Server

    Ikhsanov, N R; Beskrovnaya, N G; 10.1088/1742-6596/372/1/012062

    2012-01-01

    An accretion scenario in which the material captured by a black hole from its environment is assumed to be magnetized (\\beta ~ 1) is discussed. We show that the accretion picture in this case is strongly affected by the magnetic field of the flow itself. The accretion power within this Magnetically Controlled Accretion (MCA) scenario is converted predominantly into the magnetic energy of the accretion flow. The rapidly amplified field prevents the accretion flow from forming a homogeneous Keplerian disk. Instead, the flow is decelerated by its own magnetic field at a large distance (Shvartsman radius) from the black hole and switches into a non-Keplerian dense magnetized slab. The material in the slab is confined by the magnetic field and moves towards the black hole on the time scale of the magnetic field annihilation. The basic parameters of the slab are evaluated. Interchange instabilities in the slab may lead to a formation of Z-pinch type configuration of the magnetic field over the slab in which the acc...

  15. Moving mesh cosmology: tracing cosmological gas accretion

    CERN Document Server

    Nelson, Dylan; Genel, Shy; Sijacki, Debora; Keres, Dusan; Springel, Volker; Hernquist, Lars; 10.1093/mnras/sts595

    2013-01-01

    We investigate the nature of gas accretion onto haloes and galaxies at z=2 using cosmological hydrodynamic simulations run with the moving mesh code AREPO. Implementing a Monte Carlo tracer particle scheme to determine the origin and thermodynamic history of accreting gas, we make quantitative comparisons to an otherwise identical simulation run with the smoothed particle hydrodynamics (SPH) code GADGET-3. Contrasting these two numerical approaches, we find significant physical differences in the thermodynamic history of accreted gas in haloes above 10^10.5 solar masses. In agreement with previous work, GADGET simulations show a cold fraction near unity for galaxies forming in massive haloes, implying that only a small percentage of accreted gas heats to an appreciable fraction of the virial temperature during accretion. The same galaxies in AREPO show a much lower cold fraction, <20% in haloes above 10^11 solar masses. This results from a hot gas accretion rate which, at this same halo mass, is an order o...

  16. Accretion stream mapping with genetically modified "fire-flies"

    CERN Document Server

    Bridge, C M; Cropper, M; Ramsay, G

    2004-01-01

    We apply an eclipse mapping technique using `genetically modified fire-flies' to the eclipse light curves of HU Aqr and EP Dra. The technique makes as few assumptions as possible about the location of accretion stream material, allowing the emission to be located anywhere within the Roche lobe of the white dwarf. We model two consecutive eclipses in the UBVR_c band for HU Aqr, and four consecutive `white'-light eclipses for EP Dra, to determine the changing brightness distribution of stream material. We find fire-fly distributions which are consistent with accretion through a curtain of material in both HU Aqr and EP Dra, and show that the previously assumed two part ballistic and magnetic trajectory is a good approximation for polars. Model fits to the colour band data of HU Aqr indicate that the material confined to the magnetic field lines is brightest, and most of the emission originates from close to the white dwarf. There is evidence for emission from close to a calculated ballistic stream in both HU Aq...

  17. Circumstellar Disks of the Most Vigorously Accreting Young Stars

    CERN Document Server

    Liu, Hauyu Baobab; Kudo, Tomoyuki; Hashimoto, Jun; Dong, Ruobing; Vorobyov, Eduard I; Pyo, Tae-Soo; Fukagawa, Misato; Tamura, Motohide; Henning, Thomas; Dunham, Michael M; Karr, Jennifer; Kusakabe, Nobuhiko; Tsuribe, Toru

    2016-01-01

    Young stellar objects (YSOs) may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. We report our high angular resolution, coronagraphic near-infrared polarization imaging observations using the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) of the Subaru 8.2 m Telescope, towards four YSOs which are undergoing luminous accretion outbursts. The obtained infrared images have verified the presence of several hundred AUs scale arms and arcs surrounding these YSOs. In addition, our hydrodynamics simulations and radiative transfer models further demonstrate that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation p...

  18. The magnetic field evolution of ULX NuSTAR J095551+6940.8 in M82 - a legacy of accreting magnetar

    Science.gov (United States)

    Pan, Y. Y.; Song, L. M.; Zhang, C. M.; Tong, H.

    2016-09-01

    Ultraluminous X-ray sources are usually believed to be black holes with mass about 102-3 M⊙. However, the recent discovery of NuSTAR J095551+6940.8 in M82 by Bachetti et al. shows that it holds the spin period P = 1.37 s and period derivative dot{P}≈ -2× 10^{-10} s s^{-1}, which provides a strong evidence that some ultraluminous X-ray sources could be neutron stars. We obtain that the source may be an evolved magnetar according to our simulation by employing the model of accretion induced the polar magnetic field decay and standard spin-up torque of an accreting neutron star. The results show that NuSTAR J095551+6940.8 is still in the spin-up process, and the polar magnetic field decays to about 4.5 × 1012 G after accreting ˜10-2.5 M⊙, while the strong magnetic field exists in the out-polar region, which could be responsible for the observed low field magnetar. The ultra luminosity of the source can be explained by the beaming effect and two kinds of accretion-radial random accretion and disc accretion. Since the birth rate of magnetars is about ten per cent of the normal neutron stars, we guess that several ultraluminous X-ray sources should share the similar properties to that of NuSTAR J095551+6940.8.

  19. On the effect of injection of gas in the numerical simulation of accretion flows

    Institute of Scientific and Technical Information of China (English)

    2010-01-01

    We investigate the effects of various ways of injection of gas at the outer boundary in the numerical simulations of non-viscous accretion flows.We study three models.In Model A,we inject material around the equatorial plane.In Models B and C,fullrange θ injection is used(we employ spherical coordinates).In all three models,the injected material has the same density distribution with polar angle θ.From the equatorial region to the polar regions,angular momentum of the injected material of Model B decreases faster than that in Model C.For all of the models,after a transient episode of infall at the beginning of the simulations,the gas piles up in the equatorial regions outside the black hole and forms a thick torus bounded by a centrifugal barrier.We find that the accretion rates of Models B and C are more than ten times higher than that in Model A.In Model A,there is weak accretion only in the torus and outflows are found on the surface of the torus.In Model B,we find strong inflows on the surface of its torus,and the accretion in the torus is weak.In Model C,strong inflows also occur on the surface of its torus,but the accretion regions are narrower and there are strong outflows in its torus.In all of our models,the time-averaged density,pressure and angular momentum in the equatorial region can be described by a radial power law,with P ∝r-3/2,P ∝r-2 and l∝r0.

  20. Three Dimensional MHD Simulation of Circumbinary Accretion Disks -2. Net Accretion Rate

    CERN Document Server

    Shi, Ji-Ming

    2015-01-01

    When an accretion disk surrounds a binary rotating in the same sense, the binary exerts strong torques on the gas. Analytic work in the 1D approximation indicated that these torques sharply diminish or even eliminate accretion from the disk onto the binary. However, recent 2D and 3D simulational work has shown at most modest diminution. We present new MHD simulations demonstrating that for binaries with mass ratios of 1 and 0.1 there is essentially no difference between the accretion rate at large radius in the disk and the accretion rate onto the binary. To resolve the discrepancy with earlier analytic estimates, we identify the small subset of gas trajectories traveling from the inner edge of the disk to the binary and show how the full accretion rate is concentrated onto them.

  1. Chaotic cold accretion on to black holes

    Science.gov (United States)

    Gaspari, M.; Ruszkowski, M.; Oh, S. Peng

    2013-07-01

    Bondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when tcool/tff ≲ 10. The clouds decouple from the hot gas, `raining' on to the centre. Subsonic turbulence of just over 100 km s-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (tturb/tcool ≲ 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in

  2. The Final Fates of Accreting Supermassive Stars

    CERN Document Server

    Umeda, Hideyuki; Omukai, Kazuyuki; Yoshida, Naoki

    2016-01-01

    The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have less concentrated structure than fully-convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit $\\gtrsim 10^5~M_\\odot$ derived for the fully-convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with $\\dot M \\lesssim 0.1~M_\\odot~{\\rm yr}^{-1}$. With $\\dot{M} \\simeq 0.3 - 1~M_\\odot~{\\rm yr}^{-1}$, the star becomes GR-unstable during the helium-burning stage ...

  3. MHD of accretion-disk flows

    Science.gov (United States)

    Yankova, Krasimira

    2015-01-01

    Accretion is one of the most important problems of astrophysics concerning the transfer of matter and the transformation of energy into space. Process represents a falling of the substance on a cosmic object from the surrounding area and is a powerful gravitational mechanism for the production of radiation. Accretion disc effectively converts the mass of the substance by viscous friction and released potential energy transformed into radiation by particle collisions. Accretion onto compact object shows high energy efficiency and temporal variability in a broad class of observational data in all ranges. In the disks of these objects are developed a series instabilities and structures that govern the distribution of the energy. They are expressed in many variety non-stationary phenomena that we observe. That is why we propose generalized model of magnetized accretion disk with advection, which preserves the nonlinearity of the problem. We study interaction of the plasmas flow with the magnetic field, and how this affects the self-organizing disk. The aim of the work is to describe the accretion flow in detail, in his quality of the open astrophysical system, to investigate the evolution and to reveal the mechanisms of the structuring the disk-corona system for to interpret correctly the high energy behavior of such sources.

  4. Mixing and Accretion in lambda Bootis Stars

    CERN Document Server

    Turcotte, S

    2002-01-01

    Strong evidence for deep mixing has been uncovered for slowly rotating F, and A stars of the main sequence. As the accretion/diffusion model for the formation of lboo stars is heavily dependent on mixing in superficial regions, such deep mixing may have important repercussions on our understanding of these stars. It is shown that deep mixing at a level similar to that of FmAm stars increases the amount of matter that needs to be accreted by the stars with respect with the standard models by some three orders of magnitude. It is also shown that significantly larger accretion rates have to be maintained, as high as $10^{-11}$~M$_\\sun yr^{-1}$, to prevent meridional circulation from canceling the effect of accretion. The existence of old ($\\approx 1$~Gyr) is not a likely outcome of the present models for accretion/diffusion with or without deep mixing. It is argued that lboo stars are potentially very good diagnostics of mixing mechanisms in moderately fast rotators.

  5. The accretion of migrating giant planets

    CERN Document Server

    Dürmann, Christoph

    2016-01-01

    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 each other. We modeled a 2-dimensional disk with a steady accretion flow onto the central star and embed a Jupiter mass planet at 5.2 au. The disk is locally isothermal and viscosity is modeled using a constant $\\alpha$. 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. 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 which comes pred...

  6. Bondi accretion in early-type galaxies

    CERN Document Server

    Korol, V; Pellegrini, S

    2016-01-01

    Accretion onto central massive black holes in galaxies is often modelled with the Bondi solution. In this paper we study a generalization of the classical Bondi accretion theory, considering the additional effects of the gravitational potential of the host galaxy, and of electron scattering in the optically thin limit. We provide a general analysis of the bias in the estimates of the Bondi radius and mass accretion rate, when adopting as fiducial values for the density and temperature at infinity the values of these quantities measured at finite distance from the central black hole. We also give general formulae to compute the correction terms of the critical accretion parameter in relevant asymptotic regimes. A full analytical discussion is presented in the case of an Hernquist galaxy, when the problem reduces to the discussion of a cubic equation, therefore allowing for more than one critical point in the accretion structure. The results are useful for observational works (especially in the case of low-lumi...

  7. Optically-thick accretion discs with advection

    Institute of Scientific and Technical Information of China (English)

    陈林红; 吴枚; 尚仁成

    2002-01-01

    The structures of optically-thick accretion discs with radial advection have been investigated by the iteration and integration algorithms. The advective cooling term changes mostly the inner part of disc solution, and even results in an optically-thick advection-dominated accretion flow (ADAF). Three distinct branches-the outer Shakura-Sunyaev disc (SSD), the inner ADAF and the middle transition layer-are found for a super-Eddington disc. The SSD-ADAF transition radius can be estimated as 18(M/ME)RG where RG is the Schwarzschild radius, M is the mass accretion rate and ME is the Eddington accretion rate. SSD solutions calculated with the iteration and integration methods are identical, while ADAF solutions obtained by these two methods differ greatly. Detailed algorithms and their differences have been analysed. The iteration algorithm is not self-consistent, since it implies that the dimensionless advection factor ξ is invariant, but in the inner ADAF region the variation of ξ is not negligible. The integration algorithm is always effective for the whole region of an optically-thick disc if the accretion rate is no smaller than 10-4ME. For optically-thin discs, the validity of these two algorithms is different. We suggest that the integration method be employed to calculate the global solution of a disc model without assuming ξ to be a constant. We also discuss its application to the emergent continuum spectrum in order to explain observational facts.

  8. Bondi accretion in the early universe

    CERN Document Server

    Ricotti, Massimo

    2007-01-01

    This paper presents a study of quasi-steady spherical accretion in the early Universe, before the formation of the first stars and galaxies. The main motivation is to derive the basic formulas that will be used in a companion paper to calculate the accretion luminosity of primordial black holes and their effect on the cosmic ionization history. The following cosmological effects are investigated: the coupling of the gas to the CMB photon fluid (i.e., Compton drag), Hubble expansion, and the growth of the dark matter halo seeded by the gravitational potential of the central point mass. The gas equations of motion are solved assuming either a polytropic or an isothermal equation of state. We consider the cases in which the accreting object is a point mass or a spherical dark matter halo with power-law density profile, as predicted by the theory of "secondary infall''. Analytical solutions for the sonic radius and fitting formulas for the accretion rate are provided. Different accretion regimes exist depending o...

  9. Accretion Rates in Herbig Ae stars

    CERN Document Server

    López, R G; Testi, L; Habart, E

    2006-01-01

    Accretion rates from disks around pre-main sequence stars are of importance for our understanding of planetary formation and disk evolution. We provide in this paper estimates of the mass accretion rates in the disks around a large sample of Herbig Ae stars. We obtained medium resolution 2 micron spectra and used the results to compute values of Macc from the measured luminosity of the Br_gamma emission line, using a well established correlation between L(Br_gamma) and the accretion luminosity Lacc. We find that 80% of the stars, all of which have evidence of an associated circumstellar disk, are accreting matter, with rates 3x10^{-9} 10^{-7} Msun/yr. In most HAe stars the accretion rate is sufficiently low that the gas in the inner disk, inside the dust evaporation radius, is optically thin and does not prevent the formation of a puffed-up rim, where dust is directly exposed to the stellar radiation. When compared to the Macc values found for lower-mass stars in the star forming regions Taurus and Ophiuchus,...

  10. Polarization developments

    Energy Technology Data Exchange (ETDEWEB)

    Prescott, C.Y.

    1993-07-01

    Recent developments in laser-driven photoemission sources of polarized electrons have made prospects for highly polarized electron beams in a future linear collider very promising. This talk discusses the experiences with the SLC polarized electron source, the recent progress with research into gallium arsenide and strained gallium arsenide as a photocathode material, and the suitability of these cathode materials for a future linear collider based on the parameters of the several linear collider designs that exist.

  11. Simulation of the X-Ray Lightcurves of Intermediate Polars

    Science.gov (United States)

    Norton, A. J.

    1993-11-01

    A technique is presented for simulation of the X-ray light curves of intermediate polars. The model includes the effects of self-occultation of the emission regions by the body of the white dwarf, and variation of photoelectric absorption and electron scattering as the aspect angles of the emission regions change, and the effect of accretion occurring through a disc, directly via the accretion stream or by a combination of both. Both the variation of the rate of accretion on to a given emission region and its effective `migration' around the magnetic pole are taken into account. These features naturally arise in a system that is fed (at least in part) directly by the stream. Results are considered for two specific emission-region geometries, namely a filled circle and a semicircular arc, over a wide range of parameter space in each case. Disc-fed, non-accretion disc and discless accretion are each considered, both in the low and high X-ray energy regime and at a range of inclination angles and magnetic axis offset angles. The main conclusion of this work is that very complex pulse profiles can result from innately symmetrical geometrical and physical situations. The results obtained in the case of discless accretion feeding arc-shaped emission regions are compared with recent observational data and shown to produce qualitatively similar profiles. Finally, ways are considered in which this simple approach may be extended to include a more realistic description of the conditions that exist in intermediate polars.

  12. New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables

    CERN Document Server

    Schmidt, G D; Vanlandingham, K M; Anderson, S F; Barentine, J C; Brewington, H J; Hall, P B; Harvanek, M; Kleinman, S J; Krzesínski, J; Long, D; Margon, B; Neilsen, E H; Newman, P R; Nitta, A; Schneider, D P; Snedden, S A

    2005-01-01

    Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to six the total number of X-ray faint, magnetic accretion binaries that accrete at rates 3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors (in addition to the asynchronous Intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG.

  13. Accretion disks in luminous young stellar objects

    CERN Document Server

    Beltran, M T

    2015-01-01

    An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and therefore predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.

  14. Accretion disks in Algols: progenitors and evolution

    CERN Document Server

    Van Rensbergen, W

    2016-01-01

    There are only a few Algols with measured accretion disk parameters. These measurements provide additional constraints for tracing the origin of individual systems, narrowing down the initial parameter space. We investigate the origin and evolution of 6 Algol systems with accretion disks to find the initial parameters and evolutionary constraints for them. With a modified binary evolution code, series of close binary evolution are calculated to obtain the best match for observed individual systems. Initial parameters for 6 Algol systems with accretion disks were determined matching both the present system parameters and the observed disk characteristics. When RLOF starts during core hydrogen burning of the donor, the disk lifetime was found to be short. The disk luminosity is comparable to the luminosity of the gainer during a large fraction of the disk lifetime.

  15. MHD Turbulence in Accretion Disk Boundary Layers

    CERN Document Server

    Chan, Chi-kwan

    2012-01-01

    The physical modeling of the accretion disk boundary layer, the region where the disk meets the surface of the accreting star, usually relies on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity, widely adopted in astrophysics, satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability is inefficient in this inner disk region. I will discuss the results of a recent study on the generation of hydromagnetic stresses and energy density in the boundary layer around a weakly magnetized star. Our findings suggest that although magnetic energy density can be significantly amplified in this region, angular momentum transport is rather inefficient. This seems consistent with the results obtained in numerical simulations...

  16. Supermassive blackholes without super Eddington accretion

    Science.gov (United States)

    Christian, Damian Joseph; Kim, Matt I.; Garofalo, David; D'Avanzo, Jaclyn; Torres, John

    2017-08-01

    We explore the X-ray luminosity function at high redshift for active galactic nuclei using an albeit simplified model for mass build-up using a combination of mergers and mass accretion in the gap paradigm (Garofalo et al. 2010). Using a retrograde-dominated configuration we find an interesting low probability channel for the growth of one billion solar mass black holes within hundreds of millions of years of the big bang without appealing to super Eddington accretion (Kim et al. 2016). This result is made more compelling by the connection between this channel and an end product involving active galaxies with FRI radio morphology but weaker jet powers in mildly sub-Eddington accretion regimes. We will discuss our connection between the unexplained paucity of a given family of AGNs and the rapid growth of supermassive black holes, two heretofore seemingly unrelated aspects of the physics of AGNs that will help further understand their properties and evolution.

  17. Spherical Accretion in Nearby Weakly Active Galaxies

    CERN Document Server

    Moscibrodzka, M A

    2005-01-01

    We consider the sample of weakly active galaxies situated in 'Local Universe' collected in the paper of Pellegrini (2005) with inferred accretion efficiencies from $10^{-2}$ to $10^{-7}$. We apply a model of spherically symmetrical Bondi accretion for given parameters ($M_{BH}$,$T_{\\infty}$,$\\rho_{\\infty}$,) taken from observation. We calculate spectra emitted by the gas accreting onto its central objects using Monte Carlo method including synchrotron and bremsstrahlung photons as seed photons. We compare our results with observed nuclear X-ray luminosities $L_{X,nuc}$ (0.3-10 keV) of the sample. Model is also tested for different external medium parameters ($\\rho_{\\infty}$ and $T_{\\infty}$) and different free parameters of the model. Our model is able to explain most of the observed nuclear luminosities $L_X$ under an assumption that half of the compresion energy is transfered directly to the electrons.

  18. Magnetohydrodynamic Origin of Jets from Accretion Disks

    CERN Document Server

    Lovelace, R V E; Koldoba, A V

    1999-01-01

    A review is made of recent magnetohydrodynamic (MHD) theory and simulations of origin of jets from accretion disks. Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are the extra galactic radio jets of active galaxies and quasars, and old compact stars in binaries, and emission line jets in young stellar objects. It is widely thought that these different jets arise from rotating, conducting accretion disks threaded by an ordered magnetic field. The twisting of the magnetic field by the rotation of the disk drives the jets by magnetically extracting matter, angular momentum, and energy from the accretion disk. Two main regimes have been discussed theoretically, hydromagnetic winds which have a significant mass flux, and Poynting flux jets where the mass flux is negligible. Over the past several years, exciting new developments on models of jets have come from progress in MHD simulations which now allow the study of the origin - the acceleration and collima...

  19. Earth, Moon, Sun, and CV Accretion Disks

    CERN Document Server

    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 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 differential rotation and effects on the disk by the accretion stream must be addressed. Our a...

  20. Strongly magnetized accretion discs require poloidal flux

    Science.gov (United States)

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

    2016-08-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  1. Minidisks in Binary Black Hole Accretion

    CERN Document Server

    Ryan, Geoffrey

    2016-01-01

    Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole ("minidisks"), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using two-dimensional hydrodynamical simulations performed with a new general relativistic version of the moving mesh code Disco. We introduce a co-moving energy variable which enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the ISCO providing a Reynolds stress which causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov-Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling....

  2. Disks, accretion and outflows of brown dwarfs

    CERN Document Server

    Joergens, V; Liu, Y; Pascucci, I; Whelan, E; Alcala, J; Biazzo, K; Costigan, G; Gully-Santiago, M; Henning, Th; Natta, A; Rigliaco, E; Rodriguez-Ledesma, V; Sicilia-Aguilar, A; Tottle, J; Wolf, S

    2012-01-01

    Characterization of the properties of young brown dwarfs are important to constraining the formation of objects at the extreme low-mass end of the IMF. While young brown dwarfs share many properties with solar-mass T Tauri stars, differences may be used as tests of how the physics of accretion/outflow and disk chemistry/dissipation depend on the mass of the central object. This article summarizes the presentations and discussions during the splinter session on 'Disks, accretion and outflows of brown dwarfs' held at the CoolStars17 conference in Barcelona in June 2012. Recent results in the field of brown dwarf disks and outflows include the determination of brown dwarf disk masses and geometries based on Herschel far-IR photometry (70-160 um), accretion properties based on X-Shooter spectra, and new outflow detections in the very low-mass regime.

  3. Strongly magnetized accretion discs require poloidal flux

    CERN Document Server

    Salvesen, Greg; Simon, Jacob B; Begelman, Mitchell C

    2016-01-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  4. Global Models for Embedded, Accreting Protostellar Disks

    CERN Document Server

    Kratter, Kaitlin M; Krumholz, Mark R

    2007-01-01

    Most analytic work to date on protostellar disks has focused on disks in isolation from their environments. However, observations are now beginning to probe the earliest, most embedded phases of star formation, during which disks are rapidly accreting from their parent cores and cannot be modeled in isolation. We present a simple, one-zone model of protostellar accretion disks with high mass infall rates. Our model combines a self-consistent calculation of disk temperatures with an approximate treatment of angular momentum transport via several mechanisms. We use this model to survey the properties of protostellar disks across a wide range of stellar masses and evolutionary times, and make predictions for disks' masses, sizes, spiral structure, and fragmentation that will be directly testable by future large-scale surveys of deeply embedded disks. We define a dimensionless accretion-rotation parameter which, in conjunction with the disk's temperature, controls the disk evolution. We track the dominant mode of...

  5. Reverberation Mapping of AGN Accretion Disks

    Science.gov (United States)

    Fausnaugh, Michael; AGN STORM Collaboration

    2017-01-01

    I will discuss new reverberation mapping results that allow us to investigate the temperature structure of AGN accretion disks. By measuring time-delays between broad-band continuum light curves, we can determine the size of the disk as a function of wavelength. I will discuss the detection of continuum lags in NGC 5548 reported by the AGN STORM project and implications for the accretion disk. I will also present evidence for continuum lags in two other AGN for which we recently measured black hole masses from continuum-Hbeta reverberations. The mass measurements allow us to compare the continuum lags to predictions from standard thin disk theory, and our results indicate that the accretion disks are larger than the simplest expectations.

  6. Numerical Solution of the Radiative Transfer Equation: X-Ray Spectral Formation from Cylindrical Accretion onto a Magnetized Neutron Star

    Science.gov (United States)

    Fairnelli, R.; Ceccobello, C.; Romano, P.; Titarchuk, L.

    2011-01-01

    Predicting the emerging X-ray spectra in several astrophysical objects is of great importance, in particular when the observational data are compared with theoretical models. This requires developing numerical routines for the solution of the radiative transfer equation according to the expected physical conditions of the systems under study. Aims. We have developed an algorithm solving the radiative transfer equation in the Fokker-Planck approximation when both thermal and bulk Comptonization take place. The algorithm is essentially a relaxation method, where stable solutions are obtained when the system has reached its steady-state equilibrium. Methods. We obtained the solution of the radiative transfer equation in the two-dimensional domain defined by the photon energy E and optical depth of the system pi using finite-differences for the partial derivatives, and imposing specific boundary conditions for the solutions. We treated the case of cylindrical accretion onto a magnetized neutron star. Results. We considered a blackbody seed spectrum of photons with exponential distribution across the accretion column and for an accretion where the velocity reaches its maximum at the stellar surface and at the top of the accretion column, respectively. In both cases higher values of the electron temperature and of the optical depth pi produce flatter and harder spectra. Other parameters contributing to the spectral formation are the steepness of the vertical velocity profile, the albedo at the star surface, and the radius of the accretion column. The latter parameter modifies the emerging spectra in a specular way for the two assumed accretion profiles. Conclusions. The algorithm has been implemented in the XPEC package for X-ray fitting and is specifically dedicated to the physical framework of accretion at the polar cap of a neutron star with a high magnetic field (approx > 10(exp 12) G). This latter case is expected to be of typical accreting systems such as X

  7. Accretion-powered Compact Binaries

    Science.gov (United States)

    Mauche, Christopher W.

    2003-12-01

    Preface; The workshop logo; A short history of the CV workshop F. A. Córdova; Part I. Observations: 1. Low mass x-ray binaries A. P. Cowley, P. C. Schmidtke, D. Crampton, J. B. Hutchings, C. A. Haswell, E. L. Robinson, K. D. Horne, H. M. Johnston, S. R. Kulkarni, S. Kitamoto, X. Han, R. M. Hjellming, R. M. Wagner, S. L. Morris, P. Hertz, A. N. Parmar, L. Stella, P. Giommi, P. J. Callanan, T. Naylor, P. A. Charles, C. D. Bailyn, J. N. Imamura, T. Steiman-Cameron, J. Kristian, J. Middleditch, L. Angelini and J. P. Noris; 2. Nonmagnetic cataclysmic variables R. S. Polidan, C. W. Mauche, R. A. Wade, R. H. Kaitchuck, E. M. Schlegel, P. A. Hantzios, R. C. Smith, J. H. Wood, F. Hessman, A. Fiedler, D. H. P. Jones, J. Casares, P. A. Charles, J. van Paradijs, E. Harlaftis, T. Naylor, G. Sonneborn, B. J. M. Hassall, K. Horne, C. A. la Dous, A. W. Shafter, N. A. Hawkins, D. A. H. Buckley, D. J. Sullivan, F. V. Hessman, V. S. Dhillon, T. R. Marsh, J. Singh, S. Seetha, F. Giovannelli, A. Bianchini, E. M. Sion, D. J. Mullan, H. L. Shipman, G. Machin, P. J. Callanan, S. B. Howell, P. Szkody, E. M. Schlegel and R. F. Webbink; 3. Magnetic cataclysmic variables C. Hellier, K. O. Mason, C. W. Mauche, G. S. Miller, J. C. Raymond, F. K. Lamb, J. Patterson, A. J. Norton, M. G. Watson, A. R. King, I. M. McHardy, H. Lehto, J. P. Osborne, E. L. Robinson, A. W. Shafter, S. Balachandran, S. R. Rosen, J. Krautter, W. Buchholz, D. A. H. Buckley, I. R. Tuoly, D. Crampton, B. Warner, R. M. Prestage, B. N. Ashoka, M. Mouchet, J. M. Bonnet-Bidaud, J. M. Hameury, P. Szkody, P. Garnavich, S. Howell, T. Kii, M. Cropper, K. Mason, J. Bailey, D. T. Wickramasinghe, L. Ferrario, K. Beuermann, A. D. Schwope, H.-C. Thomas, S. Jordan, J. Schachter, A. V. Filippenko, S. M. Kahn, F. B. S. Paerels, K. Mukai, M. L. Edgar, S. Larsson, R. F. Jameson, A. R. King, A. Silber, R. Remillard, H. Bradt, M. Ishida, T. Ohashi and G. D. Schmidt; Part II. Accretion Theory: 4. Nonmagnetic W. Kley, F. Geyer, H. Herold, H

  8. Simulating a Thin Accretion Disk Using PLUTO

    Science.gov (United States)

    Phillipson, Rebecca; Vogeley, Michael S.; Boyd, Patricia T.

    2017-08-01

    Accreting black hole systems such as X-ray binaries and active galactic nuclei exhibit variability in their luminosity on many timescales ranging from milliseconds to tens of days, and even hundreds of days. The mechanism(s) driving this variability and the relationship between short- and long-term variability is poorly understood. Current studies on accretion disks seek to determine how the changes in black hole mass, the rate at which mass accretes onto the central black hole, and the external environment affect the variability on scales ranging from stellar-mass black holes to supermassive black holes. Traditionally, the fluid mechanics equations governing accretion disks have been simplified by considering only the kinematics of the disk, and perhaps magnetic fields, in order for their phenomenological behavior to be predicted analytically. We seek to employ numerical techniques to study accretion disks including more complicated physics traditionally ignored in order to more accurately understand their behavior over time. We present a proof-of-concept three dimensional, global simulation using the astrophysical hydrodynamic code PLUTO of a simplified thin disk model about a central black hole which will serve as the basis for development of more complicated models including external effects such as radiation and magnetic fields. We also develop a tool to generate a synthetic light curve that displays the variability in luminosity of the simulation over time. The preliminary simulation and accompanying synthetic light curve demonstrate that PLUTO is a reliable code to perform sophisticated simulations of accretion disk systems which can then be compared to observational results.

  9. Generalized Similarity for Accretion/Decretion Disks

    Science.gov (United States)

    Rafikov, Roman R.

    2016-10-01

    Decretion (or external) disks are gas disks freely expanding to large radii due to their internal stresses. They are expected to naturally arise in tidal disruption events, around Be stars, in mass-losing post-main-sequence binaries, as a result of supernova fallback, etc. Their evolution is theoretically understood in two regimes: when the central object does not exert torque on the disk (a standard assumption for conventional accretion disks) or when no mass inflow (or outflow) occurs at the disk center. However, many astrophysical objects—circumbinary disks, Be stars, neutron stars accreting in a propeller regime, etc.—feature non-zero torque simultaneously with the non-zero accretion (or ejection of mass) at the disk center. We provide a general description for the evolution of such disks (both linear and nonlinear) in the self-similar regime, to which the disk should asymptotically converge with time. We identify a similarity parameter λ, which is uniquely related to the degree, to which the central mass accretion is suppressed by the non-zero central torque. The known decretion disk solutions correspond to the two discrete values of λ, while our new solutions cover a continuum of its physically allowed values, corresponding to either accretion or mass ejection by the central object. A direct relationship between λ and central \\dot{M} and torque is also established. We describe the time evolution of the various disk characteristics for different λ, and show that the observable properties (spectrum and luminosity evolution) of the decretion disks, in general, are different from the standard accretion disks with no central torque.

  10. Planetesimal accretion in binary star systems

    CERN Document Server

    Marzari, Francesco; Scholl, Hans

    2007-01-01

    Numerical simulations of planetesimal accretion in circumprimary and circumbinary orbits are described. The secular perturbations by the com- panion star and gas drag are included in our models. We derive limits on the parameters of the binary system for which accretion and then planetary forma- tion are possible. In the circumbinary case we also outline the radial distance from the baricenter of the stars beyond which accumulation always occurs. Hy- drodynamical simulations are also presented to validate our N-body approach based on the axisymmetric approximation for the gas of the disk.

  11. Accretion disc flows around FU Orionis stars

    Science.gov (United States)

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

    1989-01-01

    The accretion disk model of FU Orionis systems in outburst is investigated by examining the time-dependent behavior of a disk around a low-mass protostar that accretes at 0.00001-0.0001 solar masses/yr. It is found that the disk may be stabilized against the thermal ionization instability by the effect of advective heat transport and that it may therefore exist in the quasi-steady-state observed in post-outburst FU Orionis systems. The disk models are used to discuss the cosmochemical consequences of possible FU Ori events during the evolution of the primordial solar nebula.

  12. Massive Star Formation: Accreting from Companion

    Indian Academy of Sciences (India)

    X. Chen; J. S. Zhang

    2014-09-01

    We report the possible accretion from companion in the massive star forming region (G350.69–0.49). This region seems to be a binary system composed of a diffuse object (possible nebulae or UC HII region) and a Massive Young Stellar Object (MYSO) seen in Spitzer IRAC image. The diffuse object and MYSO are connected by the shock-excited 4.5 m emission, suggesting that the massive star may form through accreting material from the companion in this system.

  13. Polarization, political

    NARCIS (Netherlands)

    M. Wojcieszak

    2015-01-01

    Polarization has been studied in three different forms: on a social, group, and individual level. This entry first focuses on the undisputed phenomenon of elite polarization (i.e., increasing adherence of policy positions among the elites) and also outlines different approaches to assessing mass pol

  14. Polar Codes

    Science.gov (United States)

    2014-12-01

    QPSK Gaussian channels . .......................................................................... 39 vi 1. INTRODUCTION Forward error correction (FEC...Capacity of BSC. 7 Figure 5. Capacity of AWGN channel . 8 4. INTRODUCTION TO POLAR CODES Polar codes were introduced by E. Arikan in [1]. This paper...Under authority of C. A. Wilgenbusch, Head ISR Division EXECUTIVE SUMMARY This report describes the results of the project “More reliable wireless

  15. Magnetospheric accretion on the fully-convective classical T Tauri star DN Tau

    CERN Document Server

    Donati, JF; Alencar, SHP; Hussain, G; Bouvier, J; Jardine, MM; Menard, F; Dougados, C; Romanova, MM

    2013-01-01

    We report here results of spectropolarimetric observations of the classical T Tauri star DN Tau carried out (at 2 epochs) with ESPaDOnS at the Canada-France-Hawaii Telescope within the `Magnetic Protostars and Planets' programme. We infer that DN Tau, with a photospheric temperature of 3,950+-50 K, a luminosity of 0.8+-0.2 Lsun and a rotation period of 6.32 d, is a ~2Myr-old fully-convective 0.65+-0.05 Msun star with a radius of 1.9+-0.2 Dsun, viewed at an inclination of 35+-10degr. Clear circularly-polarized Zeeman signatures are detected in both photospheric and accretion-powered emission lines, probing longitudinal fields of up to 1.8 kG (in the He1 D3 accretion proxy). Rotational modulation of Zeeman signatures, detected both in photospheric and accretion lines, is different between our 2 runs, providing further evidence that fields of cTTSs are generated by non-stationary dynamos. Using tomographic imaging, we reconstruct maps of the large-scale field, of the photospheric brightness and of the accretion-...

  16. Constraint on the black hole spin of M87 from the accretion-jet model

    Science.gov (United States)

    Feng, Jianchao; Wu, Qingwen

    2017-09-01

    The millimetre bump, as found in high-resolution multiwaveband observations of M87 by Prieto et al., most possibly comes from the synchrotron emission of thermal electrons in advection-dominated accretion flow (ADAF). It is possible to constrain the accretion rate near the horizon if both the nuclear millimetre emission and its polarization are produced by the hot plasma in the accretion flow. The jet power of M87 has been extensively explored, which is around 8_-3^{+7}× 10^{42} erg s-1 based on the analysis of the X-ray cavity. The black hole (BH) spin can be estimated if the jet power and the accretion rate near the horizon are known. We model the multiwavelength spectral energy distribution (SED) of M87 with a coupled ADAF-jet model surrounding a Kerr BH, where the full set of relativistic hydrodynamical equations of the ADAF are solved. The hybrid jet formation model, as a variant of the Blandford-Znajek model, is used to model the jet power. We find that the SMBH should be fast rotating with a dimensionless spin parameter a_{*}˜eq 0.98_-0.02^{+0.012}.

  17. Polarization holography

    DEFF Research Database (Denmark)

    Nikolova, L.; Ramanujam, P.S.

    Current research into holography is concerned with applications in optically storing, retrieving, and processing information. Polarization holography has many unique properties compared to conventional holography. It gives results in high efficiency, achromaticity, and special polarization...... properties. This books reviews the research carried out in this field over the last 15 years. The authors provide basic concepts in polarization and the propagation of light through anisotropic materials, before presenting a sound theoretical basis for polarization holography. The fabrication...... and characterization of azobenzene based materials, which remain the most efficient for the purpose, is described in detail. This is followed by a description of other materials that are used in polarization holography. An in-depth description of various applications, including display holography and optical storage...

  18. Polarization holography

    DEFF Research Database (Denmark)

    Nikolova, L.; Ramanujam, P.S.

    properties. This books reviews the research carried out in this field over the last 15 years. The authors provide basic concepts in polarization and the propagation of light through anisotropic materials, before presenting a sound theoretical basis for polarization holography. The fabrication......Current research into holography is concerned with applications in optically storing, retrieving, and processing information. Polarization holography has many unique properties compared to conventional holography. It gives results in high efficiency, achromaticity, and special polarization...... and characterization of azobenzene based materials, which remain the most efficient for the purpose, is described in detail. This is followed by a description of other materials that are used in polarization holography. An in-depth description of various applications, including display holography and optical storage...

  19. On the resonant behavior of longitudinally vibrating accreting rods

    CSIR Research Space (South Africa)

    Shatalov, M

    2012-09-01

    Full Text Available The theory of accreting structures is a new and fast developing branch of analytical mechanics basing on the theory of partial differential and integral equations. In the present paper the authors analyze qualitative properties of accreting rods...

  20. Bondi-Hoyle-Lyttleton Accretion onto a Protoplanetary Disk

    CERN Document Server

    Moeckel, Nickolas

    2009-01-01

    Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this `tail-end' accretion for planet formation was recently highlighted by Throop & Bally (2008). While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such ...

  1. Spin equilibrium in strongly-magnetized accreting stars

    CERN Document Server

    D'Angelo, Caroline

    2016-01-01

    The spin rate of a strongly-magnetized accreting star is regulated by the interaction between the star's magnetic field and the accreting gas. These systems are often hypothesized to be in `spin equilibrium' with their surrounding accretion flows such that the net spin change of the star as a result of accretion is very small. This condition requires that the accretion rate changes more slowly than it takes the star to reach spin equilibrium. However, this is not true for most magnetically accreting stars, which have strongly variable accretion outbursts (by one to many orders of magnitude) on timescales much shorter than the time it would take to reach spin equilibrium. This paper examines how accretion outbursts affect the time a star takes to reach spin equilibrium and its final equilibrium spin period. I consider several different models for angular momentum loss -- where angular momentum is carried away in an outflow (the standard `propeller', centrifugally-launched outflow), where most angular momentum ...

  2. 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.

  3. Accretion Models for Young Neutron Stars

    OpenAIRE

    Alpar, M. Ali

    2003-01-01

    Interaction with possible fallback material, along with the magnetic fields and rotation rates at birth should determine the fates and categories of young neutron stars. This paper addresses some issues related to pure or hybrid accretion models for explaining the properties of young neutron stars.

  4. Realizability of stationary spherically symmetric transonic accretion

    CERN Document Server

    Ray, A K; Ray, Arnab K.

    2002-01-01

    The spherically symmetric stationary transonic (Bondi) flow is considered a classic example of an accretion flow. This flow, however, is along a separatrix, which is usually not physically realizable. We demonstrate, using a pedagogical example, that it is the dynamics which selects the transonic flow.

  5. Viscosity in accretion discs. [for binary stars

    Science.gov (United States)

    Katz, J. I.

    1980-01-01

    Both HerX-1 and SS433 may contain accretion disks slaved to a precessing companion star. If so, it is possible to bound the effective viscosity in these disks. The results, in terms of the disk parameter alpha, are lower bounds of 0.01 for HerX-1 and of 0.1 for SS433.

  6. Radiation of accretion discs: the eclipses

    Energy Technology Data Exchange (ETDEWEB)

    Schwarzenberg-Czerny, A.

    1984-05-01

    Light curves have been calculated for eclipses of the accretion disc in a cataclysmic binary. The Roche geometry of the cool component was taken into account and the stellar atmospheres were interpolated to provide the local spectrum of the radiation from the disc. The dependence of the light curve on the parameters of the disc is discussed.

  7. Jets at lowest mass accretion rates

    NARCIS (Netherlands)

    Maitra, D.; Cantrell, A.; Markoff, S.; Falcke, H.; Miller, J.; Bailyn, C.

    2011-01-01

    We present results of recent observations and theoretical modeling of data from black holes accreting at very low luminosities (L/LEdd ≲ 10−8). We discuss our newly developed time-dependent model for episodic ejection of relativistic plasma within a jet framework, and a successful application of thi

  8. Turbulent Comptonization in Relativistic Accretion Disks

    CERN Document Server

    Socrates, A; Blaes, Omer M; Socrates, Aristotle; Davis, Shane W.; Blaes, Omer

    2006-01-01

    Turbulent Comptonization, a potentially important damping and radiation mechanism in relativistic accretion flows, is discussed. Particular emphasis is placed on the physical basis, relative importance, and thermodynamics of turbulent Comptonization. The effects of metal-absorption opacity on the spectral component resulting from turbulent Comptonization is considered as well.

  9. Two-dimensional vortices and accretion disks

    NARCIS (Netherlands)

    Nauta, Michiel Doede

    2001-01-01

    Observations show that there are disks around certain stars that slowly rain down on the central (compact) object: accretion disks. The rate of depletion of the disk might be slow but is still larger than was expected on theoretical grounds. That is why it has been suggested that the disks are tu

  10. Oligarchic planetesimal accretion and giant planet formation

    CERN Document Server

    Fortier, A; Brunini, A

    2007-01-01

    Aims. In the context of the core instability model, we present calculations of in situ giant planet formation. The oligarchic growth regime of solid protoplanets is the model adopted for the growth of the core. Methods. The full differential equations of giant planet formation were numerically solved with an adaptation of a Henyey-type code. The planetesimals accretion rate was coupled in a self-consistent way to the envelope's evolution. Results. We performed several simulations for the formation of a Jupiter-like object by assuming various surface densities for the protoplanetary disc and two different sizes for the accreted planetesimals. We find that the atmospheric gas drag gives rise to a major enhancement on the effective capture radius of the protoplanet, thus leading to an average timescale reduction of 30% -- 55% and ultimately to an increase by a factor of 2 of the final mass of solids accreted as compared to the situation in which drag effects are neglected. With regard to the size of accreted pla...

  11. Probing thermonuclear burning on accreting neutron stars

    NARCIS (Netherlands)

    Keek, L.

    2008-01-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes

  12. Cooling of Accretion-Heated Neutron Stars

    Indian Academy of Sciences (India)

    Rudy Wijnands; Nathalie Degenaar; Dany Page

    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.

  13. Millisecond phenomena in mass accreting neutron stars

    NARCIS (Netherlands)

    van der Klis, M.; Cohen, L.

    2007-01-01

    The past twelve years have seen the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), of several long-predicted phenomena associated with the accretion of matter onto a neutron star in a binary (double) star system. These phenomena are observed in the strong X-ray emission produced by these

  14. Accretion from debris disks onto white dwarfs : Fingering (thermohaline) instability and derived accretion rates

    CERN Document Server

    Deal, M; Vauclair, G; Vauclair, S; Wachlin, F C

    2013-01-01

    Recent observations of a large number of DA and DB white dwarfs show evidence of debris disks, which are the remnants of old planetary systems. The infrared excess detected with \\emph{Spitzer} and the lines of heavy elements observed in their atmospheres with high-resolution spectroscopy converge on the idea that planetary material accretes onto these stars. Accretion rates have been derived by several authors with the assumption of a steady state between accretion and gravitational settling. The results are unrealistically different for DA and DB white dwarfs. When heavy matter is accreted onto stars, it induces an inverse $\\mu$-gradient that leads to fingering (thermohaline) convection. The aim of this letter is to study the impact of this specific process on the derived accretion rates in white dwarfs and on the difference between DA and DB. We solve the diffusion equation for the accreted heavy elements with a time-dependent method. The models we use have been obtained both with the IRAP code, which compu...

  15. Polarizing cues.

    Science.gov (United States)

    Nicholson, Stephen P

    2012-01-01

    People categorize themselves and others, creating ingroup and outgroup distinctions. In American politics, parties constitute the in- and outgroups, and party leaders hold sway in articulating party positions. A party leader's endorsement of a policy can be persuasive, inducing co-partisans to take the same position. In contrast, a party leader's endorsement may polarize opinion, inducing out-party identifiers to take a contrary position. Using survey experiments from the 2008 presidential election, I examine whether in- and out-party candidate cues—John McCain and Barack Obama—affected partisan opinion. The results indicate that in-party leader cues do not persuade but that out-party leader cues polarize. This finding holds in an experiment featuring President Bush in which his endorsement did not persuade Republicans but it polarized Democrats. Lastly, I compare the effect of party leader cues to party label cues. The results suggest that politicians, not parties, function as polarizing cues.

  16. Polarization modulation from Lense-Thirring precession in X-ray binaries

    CERN Document Server

    Ingram, Adam; Poutanen, Juri; Krawczynski, Henric

    2015-01-01

    It has long been recognised that quasi-periodic oscillations (QPOs) in the X-ray light curves of accreting black hole and neutron star binaries have the potential to be powerful diagnostics of strong field gravity. However, this potential cannot be fulfilled without a working theoretical model, which has remained elusive. Perhaps the most promising model associates the QPO with Lense-Thirring precession of the inner accretion flow, with the changes in viewing angle and Doppler boosting modulating the flux over the course of a precession cycle. Here, we consider the polarization signature of a precessing inner accretion flow. We use simple assumptions about the Comptonization process generating the emitted spectrum and take all relativistic effects into account, parallel transporting polarization vectors towards the observer along null geodesics in the Kerr metric. We find that both the degree of linear polarization and the polarization angle should be modulated on the QPO frequency. We calculate the predicted...

  17. Spherical Accretion in a Uniformly Expanding Universe

    Science.gov (United States)

    Colpi, Monica; Shapiro, Stuart L.; Wasserman, Ira

    1996-10-01

    We consider spherically symmetric accretion of material from an initially homogeneous, uniformly expanding medium onto a Newtonian point mass M. The gas is assumed to evolve adiabatically with a constant adiabatic index F, which we vary over the range Γ ɛ [1, 5/3]. We use a one-dimensional Lagrangian code to follow the spherical infall of material as a function of time. Outflowing shells gravitationally bound to the point mass fall back, giving rise to a inflow rate that, after a rapid rise, declines as a power law in time. If there were no outflow initially, Bondi accretion would result, with a characteristic accretion time-scale ta,0. For gas initially expanding at a uniform rate, with a radial velocity U = R/t0 at radius R, the behavior of the flow at all subsequent times is determined by ta,0/t0. If ta,0/t0 ≫ 1, the gas has no time to respond to pressure forces, so the fluid motion is nearly collisionless. In this case, only loosely bound shells are influenced by pressure gradients and are pushed outward. The late-time evolution of the mass accretion rate Mdot is close to the result for pure dust, and we develop a semianalytic model that accurately accounts for the small effect of pressure gradients in this limit. In the opposite regime, ta,0/t0 ≪ 1, pressure forces significantly affect the motion of the gas. At sufficiently early times, t ≤ ttr, the flow evolved along a sequence of quasi-stationary, Bondi-like states, with a time-dependent Mdot determined by the slowly varying gas density at large distances. However, at later times, t ≥ ttr, the fluid flow enters a dustllke regime; ttr is the time when the instantaneous Bondi accretion radius reaches the marginally bound radius. The transition time ttr depends sensitively on ta,0/t0 for a given Γ and can greatly exceed t0. We show that there exists a critical value Γ = 11/9, below which the transition from fluid to ballistic motion disappears. As one application of our calculations, we consider the

  18. AGN Accretion Physics: Insights from K2

    Science.gov (United States)

    Vogeley, Michael

    We propose to use Kepler K2 mission observations of 1800 supermassive black holes at the centers of galaxies (Active Galactic Nuclei; AGN) to test models for accretion physics, to study the relationship between variability and other AGN properties such as accretion rate, and to guide methods for detecting and classifying AGN in future time-domain surveys. AGN exhibit optical brightness fluctuations on timescales from below an hour up to many years. These fluctuations are determined by the physics of accretion of matter onto black holes from their galactic environment. By observing variability on timescales down to below an hour, Kepler probes the accretion region on length scales that are too small to be directly imaged using conventional telescopes. These data allow us to test competing models for accretion physics that make different predictions for the statistics of variability. Our previous work provides strong evidence that models of AGN variability that work on long timescale data are not adequate to describe the full range of fluctuation timescales probed by Kepler. We will analyze the light curves of 1800 AGN that have been monitored by Kepler during recent and ongoing K2 campaigns. These objects span a large range of luminosity and AGN type, thus allowing study of the relationship between variability and other physical properties. We will characterize the statistics of AGN variability using state-of-the-art methods of time series analysis that are appropriate for quantifying the stochastic behavior of AGN. This analysis builds on our previous work in which we developed and tested new analysis software that extracts the full information content of these light curves and will enable several key outcomes: (1) Measurement of the relationship between types of AGN and their variability. (2) Tests for dependence of variability on accretion rate. (3) Investigation of changes in variability behavior that point to changes in the mode of accretion. (4) Correlations

  19. 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.

  20. In-flight photogrammetric measurement of wing ice accretions

    Science.gov (United States)

    Mcknight, R. C.; Palko, R. L.; Humes, R. L.

    1986-01-01

    A photographic instrumentation system was developed for the Lewis icing research aircraft to measure wind ice accretions during flight. The system generates stereo photographs of the accretions which are then photogrammetrically measured by the Air Force Arnold Engineering and Development Center. The measurements yield a survey of spatial coordinates of an accretion's surface to an accuracy of at least + or - 0.08 cm. The accretions can then be matched to corresponding icing cloud and aerodynamic measurements. The system is being used to measure rime, mixed, and clear natural ice accretions.

  1. Theory of disk accretion onto supermassive black holes

    CERN Document Server

    Armitage, P J

    2004-01-01

    Accretion onto supermassive black holes produces both the dramatic phenomena associated with active galactic nuclei and the underwhelming displays seen in the Galactic Center and most other nearby galaxies. I review selected aspects of the current theoretical understanding of black hole accretion, emphasizing the role of magnetohydrodynamic turbulence and gravitational instabilities in driving the actual accretion and the importance of the efficacy of cooling in determining the structure and observational appearance of the accretion flow. Ongoing investigations into the dynamics of the plunging region, the origin of variability in the accretion process, and the evolution of warped, twisted, or eccentric disks are summarized.

  2. Measuring Mass Accretion Rate onto the Supermassive Black Hole in M 87 Using Faraday Rotation Measure with the Submillimeter Array

    CERN Document Server

    Kuo, C Y; Rao, R; Nakamura, M; Algaba, J C; Liu, H B; Inoue, M; Koch, P M; Ho, P T P; Matsushita, S; Pu, H -Y; Akiyama, K; Nishioka, H; Pradel, N

    2014-01-01

    We present the first constraint on Faraday rotation measure (RM) at submillimeter wavelengths for the nucleus of M 87. By fitting the polarization position angles ($\\chi$) observed with the SMA at four independent frequencies around $\\sim$230 GHz and interpreting the change in $\\chi$ as a result of \\emph{external} Faraday rotation associated with accretion flow, we determine the rotation measure of the M 87 core to be between $-$7.5$\\times$10$^{5}$ and 3.4$\\times$10$^{5}$ rad/m$^{2}$. Assuming a density profile of the accretion flow that follows a power-law distribution and a magnetic field that is ordered, radial, and has equipartition strength, the limit on the rotation measure constrains the mass accretion rate $\\dot{M}$ to be below 9.2$\\times$10$^{-4}$ M$_{\\odot}$~yr$^{-1}$ at a distance of 21 Schwarzchild radii from the central black hole. This value is at least two orders of magnitude smaller than the Bondi accretion rate, suggesting significant suppression of the accretion rate in the inner region of t...

  3. The mass accretion rate of galaxy clusters: a measurable quantity

    CERN Document Server

    De Boni, Cristiano; Diaferio, Antonaldo; Giocoli, Carlo; Baldi, Marco

    2015-01-01

    We explore the possibility of measuring the mass accretion rate of galaxy clusters by using dense galaxy redshift surveys of their outer regions. By approximating the accretion with the infall of a spherical shell, the mass accretion rate only depends on the mass profile of the cluster in a thin shell at radii larger than $R_{200}$. This approximation is rather crude in hierarchical clustering scenarios, where both smooth accretion and aggregation of smaller dark matter haloes contribute to the mass accretion of clusters. Nevertheless, in the redshift range $z=[0,1]$, our prescription returns an average mass accretion rate within $20 \\%$ of the average rate derived with the more realistic merger trees of dark matter haloes extracted from $N$-body simulations. The mass accretion rate of galaxy clusters has been the topic of numerous detailed numerical and theoretical investigations, but so far it has remained inaccessible to measurements in the real Universe. Our result suggests that measuring the mass accreti...

  4. Thermally Driven Winds from Radiatively Inefficient Accretion Flows

    CERN Document Server

    Kawabata, Ryoji

    2009-01-01

    Radiatively inefficient accretion flows (RIAFs) are common feature of low-luminosity accretion flows, including quiescent states of X-ray binaries and low-lunimosity active galactic nuclei. Thermally driven winds are expected from such hot accretion flows. By assuming that the flow has self-similarity structure in the radial direction, we solve the vertical structure of the wind and accretion flows simultaneously and evaluate the mass loss rates by wind. We find that the ratio of the outflow rate to the accretion rate is approximately unity for a viscosity parameter, alpha lesssim 0.1, despite some uncertainties in the angular momentum and temperature distributions. That is, the accretion rate in the RIAFs is roughly proportional to the radius. Moreover, we elucidate the effect of cooling by wind on the underneath accretion flow, finding that this effect could be important for calculating energy spectrum of the RIAF. Observational implications are briefly discussed in the context of Sgr A*.

  5. Effects of ice accretion on the aerodynamics of bridge cables

    DEFF Research Database (Denmark)

    Demartino, C.; Koss, Holger; Georgakis, Christos T.;

    2015-01-01

    Undesirable wind induced vibrations of bridge cables can occur when atmospheric conditions are such to generate ice accretion. This paper contains the results of an extensive investigation of the effects of ice accretion due to in-cloud icing, on the aerodynamic characteristics of bridge hangers...... 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 the ice accretions is given in the paper. Only for the bridge hanger case, a short description of the evolution of the ice accretions is given. The aerodynamic force coefficients were then measured with varying yaw angle, angle of attack and wind speed, and are presented and discussed in the paper...

  6. Accretion onto black holes and relativistic jets

    CERN Document Server

    Belloni, Tomaso

    2007-01-01

    Relativistic jets from Active Galactic Nuclei are known since decades, but the study of the connection between accretion and ejection in these systems is hampered by the long time scales associated to these events. The past decade has seen a rapid advancement due to the observation of similar radio jets in galactic X-ray binaries, where the time scales are much shorter. A clear connection between accretion and ejection has been found for these systems, together with a solid characterization of the phenomenological properties of their outbursts. This wealth of new results has led to a detailed comparison between X-ray binaries and AGN, from which a number of correlations and scaling laws has been established. Here I briefly review the current observational status.

  7. Magnetised accretion discs in Kerr spacetimes

    CERN Document Server

    Ranea-Sandoval, Ignacio F

    2014-01-01

    We study the effect caused by external magnetic fields on the observed thermal spectra and iron line profiles of thin accretion discs formed around Kerr black holes and naked singularities. We aim to provide a tool that can be used to estimate the presence of magnetic fields in the neighbourhood of a compact object and to probe the cosmic censorship conjecture in these particular astrophysical environments. We developed a numerical scheme able to calculate thermal spectra of magnetised Page-Thorne accretion discs formed around rotating black holes and naked singularities as seen by an arbitrary distant observer. We incorporated two different magnetic field configurations: uniform and dipolar, using a perturbative scheme in the coupling constant between matter and magnetic field strength. Under the same assumptions, we obtained observed synthetic line profiles of the 6.4 keV fluorescent iron line. We show that an external magnetic field produces potentially observable modifications on the thermal energy spectr...

  8. Quasistars: Accreting black holes inside massive envelopes

    CERN Document Server

    Begelman, Mitchell C; Armitage, Philip J

    2007-01-01

    We study the structure and evolution of "quasistars," accreting black holes embedded within massive hydrostatic gaseous envelopes. These configurations may model the early growth of supermassive black hole seeds. The accretion rate onto the black hole adjusts so that the luminosity carried by the convective envelope equals the Eddington limit for the total mass. This greatly exceeds the Eddington limit for the black hole mass alone, leading to rapid growth of the black hole. We use analytic models and numerical stellar structure calculations to study the structure and evolution of quasistars. We derive analytically the scaling of the photospheric temperature with the black hole mass and envelope mass, and show that it decreases with time as the black hole mass increases. Once the photospheric temperature becomes lower than 10000 K, the photospheric opacity drops precipitously and the photospheric temperature hits a limiting value, analogous to the Hayashi track for red giants and protostars, below which no hy...

  9. Ice Accretion on Wind Turbine Blades

    DEFF Research Database (Denmark)

    Hudecz, Adriána; Koss, Holger; Hansen, Martin Otto Laver

    2013-01-01

    In this paper, both experimental and numerical simulations of the effects of ice accretion on a NACA 64-618 airfoil section with 7° angle of attack are presented. The wind tunnel tests were conducted in a closed-circuit climatic wind tunnel at Force Technology in Denmark. The changes of aerodynamic...... forces were monitored as ice was building up on the airfoil for glaze, rime and mixed ice. In the first part of the numerical analysis, the resulted ice profiles of the wind tunnel tests were compared to profiles estimated by using the 2D ice accretion code TURBICE. In the second part, Ansys Fluent...... of the rime iced ice profile follows the streamlines quite well, disturbing the flow the least. The TURBICE analysis agrees fairly with the profiles produced during the wind tunnel testing....

  10. Volatile accretion history of the Earth.

    Science.gov (United States)

    Wood, B J; Halliday, A N; Rehkämper, M

    2010-10-28

    It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albarède paints a different picture, proposing that the Earth first formed as a dry planet which, like the Moon, was devoid of volatile constituents. He suggests that the Earth's complement of volatile elements was only established later, by the addition of a small veneer of volatile-rich material at ∼100 Myr (here and elsewhere, ages are relative to the origin of the Solar System). Here we argue that the Earth's mass balance of moderately volatile elements is inconsistent with Albarède's hypothesis but is well explained by the standard model of accretion from partially volatile-depleted material, accompanied by core formation.

  11. The properties of external accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Pringle, J.E. (Space Telescope Science Inst., Baltimore, MD (USA))

    1991-02-15

    The properties of external accretion discs (discs with a central source of angular momentum) are explored both analytically and numerically. An illustrative example of the effect of a disc of material around a binary star on the stellar separation is considered. We consider a Greens-function-type solution in which an initial ring of matter is put in orbit around the central binary. We find that the solution splits temporally into three parts. First, the disc evolves as a standard accretion disc unaware of the inner boundary condition. Secondly, the disc interacts with the inner boundary and changes its character to become, thirdly, an outflowing disc which is propelled outwards by the source of angular momentum at the centre. (author).

  12. Eclipse Mapping: Astrotomography of Accretion Discs

    CERN Document Server

    Baptista, Raymundo

    2015-01-01

    The Eclipse Mapping Method is an indirect imaging technique that transforms the shape of the eclipse light curve into a map of the surface brightness distribution of the occulted regions. Three decades of application of this technique to the investigation of the structure, the spectrum and the time evolution of accretion discs around white dwarfs in cataclysmic variables have enriched our understanding of these accretion devices with a wealth of details such as (but not limited to) moving heating/cooling waves during outbursts in dwarf novae, tidally-induced spiral shocks of emitting gas with sub-Keplerian velocities, elliptical precessing discs associated to superhumps, and measurements of the radial run of the disc viscosity through the mapping of the disc flickering sources. This chapter reviews the principles of the method, discusses its performance, limitations, useful error propagation procedures, as well as highlights a selection of applications aimed at showing the possible scientific problems that ha...

  13. Energy-dependent variability from accretion flows

    CERN Document Server

    Zdziarski, A A

    2005-01-01

    We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependencies of the level of local variability (as expected from propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As example local emission, we consider blackbody, Wien and thermal Comptonization spectra. In addition to numerical results, we present a number of analytical formulae for the resulting rms. We also find an analytical formula for the disc Wien spectrum, which we find to be a very good approximation to the disc blackbody. We compare our results to the rms spectrum observed in an ultrasoft state of GRS 1915+105.

  14. Satellites in discs regulating the accretion luminosity

    CERN Document Server

    Syer, D; Syer, Dave; Clarke, Cathie

    1995-01-01

    We demonstrate, using a simple analytic model, that the presence of a massive satellite can globally modify the structure and emission properties of an accretion disc to which it is tidally coupled. We show, using two levels of numerical approximation, that the analytic model gives reasonable results. The results are applicable to two astrophysical situations. In the case of an active galactic nucleus, we consider the case of a \\sim 10^3\\Msun compact companion to the central black-hole and show that it could modulate the emitted spectrum on a timescale of \\sim10^5 years. In the case of a T Tauri accretion disc, a satellite such as a sub-dwarf or giant planet could modify the disc spectral energy distribution over a substantial fraction of the T Tauri star lifetime.

  15. Probing General Relativity with Accreting Black Holes

    CERN Document Server

    Fabian, A C

    2012-01-01

    Most of the X-ray emission from luminous accreting black holes emerges from within 20 gravitational radii. The effective emission radius is several times smaller if the black hole is rapidly spinning. General Relativistic effects can then be very important. Large spacetime curvature causes strong lightbending and large gravitational redshifts. The hard X-ray, power-law-emitting corona irradiates the accretion disc generating an X-ray reflection component. Atomic features in the reflection spectrum allow gravitational redshifts to be measured. Time delays between observed variations in the power-law and the reflection spectrum (reverberation) enable the physical scale of the reflecting region to be determined. The relative strength of the reflection and power-law continuum depends on light bending. All of these observed effects enable the immediate environment of the black hole where the effects of General Relativity are on display to be probed and explored.

  16. Relativistic Accretion Mediated by Turbulent Comptonization

    CERN Document Server

    Socrates, Aristotle

    2008-01-01

    Black hole and neutron star accretion flows display unusually high levels of hard coronal emission in comparison to all other optically thick, gravitationally bound, turbulent astrophysical systems. Since these flows sit in deep relativistic gravitational potentials, their random bulk motions approach the speed of light, therefore allowing turbulent Comptonization to be an important effect. We show that the inevitable production of hard X-ray photons results from turbulent Comptonization in the limit where the turbulence is trans-sonic and the accretion power approaches the Eddington Limit. In this regime, the turbulent Compton y-parameter approaches unity and the turbulent Compton temperature is a significant fraction of the electron rest mass energy, in agreement with the observed phenomena.

  17. Turbulent Comptonization in Black Hole Accretion Disks

    CERN Document Server

    Socrates, A; Blaes, Omer M; Socrates, Aristotle; Davis, Shane W.; Blaes, Omer

    2004-01-01

    In the inner-most regions of radiation pressure supported accretion disks, the turbulent magnetic pressure may greatly exceed that of the gas. If this is the case, it is possible for bulk Alfvenic motions driven by the magnetorotational instability (MRI) to surpass the electron thermal velocity. Bulk rather than thermal Comptonization may then be the dominant radiative process which mediates gravitational energy release. For sufficiently large turbulent stresses, we show that turbulent Comptonization produces a significant contribution to the far-UV and X-ray emission of black hole accretion disks. The existence of this spectral component provides a means of obtaining direct observational constraints on the nature of the turbulence itself. We describe how this component may affect the spectral energy distributions and variability properties of X-ray binaries and active galactic nuclei.

  18. FITDisk: Cataclysmic Variable Accretion Disk Demonstration Tool

    Science.gov (United States)

    Wood, Matthew A.; Dolence, J.

    2013-05-01

    FITDisk models accretion disk phenomena using a fully three-dimensional hydrodynamics calculation, and data can either be visualized as they are computed or stored to hard drive for later playback at a fast frame rate. Simulations are visualized using OpenGL graphics and the viewing angle can be changed interactively. Pseudo light curves of simulated systems can be plotted along with the associated Fourier amplitude spectrum. It provides an easy to use graphical user interface as well as 3-D interactive graphics. The code computes the evolution of a CV accretion disk, visualizes results in real time, records and plays back simulations, and generates and plots pseudo light curves and associated power spectra.

  19. Compositional evolution during rocky protoplanet accretion

    CERN Document Server

    Carter, Philip J; Elliott, Tim; Walter, Michael J; Stewart, Sarah T

    2015-01-01

    The Earth appears non-chondritic in its abundances of refractory lithophile elements, posing a significant problem for our understanding of its formation and evolution. It has been suggested that this non-chondritic composition may be explained by collisional erosion of differentiated planetesimals of originally chondritic composition. In this work, we present N-body simulations of terrestrial planet formation that track the growth of planetary embryos from planetesimals. We simulate evolution through the runaway and oligarchic growth phases under the Grand Tack model and in the absence of giant planets. These simulations include a state-of-the-art collision model which allows multiple collision outcomes, such as accretion, erosion, and bouncing events, that enables tracking of the evolving core mass fraction of accreting planetesimals. We show that the embryos grown during this intermediate stage of planet formation exhibit a range of core mass fractions, and that with significant dynamical excitation, enoug...

  20. On the Stability of Cubic Galileon Accretion

    CERN Document Server

    Bergliaffa, Santiago P E

    2016-01-01

    We examine the stability of steady-state galileon accretion for the case of a Schwarzshild black hole. Considering the galileon action up to the cubic term in a static and spherically symmetric background we obtain the general solution for the equation of motion which is divided in two branches. By perturbing this solution we define an effective metric which determines the propagation of fluctuations. In this general picture we establish the position of the sonic horizon together with the matching condition of the two branches on it. Restricting to the case of a Schwarzschild background, we show, via the analysis of the energy of the perturbations and its time derivative, that the accreting field is linearly stable.

  1. Variable protostellar accretion with episodic bursts

    CERN Document Server

    Vorobyov, Eduard I

    2015-01-01

    We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur...

  2. Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks

    Energy Technology Data Exchange (ETDEWEB)

    Fragile, P C; Lindner, C C; Anninos, P; Salmonson, J D

    2008-09-24

    In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing 'plunging streams' and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured 'cubed-sphere' grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.

  3. Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks

    Energy Technology Data Exchange (ETDEWEB)

    Fragile, P C; Lindner, C C; Anninos, P; Salmonson, J D

    2008-09-24

    In recent work we presented the first results of global general relativistic magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion disks around rotating black holes. The simulated tilted disks showed dramatic differences from comparable untilted disks, such as asymmetrical accretion onto the hole through opposing 'plunging streams' and global precession of the disk powered by a torque provided by the black hole. However, those simulations used a traditional spherical-polar grid that was purposefully underresolved along the pole, which prevented us from assessing the behavior of any jets that may have been associated with the tilted disks. To address this shortcoming we have added a block-structured 'cubed-sphere' grid option to the Cosmos++ GRMHD code, which will allow us to simultaneously resolve the disk and polar regions. Here we present our implementation of this grid and the results of a small suite of validation tests intended to demonstrate that the new grid performs as expected. The most important test in this work is a comparison of identical tilted disks, one evolved using our spherical-polar grid and the other with the cubed-sphere grid. We also demonstrate an interesting dependence of the early-time evolution of our disks on their orientation with respect to the grid alignment. This dependence arises from the differing treatment of current sheets within the disks, especially whether they are aligned with symmetry planes of the grid or not.

  4. Early dust evolution in protostellar accretion disks

    OpenAIRE

    2000-01-01

    We investigate dust dynamics and evolution during the formation of a protostellar accretion disk around intermediate mass stars via 2D numerical simulations. Using three different detailed dust models, compact spherical particles, fractal BPCA grains, and BCCA grains, we find that even during the early collapse and the first 10,000 yr of dynamical disk evolution, the initial dust size distribution is strongly modified. Close to the disk's midplane coagulation produces dust particles of sizes ...

  5. Broken discs: warp propagation in accretion discs

    OpenAIRE

    Nixon, Chris; King, Andrew

    2012-01-01

    We simulate the viscous evolution of an accretion disc around a spinning black hole. In general any such disc is misaligned, and warped by the Lense-Thirring effect. Unlike previous studies we use effective viscosities constrained to be consistent with the internal fluid dynamics of the disc. We find that nonlinear fluid effects, which reduce the effective viscosities in warped regions, can promote the breaking of the disc into two distinct planes. This occurs when the Shakura & Sunyaev dimen...

  6. Satellites in Discs: Regulating the Accretion Luminosity

    OpenAIRE

    Syer, Dave; Clarke, Cathie

    1995-01-01

    We demonstrate, using a simple analytic model, that the presence of a massive satellite can globally modify the structure and emission properties of an accretion disc to which it is tidally coupled. We show, using two levels of numerical approximation, that the analytic model gives reasonable results. The results are applicable to two astrophysical situations. In the case of an active galactic nucleus, we consider the case of a $\\sim 10^3\\Msun$ compact companion to the central black-hole and ...

  7. Probing thermonuclear burning on accreting neutron stars

    OpenAIRE

    Keek, L.

    2008-01-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes thermonuclear burning in the neutron star envelope, creating carbon and heavier elements. The fusion process may proceed in an unstable manner, resulting in a thermonuclear runaway. Within one seco...

  8. Interaction of Accretion Shocks with Winds

    Indian Academy of Sciences (India)

    Kinsuk Acharya; Sandip K. Chakrabarti; D. Molteni

    2002-03-01

    Accretion shocks are known to oscillate in presence of cooling processes in the disk. This oscillation may also cause quasi-periodic oscillations of black holes. In the presence of strong winds, these shocks have oscillations in vertical direction as well.We show examples of shock oscillations under the influence of both the effects. When the shocks are absent and the flow is cooler, the wind becomes weaker and the vertical oscillation becomes negligible.

  9. Numerical simulations of dissipationless disk accretion

    Science.gov (United States)

    Bogovalov, S. V.; Tronin, I. V.

    2017-09-01

    Our goal is to study the regime of disk accretion in which almost all of the angular momentum and energy is carried away by the wind outflowing from the disk in numerical experiments. For this type of accretion the kinetic energy flux in the outflowing wind can exceed considerably the bolometric luminosity of the accretion disk, what is observed in the plasma flow from galactic nuclei in a number of cases. In this paper we consider the nonrelativistic case of an outflow from a cold Keplerian disk. All of the conclusions derived previously for such a system in the self-similar approximation are shown to be correct. The numerical results agree well with the analytical predictions. The inclination angle of the magnetic field lines in the disk is less than 60°, which ensures a free wind outflow from the disk, while the energy flux per wind particle is greater than the particle rotation energy in its Keplerian orbit by several orders of magnitude, provided that the ratio r A/ r ≫ 1, where r A is the Alfvénic radius and r is the radius of the Keplerian orbit. In this case, the particle kinetic energy reaches half the maximum possible energy in the simulation region. The magnetic field collimates the outflowing wind near the rotation axis and decollimates appreciably the wind outflowing from the outer disk periphery.

  10. Magnetic Field Transport in Accretion Disks

    Science.gov (United States)

    Jafari, Amir; Vishniac, Ethan

    2017-06-01

    The most plausible theories for launching astrophysical jets rely on strong magnetic fields at the inner parts of some accretion disks. An internal dynamo can in principle generate small scale magnetic fields in situ but generating a large scale field in a disk seems a difficult task in the dynamo theories. In fact, as far as numerous numerical experiments indicate, a dynamo-generated field in general would not be coherent enough over the large length scales of order the disk's radius. Instead, a large scale poloidal field dragged in from the environment, and compressed by the accretion, provides a more promising possibility. The difficulty in the latter picture, however, arises from the reconnection of the radial field component across the mid-plane which annihilates the field faster than it is dragged inward by the accretion. We suggest that a combination of different effects, including magnetic buoyancy and turbulent pumping, is responsible for the vertical transport of the field lines toward the surface of the disk. The radial component of the poloidal field vanishes at the mid-plane, which efficiently impedes reconnection, and grows exponentially toward the surface where it can become much larger than the vertical field component. This allows the poloidal field to be efficiently advected to small radii until the allowed bending angle drops to of order unity, and the field can drive a strong outflow.

  11. Minidisks in Binary Black Hole Accretion

    Science.gov (United States)

    Ryan, Geoffrey; MacFadyen, Andrew

    2017-02-01

    Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole (“minidisks”), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using 2D hydrodynamical simulations performed with a new general relativistic version of the moving-mesh code Disco. We introduce a comoving energy variable that enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the innermost stable circular orbit, providing a Reynolds stress that causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov-Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly wound linear waves. We measure the shock-induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing-angle-dependent images for comparison with observations.

  12. Water Masers in AGN Accretion Disks

    Science.gov (United States)

    Braatz, J. A.; Reid, M. J.; Greenhill, L. J.; Kuo, C.-Y.; Condon, J. J.; Lo, K.-Y.; Henkel, C.

    2009-08-01

    Water vapor masers at 22 GHz have been detected in over 100 galaxies, most of them AGNs. High resolution VLBI observations of these masers provide the only opportunity for direct imaging of sub-parsec structure in AGN accretion disks. The key science goals associated with such observations are concentrated in two areas. First, observations of nearby, bright sources, exemplified by NGC 4258, enable unique investigations of accretion disk geometry, substructure, thickness, and rotation properties. Second, when combined with spectral line monitoring, VLBI imaging and subsequent disk modeling enables the estimation of a distance to the host galaxy independent of standard candle arguments. In this contribution we present VLBI observations of two maser disk systems in galaxies well into the Hubble flow, UGC 3789 and NGC 6323. A long term goal in these studies is to measure the Hubble constant with high precision and, as a complement to CMB observations, constrain several key cosmological parameters, including the equation of state for dark energy. Observations with VSOP-2 at 22 GHz will have the resolution critical for mapping substructure in these accretion disks and will contribute to reducing systematic errors in the measurement of distances to galaxies.

  13. Magnetically driven accretion in protoplanetary discs

    CERN Document Server

    Simon, Jacob B; Kunz, Matthew W; Armitage, Philip J

    2015-01-01

    We characterize magnetically driven accretion at radii between 1 au and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary di...

  14. Modeling the Accretion Structure of AU Mon

    CERN Document Server

    Atwood-Stone, Corwin; Richards, Mercedes T; Budaj, Jan; Peters, Geraldine J

    2012-01-01

    AU Mon is a long-period (11.113 d) Algol-type binary system with a persistent accretion disk that is apparent as double-peaked H-alpha emission. We present previously unpublished optical spectra of AU Mon which were obtained over several years with dense orbital phase coverage. We utilize these data, along with archival UV spectra, to model the temperature and structure of the accretion disk and the gas stream. Synthetic spectral profiles for lines including H-alpha, H-beta, and the Al III and Si IV doublets were computed with the Shellspec program. The best match between the model spectra and the observations is obtained for an accretion disk of inner/outer radius 5.1/23 R_sun, thickness of 5.2 R_sun, density of 1.0e-13 g/cm^3, and maximum temperature of 14000 K, along with a gas stream at a temperature of ~8000 K transferring ~2.4e-9 M_sun/yr. We show H-alpha Doppler tomograms of the velocity structure of the gas, constructed from difference profiles calculated through sequentially subtracting contributions...

  15. Generalized Similarity for Accretion/Decretion Disks

    CERN Document Server

    Rafikov, Roman R

    2016-01-01

    Decretion (or external) disks are gas disks freely expanding to large radii due to their internal stresses. They are expected to naturally arise in tidal disruption events, around Be stars, in mass-losing post main sequence binaries, as a result of supernova fallback, etc. Their evolution is theoretically understood in two regimes: when the central object does not exert torque on the disk (a standard assumption for conventional accretion disks) or when no mass inflow (or outflow) occurs at the disk center. However, many astrophysical objects - circumbinary disks, Be stars, neutron stars accreting in a propeller regime, etc. - feature non-zero torque simultaneously with the non-zero accretion (or ejection of mass) at the disk center. We provide a general description for the evolution of such disks (both linear and non-linear) in the self-similar regime, to which the disk should asymptotically converge with time. We identify a similarity parameter $\\lambda$, which is uniquely related to the degree, to which the...

  16. Dynamical Masses of Accreting White Dwarfs

    Science.gov (United States)

    Pala, A. F.; Gänsckie, B. T.

    2017-03-01

    The mass retention efficiency is a key question in both the theoretical and observational study of accreting white dwarfs in interacting binaries, with important implications for their potential as progenitors for type Ia supernovae (SNe Ia). Canonical wisdom is that classical nova eruptions erode the white dwarf mass, and consequently, cataclysmic variables (CVs) have been excluded from the SN Ia progenitor discussion. However the average mass of white dwarfs in CVs is substantially higher (≃ 0.83 M⊙) than that of single white dwarfs (≃ 0.64 M ⊙), in stark contrast to expectations based on current classical nova models. This finding is based on a sample of ≃ 30 CV white dwarfs with accurate mass measurements, most of them in eclipsing systems. Given the fundamental importance of the mass evolution of accreting white dwarfs, it is necessary to enlarge this sample and to diversify the methods used for measuring masses. We have begun a systematic study of 27 CVs to almost double the number of CV white dwarfs with an accurate mass measurement. Using VLT/X-shooter phase-resolved observations, we can measure the white dwarf masses to a few percent, and will be able to answer the question whether accreting CV white dwarfs grow in mass.

  17. Sub-Keplerian accretion onto circumstellar disks

    CERN Document Server

    Visser, R

    2010-01-01

    Models of the formation, evolution and photoevaporation of circumstellar disks are an essential ingredient in many theories of the formation of planetary systems. The ratio of disk mass over stellar mass in the circumstellar phase of a disk is largely determined by the angular momentum of the original cloud core from which the system was formed. While full 3D or 2D axisymmetric hydrodynamical models of accretion onto the disk automatically treat all aspects of angular momentum, this is not so trivial for 1D and semi-2D viscous disk models. Since 1D and semi-2D disk models are still very useful for long-term evolutionary modelling of disks with relatively little numerical effort, we investigate how the 2D nature of accretion affects the formation and evolution of the disk in such models. A proper treatment of this problem requires a correction for the sub-Keplerian velocity at which accretion takes place. We develop an update of our semi-2D time-dependent disk evolution model to properly treat the effects of s...

  18. Terrane accretion: Insights from numerical modelling

    Science.gov (United States)

    Vogt, Katharina; Gerya, Taras

    2016-04-01

    The oceanic crust is not homogenous, but contains significantly thicker crust than norm, i.e. extinct arcs, spreading ridges, detached continental fragments, volcanic piles or oceanic swells. These (crustal) fragments may collide with continental crust and form accretionary complexes, contributing to its growth. We analyse this process using a thermo-mechanical computer model (i2vis) of an ocean-continent subduction zone. In this model the oceanic plate can bend spontaneously under the control of visco-plastic rheologies. It moreover incorporates effects such as mineralogical phase changes, fluid release and consumption, partial melting and melt extraction. Based on our 2-D experiments we suggest that the lithospheric buoyancy of the downgoing slab and the rheological strength of crustal material may result in a variety of accretionary processes. In addition to terrane subduction, we are able to identify three distinct modes of terrane accretion: frontal accretion, basal accretion and underplating plateaus. We show that crustal fragments may dock onto continental crust and cease subduction, be scrapped off the downgoing plate, or subduct to greater depth prior to slab break off and subsequent exhumation. Direct consequences of these processes include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes, partial melting and crustal growth.

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

    Energy Technology Data Exchange (ETDEWEB)

    Matt, Sean P. [Laboratoire AIM Paris-Saclay, CEA/Irfu Universite Paris-Diderot CNRS/INSU, 91191 Gif-sur-Yvette (France); Pinzon, Giovanni [Observatorio Astronomico Nacional, Facultad de Ciencias, Universidad Nacional de Colombia, Bogota (Colombia); Greene, Thomas P. [NASA Ames Research Center, M.S. 245-6, Moffett Field, CA 94035-1000 (United States); Pudritz, Ralph E., E-mail: sean.matt@cea.fr, E-mail: thomas.p.greene@nasa.gov, E-mail: gapinzone@unal.edu.co, E-mail: pudritz@physics.mcmaster.ca [Physics and Astronomy Department, McMaster University, Hamilton, ON L8S 4M1 (Canada)

    2012-01-20

    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.

  20. Effect of Gas Accretion Disc Profile on Orbital Parameters of the Accreted Stars

    Science.gov (United States)

    Shukirgaliyev, Bekdaulet T.; Panamarev, Taras P.; Naurzbaeva, Aisha Zh.; Kalambay, Mukhagali T.; Makukov, Maxim A.; Vilkoviskij, Emmanuil Y.; Omarov, Chingis T.; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-10-01

    The results of studies of the effect of the gas disk and its profile on the dynamics of active galactic nuclei are presented. The study was conducted with a numerical model of galactic nucleus based on phiGRAPE+GPU comprising three subsystems - a central supermassive black hole, gaseous accretion disc, and compact stellar cluster. The evolution of the compact stellar cluster is modeled with direct integration (N-body simulation), while the black hole and gaseous disc are represented phenomenologically: the black hole is introduced as an external potential (fixed in space but variable in time due to black hole mass growth), and the gaseous disc is introduced as spatial time-independent density distribution. We examined and compared with each other orbital parameters of accreting stars for model of the galactic nucleus with gas disc of constant and variable thickness, as well as without gas. It was found that in the presence of a gaseous disk almost half of the accreted particles interact strongly with the gas and are captured by the disc before accretion, while more than 85% of particles are affected to some extent by the disc prior to accretion. This suggests that interaction of the stellar cluster with the gas disk in the galactic nucleus might lead to the formation of stellar disk in the central part of the nucleus.

  1. Core Forensics: Earth's Accretion and Differentiation

    Science.gov (United States)

    Badro, J.; Brodholt, J. P.; Siebert, J.; Piet, H.; Ryerson, F. J.

    2013-12-01

    Earth's accretion and its primitive differentiation are intimately interlinked processes. One way to constrain accretionary processes is by looking at the major differentiation event that took place during accretion: core formation. Understanding core formation and core composition can certainly shed a new light on early and late accretionary processes. On the other hand, testing certain accretionary models and hypothesis (fluxes, chemistries, timing) allows -short of validating them- at the very least to unambiguously refute them, through the 'filter'' of core formation and composition. Earth's core formed during accretion as a result of melting, phase-separation, and segregation of accretionary building blocks (from meteorites to planetesimals). The bulk composition of the core and mantle depends on the evolution (pressure, temperature, composition) of core extraction during accretion. The entire process left a compositional imprint on both reservoirs: (1) in the silicate Earth, in terms of siderophile trace-element (Ni, Co, V, Cr, among others) concentrations and isotopic fractionation (Si, Cu, among others), a record that is observed in present-day mantle rocks; and (2) on the core, in terms of major element composition and light elements dissolved in the metal, a record that is observed by seismology through the core density-deficit. This imprint constitutes actually a fairly impressive set of evidence (siderophile element concentration and fractionation, volatile and siderophile element isotopic fractionation), can be used today to trace back the primordial processes that occurred 4.5 billion years ago. We are seeking to provide an overhaul of the standard core formation/composition models, by using a new rationale that bridges geophysics and geochemistry. The new ingredients are (1) new laser-heated diamond anvil cell partitioning data, dramatically extending the previous P-T conditions for experimental work, (2) ab initio molecular dynamics calculations to

  2. Phantom Accretion onto the Schwarzschild de-Sitter Black Hole

    Institute of Scientific and Technical Information of China (English)

    M Sharif; G Abbas

    2011-01-01

    We deal with phantom energy accretion onto the Schwarzschild de-Sitter black hole. The energy flux conservation, relativistic Bernoulli equation and mass Bux conservation equation are formulated to discuss the phantom accretion. We discuss the conditions for critical accretion. It is found that the mass of the black hole decreases due to phantom accretion. There exist two critical points which lie in the exterior of horizons (black hole and cosmological horizons). The results for the phantom energy accretion onto the Schwarzschild black hole can be recovered by taking A → 0.%@@ We deal with phantom energy accretion onto the Schwarzschild de-Sitter black hole.The energy flux conserva-tion,relativistic Bernoulli equation and mass flux conservation equation are formulated to discuss the phantom accretion.We discuss the conditions for critical accretion.It is found that the mass of the black hole decreases due to phantom accretion.There exist two critical points which lie in the exterior of horizons(black hole and cosmological horizons).The results for the phantom energy accretion onto the Schwarzschild black hole can be recovered by taking ∧→0.

  3. Polarized Campuses.

    Science.gov (United States)

    Parr, Susan Resneck

    1991-01-01

    On college campuses, the climate is polarized because of intolerance and discrimination, censorship, factionalism, and anger among students and faculty. As a result, the campus is in danger of becoming dominated by political issues and discouraging the exchange of ideas characteristic of a true liberal arts education. (MSE)

  4. The Formation of Polar Disk Galaxies

    CERN Document Server

    Brook, Chris B; Quinn, Thomas; Wadsley, James; Brooks, Alyson M; Willman, Beth; Stilp, Adrienne; Jonsson, Patrik

    2008-01-01

    Polar Ring Galaxies, such as NGC4650A, are a class of galaxy which have two kinematically distinct components that are inclined by almost 90 degrees to each other. These striking galaxies challenge our understanding of how galaxies form; the origin of their distinct components has remained uncertain, and the subject of much debate. We use high-resolution cosmological simulations of galaxy formation to show that Polar Ring Galaxies are simply an extreme example of the angular moment misalignment that occurs during the hierarchical structure formation characteristic of Cold Dark Matter cosmology. In our model, Polar Ring Galaxies form through the continuous accretion of gas whose angular momentum is misaligned with the central galaxy.

  5. The quiescent state of the accreting X-ray pulsar SAX J2103.5+4545

    CERN Document Server

    Reig, P; Zezas, A

    2014-01-01

    We present an X-ray timing and spectral analysis of the Be/X-ray binary SAX J2103.5+4545 at a time when the Be star's circumstellar disk had disappeared and thus the main reservoir of material available for accretion had extinguished. In this very low optical state, pulsed X-ray emission was detected at a level of L_X~10^{33} erg/s. This is the lowest luminosity at which pulsations have ever been detected in an accreting pulsar. The derived spin period is 351.13 s, consistent with previous observations. The source continues its overall long-term spin-up, which reduced the spin period by 7.5 s since its discovery in 1997. The X-ray emission is consistent with a purely thermal spectrum, represented by a blackbody with kT=1 keV. We discuss possible scenarios to explain the observed quiescent luminosity and conclude that the most likely mechanism is direct emission resulting from the cooling of the polar caps, heated either during the most recent outburst or via intermittent accretion in quiescence.

  6. Radiation Pressure-supported Accretion Disks: Vertical Structure, Energy Advection, and Convective Stability

    CERN Document Server

    Gu, Wei-Min

    2012-01-01

    By taking into account the local energy balance per unit volume between the viscous heating and the advective cooling plus the radiative cooling, we investigate the vertical structure of radiation pressure-supported accretion disks in spherical coordinates. Our solutions show that the photosphere of the disk is close to the polar axis and therefore the disk seems to be extremely thick. However, the profile of density implies that most of the accreted matter exists in a moderate range around the equatorial plane. We show that the well-known polytropic relation between the pressure and the density is unsuitable for describing the vertical structure of radiation pressure-supported disks. More importantly, we find that the energy advection is significant even for slightly sub-Eddington accretion disks. We argue that the non-negligible advection may help to understand why the standard thin disk model is likely to be inaccurate above \\sim 0.3 Eddington luminosity, which was found by some works on the black hole spi...

  7. Magnetocentrifugally Driven Flows from Young Stars and Disks. VI. Accretion with a Multipole Stellar Field

    CERN Document Server

    Mohanty, Subhanjoy

    2008-01-01

    Previous analyses of magnetospheric accretion and outflow in classical T Tauri stars (CTTSs), within the context of both the X-wind model and other theoretical scenarios, have assumed a dipolar geometry for the stellar magnetic field if it were not perturbed by the presence of an accreting, electrically conducting disk. However, CTTS surveys reveal that accretion hot spots cover a small fraction of the stellar surface, and that the net field polarization on the stellar surface is small. Both facts imply that the magnetic field generated by the star has a complex non-dipolar structure. To address this discrepancy between theory and observations, we re-examine X-wind theory without the dipole constraint. Using simple physical arguments based on the concept of trapped flux, we show that a dipole configuration is in fact not essential. Independent of the precise geometry of the stellar magnetosphere, the requirement for a certain level of trapped flux predicts a definite relationship among various CTTS observable...

  8. Jet Luminosity from Neutrino-Dominated Accretion Flows in GRBs

    CERN Document Server

    Kawanaka, Norita

    2013-01-01

    A hyperaccretion disk around a stellar-mass black hole is a plausible model for the central engine that powers gamma-ray bursts (GRBs). We estimate the luminosity of a jet driven by magnetohydrodynamic processes such as the Blandford-Znajek (BZ) mechanism as a function of mass accretion rate, the black hole mass, and other accretion parameters. We show that the jet is most efficient when the accretion flow is cooled via optically-thin neutrino emission, and that its luminosity is much larger than the energy deposition rate through neutrino annihilation provided that the black hole is spinning rapidly enough. Also, we find a significant jump in the jet luminosity at the transition mass accretion rate between the advection dominated accretion flow (ADAF) regime and the neutrino-dominated accretion flow (NDAF) regime. This may cause the large variability observed in the prompt emission of GRBs.

  9. Spin Period Evolution of Recycled Pulsar in Accreting Binary

    CERN Document Server

    Wang, J; Zhao, Y H; Kojima, Y; Yin, H X; SOng, L M

    2010-01-01

    We investigate the spin-period evolutions of recycled pulsars in binary accreting systems. Taking both the accretion induced field decay and spin-up into consideration, we calculate their spin-period evolutions influenced by the initial magnetic-field strengths, initial spin-periods and accretion rates, respectively. The results indicate that the minimum spin-period (or maximum spin frequency) of millisecond pulsar (MSP) is independent of the initial conditions and accretion rate when the neutron star (NS) accretes $\\sim> 0.2\\ms$. The accretion torque with the fastness parameter and gravitational wave (GW) radiation torque may be responsible for the formation of the minimum spin-period (maximum spin frequency). The fastest spin frequency (716 Hz) of MSP can be inferred to associate with a critical fastness parameter about $\\omega_{c}=0.55$. Furthermore, the comparisons with the observational data are presented in the field-period ($B-P$) diagram.

  10. Riverbank erosion induced by gravel bar accretion

    Science.gov (United States)

    Klösch, Mario; Habersack, Helmut

    2010-05-01

    Riverbank erosion is known to be strongly fluvially controlled and determination of shear stresses at the bank surface and at the bank toe is a crucial point in bank erosion modeling. In many modeling attempts hydraulics are simulated separately in a hydrodynamic-numerical model and the simulated shear stresses are further applied onto the bank surface in a bank erosion model. Hydrodynamics are usually simulated at a constant geometry. However, in some cases bed geometry may vary strongly during the event, changing the conditions for hydrodynamics along the bank. This research seeks to investigate the effect of gravel bar accretion during high discharges on final bank retreat. At a restored section of the Drava River bed widenings have been implemented to counter bed degradation. There, in an initiated side-arm, self-dynamic widening strongly affects bed development and long-term connectivity to the main channel. Understanding the riverbank erosion processes there would help to improve planning of future restoration measures. At one riverbank section in the side-arm large bank retreat was measured repeatedly after several flow events. This section is situated between two groins with a distance of 60 m, which act as lateral boundaries to the self-widening channel. In front of this bank section a gravel bar developed. During low flow condition most discharge of the side-arm flows beside the gravel bar along the bank, but shear stresses are too low for triggering bank erosion. For higher discharges results from a two-dimensional hydrodynamic-numerical model suggested shear stresses there to be generally low during the entire events. At some discharges the modeled flow velocities even showed to be recirculating along the bank. These results didn't explain the observed bank retreat. Based on the modeled shear stresses, bank erosion models would have greatly underestimated the bank retreat induced by the investigated events. Repeated surveys after events applying

  11. Phase-resolved Spectroscopy of the Intermediate Polars -- TV Col and V1223 Sgr

    Science.gov (United States)

    Long, K.

    The cataclysmic variables called intermediate polars are characterized by magnetic fields that rip material from an accretion disk and funnel it to a WD that is not phase-locked to the binary period of the system. This is a proposal to use FUSE to conduct a time-resolved spectroscopic study to dissect the emission of two long-period intermediate polars, V1223 Sgr and TV Col, with very different inclination angles. These, along with the short-period high-inclination IP EX Hya (already observed with FUSE), comprise the only IPs with accurate distances derived from HST astrometry. We will isolate emission from the photosphere of the WD, the magnetically dominated accretion curtain, and the accretion stream. Having characterized the emission sources, we will explore the physical conditions in these same regions, and develop an integrated picture of these two intermediate polars.

  12. Volatile loss from accreting icy protoplanets

    Science.gov (United States)

    Stevenson, D. J.

    1993-03-01

    A large self-gravitating body does not easily lose significant mass because the escape velocity is much larger than the sound speed of atmosphere-forming species under ambient thermal conditions. The most significant exceptions to this are giant impacts or impact jetting by fast-moving projectiles. A very small object (e.g. a comet) also does not easily lose significant volatile mass upon formation because the energy release associated with its accretion is so small. (It can however lose a great deal of mass if it is subsequently moved closer to the Sun.) I argue that there is an intermediate mass range (corresponding to bodies with radii of approximately 300-800 km) for which the ambient steady-state mass loss is a maximum. By ambient, I mean those conditions pertaining to the formation region of the body. By steady state, I mean to exclude infrequent traumas (giant impacts). The existence of a preferred intermediate mass arises through the competition of growing gravitational containment and growing energy release by accretion; it corresponds typically to GM/(Rcs2) approximately equals 2 to 4, where M is the protoplanet mass of radius R, and cs is the sound speed. Several factors determine the amount of volatile loss is this vulnerable zone during accretion but in general the loss is a substantial fraction of the volatiles, sometimes approaching 100 percent. The principal implication is that bodies larger than a few hundred kilometers in radius will not have a 'primitive' (i.e. cometary) composition. This is relevant for understanding Triton, Pluto, Charon, and perhaps Chiron.

  13. Accretion-disc model spectra for dwarf-nova stars

    OpenAIRE

    Idan, Irit; Lasota, Jean-Pierre; Hameury, Jean-Marie; Shaviv, Giora

    2008-01-01

    Radiation from accretion discs in cataclysmic variable stars (CVs) provides fundamental information about the properties of these close binary systems and about the physics of accretion in general. The detailed diagnostics of accretion disc structure can be achieved by including in its description all the relevant heating and cooling physical mechanism, in particular the convective energy transport that, although dominant at temperatures less than about 10 000 K, is usually not taken into acc...

  14. Polar Diving

    Science.gov (United States)

    2006-01-01

    3 July 2006 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows layers exposed by erosion in a trough within the north polar residual cap of Mars, diving beneath a younger covering of polar materials. The layers have, since the Mariner 9 mission in 1972, been interpreted to be composed of a combination of dust and ice in unknown proportions. In this scene, a layer of solid carbon dioxide, which was deposited during the previous autumn and winter, blankets the trough as well as the adjacent terrain. Throughout northern spring, the carbon dioxide will be removed; by summer, the layers will be frost-free. Location near: 81.4oN, 352.2oW Image width: 3 km (1.9 mi) Illumination from: lower left Season: Northern Spring

  15. Slim accretion discs with different viscosity prescriptions

    Energy Technology Data Exchange (ETDEWEB)

    Szuszkiewicz, E. (Max-Planck-Institut fuer Physik und Astrophysik, Garching (Germany, F.R.). Inst. fuer Astrophysik)

    1990-05-15

    The variability of X-ray sources powered by accretion may be connected to thermal instabilities in the innermost parts of slim discs. The time-scales of variability predicted by the theory with the standard {alpha}-viscosity prescription agree with those observed in a wide range of sources. The amplitudes (3-4 orders of magnitude in luminosity) are correctly predicted for X-ray transient sources, but in general are too big for quasars, Seyferts, galactic black hole candidates and LMXBs. We show here that a slight modification of the viscosity prescription can offer a much better agreement with observations. (author).

  16. Fundamental Ice Crystal Accretion Physics Studies

    Science.gov (United States)

    Struk, Peter M.; Broeren, Andy P.; Tsao, Jen-Ching; Vargas, Mario; Wright, William B.; Currie, Tom; Knezevici, Danny; Fuleki, Dan

    2012-01-01

    Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. In November 2010, two weeks of testing occurred at the NRC Research Altitude Facility utilizing a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations. Icing tests were performed at Mach numbers of 0.2 to 0.3, total pressures from 93 to 45 kPa, and total temperatures from 5 to 15 C. Ice and liquid water contents ranged up to 20 and 3 g/m3, respectively. The ice appeared well adhered to the surface in the lowest pressure tests (45 kPa) and, in a particular case, showed continuous leading-edge ice growth to a thickness greater than 15 mm in 3 min. Such widespread deposits were not observed in the highest pressure tests, where the accretions were limited to a small area around the leading edge. The suction surface was typically ice-free in the tests at high pressure, but not at low pressure. The icing behavior at high and low pressure appeared to be correlated with the wet-bulb temperature, which was estimated to be above 0 C in tests at 93 kPa and below 0 C in tests at lower pressure, the latter enhanced by more evaporative cooling of water. The authors believe that the large ice accretions observed in the low pressure tests would undoubtedly cause the aerodynamic performance of a compressor component

  17. New mechanism of radiation polarization in type 1 Seyfert active galactic nuclei

    Science.gov (United States)

    Silant'ev, N. A.; Gnedin, Yu. N.; Piotrovich, M. Yu.; Natsvlishvili, T. M.; Buliga, S. D.

    2016-10-01

    In most type 1 Seyfert active galactic nuclei (AGNs), the optical linear continuum polarization degree is usually small (less than 1 per cent) and the polarization position angle is nearly parallel to the AGN radio axis. However, there are many type 1 AGNs with unexplained intermediate values for both positional angles and polarization degrees. Our explanation of polarization degree and positional angle of type 1 Seyfert AGNs focuses on the reflection of non-polarized radiation from sub-parsec jets in optically thick accretion discs. The presence of a magnetic field surrounding the scattering media will induce Faraday rotation of the polarization plane, which may explain the intermediate values of positional angles if there is a magnetic field component normal to the accretion disc. The Faraday rotation depolarization effect in the disc diminishes the competition between polarization of the reflected radiation with the parallel component of polarization and the perpendicular polarization from internal radiation of the disc (the Milne problem) in favour of polarization of the reflected radiation. This effect allows us to explain the observed polarization of type 1 Seyfert AGN radiation even though the jet optical luminosity is much lower than the luminosity of the disc. We present the calculation of polarization degrees for a number of type 1 Seyfert AGNs.

  18. Accretion and evaporation of modified Hayward black hole

    Energy Technology Data Exchange (ETDEWEB)

    Debnath, Ujjal [Indian Institute of Engineering Science and Technology, Department of Mathematics, Howrah (India)

    2015-03-01

    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.)

  19. Waves and Instabilities in Accretion Disks MHD Spectroscopic Analysis

    CERN Document Server

    Keppens, R; Goedbloed, J P

    2002-01-01

    A complete analytical and numerical treatment of all magnetohydrodynamic waves and instabilities for radially stratified, magnetized accretion disks is presented. The instabilities are a possible source of anomalous transport. While recovering results on known hydrodynamicand both weak- and strong-field magnetohydrodynamic perturbations, the full magnetohydrodynamic spectra for a realistic accretion disk model demonstrates a much richer variety of instabilities accessible to the plasma than previously realized. We show that both weakly and strongly magnetized accretion disks are prone to strong non-axisymmetric instabilities.The ability to characterize all waves arising in accretion disks holds great promise for magnetohydrodynamic spectroscopic analysis.

  20. Convection-Dominated Accretion Flows with Radiative Cooling

    Institute of Scientific and Technical Information of China (English)

    LI Shuang-Liang; XUE Li; LU Ju-Fu

    2007-01-01

    @@ By numerically solving the set of basic equations describing black hole accretion flows with low accretion rates,we show that although the dynamical structure of these flows is essentially unaffected by radiative processes in comparison with the case in which the radiation is not considered, the radiative cooling can be more important than the advective cooling in the flow's convection-dominated zone, and this result may have implications to distinguish observationally convection-dominated accretion flows from advection-dominated accretion flows.

  1. Accretion Onto a Charged Higher-Dimensional Black Hole

    CERN Document Server

    Sharif, M

    2016-01-01

    This paper deals with the steady-state polytropic fluid accretion onto a higher-dimensional Reissner-Nordstr$\\ddot{o}$m 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 critical radius, critical sound velocity and critical flow velocity. We also explore gas compression and temperature profiles to analyze the asymptotic behavior. It is found that the results for Schwarzschild black hole are recovered when $q=0$ in four dimensions. We conclude that accretion process in higher dimensions becomes slower in the presence of charge.

  2. Deceleration Effect of Magnetic Field on Black Hole Accretion Disks

    Institute of Scientific and Technical Information of China (English)

    WANG Ding-Xiong

    2000-01-01

    The deceleration effect of magnetic field near the horizon of a spinning black hole (BH) of accretion disk is investigated in the Blandford-Znajek (BZ) process. It is shown that rates of change with respect to time for both the angular velocities of BH horizon and accreting particles at the inner edge of an accretion disk are reduced in the BZ process, behaving with non-monotonous evolution characteristics. This result implies that the magnetic field near the BH horizon has & deceleration effect not only on the spinning BH but also on the surrounding accretion disk.

  3. 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.

  4. ACCRETION DISKS IN TWO-DIMENSIONAL HOYLE-LYTTLETON FLOW

    Energy Technology Data Exchange (ETDEWEB)

    Blondin, John M., E-mail: John_Blondin@ncsu.edu [Department of Physics, North Carolina State University, Raleigh, NC 27695-8202 (United States)

    2013-04-20

    We investigate the flip-flop instability observed in two-dimensional planar hydrodynamic simulations of Hoyle-Lyttleton accretion in the case of an accreting object with a radius much smaller than the nominal accretion radius, as one would expect in astrophysically relevant situations. Contrary to previous results with larger accretors, accretion from a homogenous medium onto a small accretor is characterized by a robust, quasi-Keplerian accretion disk. For gas with a ratio of specific heats of 5/3, such a disk remains locked in one direction for a uniform ambient medium. The accretion flow is more variable for gas with a ratio of specific heats of 4/3, with more dynamical interaction of the disk flow with the bow shock leading to occasional flips in the direction of rotation of the accretion disk. In both cases the accretion of angular momentum is determined by the flow pattern behind the accretion shock rather than by the parameters of the upstream flow.

  5. Spin Evolution of Accreting Young Stars. II. Effect of Accretion-Powered Stellar Winds

    CERN Document Server

    Matt, Sean P; Greene, Thomas P; Pudritz, Ralph E

    2011-01-01

    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. 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 s...

  6. Physical processes in the strong magnetic fields of accreting neutron stars

    Science.gov (United States)

    Meszaros, P.

    1984-01-01

    Analytical formulae are fitted to observational data on physical processes occurring in strong magnetic fields surrounding accreting neutron stars. The propagation of normal modes in the presence of a quantizing magnetic field is discussed in terms of a wave equation in Fourier space, quantum electrodynamic effects, polarization and mode ellipticity. The results are applied to calculating the Thomson scattering, bremsstrahlung and Compton scattering cross-sections, which are a function of the frequency, angle and polarization of the magnetic field. Numerical procedures are explored for solving the radiative transfer equations. When applied to modeling X ray pulsars, a problem arises in the necessity to couple the magnetic angle and frequency dependence of the cross-sections with the hydrodynamic equations. The use of time-dependent averaging and approximation techniques is indicated.

  7. Towards self-consistent modelling of the Sgr A* accretion flow: linking theory and observation

    Science.gov (United States)

    Roberts, Shawn R.; Jiang, Yan-Fei; Wang, Q. Daniel; Ostriker, Jeremiah P.

    2017-04-01

    The interplay between supermassive black holes (SMBHs) and their environments is believed to command an essential role in galaxy evolution. The majority of these SMBHs are in the radiative inefficient accretion phase where this interplay remains elusive, but suggestively important, due to few observational constraints. To remedy this, we directly fit 2D hydrodynamic simulations to Chandra observations of Sgr A* with Markov chain Monte Carlo sampling, self-consistently modelling the 2D inflow-outflow solution for the first time. We find the temperature and density at flow onset are consistent with the origin of the gas in the stellar winds of massive stars in the vicinity of Sgr A*. We place the first observational constraints on the angular momentum of the gas and estimate the centrifugal radius, rc ≈ 0.056 rb ≈ 8 × 10-3 pc, where rb is the Bondi radius. Less than 1 per cent of the inflowing gas accretes on to the SMBH, the remainder being ejected in a polar outflow. We decouple the quiescent point-like emission from the spatially extended flow. We find this point-like emission, accounting for ˜4 per cent of the quiescent flux, is spectrally too steep to be explained by unresolved flares, nor bremsstrahlung, but is likely a combination of a relatively steep synchrotron power law and the high-energy tail of inverse-Compton emission. With this self-consistent model of the accretion flow structure, we make predictions for the flow dynamics and discuss how future X-ray spectroscopic observations can further our understanding of the Sgr A* accretion flow.

  8. Seismology of Rapidly Rotating Accreting White Dwarfs

    CERN Document Server

    Townsley, Dean M; Bildsten, Lars

    2016-01-01

    A number of White Dwarfs (WDs) in cataclysmic binaries have shown brightness variations consistent with non-radial oscillations as observed in isolated WDs. A few objects have been well-characterized with photometric campaigns in the hopes of gleaning information about the mass, spin, and possibly internal structural characteristics. The novel aspect of this work is the possiblity to measure or constrain the interior structure and spin rate of WDs which have spent gigayears accreting material from their companion, undergoing thousands of nova outbursts in the process. In addition, variations in the surface temperature affect the site of mode driving, and provide unique and challenging tests for mode driving theories previously applied to isolated WD's. Having undergone long-term accretion, these WDs are expected to have been spun up. Spin periods in the range 60-100 seconds have been measured by other means for two objects, GW Lib and V455 And. Compared to typical mode frequencies, the spin frequency may be s...

  9. Formation of redbacks via accretion induced collapse

    CERN Document Server

    Smedley, Sarah L; Ferrario, Lilia; Wickramasinghe, Dayal T

    2014-01-01

    We examine the growing class of binary millisecond pulsars known as redbacks. In these systems the pulsar's companion has a mass between 0.1 and about 0.5 solar masses in an orbital period of less than 1.5 days. All show extended radio eclipses associated with circumbinary material. They do not lie on the period-companion mass relation expected from the canonical intermediate-mass X-ray binary evolution in which the companion filled its Roche lobe as a red giant and has now lost its envelope and cooled as a white dwarf. The redbacks lie closer to, but usually at higher period than, the period-companion mass relation followed by cataclysmic variables and low-mass X-ray binaries. In order to turn on as a pulsar mass accretion on to a neutron star must be sufficiently weak, considerably weaker than expected in systems with low-mass main-sequence companions driven together by magnetic braking or gravitational radiation. If a neutron star is formed by accretion induced collapse of a white dwarf as it approaches th...

  10. NUMERICAL SIMULATION OF ICE ACCRETION ON AIRFOIL

    Directory of Open Access Journals (Sweden)

    Nicusor ALEXANDRESCU

    2009-09-01

    Full Text Available This work consists in the simulation of the ice accretion in the leading edge of aerodynamic profiles and our proposed model encompasses: geometry generation, calculation of the potential flow around the body, boundary layer thickness computation, water droplet trajectory computation, heat and mass balances and the consequent modification of the geometry by the ice growth. The flow calculation is realized with panel methods, using only segments defined over the body contour. The viscous effects are considered using the Karman-Pohlhausen method for the laminar boundary layer. The local heat transfer coefficient is obtained by applying the Smith-Spalding method for the thermal boundary layer. The ice accretion limits and the collection efficiency are determined by computing water droplet trajectories impinging the surface. The heat transfer process is analyzed with an energy and a mass balance in each segment defining the body. Finally, the geometry is modified by the addition of the computed ice thickness to the respective panel. The process by repeating all the steps. The model validation is done using a selection of problems with experimental solution, CIRA (the CESAR project. Hereinafter, results are obtained for different aerodynamic profiles, angles of attack and meteorological parameters

  11. Magnetic fields in primordial accretion disks

    Science.gov (United States)

    Latif, M. A.; Schleicher, D. R. G.

    2016-01-01

    Magnetic fields are considered a vital ingredient of contemporary star formation and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations and are subsequently amplified by the small-scale dynamo, leading to a strong, tangled magnetic field. We explore how the magnetic field provided by the small-scale dynamo is further amplified via the α-Ω dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop. III star with 10M⊙ and an accretion rate of 10-3M⊙ yr-1, and a supermassive star with 105M⊙ and an accretion rate of 10-1M⊙ yr-1. For the 10M⊙ Pop. III star, we find that coherent magnetic fields can be produced on scales of at least 100 AU, which are sufficient to drive a jet with a luminosity of 100L⊙ and a mass outflow rate of 10-3.7M⊙ yr-1. For the supermassive star, the dynamical timescales in its environment are even shorter, implying smaller orbital timescales and an efficient magnetization out to at least 1000 AU. The jet luminosity corresponds to ~106.0L⊙ and a mass outflow rate of 10-2.1M⊙ yr-1. We expect that the feedback from the supermassive star can have a relevant impact on its host galaxy.

  12. Magnetic fields in primordial accretion disks

    CERN Document Server

    Latif, Muhammad A

    2016-01-01

    Magnetic fields are considered as a vital ingredient of contemporary star formation, and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations, and are subsequently amplified by the small-scale dynamo, leading to a strong tangled magnetic field. Here we explore how the magnetic field provided by the small-scale dynamo is further amplified via the $\\alpha-\\Omega$ dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop.~III star with $10$~M$_\\odot$ and an accretion rate of $10^{-3}$~M$_\\odot$~yr$^{-1}$, and a supermassive star with $10^5$~M$_\\odot$ and an accretion rate of $10^{-1}$~M$_\\odot$~yr$^{-1}$. For the $10$~M$_\\odot$ Pop.~III star, we find that coherent magnetic fields can be produced on scales of at least $100$~AU, which are sufficient to drive a jet with a luminosity of $100$~L$_\\odot$ and a mass outflow ra...

  13. Spiral waves in accretion discs - observations

    CERN Document Server

    Steeghs, D

    2000-01-01

    I review the observational evidence for spiral structure in the accretion discs of cataclysmic variables (CVs). Doppler tomography is ideally suited to resolve and map such co-rotating patterns and allows a straightforward comparison with theory. The dwarf nova IP Pegasi presents the best studied case, carrying two spiral arms in a wide range of emission lines throughout its outbursts. Both arms appear at the locations where tidally driven spiral waves are expected, with the arm closest to the gas stream weaker in the lines compared to the arm closest to the companion. Eclipse data indicates sub-Keplerian velocities in the outer disc. The dramatic disc structure changes in dwarf novae on timescales of days to weeks, provide unique opportunities for our understanding of angular momentum transport and the role of density waves on the structure of accretion discs. I present an extension to the Doppler tomography technique that relaxes one of the basic assumptions of tomography, and is able to map modulated emiss...

  14. A Wind Accretion Model for HLX-1

    CERN Document Server

    Miller, M Coleman; Maccarone, Thomas J

    2014-01-01

    The brightest ultraluminous X-ray source currently known, HLX-1, has been observed to undergo five outburst cycles. The periodicity of these outbursts, and their high inferred maximum accretion rates of $\\sim{\\rm few}\\times 10^{-4} M_\\odot {\\rm yr}^{-1}$, naturally suggest Roche lobe overflow at the pericenter of an eccentric orbit. It is, however, difficult for the Roche lobe overflow model to explain the apparent trend of decreasing decay times over the different outbursts while the integrated luminosity also drops. Thus if the trend is real rather than simply being a reflection of the complex physics of accretion disks, a different scenario may be necessary. We present a speculative model in which, within the last decade, a high-mass giant star had most of its envelope tidally stripped by the $\\sim 10^{4-5} M_\\odot$ black hole in HLX-1, and the remaining core plus low-mass hydrogen envelope now feeds the hole with a strong wind. This model can explain the short decay time of the disk, and could explain the...

  15. Accretion, Outflows, and Winds of Magnetized Stars

    CERN Document Server

    Romanova, M M

    2016-01-01

    Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars...

  16. Quantifying Rapid Variability in Accreting Compact Objects

    CERN Document Server

    Van der Klis, M

    1997-01-01

    I discuss some practical aspects of the analysis of millisecond time variability X-ray data obtained from accreting neutron stars and black holes. First I give an account of the statistical methods that are at present commonly applied in this field. These are mostly based on Fourier techniques. To a large extent these methods work well: they give astronomers the answers they need. Then I discuss a number of statistical questions that astronomers don't really know how to solve properly and that statisticians may have ideas about. These questions have to do with the highest and the lowest frequency ranges accessible in the Fourier analysis: how do you determine the shortest time scale present in the variability, how do you measure steep low-frequency noise. The point is stressed that in order for any method that resolves these issues to become popular, it is necessary to retain the capabilities the current methods already have in quantifying the complex, concurrent variability processes characteristic of accret...

  17. Eclipse Mapping: Astrotomography of Accretion Discs

    Science.gov (United States)

    Baptista, Raymundo

    The Eclipse Mapping Method is an indirect imaging technique that transforms the shape of the eclipse light curve into a map of the surface brightness distribution of the occulted regions. Three decades of application of this technique to the investigation of the structure, the spectrum and the time evolution of accretion discs around white dwarfs in cataclysmic variables have enriched our understanding of these accretion devices with a wealth of details such as (but not limited to) moving heating/cooling waves during outbursts in dwarf novae, tidally-induced spiral shocks of emitting gas with sub-Keplerian velocities, elliptical precessing discs associated to superhumps, and measurements of the radial run of the disc viscosity through the mapping of the disc flickering sources. This chapter reviews the principles of the method, discusses its performance, limitations, useful error propagation procedures, as well as highlights a selection of applications aimed at showing the possible scientific problems that have been and may be addresses with it.

  18. Probing thermonuclear burning on accreting neutron stars

    Science.gov (United States)

    Keek, L.

    2008-12-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes thermonuclear burning in the neutron star envelope, creating carbon and heavier elements. The fusion process may proceed in an unstable manner, resulting in a thermonuclear runaway. Within one second the entire surface is burned, which is observable as a sharp rise in the emitted X-ray flux: a type I X-ray burst. Afterwards the neutron star surface cools down on a timescale of ten to one hundred seconds. During these bursts the surface of an accreting neutron star can be observed directly, which makes them instrumental for studying this type of stars. We have studied rare kinds of X-ray bursts. One such rare burst is the superburst, which lasts a thousand times longer than an ordinary burst. Superbursts are thought to result from the explosive burning of a thick carbon layer, which lies deeper inside the neutron star, close to a layer known as the crust. A prerequisite for the occurrence of a superburst is a high enough temperature, which is set by the temperature of the crust and the heat conductivity of the envelope. The latter is lowered by the presence of heavy elements that are produced during normal X-ray bursts. Using a large set of observations from the Wide Field Camera's onboard the BeppoSAX satellite, we find that, at high accretion rate, sources which do not exhibit normal bursts likely have a longer superburst recurrence time, than the observed superburst recurrence time of one burster. We analyze in detail the first superburst from a transient source, which went into outburst only 55 days before the superburst. Recent models of the neutron star crust predict that this is too small a time to heat the crust sufficiently for superburst ignition, indicating

  19. Using Simulations of Black Holes to Study General Relativity and the Properties of Inner Accretion Flow

    Science.gov (United States)

    Hoormann, Janie Katherine

    2016-06-01

    While Albert Einstein's theory of General Relativity (GR) has been tested extensively in our solar system, it is just beginning to be tested in the strong gravitational fields that surround black holes. As a way to study the behavior of gravity in these extreme environments, I have used and added to a ray-tracing code that simulates the X-ray emission from the accretion disks surrounding black holes. In particular, the observational channels which can be simulated include the thermal and reflected spectra, polarization, and reverberation signatures. These calculations can be performed assuming GR as well as four alternative spacetimes. These results can be used to see if it is possible to determine if observations can test the No-Hair theorem of GR which states that stationary, astrophysical black holes are only described by their mass and spin. Although it proves difficult to distinguish between theories of gravity, it is possible to exclude a large portion of the possible deviations from GR using observations of rapidly spinning stellar mass black holes such as Cygnus X-1. The ray-tracing simulations can furthermore be used to study the inner regions of black hole accretion flows. I examined the dependence of X-ray reverberation observations on the ionization of the disk photosphere. My results show that X-ray reverberation and X-ray polarization provides a powerful tool to constrain the geometry of accretion disks which are too small to be imaged directly. The second part of my thesis describes the work on the balloon-borne X-Calibur hard X-ray polarimetry mission and on the space-borne PolSTAR polarimeter concept.

  20. Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW Hydrae

    CERN Document Server

    Johns-Krull, C M; Valenti, J A; Jeffers, S V; Piskunov, N E; Kuchukhov, O; Makaganiuk, V; Stempels, H C; Snik, F; Keller, C; Rodenhuis, M

    2013-01-01

    We present high spectral resolution ($R\\approx108,000$) Stokes $V$ polarimetry of the Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the \\ion{He}{1} emission lines at 5876 \\AA\\ and 6678 \\AA. The \\ion{He}{1} lines in these CTTSs contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large scale magnetospheric accretion flow. We detect strong polarization in the narrow component of the two \\ion{He}{1} emission lines in both stars. We observe a maximum implied field strength of $6.05 \\pm 0.24$ kG in the 5876 \\AA\\ line of GQ Lup, making it the star with the highest field strength measured in this line for a CTTS. We find field strengths in the two \\ion{He}{1} lines that are c...

  1. Modeling Polarized Emission from Black Hole Jets: Application to M87 Core Jet

    Directory of Open Access Journals (Sweden)

    Monika Mościbrodzka

    2017-09-01

    Full Text Available We combine three-dimensional general-relativistic numerical models of hot, magnetized Advection Dominated Accretion Flows around a supermassive black hole and the corresponding outflows from them with a general relativistic polarized radiative transfer model to produce synthetic radio images and spectra of jet outflows. We apply the model to the underluminous core of M87 galaxy. The assumptions and results of the calculations are discussed in context of millimeter observations of the M87 jet launching zone. Our ab initio polarized emission and rotation measure models allow us to address the constrains on the mass accretion rate onto the M87 supermassive black hole.

  2. Circular polarization in the optical afterglow of GRB 121024A

    CERN Document Server

    Wiersema, K; Toma, K; van der Horst, A J; Varela, K; Min, M; Greiner, J; Starling, R L C; Tanvir, N R; Wijers, R A M J; Campana, S; Curran, P A; Fan, Y; Fynbo, J P U; Gorosabel, J; Gomboc, A; Gotz, D; Hjorth, J; Jin, Z P; Kobayashi, S; Kouveliotou, C; Mundell, C; O'Brien, P T; Pian, E; Rowlinson, A; Russell, D M; Salvaterra, R; Alighieri, S di Serego; Tagliaferri, G; Vergani, S D; Elliott, J; Farina, C; Hartoog, O E; Karjalainen, R; Klose, S; Knust, F; Levan, A J; Schady, P; Sudilovski, V; Willingale, R

    2014-01-01

    Gamma-ray bursts (GRBs) are most probably powered by collimated relativistic outflows (jets) from accreting black holes at cosmological distances. Bright afterglows are produced when the outflow collides with the ambient medium. Afterglow polarization directly probes the magnetic properties of the jet, when measured minutes after the burst, and the geometric properties of the jet and the ambient medium when measured hours to days after the burst. High values of optical polarization detected minutes after burst in GRB 120308A indicate the presence of large-scale ordered magnetic fields originating from the central engine (the power source of the GRB). Theoretical models predict low degrees of linear polarization and negligable circular polarization at late times, when the energy in the original ejecta is quickly transferred to the ambient medium and propagates farther into the medium as a blastwave. Here we report the detection of circularly polarized optical light in the afterglow of GRB 121024A, measured 0.1...

  3. Evidence for Low Black Hole Spin and Physically Motivated Accretion Models from Millimeter VLBI Observations of Sagittarius A*

    CERN Document Server

    Broderick, Avery E; Doeleman, Sheperd S; Loeb, Abraham

    2010-01-01

    Millimeter very-long baseline interferometry (mm-VLBI) provides the novel capacity to probe the emission region of a handful of supermassive black holes on sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, this provides access to the region in the immediate vicinity of the horizon. Broderick et al. (2009) have already shown that by leveraging spectral and polarization information as well as accretion theory, it is possible to extract accretion-model parameters (including black hole spin) from mm-VLBI experiments containing only a handful of telescopes. Here we repeat this analysis with the most recent mm-VLBI data, considering a class of aligned, radiatively inefficient accretion flow (RIAF) models. We find that the combined data set rules out symmetric models for Sgr A*'s flux distribution at the 3.9-sigma level, strongly favoring length-to-width ratios of roughly 2.4:1. More importantly, we find that physically motivated accretion flow models provid...

  4. Ram-Pressure Stripping of Gas from Companions and Accretion onto a Spiral Galaxy A Gaseous Merger

    CERN Document Server

    Sofue, Y

    1993-01-01

    We simulated the behavior of interstellar gas clouds in a companion galaxy during a gas-dynamical interaction with the halo and disk of a spiral galaxy. By ram pressure, the gas clouds are stripped from the companion, and accreted to ward the disk of the spiral galaxy. If the companion's orbit is retrograde with respect to the rotation of the spiral galaxy, infalling clouds hit the nuclear region. Angular momentum transfer causes disruption of the inner gaseous disk, and makes a void of interstellar gas in the bulge. If the companion's orbit is either prograde or polar, infalling clouds are accreted by the outer disk, and form a rotating gas ring. We show that the ram-pressure stripping-and-accretion is one way from the companion to a gas-rich larger galaxy, which causes disposal of interstellar gas from the companion and effectively changes its galaxy type into earlier (redder). The ram-pressure process is significant durig merger of galaxies, in which interstellar gas is stripped and accreted prior to the s...

  5. On the spin modulated circular polarization from the intermediate polars NY Lup and IGRJ1509-6649

    CERN Document Server

    Potter, Stephen B; Kotze, Marissa; Zietsman, Ewald; Butters, O W; Pekeur, Nikki; Buckley, David A H

    2011-01-01

    We report on high time resolution, high signal/noise, photo-polarimetry of the intermediate polars NY Lup and IGRJ1509-6649. Our observations confirm the detection and colour dependence of circular polarization from NY Lup and additionally show a clear white dwarf, spin modulated signal. From our new high signal/noise photometry we have unambiguously detected wavelength dependent spin and beat periods and harmonics thereof. IGRJ1509-6649 is discovered to also have a particularly strong spin modulated circularly polarized signal. It appears double peaked through the I filter and single peaked through the B filter, consistent with cyclotron emission from a white dwarf with a relatively strong magnetic field. We discuss the implied accretion geometries in these two systems and any bearing this may have on the possible relationship with the connection between polars and soft X-ray-emitting IPs. The relatively strong magnetic fields is also suggestive of them being polar progenitors.

  6. Multi-dimensional structure of accreting young stars

    CERN Document Server

    Geroux, C; Viallet, M; Goffrey, T; Pratt, J; Constantino, T; Folini, D; Popov, M V; Walder, R

    2016-01-01

    This work is the first attempt to describe the multi-dimensional structure of accreting young stars based on fully compressible time implicit multi-dimensional hydrodynamics simulations. One major motivation is to analyse the validity of accretion treatment used in previous 1D stellar evolution studies. We analyse the effect of accretion on the structure of a realistic stellar model of the young Sun. Our work is inspired by the numerical work of Kley \\& Lin (1996, ApJ, 461, 933) devoted to the structure of the boundary layer in accretion disks. We analyse the redistribution of accreted material with a range of values of specific entropy relative to the bulk specific entropy of the material in the accreting object's convective envelope. A primary goal is to understand whether and how accreted energy deposited onto a stellar surface is redistributed in the interior. This study focusses on the high accretion rates characteristic of FU Ori systems. We find that the highest entropy cases produce a distinctive ...

  7. X-ray Photoevaporation-starved T Tauri Accretion

    CERN Document Server

    Drake, Jeremy J; Flaccomio, Ettore; Micela, Giusi

    2009-01-01

    X-ray luminosities of accreting T Tauri stars are observed to be systematically lower than those of non-accretors. There is as yet no widely accepted physical explanation for this effect, though it has been suggested that accretion somehow suppresses, disrupts or obscures coronal X-ray activity. Here, we suggest that the opposite might be the case: coronal X-rays modulate the accretion flow. We re-examine the X-ray luminosities of T Tauri stars in the Orion Nebula Cluster and find that not only are accreting stars systematically fainter, but that there is a correlation between mass accretion rate and stellar X-ray luminosity. We use the X-ray heated accretion disk models of Ercolano et al. to show that protoplanetary disk photoevaporative mass loss rates are strongly dependent on stellar X-ray luminosity and sufficiently high to be competitive with accretion rates. X-ray disk heating appears to offer a viable mechanism for modulating the gas accretion flow and could be at least partially responsible for the o...

  8. On the high frequency spectrum of a classical accretion disc

    CERN Document Server

    Balbus, Steven A

    2014-01-01

    We derive simple and explicit expressions for the high frequency spectrum of a classical accretion disc. Both stress-free and finite stress inner boundaries are considered. A classical accretion disc spectrum with a stress-free inner boundary departs from a Wien spectrum at large $\

  9. 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.

  10. Accretion Discs Show Their True Colours

    Science.gov (United States)

    2008-07-01

    Quasars are the brilliant cores of remote galaxies, at the hearts of which lie supermassive black holes that can generate enough power to outshine the Sun a trillion times. These mighty power sources are fuelled by interstellar gas, thought to be sucked into the hole from a surrounding 'accretion disc'. A paper in this week's issue of the journal Nature, partly based on observations collected with ESO's Very Large Telescope, verifies a long-standing prediction about the intensely luminous radiation emitted by these accretion discs. Uncovering the disc ESO PR Photo 21/08 Uncovering the inner disc "Astronomers were puzzled by the fact that the best models of these discs couldn't quite be reconciled with some of the observations, in particular, with the fact that these discs did not appear as blue as they should be," explains lead-author Makoto Kishimoto. Such a discrepancy could be the signal that there was something very wrong with the models. With his colleagues, he investigated this discrepancy by studying the polarised light from six quasars. This enabled them to demonstrate that the disc spectrum is as blue as predicted. "The crucial observational difficulty here has been that the disc is surrounded by a much larger torus containing hot dust, whose light partly outshines that of the disc," says Kishimoto. "Because the light coming from the disc is scattered in the disc vicinity and thus polarised, by observing only polarised light from the quasars, one can uncover the buried light from the disc." In a similar way that a fisherman would wear polarised sunglasses to help get rid of the glare from the water surface and allow him to see more clearly under the water, the filter on the telescope allowed the astronomers to see beyond surrounding clouds of dust and gas to the blue colour of the disc in infrared light. The observations were done with the FORS and ISAAC instruments on one of the 8.2-m Unit Telescopes of ESO's Very Large Telescope, located in the Atacama

  11. Power Spectrum Density of Stochastic Oscillating Accretion Disk

    Indian Academy of Sciences (India)

    G. B. Long; J. W. Ou; Y. G. Zheng

    2016-06-01

    In this paper, we employ a stochastic oscillating accretion disk model for the power spectral index and variability of BL Lac object S5 0716+714. In the model, we assume that there is a relativistic oscillation of thin accretion disks and it interacts with an external thermal bath through a friction force and a random force. We simulate the light curve and the power spectrum density (PSD) at (i) over-damped, (ii) critically damped and (iii) under-damped cases, respectively. Our results show that the simulated PSD curves depend on the intrinsic property of the accretion disk, and it could be produced in a wide interval ranging from 0.94 to 2.05 by changing the friction coefficient in a stochastic oscillating accretion disk model. We argue that accretion disk stochastic oscillating could be a possible interpretation for observed PSD variability.

  12. Rossby Wave Instability in the Accretion Flows around Black Holes

    Science.gov (United States)

    Gholipour, Mahmoud

    2017-01-01

    The roles of the Rossby wave instability (RWI) have been significantly developed in some important processes, such as planet formation and angular momentum transport through thin accretion disks. However, their development on accretion flows with advection is insignificant. In this paper, we investigate the effect of advection in the occurrence of RWI through accretion flows around black holes (BHs). In the absence of advection, the occurrence of RWI is extremely low because of high viscosity in the accretion flows around BHs. The results of this paper show that there is a significant chance for the occurrence of RWI in some wavelengths if we consider advection even in low amounts. Therefore, the RWI can be a suitable candidate for angular momentum transport in the accretion flows around BHs. Also, the results show that the advection parameter and the ratio of heat capacity, which are special characters of advection flows, play important roles in the occurrence of RWI.

  13. Accretion onto a noncommutative geometry inspired black hole

    Science.gov (United States)

    Kumar, Rahul; Ghosh, Sushant G.

    2017-09-01

    The spherically symmetric accretion onto a noncommutative (NC) inspired Schwarzschild black hole is treated for a polytropic fluid. The critical accretion rate \\dot{M}, sonic speed a_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 \\dot{M} ≈ {M^2} is still achievable but r_s seems to be substantially decreased due to the NC effects. They in turn do affect the accretion process.

  14. On the Eddington limit for relativistic accretion discs

    CERN Document Server

    Abolmasov, Pavel

    2015-01-01

    Standard accretion disc model relies upon several assumptions, the most important of which is geometrical thinness. Whenever this condition is violated, new physical effects become important such as radial energy advection and mass loss from the disc. These effects are important, for instance, for large mass accretion rates when the disc approaches its local Eddington limit. In this work, we study the upper limits for standard accretion disc approximation and find the corrections to the standard model that should be considered in any model aiming on reproducing the transition to super-Eddington accretion regime. First, we find that for thin accretion disc, taking into account relativistic corrections allows to increase the local Eddington limit by about a factor of two due to stronger gravity in General Relativity (GR). However, violation of the local Eddington limit also means large disc thickness. To consider consequently the disc thickness effects, one should make assumptions upon the two-dimensional rotat...

  15. Accreting planets as dust dams in `transition' discs

    CERN Document Server

    Owen, James E

    2014-01-01

    We investigate under what circumstances an embedded planet in a protoplanetary disc may sculpt the dust distribution such that it observationally presents as a `transition' disc. We concern ourselves with `transition' discs that have large holes ($\\gtrsim 10$ AU) and high accretion rates ($\\sim 10^{-9}-10^{-8}$ M$_\\odot$ yr$^{-1}$). Particularly, those discs which photoevaporative models struggle to explain. Assuming the standard picture for how massive planets sculpt their parent discs, along with the observed accretion rates in `transition' discs, 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 ($s\\lesssim 1\\mu$m) 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 disc with an embedded, accreting planet, show that only ...

  16. Accretion onto the First Stellar Mass Black Holes

    CERN Document Server

    Alvarez, Marcelo A; Abel, Tom

    2008-01-01

    The first stars in the universe, forming at redshifts z>15 in minihalos with masses of order 10^6 Msun, may leave behind black holes as their remnants. These objects could conceivably serve as "seeds" for much larger black holes observed at redshifts z~6. We study the growth of the remnant black holes through accretion including for the first time the emitted accretion radiation with adaptive mesh refinement cosmological radiation-hydrodynamical simulations. The effects of photo-ionization and heating dramatically affect the accretion flow from large scales, resulting in negligible mass growth of the black hole. We compare cases with the accretion luminosity included and neglected to show that the accretion radiation drastically changes the environment within 100 pc of the black hole, where gas temperatures are increased by an order of magnitude. The gas densities are reduced and further star formation in the same minihalo prevented for the two hundred million years of evolution we followed. These calculation...

  17. The impact of accretion disk winds on the X-ray spectrum of AGN: Part 2 - XSCORT + Hydrodynamic Simulations

    CERN Document Server

    Schurch, N J; Proga, D

    2008-01-01

    abridged: We use XSCORT, together with the hydrodynamic accretion disc wind simulation from Proga & Kallman (2004), to calculate the impact that the accretion disk wind has on the X-ray spectrum from a 1E8 solar mass black hole Active Galactic Nuclei (AGN) accreting at 0.5 L/L_Edd. The properties of the resulting spectra depend on viewing angle and clearly reflect the distinct regions apparent in the original hydrodynamic simulation. Very equatorial lines-of-sight (l.o.s) are dominated by Compton scattering and nearly-neutral absorption. Polar l.o.s result in largely featureless spectra. Finally, l.o.s that intersect the transition region between these extremes have a wide range of absorption features imprinted on the spectrum. Both polar and transition region l.o.s produce spectra that show highly-ionized, blue-shifted, Fe absorption features that are qualitatively similar to features observed in the X-ray spectra of a growing number of AGN. The spectra presented here clearly demonstrate that current sim...

  18. Accreting Millisecond Pulsars and Fundamental Physics

    Science.gov (United States)

    Strohmayer, Tod

    2005-01-01

    X-ray emission from the surfaces of rapidly rotating neutron stars encodes information about their global properties as well as physical conditions locally. Detailed modelling of, for example, the energy dependent pulse profiles observed from accreting millisecond pulsars and thermonuclear burst oscillations can be used to derive constraints on the masses and radii of neutron stars. These measurements provide direct information on the properties of the dense matter equation of state of the supranuclear density matter in their interiors. Study of absorption lines created in the surface layers can also provide measurements of masses and radii, and may be able to probe aspects of relativistic gravity, such as frame dragging. I will discuss the results of recent efforts to carry out such measurements and their implications for the properties of dense matter.

  19. Magnetohydrodynamic stability of stochastically driven accretion flows

    CERN Document Server

    Nath, Sujit K; Chattopadhyay, Amit K

    2013-01-01

    We investigate the evolution of magnetohydrodynamic/hydromagnetic perturbations in the presence of stochastic noise in rotating shear flows. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, however, are Rayleigh stable, but must be turbulent in order to explain astrophysical observed data and, hence, reveal a mismatch between the linear theory and observations/experiments. The mismatch seems to have been resolved, atleast in certain regimes, in the presence of weak magnetic field revealing magnetorotational instability. The present work explores the effects of stochastic noise on such magnetohydrodynamic flows, in order to resolve the above mismatch generically for the hot flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect, mimicking a small section of an astrophysical accretion disk around a compact object. It ...

  20. Bulk Comptonization by Turbulence in Accretion Disks

    CERN Document Server

    Kaufman, J

    2016-01-01

    Radiation pressure dominated accretion discs around compact objects may have turbulent velocities that greatly exceed the electron thermal velocities within the disc. Bulk Comptonization by the turbulence may therefore dominate over thermal Comptonization in determining the emergent spectrum. Bulk Comptonization by divergenceless turbulence is due to radiation viscous dissipation only. It can be treated as thermal Comptonization by solving the Kompaneets equation with an equivalent "wave" temperature, which is a weighted sum over the power present at each scale in the turbulent cascade. Bulk Comptonization by turbulence with non-zero divergence is due to both pressure work and radiation viscous dissipation. Pressure work has negligible effect on photon spectra in the limit of optically thin turbulence, and in this limit radiation viscous dissipation alone can be treated as thermal Comptonization with a temperature equivalent to the full turbulent power. In the limit of extremely optically thick turbulence, ra...

  1. Probing Sagittarius A* accretion with ALMA

    Science.gov (United States)

    Murchikova, Elena

    2017-01-01

    The submm Hydrogen recombination line technique can be used as a probe of the Galactic Center. We present the results of our H30α observations of ionized gas from within 0.015 pc around SgrA*. The observations were obtained on ALMA in cycle 3. The line was not detected, but we were able to set a limit on the mass of the cool gas (T~ 104 K) at 2 × 10-3 M ⊙. This is the unique probe of gas cooler than T ~106 K traced by X-ray emission. The total amount of gas near SgrA* gives us clues to understanding the accretion rate of SgrA*.

  2. Angular Momentum Transport in Accretion Disks

    DEFF Research Database (Denmark)

    E. Pessah, Martin; Chan, Chi-kwan; Psaltis, Dimitrios

    2007-01-01

    We present a scaling law that predicts the values of the stresses obtained in numerical simulations of saturated MRI-driven turbulence in non-stratified shearing boxes. It relates the turbulent stresses to the strength of the vertical magnetic field, the sound speed, the vertical size of the box...... vertical magnetic field and the turbulent magnetic energy must be in near equipartition with the thermal energy. This result has important implications for the spectra of accretion disks and their stability......., and the numerical resolution and predicts accurately the results of 35 numerical simulations performed for a wide variety of physical conditions. We use our result to show that the saturated stresses in simulations with zero net magnetic flux depend linearly on the numerical resolution and would become negligible...

  3. Structures of magnetized thin accretion disks

    Institute of Scientific and Technical Information of China (English)

    LI; xiaoqing(李晓卿); JI; Haisheng(季海生)

    2002-01-01

    We investigate the magnetohydrodynamic (MHD) process in thin accretion disks. Therelevant momentum as well as magnetic reduction equations in the thin disk approximation areincluded. On the basis of these equations, we examine numerically the stationary structures, includingdistributions of the surface mass density, temperature and flow velocities of a disk around a youngstellar object (YSO). The numerical results are as follows: (i) There should be an upper limit to themagnitude of magnetic field, such an upper limit corresponds to the equipartition field. For relevantmagnitude of magnetic field of the disk's interior the disk remains approximately Keplerian. (ii) Thedistribution of effective temperature T(r) is a smoothly decreasing function of radius with power 1 corresponding to the observed radiation flux density, provided that the magnetic fieldindex γ= -1/2,is suitably chosen.

  4. Stability of Self-Similar Spherical Accretion

    CERN Document Server

    Gaite, J

    2006-01-01

    Spherical accretion flows are simple enough for analytical study, by solution of the corresponding fluid dynamic equations. The solutions of stationary spherical flow are due to Bondi. The questions of the choice of a physical solution and of stability have been widely discussed. The answer to these questions is very dependent on the problem of boundary conditions, which vary according to whether the accretor is a compact object or a black hole. We introduce a particular, simple form of stationary spherical flow, namely, self-similar Bondi flow, as a case with physical interest in which analytic solutions for perturbations can be found. With suitable no matter-flux-perturbation boundary conditions, we will show that acoustic modes are stable in time and have no spatial instability at r=0. Furthermore, their evolution eventually becomes ergodic-like and shows no trace of instability or of acquiring any remarkable pattern.

  5. Evolution and precession of accretion disk in tidal disruption events

    Directory of Open Access Journals (Sweden)

    Matzner C.D.

    2012-12-01

    Full Text Available In a supermassive black hole (BH tidal disruption event (TDE, the tidally disrupted star feeds the BH via an accretion disk. Most often it is assumed that the accretion rate history, hence the emission light curve, tracks the rate at which new debris mass falls back onto the disk, notably the t−5/3 power law. But this is not the case when the disk evolution due to viscous spreading - the driving force for accretion - is carefully considered. We construct a simple analytical model that comprehensively describes the accretion rate history across 4 different phases of the disk evolution, in the presence of mass fallback and disk wind loss. Accretion rate evolves differently in those phases which are governed by how the disk heat energy is carried away, early on by advection and later by radiation. The accretion rate can decline as steeply as t−5/3 only if copious disk wind loss is present during the early advection-cooled phase. Later, the accretion rate history is t−8/7 or shallower. These have great implications on the TDE flare light curve. A TDE accretion disk is most likely misaligned with the equatorial plane of the spinning BH. Moreover, in the TDE the accretion rate is super- or near-Eddington thus the disk is geometrically thick, for which case the BH’s frame dragging effect may cause the disk precess as a solid body, which may manifest itself as quasi-periodic signal in the TDE light curve. Our disk evolution model predicts the disk precession period increases with time, typically as ∝ t. The results are applied to the recently jetted TDE flare Swift transient J1644 + 57 which shows numerous, quasi-periodic dips in its long-term X-ray light curve. As the current TDE sample increases, the identification of the disk precession signature provides a unique way of measuring BH spin and studying BH accretion physics.

  6. High-precision optical polarimetry of the accreting black hole V404 Cyg during the 2015 June outburst

    Science.gov (United States)

    Kosenkov, Ilia A.; Berdyugin, Andrei V.; Piirola, Vilppu; Tsygankov, Sergey S.; Pallé, Enric; Miles-Páez, Paulo A.; Poutanen, Juri

    2017-07-01

    Our simultaneous three-colour (BVR) polarimetric observations of the low-mass black hole X-ray binary V404 Cyg show a small but statistically significant change of polarization degree (Δp ˜ 1 per cent) between the outburst in 2015 June and the quiescence. The polarization of V404 Cyg in the quiescent state agrees within the errors with that of the visually close (1.4 arcsec) companion (pR = 7.3 ± 0.1 per cent), indicating that it is predominantly of interstellar origin. The polarization pattern of the surrounding field stars supports this conclusion. From the observed variable polarization during the outburst, we show that the polarization degree of the intrinsic component peaks in the V band, pV = 1.1 ± 0.1 per cent, at the polarization position angle of θV = -7° ± 2°, which is consistent in all three passbands. We detect significant variations in the position angle of the intrinsic polarization in the R band from -30° to ˜0° during the outburst peak. The observed wavelength dependence of the intrinsic polarization does not support non-thermal synchrotron emission from a jet as a plausible mechanism, but it is in better agreement with the combined effect of electron (Thomson) scattering and absorption in a flattened plasma envelope or outflow surrounding the illuminating source. Alternatively, the polarization signal can be produced by scattering of the disc radiation in a mildly relativistic polar outflow. The position angle of the intrinsic polarization, nearly parallel to the jet direction (i.e. perpendicular to the accretion disc plane), is in agreement with these interpretations.

  7. AGN Variability: Probing Black Hole Accretion

    Science.gov (United States)

    Moreno, Jackeline; O'Brien, Jack; Vogeley, Michael S.; Richards, Gordon T.; Kasliwal, Vishal P.

    2017-01-01

    We combine the long temporal baseline of Sloan Digital Sky Survey (SDSS) for quasars in Stripe 82 with the high precision photometry of the Kepler/K2 Satellite to study the physics of optical variability in the accretion disk and supermassive black hole engine. We model the lightcurves directly as Continuous-time Auto Regressive Moving Average processes (C-ARMA) with the Kali analysis package (Kasliwal et al. 2016). These models are extremely robust to irregular sampling and can capture aperiodic variability structure on various timescales. We also estimate the power spectral density and structure function of both the model family and the data. A Green's function kernel may also be estimated for the resulting C-ARMA parameter fit, which may be interpreted as the response to driving impulses such as hotspots in the accretion disk. We also examine available spectra for our AGN sample to relate observed and modelled behavior to spectral properties. The objective of this work is twofold: to explore the proper physical interpretation of different families of C-ARMA models applied to AGN optical flux variability and to relate empirical characteristic timescales of our AGN sample to physical theory or to properties estimated from spectra or simulations like the disk viscosity and temperature. We find that AGN with strong variability features on timescales resolved by K2 are well modelled by a low order C-ARMA family while K2 lightcurves with weak amplitude variability are dominated by outliers and measurement errors which force higher order model fits. This work explores a novel approach to combining SDSS and K2 data sets and presents recovered characteristic timescales of AGN variability.

  8. Polar Shapelets

    CERN Document Server

    Massey, R; Massey, Richard; Refregier, Alexandre

    2004-01-01

    The shapelets method for astronomical image analysis is based around the decomposition of localised objects into a series of orthogonal components with particularly convenient mathematical properties. We extend the "Cartesian shapelet" formalism from earlier work, and construct "polar shapelet" basis functions that separate an image into components with explicit rotational symmetries. This provides a more compact representation of typical galaxy shapes, and its physical interpretation is frequently more intuitive. Linear coordinate transformations can be simply expressed using this basis set, and shape measures (including object photometry, astrometry and galaxy morphology estimators) take a naturally elegant form. Particular attention is paid to the analysis of astronomical survey images, and we test shapelet techniques with real data from the Hubble Space Telescope. We present a practical method to automatically optimise the quality of an arbitrary shapelet decomposition in the presence of noise, pixellisat...

  9. Elliptical Accretion and Low Luminosity from High Accretion Rate Stellar Tidal Disruption Events

    Science.gov (United States)

    Svirski, Gilad; Piran, Tsvi; Krolik, Julian

    2017-01-01

    Models for tidal disruption events (TDEs) in which a supermassive black hole disrupts a star commonly assume that the highly eccentric streams of bound stellar debris promptly form a circular accretion disk at the pericenter scale. However, the bolometric peak luminosity of most TDE candidates, ˜ 10^{44} {erg s^{-1}}, implies that we observe only ˜1% of the energy expected from radiatively efficient accretion. Even the energy that must be lost to circularize the returning tidal flow is larger than the observed energy. Recently, Piran et al. (2015) suggested that the observed optical TDE emission is powered by shocks at the apocenter between freshly infalling material and earlier arriving matter. This model explains the small radiated energy, the low temperature, and the large radius implied by the observations as well as the t-5/3 light curve. However the question of the system's low bolometric efficiency remains unanswered. We suggest that the high orbital energy and low angular momentum of the flow make it possible for magnetic stresses to reduce the matter's already small angular momentum to the point at which it can fall ballistically into the SMBH before circularization. As a result, the efficiency is only ˜1-10% of a standard accretion disk's efficiency. Thus, the intrinsically high eccentricity of the tidal debris naturally explains why most TDE candidates are fainter than expected.

  10. Bondi-Hoyle accretion in an isothermal magnetized plasma

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Aaron T.; McKee, Christopher F.; Klein, Richard I. [Department of Astronomy, University of California Berkeley, Berkeley, CA 94720 (United States); Cunningham, Andrew J., E-mail: a.t.lee@berkeley.edu [Lawrence Livermore National Laboratory, P.O. Box 808, L-23, Livermore, CA 94550 (United States)

    2014-03-01

    In regions of star formation, protostars and newborn stars will accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma β—the ratio of thermal and magnetic pressures. Observations show that molecular clouds have β ≲ 1, so magnetic fields have the potential to play a significant role in the accretion process. We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbers of up to M≈45; magnetic fields that are either parallel, perpendicular, or oriented 45° to the flow; and β as low as 0.01. Our simulations utilize adaptive mesh refinement in order to obtain high spatial resolution where it is needed; this also allows the boundaries to be far from the accreting object to avoid unphysical effects arising from boundary conditions. Additionally, we show that our results are independent of our exact prescription for accreting mass in the sink particle. We give simple expressions for the steady-state accretion rate as a function of β and M for the parallel and perpendicular orientations. Using typical molecular cloud values of M∼5 and β ∼ 0.04 from the literature, our fits suggest that a 0.4 M {sub ☉} star accretes ∼4 × 10{sup –9} M {sub ☉} yr{sup –1}, almost a factor of two less than accretion rates predicted by hydrodynamic models. This disparity can grow to orders of magnitude for stronger fields and lower Mach numbers. We also discuss the applicability of these accretion rates versus accretion rates expected from gravitational collapse, and under what conditions a steady state is possible. The reduction in the accretion rate in a magnetized medium leads to an increase in the time required to form stars in competitive accretion models, making such models less efficient than predicted by

  11. Photometry and Multipolar Magnetic Field Modeling of Polars: BY Camelopardalis and FL Ceti

    Directory of Open Access Journals (Sweden)

    P. A. Mason

    2015-02-01

    Full Text Available We present new broad band optical photometry of two magnetic cataclysmic variable stars, the asynchronous polar BY Camelopardalis and the short period polar FL Ceti. Observations were obtained at the 2.1-m Otto Struve Telescope of McDonald Observatory with 3s and 1s integration times respectively. In an attempt to understand the observed complex changes in accretion flow geometry observed in BY Cam, we performed full 3D MHD simulations assuming a variety of white dwarf magnetic field structures. We investigate fields with increasing complexity including both aligned and non-aligned dipole plus quadrupole field components. We compare model predictions with photometry at various phases of the beat cycle and find that synthetic light curves derived from a multipolar field structure are broadly consistent with optical photometry. FL Ceti is observed to have two very small accretion regions at the foot-points of the white dwarf’s magnetic field. Both accretion regions are visible at the same time in the high state and are about 100 degrees apart. MHD modeling using a dipole plus quadrupole field structure yields quite similar accretion regions as those observed in FL Ceti. We conclude that accretion flows calculated from MHD modeling of multi-polar magnetic fields produce synthetic light curves consistent with photometry of these magnetic cataclysmic variables.

  12. 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

  13. Is TW Pictoris really an intermediate polar?

    CERN Document Server

    Norton, A J

    1998-01-01

    We present the results of a long ROSAT HRI observation of the candidate intermediate polar TW Pic. The power spectrum shows no sign of either the previously proposed white dwarf spin period or the proposed binary orbital period (1.996 hr and 6.06 hr respectively). The limits to the X-ray modulation are less than 0.3% in each case. In the absence of a coherent X-ray pulsation, the credentials of TW Pic for membership of the intermediate polar subclass must be suspect. We further suggest that the true orbital period of the binary may be the shorter of the two previously suggested, and that the longer period may represent a quasi-periodic phenomenon associated with the accretion disc.

  14. Single/Dual-Polarized Infrared Rectenna for Solar Energy Harvesting

    Directory of Open Access Journals (Sweden)

    S. H. Zainud-Deen

    2016-05-01

    Full Text Available Single and dual linearly-polarized receiving mode nanoantennas are designed for solar energy harvesting at 28.3 THz. The infrared rectennas are used to harvest the solar energy and converting it to electrical energy.  The proposed infrared rectenna is a thin dipole made of gold and printed on a silicon dioxide substrate. Different shapes of the dipole arms have been investigated for maximum collected energy. The two poles of the dipole have been determined in a rectangular, circular and rhombus shapes. The rectenna dipole is used to concentrate the electromagnetic energy into a small localized area at the inner tips of the gap between the dipole arms. The dimensions of the different dipole shapes are optimized for maximum near electric field intensity at a frequency of 28.3 THz. A Metal Insulator Metal (MIM diode is incorporated with the nanoantenna dipole to rectify the received energy. The receiving efficiency of the solar energy collector with integrated MIM diode has been investigated. A dual-polarized, four arms, rhombus shaped nanoantenna dipole for solar energy harvesting has been designed and optimized for 28.3 THz applications.

  15. Effect of Radiation Drag on Hoyle-Lyttleton Accretion

    CERN Document Server

    Nio, T; Fukue, J; Nio, Tomomi; Matsuda, Takuya; Fukue, Jun

    1998-01-01

    Hoyle-Lyttleton type accretion is investigated, by taking account of not only the effect of radiation pressure but the effect of radiation drag. We calculate the trajectories of particles for three cases: only the effect of gravity is considered (case A); the effect of radiation pressure is taken into account (case B); the effect of radiation drag as well as radiation pressure is taken into account (case C). The accretion radii for former two cases are $2GM/v_{\\infty}^2$ for case A and $2GM(1-\\Gamma)/v_{\\infty}^2$ for case B, where M is the mass of the accreted object, $v_{\\infty}$ the relative velocity, and Gamma the normalized luminosity of the accreted object. We found that the accretion radius for case C is in between those of cases A and B under the present approximation; i.e., the accretion radius decreases due to radiation pressure while it increases due to radiation drag. In addition, the accretion radius for case C becomes larger as the incident velocity becomes fast. The effect of radiation drag bec...

  16. Formation of primordial supermassive stars by burst accretion

    CERN Document Server

    Sakurai, Y; Yoshida, N; Yorke, H W

    2015-01-01

    A promising formation channel of SMBHs at redshift 6 is the so-called DC model, which posits that a massive seed BH forms through gravitational collapse of a $\\sim 10^5~M_\\odot$ SMS. We study the evolution of such a SMS growing by rapid mass accretion. In particular, we examine the impact of time-dependent mass accretion of repeating burst and quiescent phases that are expected to occur with a self-gravitating circumstellar disk. We show that the stellar evolution with such episodic accretion differs qualitatively from that expected with a constant accretion rate, even if the mean accretion rate is the same. Unlike the case of constant mass accretion, whereby the star expands roughly following $R_* \\simeq 2.6 \\times 10^3 R_\\odot (M_*/100~M_\\odot)^{1/2}$, the protostar can substantially contract during the quiescent phases between accretion bursts. The stellar effective temperature and ionizing photon emissivity increase accordingly as the star contracts, which can cause strong ionizing feedback and halt the m...

  17. Forced accretion in stochastically fed AGN and quasars

    CERN Document Server

    Nayakshin, Sergei

    2007-01-01

    Steady state accretion discs larger than ~ 0.01-0.1 pc are known to be gravitationally unstable for the accretion rates needed to explain super-massive black hole (SMBH) activity. We propose that SMBH are fed by a succession of mass deposition events with randomly directed angular momenta. Because of incomplete angular momentum cancellation a warped accretion disc forms in the inner few parsec. The orientation of the disc performs a random walk. Deposition of new material promotes SMBH accretion at rates much faster than viscous. Observational implications of this picture include: (i) lighter accretion discs that can fuel AGN and quasars and yet avoid star formation at R >> 0.1 pc; (ii) star formation inside the disc is not a function of mass accretion rate only. It can take place at high or low accretion rates, e.g., when too few clouds arrive in the inner region. An example of this might be the central parsec of our Galaxy. (iii) The discs can form Compton-thick obscuring structures of ~ parsec size as requ...

  18. Migration of accreting planets in radiative discs from dynamical torques

    CERN Document Server

    Pierens, Arnaud

    2016-01-01

    We present the results of hydrodynamical simulations of the orbital evolution of planets undergoing runaway gas accretion in radiative discs. We consider accreting disc models with constant mass flux through the disc, and where radiative cooling balances the effect of viscous heating and stellar irradiation. We assume that 20-30 $M_\\oplus$ giant planet cores are formed in the region where viscous heating dominates and migrate outward under the action of a strong corotation torque. In the case where gas accretion is neglected, we find evidence for strong dynamical torques in accreting discs with accretion rates ${\\dot M}\\gtrsim 7\\times 10^{-8} \\;M_\\odot/yr$. Their main effect is to increase outward migration rates by a factor of $\\sim 2$ typically. In the presence of gas accretion, however, runaway outward migration is observed with the planet passing through the zero-torque radius and the transition between the viscous heating and stellar heating dominated regimes. The ability for an accreting planet to enter...

  19. Accretion onto Seed Black Holes in the First Galaxies

    CERN Document Server

    Milosavljevic, Milos; Couch, Sean M; Oh, S Peng

    2008-01-01

    The validity of the hypothesis that the massive black holes in high redshift quasars grew from stellar-sized "seeds" is contingent on a seed's ability to double its mass every few ten million years. This requires that the seed accrete at approximately the Eddington-limited rate. In the specific case of radiatively efficient quasiradial accretion in a metal-poor protogalactic medium, for which the Bondi accretion rate is often prescribed in cosmological simulations of massive black hole formation, we examine the effects of the radiation emitted near the black hole's event horizon on the structure of the surrounding gas flow. We find that the radiation pressure from photoionization significantly reduces the steady-state accretion rate and renders the quasiradial accretion flow unsteady and inefficient. The time-averaged accretion rates are a small fraction of the Eddington-limited accretion rate for Thomson scattering. The pressure of Ly-alpha photons trapped near the HII region surrounding the black hole may f...

  20. Atmospheric signatures of giant exoplanet formation by pebble accretion

    Science.gov (United States)

    Madhusudhan, Nikku; Bitsch, Bertram; Johansen, Anders; Eriksson, Linn

    2017-08-01

    Atmospheric chemical abundances of giant planets lead to important constraints on planetary formation and migration. Studies have shown that giant planets that migrate through the protoplanetary disc can accrete substantial amounts of oxygen-rich planetesimals, leading to supersolar metallicities in the envelope and solar or subsolar C/O ratios. Pebble accretion has been demonstrated to play an important role in core accretion and to have growth rates that are consistent with planetary migration. The high pebble accretion rates allow planetary cores to start their growth beyond 10 au and subsequently migrate to cold (≳1 au), warm (˜0.1-1 au) or hot (≲0.1 au) orbits. In this work we investigate how the formation of giant planets via pebble accretion influences their atmospheric chemical compositions. We find that under the standard pebble accretion scenario, where the core is isolated from the envelope, the resulting metallicities (O/H and C/H ratios) are subsolar, while the C/O ratios are supersolar. Planets that migrate through the disc to become hot Jupiters accrete substantial amounts of water vapour, but still acquire slightly subsolar O/H and supersolar C/O of 0.7-0.8. The metallicity can be substantially subsolar (˜0.2-0.5 × solar) and the C/O can even approach 1.0 if the planet accretes its envelope mostly beyond the CO2 ice line, i.e. cold Jupiters or hot Jupiters that form far out and migrate in by scattering. Allowing for core erosion yields significantly supersolar metallicities and solar or subsolar C/O, which can also be achieved by other means, e.g. photoevaporation and late-stage planetesimal accretion.

  1. The geochemical constraints on Earth's accretion and core formation (Invited)

    Science.gov (United States)

    Rudge, J. F.; Kleine, T.; Bourdon, B.

    2010-12-01

    There are now a wide range of geochemical observations that can be used to place constraints on Earth's first hundred million years. During this time the Earth accreted through collisions between numerous planetary embryos, and these collisions are thought to have caused significant melting and segregation of metal, forming the Earth's core. Information on the pressure, temperature, and oxygen fugacity conditions of core formation can be obtained from the abundances of siderophile elements in Earth's mantle and high pressure partitioning experiments. Timing information can be obtained from isotopic measurements, notably Hf-W and U-Pb. Here we present a simple geochemical box model that can be used to provide constraints on Earth's accretion and core formation. A key parameter in the model is the degree of equilibration during metal-silicate segregation. Existing models have shown that the siderophile element abundances are consistent with full equilibration in a deep magma ocean, with an increase in oxygen fugacity during accretion. Here we show that the siderophile element abundances are equally consistent with scenarios involving partial equilibration. The Hf-W isotopic observations constrain the degree of equilibration to be at least 36%. The timing constraints depend strongly on the degree of equilibration, but nevertheless bounds can be placed on the timing of Earth's accretion. With full equilibration, the Hf-W observations imply a rapid early accretion stage (at least 80% of Earth accreting within 35 Myr), but with partial equilibration accretion may be much more protracted. If Pb partitions into Earth’s core, the U-Pb observations can be used to constrain the late stages of accretion, and are consistent with the final 10% of Earth’s accretion occurring during the Moon-forming giant impact at ~4.45Ga.

  2. Accretion of radiation and Primordial black holes in anisotropic universe

    CERN Document Server

    Mahapatra, Swapna

    2013-01-01

    We consider Primordial black holes (PBHs) in a LRS (locally rotationally symmetric) Bianchi-I anisotropic space time and study the effect of accretion of radiation in the radiation dominated era. We show that the life time of PBHs in anisotropic universe becomes longer by accretion of radiation. We generalize the result by including nonzero angular momentum parameter and study the PBH evolution equation. We find that the evaporation time gets prolonged and it depends on the accretion efficiency as well as angular momentum parameter. This supports the conjecture that Primordial black holes can be considered as a viable candidate for dark matter.

  3. Lambda Boo Abundance Patterns: Accretion from Orbiting Sources

    CERN Document Server

    Jura, M

    2015-01-01

    The abundance anomalies in lambda Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically-inferred bounds for interstellar accretion. Therefore, a lambda Boo star's thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt Objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some lambda Boo stars accrete from the winds of hot Jupiters.

  4. LAMBDA BOO ABUNDANCE PATTERNS: ACCRETION FROM ORBITING SOURCES

    Energy Technology Data Exchange (ETDEWEB)

    Jura, M., E-mail: jura@astro.ucla.edu [Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1562 (United States)

    2015-12-15

    The abundance anomalies in λ Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically inferred bounds for interstellar accretion. Therefore, a λ Boo star’s thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some λ Boo stars accrete from the winds of hot Jupiters.

  5. Do we see accreting magnetars in X-ray pulsars?

    Directory of Open Access Journals (Sweden)

    Postnov K.A.

    2014-01-01

    Full Text Available Strong magnetic field of accreting neutron stars (1014 G is hard to probe by Xray spectroscopy but can be indirectly inferred from spin-up/spin-down measurement in X-ray pulsars. The existing observations of slowly rotating X-ray pulsars are discussed. It is shown that magnetic fields of neutron stars derived from these observations (or lower limits in some cases fall within the standard 1012-1013 G range. Claims about the evidence for accreting magnetars are critically discussed in the light of recent progress in understanding of accretion onto slowly rotating neutron stars in the subsonic regime.

  6. Quasi-periodic oscillations in luminous accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Okuda, T.; Mineshige, S. (Cambridge Univ. (UK). Inst. of Astronomy)

    1991-04-15

    We examine the time-dependent evolution of radial pulsational instabilities in luminous accretion discs around neutron stars, using a one-dimensional hydrodynamic code. In geometrically thin and optically thick accretion discs, radial oscillations are overstable to axisymmetric perturbations. It is due to a mechanism similar to the {epsilon} mechanism in stellar pulsation. We find that this instability induces large amplitude oscillations (more than 100 per cent) in accretion rates and small modulations (a few per cent) in temperature and surface in the inner part of the disc, both with local Keplerian frequencies. (author).

  7. Tearing up a misaligned accretion disc with a binary companion

    CERN Document Server

    Doğan, Suzan; King, Andrew; Price, Daniel J

    2015-01-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. We calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. We run hydrodynamical simulations to check these results, and confirm that disc breaking is widespread and generally enhances accretion on to the central object. This applies in many cases of astrophysical accretion, e.g. supermassive black hole binaries and X--ray binaries.

  8. 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.

  9. Reconfigurable thz polarizer

    DEFF Research Database (Denmark)

    2016-01-01

    The present invention provides a polarizer. The polarizer comprises a first membrane having a first polarization region comprising a first plurality of membrane perforations; a second membrane having a second polarization region comprising a second plurality of membrane perforations; and a support...... with one or more membrane perforations in the second plurality of perforations in a direction normal to the first polarization region or normal to the second polarization region, resulting in corresponding one or more openings in said direction....

  10. Disk Accretion Onto High-Mass Planets

    CERN Document Server

    Lubow, S H; Artymowicz, P

    1999-01-01

    We analyze the nonlinear, two-dimensional response of a gaseous, viscous protoplanetary disk to the presence of a planet of one Jupiter mass (1 M_J) and greater that orbits a 1 solar mass star by using the ZEUS hydrodynamics code with high resolution near the planet's Roche lobe. The planet is assumed to be in a circular orbit about the central star and is not allowed to migrate. A gap is formed about the orbit of the planet, but there is a nonaxisymmetric flow through the gap and onto the planet. The gap partitions the disk into an inner (outer) disk that extends inside (outside) the planet's orbit. For a 1 M_J planet and typical disk parameters, the accretion through the gap onto the planet is highly efficient. For typical disk parameters, the mass doubling time scale is less than 10^5 years, considerably shorter than the disk lifetime. Following shocks near the L1 and L2 Lagrange points, disk material enters the Roche lobe in the form of two gas streams. Shocks occur within the Roche lobe as the gas stream...

  11. Accreting Binary Populations in the Earlier Universe

    Science.gov (United States)

    Hornschemeier, Ann

    2010-01-01

    It is now understood that X-ray binaries dominate the hard X-ray emission from normal star-forming galaxies. Thanks to the deepest (2-4 Ms) Chandra surveys, such galaxies are now being studied in X-rays out to z approximates 4. Interesting X-ray stacking results (based on 30+ galaxies per redshift bin) suggest that the mean rest-frame 2-10 keV luminosity from z=3-4 Lyman break galaxies (LBGs), is comparable to the most powerful starburst galaxies in the local Universe. This result possibly indicates a similar production mechanism for accreting binaries over large cosmological timescales. To understand and constrain better the production of X-ray binaries in high-redshift LBGs, we have utilized XMM-Newton observations of a small sample of z approximates 0.1 GALEX-selected Ultraviolet-Luminous Galaxies (UVLGs); local analogs to high-redshift LBGs. Our observations enable us to study the X-ray emission from LBG-like galaxies on an individual basis, thus allowing us to constrain object-to-object variances in this population. We supplement these results with X-ray stacking constraints using the new 3.2 Ms Chandra Deep Field-South (completed spring 2010) and LBG candidates selected from HST, Swift UVOT, and ground-based data. These measurements provide new X-ray constraints that sample well the entire z=0-4 baseline

  12. Viscosity prescription for gravitationally unstable accretion disks

    CERN Document Server

    Rafikov, Roman R

    2015-01-01

    Gravitationally unstable accretion disks emerge in a variety of astrophysical contexts - giant planet formation, FU Orioni outbursts, feeding of AGNs, and the origin of Pop III stars. When a gravitationally unstable disk is unable to cool rapidly it settles into a quasi-stationary, fluctuating gravitoturbulent state, in which its Toomre Q remains close to a constant value Q_0~1. Here we develop an analytical formalism describing the evolution of such a disk, which is based on the assumptions of Q=Q_0 and local thermal equilibrium. Our approach works in the presence of additional sources of angular momentum transport (e.g. MRI), as well as external irradiation. Thermal balance dictates a unique value of the gravitoturbulent stress \\alpha_{gt} driving disk evolution, which is a function of the local surface density and angular frequency. We compare this approach with other commonly used gravitoturbulent viscosity prescriptions, which specify the explicit dependence of stress \\alpha_{gt} on Toomre Q in an ad hoc...

  13. Magnetohydrodynamic stability of stochastically driven accretion flows.

    Science.gov (United States)

    Nath, Sujit Kumar; Mukhopadhyay, Banibrata; Chattopadhyay, Amit K

    2013-07-01

    We investigate the evolution of magnetohydrodynamic (or hydromagnetic as coined by Chandrasekhar) perturbations in the presence of stochastic noise in rotating shear flows. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, however, are Rayleigh stable but must be turbulent in order to explain astrophysical observed data and, hence, reveal a mismatch between the linear theory and observations and experiments. The mismatch seems to have been resolved, at least in certain regimes, in the presence of a weak magnetic field, revealing magnetorotational instability. The present work explores the effects of stochastic noise on such magnetohydrodynamic flows, in order to resolve the above mismatch generically for the hot flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect, mimicking a small section of an astrophysical accretion disk around a compact object. It is found that such stochastically driven flows exhibit large temporal and spatial autocorrelations and cross-correlations of perturbation and, hence, large energy dissipations of perturbation, which generate instability. Interestingly, autocorrelations and cross-correlations appear independent of background angular velocity profiles, which are Rayleigh stable, indicating their universality. This work initiates our attempt to understand the evolution of three-dimensional hydromagnetic perturbations in rotating shear flows in the presence of stochastic noise.

  14. Black Hole Accretion in Gamma Ray Bursts

    Directory of Open Access Journals (Sweden)

    Agnieszka Janiuk

    2017-02-01

    Full Text Available We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on 14 September 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 s and appeared about 0.4 s after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnant’s matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole.

  15. Accretion of Supersonic Winds on Boson Stars

    CERN Document Server

    Gracia-Linares, M

    2016-01-01

    We present the evolution of a supersonic wind interacting with a Boson Star (BS) and compare the resulting wind density profile with that of the shock cone formed when the wind is accreted by a non-rotating Black Hole (BH) of the same mass. The physical differences between these accretors are that a BS, unlike a BH has no horizon, it does not have a mechanical surface either and thus the wind is expected to trespass the BS. Despite these conditions, on the BS space-time the gas achieves a stationary flux with the gas accumulating in a high density elongated structure comparable to the shock cone formed behind a BH. The highest density resides in the center of the BS whereas in the case of the BH it is found on the downstream part of the BH near the event horizon. The maximum density of the gas is smaller in the BS than in the BH case. Our results indicate that the highest density of the wind is more similar on the BS to that on the BH when the BS has high self-interaction, when it is more compact and when the...

  16. The universal nature of subhalo accretion

    CERN Document Server

    Libeskind, Noam I; Hoffman, Yehuda; Gottloeber, Stefan

    2014-01-01

    We examine the angular infall pattern of subhaloes onto host haloes in the context of the large-scale structure. We find that this infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are found to be preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (~10 Mpc) scale -...

  17. Accretion mode changes in Centaurus X-3

    CERN Document Server

    Paul, B; Mukherjee, U

    2005-01-01

    We report here discovery of the existence of two different accretion modes in the high mass X-ray binary pulsar Cen X-3 during its high states. The multiband X-ray light curves of Cen X-3 lasting for more than 3400 days obtained with the All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE) shows many episodes of high and low X-ray intensities. The high intensity phases last between a few to upto 110 days and the separation between two high intensity phases also varies widely. One remarkable feature deduced from the RXTE-ASM light curves is that during these high intensity phases, Cen X-3 manifests in two very distinct spectral states. When the source makes a transition from the low intensity phase to the high intensity phase, it adopts one of these two spectral states and during the entire high intensity phase remains in that particular spectral state. During December 2000 to April 2004, all the high intensity episodes showed a hardness ratio which is significantly larger than the same during ...

  18. Accretion Dynamics on Wet Granular Materials.

    Science.gov (United States)

    Saingier, Guillaume; Sauret, Alban; Jop, Pierre

    2017-05-19

    Wet granular aggregates are common precursors of construction materials, food, and health care products. The physical mechanisms involved in the mixing of dry grains with a wet substrate are not well understood and difficult to control. Here, we study experimentally the accretion of dry grains on a wet granular substrate by measuring the growth dynamics of the wet aggregate. We show that this aggregate is fully saturated and its cohesion is ensured by the capillary depression at the air-liquid interface. The growth dynamics is controlled by the liquid fraction at the surface of the aggregate and exhibits two regimes. In the viscous regime, the growth dynamics is limited by the capillary-driven flow of liquid through the granular packing to the surface of the aggregate. In the capture regime, the capture probability depends on the availability of the liquid at the saturated interface, which is controlled by the hydrostatic depression in the material. We propose a model that rationalizes our observations and captures both dynamics based on the evolution of the capture probability with the hydrostatic depression.

  19. Lopsided spiral galaxies: evidence for gas accretion

    CERN Document Server

    Bournaud, F; Jog, C J; Puerari, I

    2005-01-01

    We quantify the degree of lopsidedness for a sample of 149 galaxies observed in the near-infrared from the OSUBGS sample, and try to explain the physical origin for the observed disk lopsidedness. We confirm previous studies, but now for a larger sample, that a large fraction of galaxies show significant lopsidedness in their stellar disks, measured as the Fourier amplitude of the m=1 component, normalised to the average or m=0 component, in the surface density. Late-type galaxies are found to be more lopsided, while the presence of m=2 spiral arms and bars is correlated. The m=1 amplitude is found to be uncorrelated with the tidal forces acting on a galaxy via nearby companions. Numerical simulations are carried out to study the generation of m=1 via different processes: galaxy tidal encounters, galaxy mergers, and external gas accretion and subsequent star formation. The simulations show that galaxy interactions and mergers can trigger strong lopsidedness, but do not explain several independent statistical ...

  20. Viscous Stability of Relativistic Keplerian Accretion Disks

    CERN Document Server

    Ghosh, P

    1998-01-01

    We investigate the viscous stability of thin, Keplerian accretion disks in regions where general relativistic (GR) effects are essential. For gas pressure dominated (GPD) disks, we show that the Newtonian conclusion that such disks are viscously stable is reversed by GR modifications in the behaviors of viscous stress and surface density over a significantly large annular region not far from the innermost stable orbit at $r=\\rms$. For slowly-rotating central objects, this region spans a range of radii $14\\lo r\\lo 19$ in units of the central object's mass $M$. For radiation pressure dominated (RPD) disks, the Newtonian conclusion that they are viscously unstable remains valid after including the above GR modifications, except in a very small annulus around $r\\approx 14M$, which has a negligible influence. Inclusion of the stabilizing effect of the mass-inflow through the disk's inner edge via a GR analogue of Roche-lobe overflow adds a small, stable region around \\rms~for RPD disks, but leaves GPD disks unchan...

  1. The tidal disruption of protoplanetary accretion discs

    CERN Document Server

    Larwood, J D

    1997-01-01

    In this paper we revisit the problem of the tidal interaction occuring between a protostellar accretion disc and a secondary point mass following a parabolic trajectory. We model the disc response analytically and we compare our results with three-dimensional SPH simulations. Inviscid as well as viscous hydrodynamics is considered. We show that in a viscous system the response derived from inviscid considerations is predominant even for the highest estimates of an anomalous disc shear viscosity. The angular momentum lost from the disc during the encounter is derived from linear theory, for distant fly-bys, as well as the changes to the disc orientation expected in non-coplanar encounters. It is shown that the target discs can become warped and precess by a small amount during non-coplanar encounters. This small precession is shown to give rise to a relative tilt of the disc which is always more important for determining its final orientation than is the change to the orbital inclination. We discuss the implic...

  2. Evolution of transonicity in an accretion disc

    CERN Document Server

    Ray, A K; Ray, Arnab K.; Bhattacharjee, Jayanta K.

    2007-01-01

    For inviscid, rotational accretion flows driven by a general pseudo-Newtonian potential on to a Schwarzschild black hole, the only possible fixed points are saddle points and centre-type points. For the specific choice of the Newtonian potential, the flow has only two critical points, of which the outer one is a saddle point while the inner one is a centre-type point. A restrictive upper bound is imposed on the admissible range of values of the angular momentum of sub-Keplerian flows through a saddle point. These flows are very unstable to any deviation from a necessarily precise boundary condition. The difficulties against the physical realisability of a solution passing through the saddle point have been addressed through a temporal evolution of the flow, which gives a non-perturbative mechanism for selecting a transonic solution passing through the saddle point. An equation of motion for a real-time perturbation about the stationary flows reveals a very close correspondence with the metric of an acoustic b...

  3. Accretion Disks around Young Low Mass Stars

    Directory of Open Access Journals (Sweden)

    Paola D´Alessio

    2001-01-01

    Full Text Available In the past decade, it has become clear that almost half of the low mass pre-main sequence stars are surrounded by disks, which are responsible for the observed infrared and optical-UV excess emission. The characterization of the structure of circumstellar disks is a crucial step towards understanding the early stellar evolution and planet formation. The thesis summarized here presents physical models of the detailed structure of accretion disks surrounding T Tauri stars. The disks are assumed to be in steady state, in vertical hydrostatic equilibrium, and with a turbulent viscosity described by the alpha-prescription. We consider different heating mechanisms: viscous dissipation, heating by cosmic rays and radioactive decay, irradiation by the central star or irradiation by an infalling envelope. The energy is transported in the vertical direction by radiation, convection and the turbulent flux. Give n the disk structure, we calculate its emission by integrating the radiative transfer equation for an arbitrary orientation of the disk relative to the line of sight. Spectral energy distributions (SEDs and images are compared with observations, and disk properties can be inferred or constrained.

  4. Stability of accretion disk around rotating black holes

    CERN Document Server

    Mukhopadhyay, B

    2004-01-01

    I discuss the stability of accretion disks when the black hole is considered to be rotating. I show, how the fluid properties get changed for different choices of angular momentum of black holes. I treat the problem in pseudo-Newtonian approach with a suitable potential from Kerr geometry. When the angular momentum of a black hole is considered to be significant, the valid disk parameter region affects and a disk may become unstable. Also the possibility of shock in an accretion disk around rotating black holes is checked. When the black hole is chosen to be rotating, the sonic locations of the accretion disk get shifted or disappear, making the disk unstable by means of loosing entropy. To bring the disk in a stable situation, the angular momentum of the accreting matter has to be reduced/enhanced (for co/counter-rotating disk) by means of some physical process.

  5. Accretion to a Magnetized Neutron Star in the "Propeller" Regime

    CERN Document Server

    Toropina, O D; Lovelace, R V E

    2006-01-01

    We investigate spherical accretion to a rotating magnetized star in the "propeller" regime using axisymmetric resistive magnetohydrodynamic simulations. The regime is predicted to occur if the magnetospheric radius is larger than the corotation radius and smaller than the light cylinder radius. The simulations show that accreting matter is expelled from the equatorial region of the magnetosphere and that it moves away from the star in a supersonic, disk-shaped outflow. At larger radial distances the outflow slows down and becomes subsonic. The equatorial matter outflow is initially driven by the centrifugal force, but at larger distances the pressure gradient force becomes significant. We find the fraction of the Bondi accretion rate which accretes to the surface of the star.

  6. The Burst Mode of Accretion in Primordial Protostars

    CERN Document Server

    Vorobyov, Eduard I; Basu, Shantanu

    2013-01-01

    We study the formation and long-term evolution of primordial protostellar disks harbored by first stars using numerical hydrodynamics simulations in the thin-disk limit. The initial conditions are specified by pre-stellar cores with distinct mass, angular momentum, and temperature. This allows us to probe several tens of thousand years of the disk's initial evolution, during which we observe multiple episodes of fragmentation leading to the formation of gravitationally bound gaseous clumps within spiral arms. These fragments are torqued inward due to gravitational interaction with the spiral arms on timescales of 10^3 - 10^4 yr and accreted onto the growing protostar, giving rise to accretion and luminosity bursts. The burst phenomenon is fueled by continuing accretion of material falling onto the disk from the collapsing parent core, which replenishes the mass lost by the disk due to accretion, and triggers repetitive episodes of disk fragmentation. We show that the burst phenomenon is expected to occur for ...

  7. Dynamically important magnetic fields near accreting supermassive black holes.

    Science.gov (United States)

    Zamaninasab, M; Clausen-Brown, E; Savolainen, T; Tchekhovskoy, A

    2014-06-05

    Accreting supermassive black holes at the centres of active galaxies often produce 'jets'--collimated bipolar outflows of relativistic particles. Magnetic fields probably play a critical role in jet formation and in accretion disk physics. A dynamically important magnetic field was recently found near the Galactic Centre black hole. If this is common and if the field continues to near the black hole event horizon, disk structures will be affected, invalidating assumptions made in standard models. Here we report that jet magnetic field and accretion disk luminosity are tightly correlated over seven orders of magnitude for a sample of 76 radio-loud active galaxies. We conclude that the jet-launching regions of these radio-loud galaxies are threaded by dynamically important fields, which will affect the disk properties. These fields obstruct gas infall, compress the accretion disk vertically, slow down the disk rotation by carrying away its angular momentum in an outflow and determine the directionality of jets.

  8. The mass accretion rate of galaxy clusters: a measurable quantity

    CERN Document Server

    De Boni, Cristiano

    2016-01-01

    We are interested in investigating the growth of structures at the nonlinear scales of galaxy clusters from an observational perspective: we explore the possibility of measuring the mass accretion rate of galaxy clusters from their mass profile beyond the virial radius. We derive the accretion rate from the mass of a spherical shell whose infall velocity is extracted from $N$-body simulations. In the redshift range $z=[0,2]$, our prescription returns an average mass accretion rate within $20-40 \\%$ of the average rate derived from the merger trees of dark matter haloes extracted from $N$-body simulations. Our result suggests that measuring the mean mass accretion rate of a sample of galaxy clusters is actually feasible, thus providing a new potential observational test of the cosmological and structure formation models.

  9. Accretion disks around black holes with account of magnetic fields

    CERN Document Server

    Bisnovatyi-Kogan, G S

    2004-01-01

    Accretion disks are observed in young stars, cataclysmic variables, binary X-ray sources et al. Accretion disk 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 disk. Important part of this theory was the phenomenological treatment of the turbulent viscosity, known the `` alpha'' prescription, where the $(r \\phi)$ component of the stress tensor was connected with the pressure as $\\alpha P$. Sources of turbulence in the accretion disk are discussed, including hydrodynamic turbulence, convection and magnetic field role. Optically thin solution and advective disks are considered. Related problems of mass ejection from magnetized accretion disks and jet formation are discussed.

  10. Dynamical structure of magnetized dissipative accretion flow around black holes

    CERN Document Server

    Sarkar, Biplob

    2016-01-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accret...

  11. Cold, clumpy accretion onto an active supermassive black hole

    CERN Document Server

    Tremblay, Grant R; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P; Baum, Stefi A; Voit, G Mark; Donahue, Megan; McNamara, Brian R; Davis, Timothy A; McDonald, Michael A; Edge, Alastair C; Clarke, Tracy E; Galván-Madrid, Roberto; Bremer, Malcolm N; Edwards, Louise O V; Fabian, Andrew C; Hamer, Stephen L; Li, Yuan; Maury, Anaëlle; Russell, Helen R; Quillen, Alice C; Urry, C Megan; Sanders, Jeremy S; Wise, Michael

    2016-01-01

    Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds - a departure from the "hot mode" accretion model - although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z=0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities can precipitate from this hot gas, producing a rain of c...

  12. Accretion Does Not Drive the Turbulence in Galactic Disks

    CERN Document Server

    Hopkins, Philip F; Murray, Norman

    2013-01-01

    Rapid accretion of cold gas plays a crucial role in getting gas into galaxies. It has been suggested that this accretion proceeds along narrow streams that might also directly drive the turbulence in galactic gas, dynamical disturbances, and bulge formation. In cosmological simulations, however, it is impossible to isolate and hence disentangle the effect of accretion from internal instabilities and mergers. Moreover, in most cosmological simulations, the phase structure and turbulence in the ISM arising from stellar feedback are treated in a sub-grid manner, so that feedback cannot generate ISM turbulence. In this paper we therefore test the effects of cold streams in extremely high-resolution simulations of otherwise isolated galaxy disks using detailed models for star formation and feedback; we then include or exclude mock cold flows falling onto the galaxies with accretion rates, velocities and geometry set to maximize their effect on the disk. We find: (1) Turbulent velocity dispersions in gas disks are ...

  13. Images of accretion discs. 1. The eclipse mapping method

    Energy Technology Data Exchange (ETDEWEB)

    Horne, K.

    1985-03-01

    A method of mapping the surface brightness distributions of accretion discs in eclipsing cataclysmic binaries is described and tested with synthetic eclipse data. Accurate synthetic light curves are computed by numerical simulation of the accretion disc eclipse, and images of the disc are reconstructed by maximum entropy methods. The conventional definition of entropy leads to a distorted image of the disc. A modified form of entropy, sensitive to the aximuthal structure of the image but not to its radial profile, suppresses azimuthal structure but correctly recovers the radial structure of the accretion disc. This eclipse mapping method permits powerful tests of accretion disc theory by deriving the spatial structure of discs from observational data with a minimum of model-dependent assumptions.

  14. Evolution of Kerr superspinars due to accretion counterrotating thin discs

    Energy Technology Data Exchange (ETDEWEB)

    StuchlIk, Zdenek; HledIk, Stanislav; Truparova, Kamila, E-mail: zdenek.stuchlik@fpf.slu.cz, E-mail: stanislav.hledik@fpf.slu.cz [Faculty of Philosophy and Science, Institute of Physics, Silesian University in Opava, Bezrucovo nam. 13, CZ-74601 Opava (Czech Republic)

    2011-08-07

    String theory predicts the existence of extremely compact objects spinning faster than Kerr black holes. The spacetime exterior to such superspinars is described by Kerr naked singularity geometry breaking the black-hole limit on the internal angular momentum. We demonstrate that the conversion of Kerr superspinars into a near-extreme black hole due to an accretion counterrotating Keplerian disc is much more effective in comparison with the case of a corotating one since both the accreted rest mass necessary for conversion and the evolution time of conversion are by orders smaller for counterrotating discs. The conversion time of Kerr superspinars is given for several accretion regimes, and it is shown that the self-regulated accretion flow implies fastest evolution to the black-hole state. In the final stages of the conversion, Kerr superspinars can serve as very efficient particle accelerators in the region where the black-hole horizon forms.

  15. Magnetically Controlled Accretion Flows onto Young Stellar Objects

    CERN Document Server

    Adams, Fred C

    2011-01-01

    (abridged) Accretion from disks onto young stars is thought to follow magnetic field lines from the inner disk edge to the stellar surface. The accretion flow thus depends on the geometry of the magnetic field. This paper extends previous work by constructing a collection of orthogonal coordinate systems, including the corresponding differential operators, where one coordinate traces the magnetic field lines. This formalism allows for an (essentially) analytic description of the geometry and the conditions required for the flow to pass through sonic points. Using this approach, we revisit the problem of magnetically controlled accretion flow in a dipole geometry, and then generalize the treatment to consider magnetic fields with multiple components, including dipole, octupole, and split monopole contributions. This approach can be generalized further to consider more complex magnetic field configurations. Observations indicate that accreting young stars have substantial dipole and octupole components, and tha...

  16. Photon Bubbles and the Vertical Structure of Accretion Disks

    CERN Document Server

    Begelman, M C

    2006-01-01

    We consider the effects of "photon bubble" shock trains on the vertical structure of radiation pressure-dominated accretion disks. These density inhomogeneities are expected to develop spontaneously in radiation-dominated accretion disks where magnetic pressure exceeds gas pressure, even in the presence of magnetorotational instability. They increase the rate at which radiation escapes from the disk, and may allow disks to exceed the Eddington limit by a substantial factor. We first generalize the theory of photon bubbles to include the effects of finite optical depths and radiation damping. Modifications to the diffusion law at low optical depth tend to fill in the low-density regions of photon bubbles, while radiation damping inhibits the formation of photon bubbles at large radii, small accretion rates, and small heights above the equatorial plane. Accretion disks dominated by photon bubble transport may reach luminosities of 10 to >100 times the Eddington limit (L_E), depending on the mass of the central ...

  17. Variability and Stability in Radiation Hydrodynamic Accretion Flows

    CERN Document Server

    Miller, G S; Miller, Guy S.; Park, Myeong-Gu

    1997-01-01

    In this paper we examine time-dependent and three-dimensional perturbations of spherical accretion flow onto a neutron star close to its Eddington limit. Our treatment assumes a Schwarzschild geometry for the spacetime outside the neutron star and is fully general relativistic. At all the accretion rates studied, the response of the accretion flow to perturbations includes weakly damped oscillatory modes. At sufficiently high luminosities --- but still well below the Eddington limit --- the flows become unstable to aspherical perturbations. These unstable radiation hydrodynamic modes resemble the onset of convection, and allow accretion to occur preferentially through more rapidly descending columns of gas, while the radiation produced escapes through neighboring columns in which the gas descends more slowly.

  18. Wind-driven Accretion in Transitional Protostellar Disks

    CERN Document Server

    Wang, Lile

    2016-01-01

    Transitional protostellar disks have inner cavities heavily depleted in dust and gas, yet most 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 imply 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 such cavities are estimated for plausible X-ray and FUV radiation fields. The gas is molecular and predominantly neutral, with a dimensionless ambipolar parameter in the right general range for wind solutions of the type developed by K\\"onigl, Wardle, and others. That is to say, the density of ions and electrons is sufficient for moderately good coupling to the magnetic field, but not so good that the magnetic flux need be dragged inward by the accreting neutrals.

  19. Impact-induced melting during accretion of the Earth

    CERN Document Server

    de Vries, Jellie; Melosh, H Jay; Jacobson, Seth A; Morbidelli, Alessandro; Rubie, David C

    2016-01-01

    Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parametrised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth's ac...

  20. On the growth of pebble-accreting planetesimals

    CERN Document Server

    Visser, Rico G

    2015-01-01

    Pebble accretion is a new mechanism to quickly grow the cores of planets. In pebble accretion, gravity and gas drag conspire to yield large collisional cross sections for small particles in protoplanetary disks. However, before pebble accretion commences, aerodynamical deflection may act to prevent planetesimals from becoming large, because particles tend to follow gas streamlines. We derive the planetesimal radius where pebble accretion is initiated and determine the growth timescales of planetesimals by sweepup of small particles. We obtain the collision efficiency factor as the ratio of the numerically-obtained collisional cross section to the planetesimal surface area, from which we obtain the growth timescales. Integrations are conducted in the potential flow limit (steady, inviscid) and in the Stokes flow regime (steady, viscid). Only particles of stopping time $t_s \\ll t_X$ where $t_X\\approx10^3$ s experience aerodynamic deflection. Even in that case, the planetesimal's gravity always ensures positive ...

  1. Cross-correlation Aided Transport in Stochastically Driven Accretion Flows

    CERN Document Server

    Nath, Sujit Kumar

    2014-01-01

    Origin of linear instability resulting in rotating sheared accretion flows has remained a controversial subject for long. While some explanations of such non-normal transient growth of disturbances in the Rayleigh stable limit were available for magnetized accretion flows, similar instabilities in absence of magnetic perturbations remained unexplained. This dichotomy was resolved in two recent publications by Chattopadhyay, {\\it et al} where it was shown that such instabilities, especially for non-magnetized accretion flows, were introduced through interaction of the inherent stochastic noise in the system (even a \\enquote{cold} accretion flow at 3000K is too \\enquote{hot} in the statistical parlance and is capable of inducing strong thermal modes) with the underlying Taylor-Couette flow profiles. Both studies, however, excluded the additional energy influx (or efflux) that could result from nonzero cross-correlation of a noise perturbing the velocity flow, say, with the noise that is driving the vorticity fl...

  2. Gas accretion from halos to disks: observations, curiosities, and problems

    CERN Document Server

    Elmegreen, Bruce G

    2015-01-01

    Accretion of gas from the cosmic web to galaxy halos and ultimately their disks is a prediction of modern cosmological models but is rarely observed directly or at the full rate expected from star formation. Here we illustrate possible large-scale cosmic HI accretion onto the nearby dwarf starburst galaxy IC10, observed with the VLA and GBT. We also suggest that cosmic accretion is the origin of sharp metallicity drops in the starburst regions of other dwarf galaxies, as observed with the 10-m GTC. Finally, we question the importance of cosmic accretion in normal dwarf irregulars, for which a recent study of their far-outer regions sees no need for, or evidence of, continuing gas buildup.

  3. POLARIZATION MODULATION FROM LENSE–THIRRING PRECESSION IN X-RAY BINARIES

    Energy Technology Data Exchange (ETDEWEB)

    Ingram, Adam [Anton Pannekoek Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam (Netherlands); Maccarone, Thomas J. [Department of Physics, Texas Tech University, Box 41051, Lubbock, TX 79409-1051 (United States); Poutanen, Juri [Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, FI-21500 Piikkiö (Finland); Krawczynski, Henric, E-mail: a.r.ingram@uva.nl [Physics Department and McDonnell Center for the Space Sciences, Washington University in St. Louis, 1 Brookings Drive, CB 1105, St. Louis, MO 63130 (United States)

    2015-07-01

    It has long been recognized that quasi-periodic oscillations (QPOs) in the X-ray light curves of accreting black hole and neutron star binaries have the potential to be powerful diagnostics of strong field gravity. However, this potential cannot be fulfilled without a working theoretical model, which has remained elusive. Perhaps, the most promising model associates the QPO with Lense–Thirring precession of the inner accretion flow, with the changes in viewing angle and Doppler boosting modulating the flux over the course of a precession cycle. Here, we consider the polarization signature of a precessing inner accretion flow. We use simple assumptions about the Comptonization process generating the emitted spectrum and take all relativistic effects into account, parallel transporting polarization vectors toward the observer along null geodesics in the Kerr metric. We find that both the degree of linear polarization and the polarization angle should be modulated on the QPO frequency. We calculate the predicted absolute rms variability amplitude of the polarization degree and angle for a specific model geometry. We find that it should be possible to detect these modulations for a reasonable fraction of parameter space with a future X-ray polarimeter such as NASA’s Polarization Spectroscopic Telescope Array (the satellite incarnation of the balloon experiment X-Calibur)

  4. TLUSTY: Stellar Atmospheres, Accretion Disks, and Spectroscopic Diagnostics

    Science.gov (United States)

    Hubeny, Ivan; Lanz, Thierry

    2011-09-01

    TLUSTY is a user-oriented package written in FORTRAN77 for modeling stellar atmospheres and accretion disks and wide range of spectroscopic diagnostics. In the program's maximum configuration, the user may start from scratch and calculate a model atmosphere of a chosen degree of complexity, and end with a synthetic spectrum in a wavelength region of interest for an arbitrary stellar rotation and an arbitrary instrumental profile. The user may also model the vertical structure of annuli of an accretion disk.

  5. On accretion of dark energy onto black and wormholes

    OpenAIRE

    Jiménez Madrid, J. A.; Martín Moruno, Prado

    2010-01-01

    We review some of the possible models that are able to describe the current Universe which point out the future singularities that could appear. We show that the study of the dark energy accretion onto black- and worm-holes phenomena in these models could lead to unexpected consequences, allowing even the avoidance of the considered singularities. We also review the debate about the approach used to study the accretion phenomenon which has appeared in literature to demonstrate the advantages ...

  6. Standing Shocks in Viscous Accretion Flows around Black Holes

    Institute of Scientific and Technical Information of China (English)

    GU Wei-Min; LU Ju-Fu

    2005-01-01

    @@ We study the problem of standing shocks in viscous accretion flows around black holes.We parameterize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K.By providing the global dependence of shock formation in the j - K parameter space, we show that a significant parameter region can ensure solutions with shocks of different types, namely Rankine-Hugoniot shocks, isothermal shocks, and more realistically, mixed shocks.

  7. Massive thin accretion discs: Pt. 3; Comparison with the observations

    Energy Technology Data Exchange (ETDEWEB)

    Laor, A. (Tel Aviv Univ. (Israel). Wise Observatory)

    1990-10-01

    This paper presents a detailed comparison of theoretical thin accretion disc models with the spectra of 101 bright AGN. The models have been described previously; they take into account all relativistic effects and use an improved approximation for the local spectrum. The comparison is made by fitting the observation with a two-component model, a thin 'bare' accretion disc and an underlying power law. (author).

  8. Observational constraints on viscosity in AGN accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Siemiginowska, A.; Czerny, B. (N. Copernicus Astronomical Centre, Warsaw (Poland))

    1989-07-15

    The optical/UV/soft X-ray big bump can be modelled as thermal emission from an accretion disc. The observed UV variability in AGN spectra may be caused by accretion-disc instabilities, and can be used to constrain the viscosity. The comparison of thermal time-scales with the observed time-scales of variability in 10 Seyfert galaxies and 16 QSOs indicates values for the parameter {alpha} of the order of 0.01 for most cases. (author).

  9. Black hole accretion discs and screened scalar hair

    CERN Document Server

    Davis, Anne-Christine; Jha, Rahul

    2016-01-01

    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.

  10. Black hole accretion discs and screened scalar hair

    Science.gov (United States)

    Davis, Anne-Christine; Gregory, Ruth; Jha, Rahul

    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.

  11. Magnetic fields and accretion discs around static black holes

    Energy Technology Data Exchange (ETDEWEB)

    Dadhich, N. (Poona Univ. (India). Dept. of Mathematics and Statistics); Wiita, P.J. (Tata Inst. of Fundamental Research, Bombay (India))

    1982-08-01

    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.

  12. Relativistic sonic geometry for isothermal accretion in the Schwarzschild metric

    OpenAIRE

    Shaikh, Md Arif; Firdousi, Ivleena; Das, Tapas K

    2016-01-01

    The velocity potential, mass accretion rate and the Bernoulli's Constant corresponding to the general relativistic isothermal accretion in the Schwarzschild metric have been linearly perturbed, both for spherical as well as the axially symmetric flow to demonstrate the emergence of the embedded curved sonic manifold. Except the conformal factors, the relativistic acoustic geometry remains invariant irrespective of the physical quantity getting perturbed. The acoustic surface gravity has been ...

  13. A New Approach to Evolution of Black Hole Accretion Disks

    Institute of Scientific and Technical Information of China (English)

    WANG Ding-Xiong; LEI Wei-Hua; XIAO Kan

    2000-01-01

    Evolution of black hole (BH) accretion disks is investigated by a new approach, in which the evolution of the central BH can be derived in terms of BH spin directly, and the evolution characteristics of the concerning BH parameters are shown more easily and obviously. As an example, the unusual evolution characteristics of angular velocity of BH horizon and that of accreting particles at the inner edge of the disk are derived by considering the Blandford-Znajek process.

  14. The accretion rate dependence of burst oscillation amplitude

    CERN Document Server

    Ootes, Laura S; Galloway, Duncan K; Wijnands, Rudy

    2016-01-01

    Neutron stars in low mass X-ray binaries exhibit oscillations during thermonuclear bursts, attributed to asymmetric brightness patterns on the burning surfaces. All models that have been proposed to explain the origin of these asymmetries (spreading hotspots, surface waves, and cooling wakes) depend on the accretion rate. By analysis of archival RXTE data of six oscillation sources, we investigate the accretion rate dependence of the amplitude of burst oscillations. This more than doubles the size of the sample analysed previously by Muno et al. (2004), who found indications for a relationship between accretion rate and oscillation amplitudes. We find that burst oscillation signals can be detected at all observed accretion rates. Moreover, oscillations at low accretion rates are found to have relatively small amplitudes ($A_\\text{rms}\\leq0.10$) while oscillations detected in bursts observed at high accretion rates cover a broad spread in amplitudes ($0.05\\leq A_\\text{rms}\\leq0.20$). In this paper we present t...

  15. Implementation and Validation of 3-D Ice Accretion Measurement Methodology

    Science.gov (United States)

    Lee, Sam; Broeren, Andy P.; Kreeger, Richard E.; Potapczuk, Mark; Utt, Lloyd

    2014-01-01

    A research program has been implemented to develop and validate the use of a commercial 3-D laser scanning system to record ice accretion geometry in the NASA Icing Research Tunnel. A main component of the program was the geometric assessment of the 3- D laser scanning system on a 2-D (straight wing) and a 3-D (swept wing) airfoil geometries. This exercise consisted of comparison of scanned ice accretion to castings of the same ice accretion. The scan data were also used to create rapid prototype artificial ice shapes that were scanned and compared to the original ice accretion. The results from geometric comparisons on the straight wing showed that the ice shape models generated through the scan/rapid prototype process compared reasonably well with the cast shapes. Similar results were obtained with the geometric comparisons on the swept wing. It was difficult to precisely compare the scans of the cast shapes to the original ice accretion scans because the cast shapes appear to have shrunk during the mold/casting process by as much as 0.10-inch. However the comparison of the local ice-shape features were possible and produced better results. The rapid prototype manufacturing process was shown to reproduce the original ice accretion scan normally within 0.01-inch.

  16. Evolution of Accretion Disks in Tidal Disruption Events

    CERN Document Server

    Shen, Rong-Feng

    2013-01-01

    In a stellar tidal disruption event (TDE), an accretion disk forms as the stellar debris returns and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. An outer spreading disk is a source of matter for re-accretion at rates which can exceed the later stellar fall-back 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. Moreover, as an angular momentum reservoir, it can broadcast its existence by affecting the disk precession rate. Because these features depend on the disk's internal viscosity and the nature of wind produced in its early, advection-dominated phase, they are useful probes of transient disk physics. To model the evolution of TDE disk size and accretion rate, we account for the possibility of thermal instability for accretion rates...

  17. Accretion onto a black hole in a string cloud background

    CERN Document Server

    Ganguly, Apratim; Maharaj, Sunil D

    2014-01-01

    We examine the accretion process onto the black hole with a string cloud background, where the horizon of the black hole has an enlarged radius $r_H=2 M/(1-\\alpha)$, due to the string cloud parameter $\\alpha\\; (0 \\leq \\alpha < 1)$. The problem of stationary, spherically symmetric accretion of a polytropic fluid is analysed to obtain an analytic solution for such a perturbation. Generalised expressions for the accretion rate $\\dot{M}$, critical radius $r_s$, and other flow parameters are found. The accretion rate $\\dot{M}$ is an explicit function of the black hole mass $M$, as well as the gas boundary conditions and the string cloud parameter $\\alpha$. We also find the gas compression ratios and temperature profiles below the accretion radius and at the event horizon. It is shown that the mass accretion rate, for both the relativistic and the non-relativistic fluid by a black hole in the string cloud model, increases with increase in $\\alpha$.

  18. Multiphase, non-spherical gas accretion onto a black hole

    CERN Document Server

    Barai, Paramita; Nagamine, Kentaro

    2011-01-01

    (Abridged) We investigate non-spherical behavior of gas accreting onto a central supermassive black hole performing simulations using the SPH code GADGET-3 including radiative cooling and heating by the central X-ray source. As found in earlier 1D studies, our 3D simulations show that the accretion mode depends on the X-ray luminosity (L_X) for a fixed density at infinity and accretion efficiency. In the low L_X limit, gas accretes in a stable, spherically symmetric fashion. In the high L_X limit, the inner gas is significantly heated up and expands, reducing the central mass inflow rate. The expanding gas can turn into a strong enough outflow capable of expelling most of the gas at larger radii. For some intermediate L_X, the accretion flow becomes unstable developing prominent non-spherical features, the key reason for which is thermal instability (TI) as shown by our analyses. Small perturbations of the initially spherically symmetric accretion flow that is heated by the intermediate L_X quickly grow to fo...

  19. A magnetic accretion switch in pre-cataclysmic binaries?

    CERN Document Server

    Drake, Jeremy J; Takei, Dai; Gaensicke, Boris

    2014-01-01

    We have investigated the mass accretion rate implied by published surface abundances of Si and C in the white dwarf component of the 3.62 hr period pre-cataclysmic binary and planet host candidate QS Vir (DA+M2-4). Diffusion timescales for gravitational settling imply $\\dot{M} \\sim 10^{-16}M_\\odot$ yr$^{-1}$ for the 1999 epoch of the observations, which is three orders of magnitude lower than measured from a 2006 {\\it XMM-Newton} observation. This is the first time that large accretion rate variations have been seen in a detached pre-CV. A third body in a 14 yr eccentric orbit suggested in a recent eclipse timing study is too distant to perturb the central binary sufficiently to influence accretion. A hypothetical coronal mass ejection just prior to the {\\it XMM-Newton} observation might explain the higher accretion rate, but the implied size and frequency of such events appear too great. We suggest accretion is most likely modulated by a magnetic cycle on the secondary acting as a wind "accretion switch", a ...

  20. Growth of massive black holes during radiatively inefficient accretion phases

    CERN Document Server

    Cao, X

    2006-01-01

    The massive black holes in most faint active galactic nuclei (AGNs) and even normal galaxies are still accreting gases, though their accretion rates are very low. Radiatively inefficient accretion flows (RIAFs) are supposed in these faint sources, which should radiate mostly in the hard X-ray band. We calculate the contribution to the X-ray background from both the bright AGNs and the RIAFs in faint AGNs/normal galaxies. Our calculations show that both the observed intensity and spectral shape of the XRB with an energy peak at ~30$ keV can be well reproduced without including the emission of Compton-thick AGNs, if the massive black holes in faint AGNs/normal galaxies are spinning rapidly with a~0.9 and accreting at rates ~1.0-3.0\\times 10^{-4}. It indicates that less than ~5 per cent of local massive black hole mass density was accreted during radiatively inefficient accretion phases, which is obviously only an upper limit, because Compton-thick AGNs have not been considered. If the same number of the Compton...

  1. Black hole growth and AGN feedback under clumpy accretion

    Science.gov (United States)

    DeGraf, C.; Dekel, A.; Gabor, J.; Bournaud, F.

    2017-04-01

    High-resolution simulations of supermassive black holes in isolated galaxies have suggested the importance of short (∼10 Myr) episodes of rapid accretion caused by interactions between the black hole and massive dense clouds within the host. Accretion of such clouds could potentially provide the dominant source for black hole growth in high-z galaxies, but it remains unresolved in cosmological simulations. Using a stochastic subgrid model calibrated by high-resolution isolated galaxy simulations, we investigate the impact that variability in black hole accretion rates has on black hole growth and the evolution of the host galaxy. We find this clumpy accretion to more efficiently fuel high-redshift black hole growth. This increased mass allows for more rapid accretion even in the absence of high-density clumps, compounding the effect and resulting in substantially faster overall black hole growth. This increased growth allows the black hole to efficiently evacuate gas from the central region of the galaxy, driving strong winds up to ∼2500 km s-1, producing outflows ∼10 × stronger than the smooth accretion case, suppressing the inflow of gas on to the host galaxy, and suppressing the star formation within the galaxy by as much as a factor of 2. This suggests that the proper incorporation of variability is a key factor in the co-evolution between black holes and their hosts.

  2. Phantom Energy Accretion by a Stringy Charged Black Hole

    Institute of Scientific and Technical Information of China (English)

    M.Sharif; G.Abbas

    2012-01-01

    We investigate the dynamical behavior of phantom energy near a stringy magnetically charged black hole. For this purpose, we derive equations of motion for steady-state spherically symmetric Row of phantom energy onto the stringy magnetically charged black hole. It is found that phantom energy accreting onto a black hole decreases its mass. Further, the location of the critical points of accretion is explored, which yields a mass to charge ratio. This ratio implies that accretion process cannot transform a black hole into an extremal black hole or a naked singularity, hence cosmic censorship hypothesis remains valid here.%We investigate the dynamical behavior of phantom energy near a stringy magnetically charged black hole.For this purpose,we derive equations of motion for steady-state spherically symmetric flow of phantom energy onto the stringy magnetically charged black hole.It is found that phantom energy accreting onto a black hole decreases its mass.Further,the location of the critical points of accretion is explored,which yields a mass to charge ratio.This ratio implies that accretion process cannot transform a black hole into an extremal black hole or a naked singularity,hence cosmic censorship hypothesis remains valid here.

  3. The accretion of galaxies into groups and clusters

    CERN Document Server

    McGee, Sean L; Bower, Richard G; Font, Andreea S; McCarthy, Ian G

    2009-01-01

    We use the galaxy stellar mass and halo merger tree information from the semi-analytic model galaxy catalogue of Font et al. (2009) to examine the accretion of galaxies into a large sample of groups and clusters, covering a wide range in halo mass (10E12.9 to 10E15.3 Msun/h), and selected from each of four redshift epochs (z=0, 0.5, 1.0 and 1.5). We find that clusters at all examined redshifts have accreted a significant fraction of their final galaxy populations through galaxy groups. A 10E14.5 Msun/h mass cluster at z=0 has, on average, accreted ~ 40% of its galaxies (Mstellar > 10E9 Msun/h) from halos with masses greater than 10E13 Msun/h. Further, the galaxies which are accreted through groups are more massive, on average, than galaxies accreted through smaller halos or from the field population. We find that at a given epoch, the fraction of galaxies accreted from isolated environments is independent of the final cluster or group mass. In contrast, we find that observing a cluster of the same halo mass a...

  4. Evolution of Gas Giant Entropy During Formation by Runaway Accretion

    CERN Document Server

    Berardo, David; Marleau, Gabriel-Dominique

    2016-01-01

    We calculate the evolution of gas giant planets during the runaway gas accretion phase of formation, to understand how the luminosity of young giant planets depends on the accretion conditions. We construct steady-state envelope models, and run time-dependent simulations of accreting planets with the Modules for Experiments in Stellar Astrophysics (MESA) code. We show that the evolution of the internal entropy depends on the contrast between the internal adiabat and the entropy of the accreted material, parametrized by the shock temperature $T_0$ and pressure $P_0$. At low temperatures ($T_0\\lesssim 300$--$1000\\ {\\rm K}$, depending on model parameters), the accreted material has a lower entropy than the interior. The convection zone extends to the surface and can drive a large luminosity, leading to rapid cooling and cold starts. For higher temperatures, the accreted material has a larger entropy than the interior, giving a radiative zone that stalls cooling. For $T_0\\gtrsim 2000\\ {\\rm K}$, the surface--inter...

  5. Atmospheric Signatures of Giant Exoplanet Formation by Pebble Accretion

    CERN Document Server

    Madhusudhan, Nikku; Johansen, Anders; Eriksson, Linn

    2016-01-01

    Atmospheric chemical abundances of giant planets lead to important constraints on planetary formation and migration. Recent studies have shown that giant planets that migrate through the protoplanetary disk can accrete substantial amounts of oxygen-rich solids, leading to super-solar metallicities in the envelope and solar or sub-solar C/O ratios. Pebble accretion has been demonstrated recently to play an important role in core accretion and to have growth rates that are consistent with planetary migration. The high pebble accretion rates allow planetary cores to start their growth beyond 10 AU and subsequently migrate to cold (>~ 1 AU) or hot (<~ 0.1 AU) orbits. In this work we investigate how the formation of giant planets via pebble accretion influences their atmospheric chemical compositions. We find that under the standard pebble accretion scenario, where the core is isolated from the envelope, the resulting metallicities (O/H and C/H ratios) are sub-solar, while the C/O ratios are super-solar. Planet...

  6. Probing neutron star physics using accreting neutron stars

    Directory of Open Access Journals (Sweden)

    Patruno A.

    2010-10-01

    Full Text Available We give an obervational overview of the accreting neutron stars systems as probes of neutron star physics. In particular we focus on the results obtained from the periodic timing of accreting millisecond X-ray pulsars in outburst and from the measurement of X-ray spectra of accreting neutron stars during quiescence. In the first part of this overview we show that the X-ray pulses are contaminated by a large amount of noise of uncertain origin, and that all these neutron stars do not show evidence of spin variations during the outburst. We present also some recent developments on the presence of intermittency in three accreting millisecond X-ray pulsars and investigate the reason why only a small number of accreting neutron stars show X-ray pulsations and why none of these pulsars shows sub-millisecond spin periods. In the second part of the overview we introduce the observational technique that allows the study of neutron star cooling in accreting systems as probes of neutron star internal composition and equation of state. We explain the phenomenon of the deep crustal heating and present some recent developments on several quasi persistent X-ray sources where a cooling neutron star has been observed.

  7. Accretion Rate and the Physical Nature of Unobscured Active Galaxies

    CERN Document Server

    Trump, Jonathan R; Kelly, Brandon C; Civano, Francesca; Gabor, Jared M; Diamond-Stanic, Aleksandar M; Merloni, Andrea; Urry, C Megan; Hao, Heng; Jahnke, Knud; Nagao, Tohru; Taniguchi, Yoshi; Koekemoer, Anton M; Lanzuisi, Giorgio; Liu, Charles; Mainieri, Vincenzo; Salvato, Mara; Scoville, Nick Z

    2011-01-01

    We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rate by using accurate accretion luminosities (L_int) from well-sampled multiwavelength SEDs from the Cosmic Evolution Survey (COSMOS), and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L_int/L_Edd > 0.01), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L_int/L_Edd 0.01 broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L_int/L_Edd < 0.01 AGNs, although there may be ...

  8. Observations on the Formation of Massive Stars by Accretion

    CERN Document Server

    Keto, E; Keto, Eric; Wood, Kenneth

    2006-01-01

    Observations of the H66a recombination line from the ionized gas in the cluster of newly formed massive stars, G10.6-0.4, show that most of the continuum emission derives from the dense gas in an ionized accretion flow that forms an ionized disk or torus around a group of stars in the center of the cluster. The inward motion observed in the accretion flow suggests that despite the equivalent luminosity and ionizing radiation of several O stars, neither radiation pressure nor thermal pressure has reversed the accretion flow. The observations indicate why the radiation pressure of the stars and the thermal pressure of the HII region are not effective in reversing the accretion flow. The observed rate of the accretion flow, 0.001 solar masses/yr, is sufficient to form massive stars within the time scale imposed by their short main sequence lifetimes. A simple model of disk accretion relates quenched HII regions, trapped hypercompact HII regions, and photo-evaporating disks in an evolutionary sequence.

  9. Close stars and accretion in Low Luminosity Active Galactic Nuclei

    CERN Document Server

    Nayakshin, S

    2004-01-01

    Quasar accretion disks are believed to form stars by self-gravity. Low Luminosity Active Galactic Nuclei (LLAGN) are much dimmer galactic centers, and are often believed to be quasars that ran out of gaseous fuel. LLAGN accretion disks should thus co-exist with thousands to millions of stars or proto-stars left from the previous stronger accretion activity. In principle, these stars may produce several important effects: (i) contribute to the optical/UV spectra of some LLAGN; (ii) reprocessing of the stellar radiation in the dusty disks could dominate the LLAGN infra-red spectra; (iii) deplete the (accretion) gas disk much faster than it can accrete onto the supper-massive black hole (SMBH); (iv) stars, individually or in groups, may slow down and modulate the accretion flow significantly due to their inertia. In this way they may produce the LLAGN cut-off disks; (v) alternatively, frequent enough stellar collisions and resulting stellar disruptions could keep the inner disk empty. Here we explore these ideas...

  10. Impact-induced melting during accretion of the Earth

    Science.gov (United States)

    de Vries, Jellie; Nimmo, Francis; Melosh, H. Jay; Jacobson, Seth A.; Morbidelli, Alessandro; Rubie, David C.

    2016-12-01

    Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parameterised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth's accretion depends strongly on the lifetime of impact-generated magma oceans compared to the time interval between large impacts. In addition, such results depend on starting parameters in the N-body simulations, such as the number and initial mass of embryos. Thus, there is the potential for combining the results, such as those presented here, with multistage core formation models to better constrain the accretional history of the Earth.

  11. Magnetospheres and Disk Accretion in Herbig Ae/Be Stars

    CERN Document Server

    Muzerolle, J; Calvet, N; Hartmann, L

    2004-01-01

    We present evidence of magnetically-mediated disk accretion in Herbig Ae/Be stars. Magnetospheric accretion models of Balmer and sodium profiles calculated with appropriate stellar and rotational parameters are in qualitative agreement with the observed profiles of the Herbig Ae star UX Ori, and yield a mass accretion rate of ~ 10^{-8} Msun/yr. If more recent indications of an extremely large rotation rate for this object are correct, the magnetic field geometry must deviate from that of a standard dipole in order to produce line emission consistent with observed flux levels. Models of the associated accretion shock qualitatively explain the observed distribution of excess fluxes in the Balmer discontinuity for a large ensemble of Herbig Ae/Be stars, and imply typically small mass accretion rates, < 10^{-7} Msun/yr. In order for accretion to proceed onto the star, significant amounts of gas must exist inside the dust destruction radius, which is potentially problematic for recently advocated scenarios of "...

  12. Are Radio AGN Powered by Accretion or Black Hole Spin?

    CERN Document Server

    McNamara, B R; Nulsen, P E J

    2010-01-01

    We compare accretion and black hole spin as potential energy sources for outbursts from AGN in brightest cluster galaxies (BCGs). We find that the distribution of AGN power estimated from X-ray cavities is consistent with a broad range of both spin parameter and accretion rate. Sufficient quantities of molecular gas are available in most BCGs to power their AGN by accretion alone. However, we find no correlation between AGN power and molecular gas mass. For a given AGN power, the BCG's gas mass and accretion efficiency vary by more than two orders of magnitude. Most of the molecular gas in BCGs is apparently consumed by star formation or is driven out of the nucleus by the AGN before it reaches the nuclear black hole. Bondi accretion from hot atmospheres is generally unable to fuel powerful AGN, unless their black holes are more massive than their bulge luminosities imply. We identify several powerful AGN that reside in relatively gas-poor galaxies, indicating an unusually efficient mode of accretion, or that...

  13. Binary interactions with high accretion rates onto main sequence stars

    Science.gov (United States)

    Shiber, Sagiv; Schreier, Ron; Soker, Noam

    2016-07-01

    Energetic outflows from main sequence stars accreting mass at very high rates might account for the powering of some eruptive objects, such as merging main sequence stars, major eruptions of luminous blue variables, e.g., the Great Eruption of Eta Carinae, and other intermediate luminosity optical transients (ILOTs; red novae; red transients). These powerful outflows could potentially also supply the extra energy required in the common envelope process and in the grazing envelope evolution of binary systems. We propose that a massive outflow/jets mediated by magnetic fields might remove energy and angular momentum from the accretion disk to allow such high accretion rate flows. By examining the possible activity of the magnetic fields of accretion disks, we conclude that indeed main sequence stars might accrete mass at very high rates, up to ≈ 10-2 M ⊙ yr-1 for solar type stars, and up to ≈ 1 M ⊙ yr-1 for very massive stars. We speculate that magnetic fields amplified in such extreme conditions might lead to the formation of massive bipolar outflows that can remove most of the disk's energy and angular momentum. It is this energy and angular momentum removal that allows the very high mass accretion rate onto main sequence stars.

  14. Data Analysis And Polarization Measurements With GEMS

    Science.gov (United States)

    Stohmayer, Tod

    2011-01-01

    The Gravity and Extreme Magnetism SMEX (GEMS) mission was selected by NASA for flight in 2014. GEMS will make the first sensitive survey of X-ray polarization across a wide range of source classes including black hole and neutron star binaries, AGN of different types, rotation and accretion-powered pulsars, magnetars, shell supernova remnants and pulsar wind nebulae. GEMS employs grazing-incidence foil mirrors and novel time-projection chamber (TPC) polarimeters leveraging the photoelectric effect. The GEMS detectors image the charge tracks of photoelectrons produced by 2 - 10 keV X-rays. The initial direction of the photoelectron is determined by the linear polarization of the photon. We present an overview of the data analysis challenges and methods for GEMS, including procedures for producing optimally filtered images of the charge tracks and estimating their initial directions. We illustrate our methods using laboratory measurements of polarized and unpolarized X-rays with flight-like detectors as well as from simulated tracks. We also present detailed simulations exploring the statistics of polarization measurements appropriate for GEMS, and make comparisons with previous work.

  15. Towards Self-Consistent Modelling of the Sgr A* Accretion Flow: Linking Theory and Observation

    CERN Document Server

    Roberts, Shawn R; Jiang, Yan-Fei; Ostriker, Jeremiah P

    2016-01-01

    The interplay between supermassive black holes (SMBHs) and their environments is believed to command an essential role in galaxy evolution. The majority of these SMBHs are in the radiative inefficient accretion phase where this interplay remains elusive, but suggestively important, due to few observational constraints. To remedy this, we directly fit 2-D hydrodynamic simulations to Chandra observations of Sgr A* with Markov Chain Monte Carlo sampling, self-consistently modelling the 2-D inflow-outflow solution for the first time. We find the temperature and density at flow onset are consistent with the origin of the gas in the stellar winds of massive stars in the vicinity of Sgr A*. We place the first observational constraints on the angular momentum of the gas and estimate the centrifugal radius, r$_c$ $\\approx$ 0.056 r$_b$ $\\approx8\\times10^{-3}$ pc, where r$_b$ is the Bondi radius. Less than 1\\% of the inflowing gas accretes onto the SMBH, the remainder being ejected in a polar outflow. For the first time...

  16. Viscous Hydrodynamics Simulations of Circumbinary Accretion Discs: Variability, Quasi-Steady State, and Angular Momentum Transfer

    CERN Document Server

    Miranda, Ryan; Lai, Dong

    2016-01-01

    Circumbinary discs are found in a variety of astrophysical contexts, including around young stellar binaries and supermassive black hole binaries. We carry out a suite of numerical simulations of circumbinary discs, solving the viscous hydrodynamics equations on a polar grid covering an extended disc outside the binary co-orbital region. We use carefully controlled outer boundary conditions and long-term integrations to ensure that the disc reaches a quasi-steady state, in which the time-averaged mass accretion rate onto the binary, $\\langle\\dot{M}\\rangle$, matches the mass supply rate at the outer disc. We focus on binaries with comparable masses, but with a wide range of eccentricities ($e_\\mathrm{B}$). For $e_\\mathrm{B} \\lesssim 0.05$, the mass accretion rate of the binary is modulated with a period of about $5$ times the binary period; otherwise it is modulated at the binary period. The inner part of the circumbinary disc generally becomes coherently eccentric. For low and high $e_\\mathrm{B}$, the disc li...

  17. Non-thermal emissions from accreting X-ray binary pulsars

    Science.gov (United States)

    Zhang, Jian-Fu; Jin, Hui; Dong, Ai-Jun

    2014-03-01

    We study non-thermal emissions from cascade processes in accreting X-ray binary pulsars. In the framework of the magnetospheric gap model, we consider three photon fields, which are respectively from the polar cap of a pulsar, its surrounding accretion disk and a massive companion star with a circumstellar disk, to shield the gap. The gap-accelerated ultra-relativistic electrons emit high-energy photons via curvature radiation and an inverse Compton scattering process, in which part of these high-energy photons absorbed by interactions with the surrounding photon fields can facilitate the following electromagnetic cascades. We first carry out numerical calculations of the cascade processes in order to obtain the predicted emission spectra. As an example, we subsequently apply this model to reproduce observations of LS I +61° 303. We find that the results can fit observations ranging from hard X-ray to γ-ray bands. In particular, they can explain the spectral cutoff feature at a few GeV. Finally, we suggest that the emissions detected by the Fermi Large Area Telescope from X-ray binary pulsars originate in the magnetosphere region of the pulsar.

  18. THE GLOBULAR CLUSTER SYSTEM OF THE MILKY WAY: ACCRETION IN A COSMOLOGICAL CONTEXT

    Energy Technology Data Exchange (ETDEWEB)

    Keller, Stefan C.; Mackey, Dougal; Da Costa, Gary S. [RSAA, Australian National University, Weston Creek, ACT 2611 (Australia)

    2012-01-01

    We examine the significance of a planar arrangement in the spatial distribution of the Milky Way (MW) globular clusters (GCs). We find that, when separated on the basis of horizontal branch morphology and metallicity, the outermost canonical young halo (YH) GC sample (at galactocentric radii in excess of 10 kpc) exhibits an anisotropic distribution that may be equated to a plane (24 {+-} 4) kpc thick (rms) and inclined at 8 Degree-Sign {+-} 5 Degree-Sign to the polar axis of the MW disk. To quantify the significance of this plane we determine the fraction of times that an isotropic distribution replicates the observed distribution in Monte Carlo trials. The plane is found to remain significant at the >95% level outside a galactocentric radius of 10 kpc, inside this radius the spatial distribution is apparently isotropic. In contrast, the spatial distribution of the old halo sample outside 10 kpc is well matched by an isotropic distribution. The plane described by the outer YH GCs is indistinguishable in orientation from that presented by the satellite galaxies of the MW. Simulations have shown that the planar arrangement of satellites can arise as filaments of the surrounding large-scale structure feed into the MW's potential. We therefore propose that our results are direct observational evidence for the accreted origin of the outer YH GC population. This conclusion confirms numerous lines of evidence that have similarly indicated an accreted origin for this set of clusters from the inferred cluster properties.

  19. Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

    CERN Document Server

    Moss, David; Suleimanov, Valery

    2016-01-01

    Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. We consider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models ...

  20. Limits on luminosity and mass accretion rate of a radiation pressure dominated accretion disc

    CERN Document Server

    Cao, Xinwu

    2015-01-01

    There is a maximum for the gravity of a black hole in the vertical direction in the accretion disc. Outflows may probably be driven from the disc if the radiation flux of the disc is greater than a critical value corresponding to the maximal vertical gravity. We find that outflows are driven by the radiation force from the disc if the accretion rate is greater than the Eddington rate. The radiation of the disc is therefore limited by such outflows. The disc luminosity, L=L_Edd\\propto ln mdot, at large-mdot cases. The Eddington ratio of the disc is ~3 for mdot~100, which is significantly lower than that of a conventional slim disc without outflows. This implies that the emission from some ultra-luminous X-ray sources with highly super Eddington luminosity should be Doppler beamed, or intermediate mass black holes are in these sources instead of stellar mass black holes. The spectra of the discs with outflows are saturated in the high frequency end provided mdot>2. We suggest that the saturated emission can be ...

  1. Initiation of continental accretion: metamorphic conditions

    Science.gov (United States)

    Clement, Conand; Frederic, Mouthereau; Gianreto, Manatschal; Adbeltif, Lahfid

    2017-04-01

    The physical processes involved at the beginning of the continental collision are largely unknown because they are transient and therefore hardly identifiable from the rock record. Despite the importance of key parameters for understanding mountain building processes, especially the formation of deep mountain roots and their impacts on earthquakes nucleation, rock/fluid transfers and oil/gas resources in the continental crust, observations from the earliest collision stages remain fragmentary. Here, we focus on the example of Taiwan, a young and active mountain belt where the transition from oceanic subduction, accretion of the first continental margin to mature collision can be followed in space and time. We present preliminary results and provide key questions regarding the reconstruction of time-pressure-temperature paths of rocks & fluids to allow discriminating between rift-related thermal/rheological inheritance and burial/heating phases during convergence. Previous studies have focused on peak temperatures analyzed by Raman Spectrometry of Carbonaceous Matter from the deeper structural layers exposed in the Central Range of Taiwan. In the pre-rift sediments, these studies reported a positive gradient from West to Est, and values from units. Cross sections and maps with high resolution peak temperatures are in process as well as pressure estimations to determine how the sediments were metamorphosed. In addition to this work, we report a few inherited temperatures in the 390-570 °C range, indicating recycling of organic matter from metasediments that recorded HT events, likely originated from higher grade metamorphic units of mainland China, which have been eroded and deposited in the post-rift sediments.

  2. Pouring 'Cold Water' on Hot Accretion

    Science.gov (United States)

    Rubin, A. E.

    1995-09-01

    The extensive recrystallization of type-6 OC has been interpreted as having resulted either from prograde thermal metamorphism of initially cold, unequilibrated material [1,2] or from autometamorphism due to slow cooling of material that accreted while still hot (1000-1200 K). Although the physical implausibility of hot accretion has been addressed [3], no comprehensive evaluation has been made of arguments in its favor. As shown below, these arguments are based on incomplete data, flawed experiments or improbable interpretations. Correlation between petrologic type and Ca in low-Ca pyroxene. Models of prograde metamorphism assume that, with increasing temperature, opx acquires Ca at the expense of diopside. Analyses of pyroxene in 10 H chondrites showed no correlation between Ca in pyroxene cores and increasing petrologic type [4], but more extensive data sets show such correlations [1,5,6]. A review of data for 51 OC [7] shows a progressive increase in the Wo content of low-Ca pyroxene with petrologic type: Wo 0.4-1.2 in type-3 and -4; Wo 1.2-1.6 in type-5; and Wo 1.6-2.2 in type-6. Striated opx. Undeformed striated opx were interpreted as having formed from inverted protopyroxene during slow cooling [8]; striated opx from H4 Quenggouk were found to convert into normal opx within 1 week during annealing at 1100 K [9]. Because prograde metamorphism probably lasted ~60 Ma [10], there should be no striated opx remaining in type-4 or -5 OC. However, samples of 99% twinned clinopyroxene (analogous to that in chondrules in type-3 OC) annealed for >3 weeks at Conquista could not have formed during single stage cooling as expected in autometamorphism; a two-stage cooling history involving rapid cooling during chondrule formation followed by parent-body annealing is more plausible. Polycrystalline taenite. Polycrystalline taenite in H/L3 Tieschitz was interpreted as a relict solidification structure that failed to anneal into monocrystalline taenite because of rapid

  3. Numerical Simulation of Hot Accretion Flows (I): A Large Radial Dynamical Range and the Density Profile of Accretion Flow

    CERN Document Server

    Yuan, Feng; Bu, Defu

    2012-01-01

    Numerical simulations of hot accretion flow have shown that the mass accretion rate decreases with decreasing radius; consequently the density profile of accretion flow becomes flatter compared to the case of a constant accretion rate. This result has important theoretical and observational implications. However, because of technical difficulties, the radial dynamic range in almost all previous simulations usually spans at most two orders of magnitude. This small dynamical range, combined with the effects of boundary conditions, makes the simulation results suspectable. Especially, the radial profiles of density and accretion rate may not be precise enough to be used to compare with observations. In this paper we present a "two-zone" approach to expand the radial dynamical range from two to four orders of magnitude. We confirm previous results and find that from $r_s$ to $ 10^4r_s$ the radial profiles of accretion rate and density can be well described by $\\dot{M}(r)\\propto r^s$ and $\\rho\\propto r^{-p}$. The ...

  4. Polarized Light in Astronomy.

    Science.gov (United States)

    King, D. J.

    1983-01-01

    The application of very sensitive electronic detecting devices during the last decade has revolutionized and revitalized the study of polarization in celestial objects. The nature of polarization, how polaroids work, interstellar polarization, dichroic filters, polarization by scattering, and modern polarimetry are among the topics discussed. (JN)

  5. Gamma-Ray Pulsar Light Curves in Offset Polar Cap Geometry

    Science.gov (United States)

    Harding, Alice K.; DeCesar, Megan; Miller, M. Coleman

    2011-01-01

    Recent studies have shown that gamma-ray pulsar light curves are very sensitive to the geometry of the pulsar magnetic field. Pulsar magnetic field geometries, such as the retarded vacuum dipole and force-free magnetospheres, used to model high-energy light curves have distorted polar caps that are offset from the magnetic axis in the direction opposite to rotation. Since this effect is due to the sweepback of field lines near the light cylinder, offset polar caps are a generic property of pulsar magnetospheres and their effects should be included in gamma-ray pulsar light curve modeling. In slot gap models (having two-pole caustic geometry), the offset polar caps cause a strong azimuthal asymmetry of the particle acceleration around the magnetic axis. We have studied the effect of the offset polar caps in both retarded vacuum dipole and force-free geometry on the model high-energy pulse profile. We find that. corn pared to the profile:-; derived from :-;ymmetric caps, the flux in the pulse peaks, which are caustics formed along the trailing magnetic field lines. increases significantly relative to the off-peak emission. formed along leading field lines. The enhanced contrast produces greatly improved slot gap model fits to Fermi pulsar light curves like Vela, which show very little off-peak emIssIon.

  6. Implications of the β Lyrae accretion disk rim Teff

    Science.gov (United States)

    Linnell, A. P.

    2000-12-01

    Photometric evidence indicates that the massive gainer in the β Lyrae system is hidden from the observer by a thick accretion disk (Linnell, Hubeny, & Harmanec, 1998, ApJ, 509, 379). It is believed that the gainer approximates a main sequence star of Teff= 30000K. Spectroscopic analysis by Balachrandan et al. (1986, MNRAS, 219, 479) establishes a Teff of 13,300K for the donor. System synthetic spectra, fitted via the BINSYN suite to spectrophotometric scan data and IUE spectra, establish a mean rim Teff of 9000K. Assuming conservative mass transfer, Harmanec & Scholz (1993, A&A, 279, 131) use the rate of period change to derive a mass transfer rate of 20x10-6M⊙ yr-1. Connecting the rim Teff to the accretion disk face Teff with the Hubeny theory (Hubeny & Plavec 1991, AJ, 102, 1156) and using the standard accretion disk relations (Frank, King & Raine), the adopted mass transfer rate predicts a rim Teff of 4500K. The BINSYN-derived 9000K rim Teff would require a mass transfer rate 30 times larger than the adopted value. The observed rate of period change excludes such a large mass transfer rate. The bolometric luminosity of the rim, from the BINSYN model, is 5.6x1036erg sec-1. The bolometric luminosity of the gainer, on the adopted model, is 9.8x1037erg sec-1. Thus, the luminosity of the rim is 6% of the luminosity of the gainer. On the BINSYN model, the accretion disk covers 26% of the sky, as seen by the gainer. Absorption of radiation from the gainer, and its reradiation by the accretion disk, could explain the derived Teff of the rim. The conclusion is that the β Lyrae accretion disk structure must be strongly affected by radiation from the hot gainer (unseen by the observer) at the center of the accretion disk.

  7. GK Per and EX Hya: Intermediate polars with small magnetospheres

    CERN Document Server

    Suleimanov, V; Ducci, L; Zhukov, G V; Werner, K

    2016-01-01

    Observed hard X-ray spectra of intermediate polars are determined mainly by the accretion flow velocity at the white dwarf surface, which is normally close to the free-fall velocity. This allows to estimate the white dwarf masses as the white dwarf mass-radius relation M-R and the expected free-fall velocities at the surface are well known. This method is widely used, however, derived white dwarf masses M can be systematically underestimated because the accretion flow is stopped at and re-accelerates from the magnetospheric boundary R_m, and therefore, its velocity at the surface will be lower than free-fall.To avoid this problem we computed a two-parameter set of model hard X-ray spectra, which allows to constrain a degenerate M - R_m dependence. On the other hand, previous works showed that power spectra of accreting X-ray pulsars and intermediate polars exhibit breaks at the frequencies corresponding to the Keplerian frequencies at the magnetospheric boundary. Therefore, the break frequency \

  8. An EUV Study of the Intermediate Polar EX Hydrae

    CERN Document Server

    Belle, K E; Sirk, M; Huber, M E; Belle, Kunegunda E.; Howell, Steve B.; Sirk, Martin; Huber, Mark E.

    2002-01-01

    On 2000 May 5, we began a large multi-wavelength campaign to study the intermediate polar, EX Hydrae. The simultaneous observations from six satellites and four telescopes were centered around a one million second observation with EUVE. Although EX Hydrae has been studied previously with EUVE, our higher signal-to-noise observations present new results and challenge the current IP models. Previously unseen dips in the light curve are reminiscent of the stream dips seen in polar light curves. Also of interest is the temporal extent of the bulge dip; approximately 0.5 in phase, implying that the bulge extends over half of the accretion disk. We propose that the magnetic field in EX Hydrae is strong enough (a few MG) to begin pulling material directly from the outer edge of the disk, thereby forming a large accretion curtain which would produce a very broad bulge dip. This would also result in magnetically controlled accretion streams originating from the outer edge of the disk. We also present a period analysis...

  9. X-ray spectroscopy and photometry of the long-period polar AI Tri with XMM-Newton

    CERN Document Server

    Traulsen, I; Schwarz, R; Dreizler, S; Beuermann, K; Schwope, A D; Burwitz, V

    2010-01-01

    Context. The energy balance of cataclysmic variables with strong magnetic fields is a central subject in understanding accretion processes on magnetic white dwarfs. With XMM-Newton, we perform a spectroscopic and photometric study of soft X-ray selected polars during their high states of accretion. Aims. On the basis of X-ray and optical observations of the magnetic cataclysmic variable AI Tri, we derive the properties of the spectral components, their flux contributions, and the physical structure of the accretion region in soft polars. Methods. We use multi-temperature approaches in our xspec modeling of the spectra to describe the physical conditions and the structures of the post-shock accretion flow and the accretion spot on the white-dwarf surface. In addition, we investigate the accretion geometry of the system by a timing analysis of the photometric data. Results. Flaring soft X-ray emission from the heated surface of the white dwarf dominates the X-ray flux during roughly 70% of the binary cycle. Thi...

  10. Quasi-periodic oscillations in accreting magnetic white dwarfs I. Observational constraints in X-ray and optical

    CERN Document Server

    Bonnet-Bidaud, J M; Busschaert, C; Falize, E; Michaut, C

    2015-01-01

    Quasi-periodic oscillations (QPOs) are observed in the optical flux of some polars with typical periods of 1 to 3 s but none have been observed yet in X-rays where a significant part of the accreting energy is released. QPOs are expected and predicted from shock oscillations. Most of the polars have been observed by the XMM-Newton satellite. We made use of the homogeneous set of observations of the polars by XMM-Newton to search for the presence of QPOs in the (0.5-10 keV) energy range and to set significant upper limits for the brightest X-ray polars. We extracted high time-resolution X-ray light curves by taking advantage of the 0.07 sec resolution of the EPIC-PN camera. Among the 65 polars observed with XMM-Newton from 1998 to 2012, a sample of 24 sources was selected on the basis of their counting rate in the PN instrument to secure significant limits. We searched for QPOs using Fast Fourier Transform (FFT) methods and defined limits of detection using statistical tools. Among the sample surveyed, none sh...

  11. Migration of accreting planets in radiative discs from dynamical torques

    Science.gov (United States)

    Pierens, A.; Raymond, S. N.

    2016-11-01

    We present the results of hydrodynamical simulations of the orbital evolution of planets undergoing runaway gas accretion in radiative discs. We consider accreting disc models with constant mass flux through the disc, and where radiative cooling balances the effect of viscous heating and stellar irradiation. We assume that 20-30 M⊕ giant planet cores are formed in the region where viscous heating dominates and migrate outward under the action of a strong entropy-related corotation torque. In the case where gas accretion is neglected and for an α viscous stress parameter α = 2 × 10-3, we find evidence for strong dynamical torques in accreting discs with accretion rates {dot{M}}≳ 7× 10^{-8} M_{⊙} yr{}^{-1}. Their main effect is to increase outward migration rates by a factor of ˜2 typically. In the presence of gas accretion, however, runaway outward migration is observed with the planet passing through the zero-torque radius and the transition between the viscous heating and stellar heating dominated regimes. The ability for an accreting planet to enter a fast migration regime is found to depend strongly on the planet growth rate, but can occur for values of the mass flux through the disc of {dot{M}}≳ 5× 10^{-8} M_{⊙} yr{}^{-1}. We find that an episode of runaway outward migration can cause an accreting planet formed in the 5-10 au region to temporarily orbit at star-planet separations as large as ˜60-70 au. However, increase in the amplitude of the Lindblad torque associated with planet growth plus change in the streamline topology near the planet systematically cause the direction of migration to be reversed. Subsequent evolution corresponds to the planet migrating inward rapidly until it becomes massive enough to open a gap in the disc and migrate in the type II regime. Our results indicate that a planet can reach large orbital distances under the combined effect of dynamical torques and gas accretion, but an alternative mechanism is required to

  12. Numerical simulations of accretion discs: Pt. 1. Superhumps: a tidal phenomenon of accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Whitehurst, Robert

    1988-05-01

    Numerical simulations of the SU UMa star ZCha in outburst are presented which demonstrate the role of the secondary's tidal influence upon the accretion flow and provide explanations for the superhumps seen in SU UMa stars. They show that for a system with an extreme mass-ratio q = 0.15, the disc is tidally unstable and can become asymmetric and slowly rotate in the inertial frame of reference. The tidal stresses raised in this disc by the secondary produce a peak in the light curve of order 30 per cent which recurs with a period 3.5 per cent longer than that of the orbit. This is interpreted as the superhump. Analytic considerations show that only for systems with mass-ratios more extreme than approximately 4::1 will such behaviour occur, which is consistent with the known mass-ratio of SU UMa stars and other cataclysmic variables.

  13. Constraining protoplanetary disc evolution using accretion rate and disc mass measurements: the usefulness of the dimensionless accretion parameter

    Science.gov (United States)

    Rosotti, Giovanni P.; Clarke, Cathie J.; Manara, Carlo F.; Facchini, Stefano

    2017-06-01

    We explore how measurements of protoplanetary disc masses and accretion rates provided by surveys of star-forming regions can be analysed via the dimensionless accretion parameter, which we define as the product of the accretion rate and stellar age divided by the disc mass. By extending and generalizing the study of Jones et al., we demonstrate that this parameter should be less than or of order unity for a wide range of evolutionary scenarios, rising above unity only during the final stages of outside-in clearing by external photoevaporation. We use this result to assess the reliability of disc mass estimates derived from CO isotopologues and sub-mm continuum emission by examining the distribution of accretion efficiencies in regions that are not subject to external photoevaporation. We find that while dust-based mass estimates produce results compatible with theoretical expectations assuming a canonical dust-to-gas ratio, the systematically lower CO-based estimates yield accretion efficiencies significantly above unity in contrast with the theory. This finding provides additional evidence that CO-based disc masses are an underestimate, in line with arguments that have been made on the basis of chemical modelling of relatively small samples. On the other hand, we demonstrate that dust-based mass estimates are sufficiently accurate to reveal distinctly higher accretion efficiencies in the Trapezium cluster, where this result is expected, given the evident importance of external photoevaporation. We therefore propose the dimensionless accretion parameter as a new diagnostic of external photoevaporation in other star-forming regions.

  14. Dynamical structure of magnetized dissipative accretion flow around black holes

    Science.gov (United States)

    Sarkar, Biplob; Das, Santabrata

    2016-09-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several supermassive black hole sources and the observational implications of our present analysis are discussed.

  15. Detecting gravitational wave emission from the known accreting neutron stars

    CERN Document Server

    Watts, Anna; Bildsten, Lars; Schutz, Bernard

    2008-01-01

    Detection of gravitational waves from accreting neutron stars (NSs) in our galaxy, due to ellipticity or internal oscillation, would be a breakthrough in our understanding of compact objects and explain the absence of NSs rotating near the break-up limit. Direct detection, however, poses a formidable challenge. Using the current data available on the properties of the accreting NSs in Low Mass X-Ray Binaries (LMXBs), we quantify the detectability for the known accreting NSs, considering various emission scenarios and taking into account the negative impact of parameter uncertainty on the data analysis process. Only a few of the persistently bright NSs accreting at rates near the Eddington limit are detectable by Advanced LIGO if they are emitting gravitational waves at a rate matching the torque from accretion. A larger fraction of the known population is detectable if the spin and orbital parameters are known in advance, especially with the narrow-band Advanced LIGO. We identify the most promising targets, a...

  16. Accretion in Radiative Equipartition (AiRE) Disks

    CERN Document Server

    Yazdi, Yasaman K

    2016-01-01

    Standard accretion disk theory (Shakura & Sunyaev 1973) 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 time-scale, our accretion models in the radiation pressure dominated regime (i.e. inner disk) need to be modified. Here, we present a modification to the SS model, where radiation pressure is in equipartition with 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 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 fu...

  17. X-Shooter study of accretion in Chamaeleon I

    CERN Document Server

    Manara, C F; Herczeg, G J; Teixeira, P

    2016-01-01

    We present the analysis of 34 new VLT/X-Shooter spectra of young stellar objects in the Chamaeleon I star forming region, together with four more spectra of stars in Taurus and two in Chamaeleon II. The broad wavelength coverage and accurate flux calibration of our spectra allow us to estimate stellar and accretion parameters for our targets by fitting the photospheric and accretion continuum emission from the Balmer continuum down to 700 nm. The dependence of accretion with stellar properties for this sample is consistent with previous results from the literature. The accretion rates for transitional disks are consistent with those of full disks in the same region. The spread of mass accretion rates at any given stellar mass is found to be smaller than in many studies, but is larger than that derived in the Lupus clouds using similar data and techniques. Differences in the stellar mass range and in the environmental conditions between our sample and that of Lupus may account for the discrepancy in scatter be...

  18. Self-Similar Hot Accretion Flow onto a Neutron Star

    CERN Document Server

    Medvedev, M V; Medvedev, Mikhail V.; Narayan, Ramesh

    2000-01-01

    We consider hot, two-temperature, viscous accretion onto a rotating, unmagnetized neutron star. We assume Coulomb coupling betweenthe protons and electrons, and free-free cooling from the electrons. We show that the accretion flow has an extended settling region which can be described by means of two analytical self-similar solutions: a two-temperature solution which is valid in an inner zone, $r10^{2.5}$. In both zones the density varies as $\\rho\\propto r^{-2}$ and the angular velocity as $\\Omega\\propto r^{-3/2}$. We solve the flow equations numerically and confirm that the analytical solutions are accurate. The self-similar settling solution differs from the advection-dominated accretion flow discussed in the context of black hole accretion. The settling flow radiates the energy dissipated by viscosity; so it is not advection-dominated. Except for the radial velocity, all other gas properties - density, angular velocity, temperature, luminosity, angular momentum flux - are independent of the mass accretion ...

  19. Kronos: a multiwavelength observatory for mapping accretion-driven sources

    Science.gov (United States)

    Peterson, Bradley M.; Polidan, Ronald S.; Robinson, Edward L.

    2003-02-01

    Kronos is a multiwavelength observatory proposed as a NASA Medium Explorer. Kronos is designed to make use of the natural variability of accreting sources to create microarcsecond-resolution maps of the environments of supermassive black holes in active galaxies and stellar-size black holes in binary systems and to characterize accretion processes in Galactic compact binaries. Kronos will obtain broad energy range spectroscopic data with co-aligned X-ray, ultraviolet, and optical spectrometers. The high-Earth orbit of Kronos enables well-sampled, high time-resolution observations, critical for the innovative and sophisticated methods that are used to understand the accretion flows, mass outflows, jets, and other phenomena found in accreting sources. By utilizing reverberation mapping analysis techniques, Kronos produces advanced high-resolution maps of unprecedented resolution of the extreme environment in the inner cores of active galaxies. Similarly, Doppler tomography and eclipse mapping techniques characterize and map Galactic binary systems, revealing the details of the physics of accretion processes in black hole, neutron star, and white dwarf binary systems. The Kronos instrument complement, sensitivity, and orbital environment make it suitable to aggressively address time variable phenomena in a wide range of astronomical objects from nearby flare stars to distant galaxies.

  20. Progenitors of the Accretion-Induced Collapse of White Dwarfs

    CERN Document Server

    Kwiatkowski, Damian

    2015-01-01

    Recent calculations of accretion-induced collapse of an oxygen-neon-magnesium white dwarf into a neutron star [Piro & Thompson 2014] allow for a potentially detectable transient electromagnetic signal. Motivated by these results, I present theoretical rates and physical properties of binary stars that can produce accretion-induced collapse. The rates are presented for various types of host galaxies (e.g. old ellipticals versus spirals) and are differentiated by the donor star type (e.g. large giant star versus compact helium-rich donor). Results presented in this thesis may help to guide near-future electromagnetic transient search campaigns to find likely candidates for accretion-induced collapse events. My predictions are based on binary evolution calculations that include the most recent updates on mass accretion and secular mass growth of white dwarfs. I find that the most likely systems that undergo accretion-induced collapse consist of an ONeMg white dwarf with a Hertzsprung gap star or a red giant ...

  1. Accretion of solid materials onto circumplanetary disks from protoplanetary disks

    Energy Technology Data Exchange (ETDEWEB)

    Tanigawa, Takayuki [Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819 (Japan); Maruta, Akito; Machida, Masahiro N., E-mail: tanigawa@pop.lowtem.hokudai.ac.jp [Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 812-8581 (Japan)

    2014-04-01

    We investigate the accretion of solid materials onto circumplanetary disks from heliocentric orbits rotating in protoplanetary disks, which is a key process for the formation of regular satellite systems. In the late stage of the gas-capturing phase of giant planet formation, the accreting gas from protoplanetary disks forms circumplanetary disks. Since the accretion flow toward the circumplanetary disks affects the particle motion through gas drag force, we use hydrodynamic simulation data for the gas drag term to calculate the motion of solid materials. We consider a wide range of size for the solid particles (10{sup –2}-10{sup 6} m), and find that the accretion efficiency of the solid particles peaks around 10 m sized particles because energy dissipation of drag with circum-planetary disk gas in this size regime is most effective. The efficiency for particles larger than 10 m becomes lower because gas drag becomes less effective. For particles smaller than 10 m, the efficiency is lower because the particles are strongly coupled with the background gas flow, which prevents particles from accretion. We also find that the distance from the planet where the particles are captured by the circumplanetary disks is in a narrow range and well described as a function of the particle size.

  2. The IMF of stellar clusters: effects of accretion and feedback

    CERN Document Server

    Dib, Sami; Padoan, Paolo; G., Maheswar; Ojha, D K; Khajenabi, Fazeleh

    2009-01-01

    (abridged) We develop a model which describes the coevolution of the mass function of dense cores and of the IMF in a protocluster clump. In the model, cores injected in the clump evolve under the effect of gas accretion. Accretion onto the cores follows a time-dependent accretion rate that describes accretion in a turbulent medium. Once the accretion timescales of cores exceed their contraction timescales, they are turned into stars. We include the effect of feedback by the newly formed massive stars through their stellar winds. A fraction of the wind's energy is assumed to counter gravity and disperse the gas from the protocluster and as a consequence, quench further star formation. The latter effect sets the final IMF of the cluster. We apply our model to a clump that is expected to resemble the progenitor clump of the Orion Nebula Cluster (ONC). Our model is able to reproduce both the shape and normalization of the ONC's IMF and the mass function of dense cores in Orion. The complex features of the ONC's ...

  3. The hunt for the Milky Way's accreted disc

    CERN Document Server

    Ruchti, Gregory R; Feltzing, Sofia; Pipino, Antonio; Bensby, Thomas

    2014-01-01

    The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive >1:10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the Milky Way's thin and thick stellar disc. However, it is interesting because: (i) its associated `dark disc' has important implications for experiments hoping to detect a dark matter particle in the laboratory; and (ii) the presence or absence of such a disc constrains the merger history of our Galaxy. In this work, we develop a chemo-dynamical template to hunt for the accreted disc. We apply our template to the high-resolution spectroscopic sample from Ruchti et al. (2011), finding at present no evidence for accreted stars. Our results are consistent with a quiescent Milky Way with no >1:10 mergers since the disc formed and a correspondingly light `dark disc'. However, we caution that while our method can robustly identify accreted stars...

  4. Bondi-Hoyle Accretion in an Isothermal Magnetized Plasma

    CERN Document Server

    Lee, Aaron T; McKee, Christopher F; Klein, Richard I

    2014-01-01

    In regions of star formation, protostars and newborn stars accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma beta---the ratio of thermal and magnetic pressures. Observations show molecular clouds have beta <= 1, so magnetic fields can play a significant role in the accretion process. We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbers of up M ~ 45, magnetic fields that are either parallel, perpendicular, or oriented 45 degrees to the flow, and beta as low as 0.01. Our simulations utilize AMR to obtain high spatial resolution where needed; this also allows the simulation boundaries to be far from the accreting object. Additionally, we show our results are independent of our exact prescription for accreting mass in the sink particle. We giv...

  5. The large scale magnetic fields of thin accretion disks

    CERN Document Server

    Cao, Xinwu

    2013-01-01

    Large scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared to the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number Pm is around unity. In this work, we revisit this problem considering the angular momentum of the disk is removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-bet...

  6. Models of Vortices and Spirals in White Dwarf's Accretion Binaries

    Science.gov (United States)

    Boneva, Daniela

    2010-11-01

    The main aim in the current survey is to suggest models of the development of structures, such as vortices and spirals, in accretion white dwarf's binaries. On the base of hydrodynamical analytical considerations it is applied numerical methods and simulations. It is suggested in the theoretical model the perturbation's parameters of the accretion flow, caused by the influences of the tidal wave over the flux of accretion matter around the secondary star. To examine such disturbed flow, the numerical code has involved in the calculations. The results reveal us an appearing of structure with spiral shape due to the tidal interaction in the close binaries. Our further simulations give the solution, which expresses the formation of vortical configurations in the accretion disc's zone. The evolution of vortices in areas of the flow's interaction is explored using single vortex and composite vortex models. Gas in the disc matter is considered to be compressible and non-ideal. The longevity of all these structures is different and each depends of time period of the rotation, density and velocity of the accretion matter.

  7. Atlas of Tilted Accretion Disks & Source to Negative Superhumps

    CERN Document Server

    Montgomery, M M

    2009-01-01

    Using smoothed particle hydrodynamics, we numerically simulate steady state accretion discs for Cataclysmic Variable Dwarf Novae systems that have a secondary-to-primary mass ratio (0.35 \\le q \\le 0.55). After these accretion discs have come to quasi-equilibrium, we rotate each disc out of the orbital plane by (\\delta = (1, 2, 3, 4, 5,) or (20)^{o}) to induce negative superhumps. For accretion discs tilted $5^{o}$, we generate light curves and associated Fourier transforms for an atlas on negative superhumps and retrograde precession. Our simulation results suggest that accretion discs need to be tilted more than three degrees for negative superhumps to be statistically significant. We also show that if the disc is tilted enough such that the gas stream strikes a disc face, then a dense cooling ring is generated near the radius of impact. In addition to the atlas, we study these artificially tilted accretion discs to find the source to negative superhumps. Our results suggest that the source is additional lig...

  8. Nucleosynthesis in the accretion disks of Type II collapsars

    CERN Document Server

    Banerjee, Indrani

    2013-01-01

    We investigate nucleosynthesis inside the gamma-ray burst (GRB) accretion disks formed by the Type II collapsars. In these collapsars, the core collapse of massive stars first leads to the formation of a proto-neutron star and a mild supernova explosion is driven. However, this supernova ejecta lack momentum and falls back onto the neutron star which gets transformed to a stellar mass black hole. In order to study the hydrodynamics and nucleosynthesis of such an accretion disk formed from the fallback material of the supernova ejecta, we use the well established hydrodynamic models. In such a disk neutrino cooling becomes important in the inner disk where the temperature and density are higher. Higher the accretion rate (dot{M}), higher is the density and temperature in the disks. In this work we deal with accretion disks with relatively low accretion rates: 0.001 M_sun s^{-1} \\lesssim dot{M} \\lesssim 0.01 M_sun s^{-1} and hence these disks are predominantly advection dominated. We use He-rich and Si-rich abu...

  9. Modeling of Disk-Star Interaction: Different Regimes of Accretion and Variability

    CERN Document Server

    Romanova, Marina M; Long, Min; Lovelace, Richard V E

    2008-01-01

    The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g. Wijnands & van der Klis 1998) depends strongly on the accretion rate, the effective viscosity and the effective magnetic diffusivity of the disk-magnetosphere boundary. The accretion rate is the main parameter which determines the location of the magnetospheric radius of the star for a given stellar magnetic field. We introduce a classification of accreting neutron stars as a function of the accretion rate and show the corresponding stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and from our axisymmetric MHD simulations. We discuss the expected variability features in each stage of accretion, both periodic and quasi-periodic (QPOs). We conclude that the periodicity may be suppressed at both very high and very low accretion rates. In addition the periodicity may disappear when ordered funnel flow accretion is replaced by disordered accretion through the interchange instability.

  10. WFPC2 Polarization Calibration

    Science.gov (United States)

    Biretta, J.; McMaster, M.

    1997-12-01

    We derive a detailed calibration for WFPC2 polarization data which is accurate to about 1.5%. We begin by computing polarizer flats, and show how they are applied to data. A physical model for the polarization effects of the WFPC2 optics is then created using Mueller matricies. This model includes corrections for the instrumental polarization (diattenuation and phase retardance) of the pick-off mirror, as well as the high cross-polarization transmission of the polarizer filter. We compare this model against the on-orbit observations of polarization calibrators, and show it predicts relative counts in the different polarizer/aperture settings to 1.5% RMS accuracy. We then show how this model can be used to calibrate GO data, and present two WWW tools which allow observers to easily calibrate their data. Detailed examples are given illustrationg the calibration and display of WFPC2 polarization data. In closing we describe future plans and possible improvements.

  11. Metasurface polarization splitter

    CERN Document Server

    Slovick, Brian A; Yu, Zhi Gang; Kravchenckou, Ivan I; Briggs, Dayrl P; Moitra, Parikshit; Krishnamurthy, Srini; Valentine, Jason

    2016-01-01

    Polarization beam splitters, devices that separate the two orthogonal polarizations of light into different propagation directions, are one of the most ubiquitous optical elements. However, traditionally polarization splitters rely on bulky optical materials, while emerging optoelectronic and photonic circuits require compact, chip-scale polarization splitters. Here we show that a subwavelength rectangular lattice of cylindrical silicon Mie resonators functions as a polarization splitter, efficiently reflecting one polarization while transmitting the other. We show that the polarization splitting arises from the anisotropic permittivity and permeability of the metasurface due to the two-fold rotational symmetry of the rectangular unit cell. The high polarization efficiency, low loss, and low profile make these metasurface polarization splitters ideally suited for monolithic integration with optoelectronic and photonic circuits.

  12. Polarized Electron Source Developments

    Energy Technology Data Exchange (ETDEWEB)

    Charles K. Sinclair

    1990-02-23

    Presently, only two methods of producing beams of polarized electrons for injection into linear accelerators are in use. Each of these methods uses optical pumping by circularly polarized light to produce electron polarization. In one case, electron polarization is established in metastable helium atoms, while in the other case, the polarized electrons are produced in the conduction band of appropriate semiconductors. The polarized electrons are liberated from the helium metastable by chemi-ionization, and from the semiconductors by lowering the work function at the surface of the material. Developments with each of these sources since the 1988 Spin Physics Conference are reviewed, and the prospects for further improvements discussed.

  13. Geographical Income Polarization

    DEFF Research Database (Denmark)

    Azhar, Hussain; Jonassen, Anders Bruun

    In this paper we estimate the degree, composition and development of geographical income polarization based on data at the individual and municipal level in Denmark from 1984 to 2002. Rising income polarization is reconfirmed when applying new polarization measures, the driving force being greater...... inter municipal income inequality. Counter factual simulations show that rising property prices to a large part explain the rise in polarization. One side-effect of polarization is tendencies towards a parallel polarization of residence location patterns, where low skilled individuals tend to live...

  14. Preliminary modelling study of ice accretion on wind turbines

    DEFF Research Database (Denmark)

    Pedersen, Marie Cecilie; Yin, Chungen

    2014-01-01

    One of the main challenges associated with cold-climate wind energy is icing on wind turbines and a series of icing-induced problems such as production loss, blade fatigue and safety issues. Because of the difficulties with on-site measurements, simulations are often used to understand and predict...... icing events. In this paper, a new methodology for prediction of icing-induced production loss is proposed, from which the fundamentals of ice accretion on wind turbines can be better understood and the operational production losses can be more reliably predicted. Computational fluid dynamics (CFD......) modelling of ice accretion on wind turbines is also performed for different ice events, resulting in a reliable framework for CFD-based ice accretion modelling which is one of the key elements in the new methodology....

  15. The Formation of Massive Star Systems by Accretion

    CERN Document Server

    Krumholz, Mark R; McKee, Christopher F; Offner, Stella S R; Cunningham, Andrew J

    2009-01-01

    Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation pressure does not halt accretion. Instead, gravitational and Rayleigh-Taylor instabilities channel gas onto the star system through non-axisymmetric disks and filaments that self-shield against radiation, while allowing radiation to escape through optically-thin bubbles. Gravitational instabilities cause the disk to fragment and form a massive companion to the primary star. Radiation pressure does not limit stellar masses, but the instabilities that allow accretion to continue lead to small multiple systems.

  16. Transport of Large Scale Poloidal Flux in Black Hole Accretion

    CERN Document Server

    Beckwith, Kris; Krolik, Julian H

    2009-01-01

    We perform a global, three-dimensional GRMHD simulation of an accretion torus embedded in a large scale vertical magnetic field orbiting a Schwarzschild black hole. This simulation investigates how a large scale vertical field evolves within a turbulent accretion disk and whether global magnetic field configurations suitable for launching jets and winds can develop. We identify a ``coronal mechanism'' of magnetic flux motion, which dominates the global flux evolution. In this coronal mechanism, magnetic stresses driven by orbital shear create large-scale half-loops of magnetic field that stretch radially inward and then reconnect, leading to discontinuous jumps in the location of magnetic flux. This mechanism is supplemented by a smaller amount of flux advection in the accretion flow proper. Because the black hole in this case does not rotate, the magnetic flux on the horizon determines the mean magnetic field strength in the funnel around the disk axis; this field strength is regulated by a combination of th...

  17. The Instability in Accretion Flows: GvMRI

    Science.gov (United States)

    Yardimci, Melis; Ebru Devlen, Doç.

    2016-07-01

    In this study, we discuss the physical instability defining the expected turbulence in Radiatively Inefficient Accretion Flows (RIAFs) around the supermassive black holes (e.g., Sagittarius A* in the center of our Galaxy). These flows, with a high probability, include weakly collisional hot, optically thin and dilute plasmas. Within these flows, gravitational potential energy brought about by turbulent stresses is trapped as heat energy. Thus, in order accretion to be realized, outward transport of heat as well as angular momentum is required. This outward heat transport may reduce the mass inflow rate on black hole. We solve MHD equations including variation of viscosity coefficients with pressure in the momentum conservation equation. We plot the wave number-frequency diagrams for the wave modes. We show that one of the most probable candidates for definition of mass accretion and the source of excess heat energy in RIAFs is the gyroviscous modified magnetorotational instabilitiy (GvMRI).

  18. Standing Rankine-Hugoniot Shocks in Black Hole Accretion Discs

    Institute of Scientific and Technical Information of China (English)

    GU Wei-Min; LU Ju-Fu

    2004-01-01

    @@ We study the problem of standing shocks in viscous disc-like accretion flows around black holes. For the first time we parametrize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K. By providing the global dependence of shock formation in the j - K parameter space, we show that a significant parameter region can ensure solutions with Rankine-Hugoniot shocks; and that the possibilities of shock formation are the largest for inviscid flows, decreasing with increasing viscosity, and ceasing to exist for a strong enough viscosity. Our results support the view that the standing shock is an essential ingredient in black hole accretion discs and is a general phenomenon in astrophysics, and that there should be a continuous change from the properties of inviscid flows to those of viscous ones.

  19. Accretion Disks and Dynamos: Toward a Unified Mean Field Theory

    CERN Document Server

    Blackman, Eric G

    2012-01-01

    Conversion of gravitational energy into radiation near stars and compact objects in accretion disks the origin of large scale magnetic fields in astrophysical rotators have long been distinct topics of active research in astrophysics. In semi-analytic work on both problems it has been useful to presume large scale symmetries, which necessarily results in mean field theories; magnetohydrodynamic turbulence makes the underlying systems locally asymmetric and highly nonlinear. Synergy between theory and simulations should aim for the development of practical, semi-analytic mean field models that capture the essential physics and can be used for observational modeling. Mean field dynamo (MFD) theory and alpha-viscosity accretion disc theory have exemplified such distinct pursuits. Both are presently incomplete, but 21st century MFD theory has nonlinear predictive power compared to 20th century MFD. in contrast, alpha-viscosity accretion theory is still in a 20th century state. In fact, insights from MFD theory ar...

  20. Star Formation and Gas Accretion in Nearby Galaxies

    CERN Document Server

    Yim, Kijeong

    2016-01-01

    In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 {\\mu}m (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and HI in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.

  1. Angular Momentum Transport in Quasi-Keplerian Accretion Disks

    Indian Academy of Sciences (India)

    Prasad Subramanian; B. S. Pujari; Peter A. Becker

    2004-03-01

    We reexamine arguments advanced by Hayashi & Matsuda (2001), who claim that several simple, physically motivated derivations based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the nature of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expression for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The derivation is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.

  2. Observational Signatures of Tilted Black Hole Accretion Disks from Simulations

    CERN Document Server

    Dexter, Jason

    2011-01-01

    Geometrically thick accretion flows may be present in black hole X-ray binaries observed in the low/hard state and in low-luminosity active galactic nuclei. Unlike in geometrically thin disks, the angular momentum axis in these sources is not expected to align with the black hole spin axis. We compute images from three-dimensional general relativistic magnetohydrodynamic simulations of misaligned (tilted) accretion flows using relativistic radiative transfer, and compare the estimated locations of the radiation edge with expectations from their aligned (untilted) counterparts. The radiation edge in the tilted simulations is independent of black hole spin for a tilt of 15 degrees, in stark contrast to the results for untilted simulations, which agree with the monotonic dependence on spin expected from thin accretion disk theory. Synthetic emission line profiles from the tilted simulations depend strongly on the observer's azimuth, and exhibit unique features such as broad "blue wings." Coupled with precession,...

  3. Gas accretion from the cosmic web in the local Universe

    Science.gov (United States)

    Sánchez Almeida, J.; Elmegreen, B. G.; Muñoz-Tuñón, C.; Elmegreen, D. M.

    2016-10-01

    Numerical simulations predict that gas accretion from the cosmic web drives star formation in disks galaxies. The process is important in low mass haloes (One of the most compelling cases for gas accretion at work in the local universe comes from the extremely metal poor (XMP) galaxies. They show metallicity inhomogeneities associated with star-forming regions, so that large starbursts have lower metallicity than the underlying galaxy. Here we put forward the case for gas accretion from the web posed by XMP galaxies. Two other observational results are discussed too, namely, the fact that the gas consumption time-scale is shorter than most stellar ages, and the systematic morphological distortions of the HI around galaxies.

  4. Holocene reef accretion: southwest Molokai, Hawaii, U.S.A.

    Science.gov (United States)

    Engels, Mary S.; Fletcher, Charles H.; Field, Michael E.; Storlazzi, Curt D.; Grossman, Eric E.; Rooney, John J.B.; Conger, Christopher L.; Glenn, Craig

    2004-01-01

    Two reef systems off south Molokai, Hale O Lono and Hikauhi (separated by only 10 km), show strong and fundamental differences in modern ecosystem structure and Holocene accretion history that reflect the influence of wave-induced near-bed shear stresses on reef development in Hawaii. Both sites are exposed to similar impacts from south, Kona, and trade-wind swell. However, the Hale O Lono site is exposed to north swell and the Hikuahi site is not. As a result, the reef at Hale O Lono records no late Holocene net accretion while the reef at Hikauhi records consistent and robust accretion over late Holocene time. Analysis and dating of 24 cores from Hale O Lono and Hikauhi reveal the presence of five major lithofacies that reflect paleo-environmental conditions. In order of decreasing depositional energy they are: (1) coral-algal bindstone; (2) mixed skeletal rudstone; (3) massive coral framestone; (4) unconsolidated floatstone; and (5) branching coral framestone-bafflestone. At Hale O Lono, 10 cores document a backstepping reef ranging from ∼ 8,100 cal yr BP (offshore) to ∼ 4,800 cal yr BP (nearshore). A depauperate community of modern coral diminishes shoreward and seaward of ∼ 15 m depth due to wave energy, disrupted recruitment activities, and physical abrasion. Evidence suggests a change from conditions conducive to accretion during the early Holocene to conditions detrimental to accretion in the late Holocene. Reef structure at Hikauhi, reconstructed from 14 cores, reveals a thick, rapidly accreting and young reef (maximum age ∼ 900 cal yr BP). Living coral cover on this reef increases seaward with distance from the reef crest but terminates at a depth of ∼ 20 m where the reef ends in a large sand field. The primary limitation on vertical reef growth is accommodation space under wave base, not recruitment activities or energy conditions. Interpretations of cored lithofacies suggest that modern reef growth on the southwest corner of Molokai, and by

  5. The influence of outflow in supercritical accretion flows

    CERN Document Server

    Zeraatgari, Fatemeh Zahra; Mosallanezhad, Amin

    2016-01-01

    We solve the radiation-hydrodynamic (RHD) equations of supercritical accretion flows in the presence of radiation force and outflow by using self similar solutions. Compare with the pioneer works, in this paper we consider power-law function for mass inflow rate as $ \\dot{M} \\propto r^{s} $. We found that $ s = 1 $ when the radiative cooling term is included in the energy equation. Correspondingly, the effective temperature profile with respect to the radius was obtained as $ T_{\\text{eff}} \\varpropto r^{-1/2} $. In addition, we investigated the influence of the outflow on the dynamics of the accretion flow. We also calculated the continuum spectrum emitted from the disk surface as well as the bolometric luminosity of the accretion flow. Furthermore, our results show that the advection parameter, $ f $, strongly depends on mass inflow rate.

  6. X-ray emission from hot accretion flows

    CERN Document Server

    Niedzwiecki, Andrzej; Stepnik, Agnieszka

    2014-01-01

    Radiatively inefficient, hot accretion flows are widely considered as a relevant accretion mode in low-luminosity AGNs. We study spectral formation in such flows using a refined model with a fully general relativistic description of both the radiative (leptonic and hadronic) and hydrodynamic processes, as well as with an exact treatment of global Comptonization. We find that the X-ray spectral index--Eddington ratio anticorrelation as well as the cut-off energy measured in the best-studied objects favor accretion flows with rather strong magnetic field and with a weak direct heating of electrons. Furthermore, they require a much stronger source of seed photons than considered in previous studies. The nonthermal synchrotron radiation of relativistic electrons seems to be the most likely process capable of providing a sufficient flux of seed photons. Hadronic processes, which should occur due to basic properties of hot flows, provide an attractive explanation for the origin of such electrons.

  7. X-ray emission from hot accretion flows

    Science.gov (United States)

    Niedźwiecki, Andrzej; Xie, Fu-Guo; Stȩpnik, Agnieszka

    2014-07-01

    Radiatively inefficient, hot accretion flows are widely considered as a relevant accretion mode in low-luminosity AGNs. We study spectral formation in such flows using a refined model with a fully general relativistic description of both the radiative (leptonic and hadronic) and hydrodynamic processes, as well as with an exact treatment of global Comptonization. We find that the X-ray spectral index-Eddington ratio anticorrelation as well as the cut-off energy measured in the best-studied objects favor accretion flows with rather strong magnetic field and with a weak direct heating of electrons. Furthermore, they require a much stronger source of seed photons than considered in previous studies. The nonthermal synchrotron radiation of relativistic electrons seems to be the most likely process capable of providing a sufficient flux of seed photons. Hadronic processes, which should occur due to basic properties of hot flows, provide an attractive explanation for the origin of such electrons.

  8. Dynamo magnetic-field generation in turbulent accretion disks

    Science.gov (United States)

    Stepinski, T. F.

    1991-01-01

    Magnetic fields can play important roles in the dynamics and evolution of accretion disks. The presence of strong differential rotation and vertical density gradients in turbulent disks allows the alpha-omega dynamo mechanism to offset the turbulent dissipation and maintain strong magnetic fields. It is found that MHD dynamo magnetic-field normal modes in an accretion disk are highly localized to restricted regions of a disk. Implications for the character of real, dynamically constrained magnetic fields in accretion disks are discussed. The magnetic stress due to the mean magnetic field is found to be of the order of a viscous stress. The dominant stress, however, is likely to come from small-scale fluctuating magnetic fields. These fields may also give rise to energetic flares above the disk surface, providing a possible explanation for the highly variable hard X-ray emission from objects like Cyg X-l.

  9. Peculiarities of the Accretion Flow in the System HL CMa

    CERN Document Server

    Semena, Andrey; Buckley, David; Lutovinov, Alexander; Breytenbach, Hannes

    2016-01-01

    The properties of the aperiodic brightness variability for the dwarf nova HL CMa are considered. The variability of the system HL CMa is shown to be suppressed at frequencies above $7\\times10^{-3}$Hz. Different variability suppression mechanisms related to the radiation reprocessing time, partial disk evaporation, and characteristic variability formation time are proposed. It has been found that the variability suppression frequency does not change when the system passes from the quiescent state to the outburst one, suggesting that the accretion flow geometry is invariable. It is concluded from the optical and X-ray luminosities of the system that the boundary layer on the white dwarf surface is optically thick in both quiescent and outburst states. The latter implies that the optically thick part of the accretion flow (disk) reaches the white dwarf surface. The accretion rate in the system, the flow geometry and temperature have been estimated from the variability power spectra and spectral characteristics i...

  10. Theoretical Models of Superbursts on Accreting Neutron Stars

    CERN Document Server

    Cooper, R L; Cooper, Randall L.; Narayan, Ramesh

    2004-01-01

    We carry out a general-relativistic global linear stability analysis of the amassed carbon fuel on the surface of an accreting neutron star to determine the conditions under which superbursts occur. By comparing our results with observations, we are able to set constraints on neutron star parameters such as the stellar radius and neutrino cooling mechanism in the core, as well as the composition of the ashes where superbursts are triggered. Specifically, we find that accreting neutron stars with ordered crusts and highly efficient neutrino emission in their cores produce extremely energetic superbursts which are inconsistent with observations. Also, because of pycnonuclear burning of carbon, they do not have superbursts in the range of accretion rates at which superbursts are actually observed. Stars with less efficient neutrino emission produce bursts that agree better with observations. Stars with highly inefficient neutrino emission in their cores produce bursts that agree best with observations. Furthermo...

  11. Accretion of jet streams and formation of asteroids

    Science.gov (United States)

    Hu, Zhong-wei; Tong, Yi

    1983-03-01

    Our basic view on the formation of asteroids, stated in [1], is that the initial physical and chemical conditions in the asteroid region led to a slow growth of planetesimals in the region and a transfer of accretable matter to the Jupitor region, resulting in the planetesimals stopping at the "half-finished" stage, eventually forming only asteroids and not major planets. In this paper, using the conditions of the nebular disk obtained in that paper and the formula for gravitational instability and regarding the rings resulting from gravitational instability as "jet streams", we apply the theory of accretion of jet streams to calculate the growth of the planetesimals and discuss the question of the transfer of accretable material, providing further confirmation of our basic view.

  12. The Hikurangi Plateau: Tectonic Ricochet and Accretion

    Science.gov (United States)

    Willis, David; Moresi, Louis; Betts, Peter; Whittaker, Joanne

    2015-04-01

    80 million years between interactions with different subduction systems provided time for the Hikurangi Plateau and Pacific Ocean lithosphere to cool, densify and strengthen. Neogene subduction of the Hikurangi Plateau occurring orthogonal to its Cretaceous predecessor, provides a unique opportunity to explore how changes to the physical properties of oceanic lithosphere affect subduction dynamics. We used Underworld to build mechanically consistent collision models to understand the dynamics of the two Hikurangi collisions. The Hikurangi Plateau is a ~112 Ma, 15km thick oceanic plateau that has been entrained by subduction zones immediately preceding the final break-up of Eastern Gondwana and currently within the active Hikurangi Margin. We explore why attempted subduction of the plateau has resulted in vastly different dynamics on two separate occasions. Slab break-off occured during the collision with Gondwana, currently there is apparent subduction of the plateau underneath New Zealand. At ~100Ma the young, hot Hikurangi Plateau, positively buoyant with respect to the underlying mantle, impacted a Gondwana Margin under rapid extension after the subduction of an mid-ocean ridge 10-15Ma earlier. Modelling of plateaus within young oceanic crust indicates that subduction of the thickened crust was unlikely to occur. Frontal accretion of the plateau and accompanying slab break-off is expected to have occured rapidly after its arrival. The weak, young slab was susceptible to lateral propagation of the ~1500 km window opened by the collision, and break-off would have progressed along the subduction zone inhibiting the "step-back" of the trench seen in older plates. Slab break-off coincided with a world-wide reorganisation of plate velocites, and orogenic collapse along the Gondwana margin characterised by rapid extension and thinning of the over-riding continental plate from ~60 to 30km. Following extension, Zealandia migrated to the NW until the Miocene allowing the

  13. 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.

  14. Disk Accretion of Tidally Disrupted Rocky Bodies onto White Dwarfs

    Science.gov (United States)

    Feng, Wanda; Desch, Steven; Turner, Neal; Kalyaan, Anusha

    2016-06-01

    About 1/3 of white dwarfs (WDs) are polluted with heavy elements (e.g., Koester et al., 2014; Zuckerman et al., 2010) that should sediment out of their atmospheres on astronomically short timescales unless replenished by accretion from a reservoir, at rates that for many WDs must exceed ~1010 g/s (Farihi et al., 2010). Direct accretion of planetesimals is too improbable and Poynting-Robertson drag of dust is too slow (due to the low luminosity of WDs) (Jura, 2003), so it is often assumed that WDs accrete from a disk of gas and solid particles, fed by tidal disruption of planeteismals inside the WD Roche limit (e.g. Debes et al., 2012; Rafikov, 2011a, 2011b). A few such gaseous disks have been directly observed, through emission from Ca II atoms in the disk (e.g. Manser et al., 2016; Wilson et al. 2014). Models successfully explain the accretion rates of metals onto the WD, provided the gaseous disk viscously spreads at rates consistent with a partially suppressed magnetorotational instability (Rafikov, 2011a, 2011b). However, these models currently do not explore the likely extent of the magnetorotational instability in disks by calculating the degree of ionization, or suppression by strong magnetic field.We present a 1-D model of a gaseous WD disk accretion, to assess the extent of the magnetorotational instability in WD disks. The composition of the disk, the ionization and recombination mechanisms, and the degree of ionization of the disk are explored. Magnetic field strengths consistent with WD dipolar magnetic fields are assumed. Elsasser numbers are calculated as a function of radius in the WD disk. The rate of viscous spreading is calculated, and the model of Rafikov (2011a, 2011b) updated to compute likely accretion rates of metals onto WDs.

  15. Subduction erosion and accretion in the Solomon Sea region

    Science.gov (United States)

    Honza, Eiichi; Miyazaki, Teruki; Lock, Jo

    1989-03-01

    The Solomon Sea region is an area of intense tectonic activity characterized by structural complexity, a high level of seismicity and volcanism, and rapid evolution of plate boundaries. There is little accretion in the eastern New Britain Trench. Accretion gradually increases westward with thick accretion in the western New Britain Trench and in the Trobriand Subduction System. The thick accretion in the western part of the New Britain Trench may be a result of collision from the north of Finisterre-Huon block with New Guinea mainland. The present boundary of the collision is along the Ram-Markham fault. Deformation structures and present day seismicity suggest that the northern block is under compression. Accretion has occurred in the sediment filled trenches in the Solomon Sea. The scale of the accretionary wedge depends on the amount of trench-fill sediment available. It is unlikely that there is no sediment supply to the eastern part of the New Britain Trench where no accretion is observed and subduction erosion may be occurring. There are two possible mechanisms for subduction erosion of sediment; either a rapid rate of subduction relative to the supply of sediment inhibiting sediment accumulation in the trench; or horizontal tensional force superimposed on both the forearc and backarc regions of the arc. Seafloor spreading in both the Manus and Woodlark basins is fan-like with nearby poles in the western margins of the basins. This may be a reflection of a horizontally compressional field in the western part and a tensional field in the eastern part of the Solomon Sea. Therefore it is possible to conclude that the consumption of sediment in the eastern New Britain Trench is related to the horizontal tensional field superimposed on both the forearc and backarc regions of the subduction system. Imbricated thrust and overthrust faults in the western New Britain Trench and Trobriand Trough are not linear over long distance, but form wavy patterns in blocks with

  16. On the Maximum Mass of Accreting Primordial Supermassive Stars

    Science.gov (United States)

    Woods, T. E.; Heger, Alexander; Whalen, Daniel J.; Haemmerlé, Lionel; Klessen, Ralf S.

    2017-06-01

    Supermassive primordial stars are suspected to be the progenitors of the most massive quasars at z ˜ 6. Previous studies of such stars were either unable to resolve hydrodynamical timescales or considered stars in isolation, not in the extreme accretion flows in which they actually form. Therefore, they could not self-consistently predict their final masses at collapse, or those of the resulting supermassive black hole seeds, but rather invoked comparison to simple polytropic models. Here, we systematically examine the birth, evolution, and collapse of accreting, non-rotating supermassive stars under accretion rates of 0.01-10 M ⊙ yr-1 using the stellar evolution code Kepler. Our approach includes post-Newtonian corrections to the stellar structure and an adaptive nuclear network and can transition to following the hydrodynamic evolution of supermassive stars after they encounter the general relativistic instability. We find that this instability triggers the collapse of the star at masses of 150,000-330,000 M ⊙ for accretion rates of 0.1-10 M ⊙ yr-1, and that the final mass of the star scales roughly logarithmically with the rate. The structure of the star, and thus its stability against collapse, is sensitive to the treatment of convection and the heat content of the outer accreted envelope. Comparison with other codes suggests differences here may lead to small deviations in the evolutionary state of the star as a function of time, that worsen with accretion rate. Since the general relativistic instability leads to the immediate death of these stars, our models place an upper limit on the masses of the first quasars at birth.

  17. Super-Eddington accretion on to a magnetized neutron star

    Science.gov (United States)

    Chashkina, Anna; Abolmasov, Pavel; Poutanen, Juri

    2017-09-01

    Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small Eddington limit. The accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. We propose a model of a nearly standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. Our calculations show that the size of the magnetosphere may be represented as the classical Alfvén radius times a dimensionless factor ξ which depends only on the disc thickness. In the case of radiation-pressure-dominated disc, the size of the magnetosphere does not depend on the mass accretion rate. In general, increasing the disc thickness leads to a larger magnetosphere size in units of the Alfvén radius. For large enough mass accretion rates and magnetic moments, it is important to take into account not only the pressure of the magnetic field and the radiation pressure inside the disc, but also the pressure of the radiation produced close to the surface of the neutron star in accretion column. The magnetospheric size may increase by up to factor of 2 as a result of the effects related to the disc thickness and the irradiation from the central source. Accounting for these effects reduces the estimate of the neutron star magnetic moment by a factor of several orders.

  18. Accretion onto some well-known regular black holes

    Energy Technology Data Exchange (ETDEWEB)

    Jawad, Abdul; Shahzad, M.U. [COMSATS Institute of Information Technology, Department of Mathematics, Lahore (Pakistan)

    2016-03-15

    In this work, we discuss the accretion onto static spherically symmetric regular black holes for specific choices of the equation of state parameter. The underlying regular black holes are charged regular black holes using the Fermi-Dirac distribution, logistic distribution, nonlinear electrodynamics, respectively, and Kehagias-Sftesos asymptotically flat regular black holes. We obtain the critical radius, critical speed, and squared sound speed during the accretion process near the regular black holes. We also study the behavior of radial velocity, energy density, and the rate of change of the mass for each of the regular black holes. (orig.)

  19. Fountain-driven gas accretion by the Milky Way

    Directory of Open Access Journals (Sweden)

    Ciotti L.

    2012-02-01

    Full Text Available Accretion of fresh gas at a rate of ∼ 1M☉yr−1 is necessary in star-forming disc galaxies, such as the Milky Way, in order to sustain their star-formation rates. In this work we present the results of a new hydrodynamic simulation supporting the scenario in which the gas required for star formation is drawn from the hot corona that surrounds the star-forming disc. In particular, the cooling of this hot gas and its accretion on to the disc are caused by the passage of cold galactic fountain clouds through the corona.

  20. Retrograde binaries of massive black holes in circumbinary accretion discs

    Science.gov (United States)

    Amaro-Seoane, Pau; Maureira-Fredes, Cristián; Dotti, Massimo; Colpi, Monica

    2016-06-01

    Context. We explore the hardening of a massive black hole binary embedded in a circumbinary gas disc under a specific circumstance: when the binary and the gas are coplanar and the gas is counter-rotating. The binary has unequal mass and the interaction of the gas with the lighter secondary black hole is the main cause of the braking torque on the binary that shrinks with time. The secondary black hole, revolving in the direction opposite to the gas, experiences a drag from gas-dynamical friction and from direct accretion of part of it. Aims: In this paper, using two-dimensional (2D) hydrodynamical grid simulations we investigate the effect of changing the accretion prescriptions on the dynamics of the secondary black hole, which in turn affect the binary hardening and eccentricity evolution. Methods: We find that realistic accretion prescriptions lead to results that differ from those inferred assuming accretion of all the gas within the Roche Lobe of the secondary black hole. Results: When considering gas accretion within the gravitational influence radius of the secondary black hole (which is smaller than the Roche Lobe radius) to better describe gas inflows, the shrinking of the binary is slower. In addition, in this case, a smaller amount of accreted mass is required to reduce the binary separation by the same amount. Different accretion prescriptions result in different discs' surface densities, which alter the black hole's dynamics back. Full 3D Smoothed-particle hydrodynamics realizations of a number of representative cases, run over a shorter interval of time, validate the general trends observed in the less computationally demanding 2D simulations. Conclusions: Initially circular black hole binaries increase their eccentricity only slightly, which then oscillates around small values (<0.1) while they harden. By contrast, initially eccentric binaries become more and more eccentric. A semi-analytical model describing the black hole's dynamics under

  1. The evolution of the core mass function by gas accretion

    CERN Document Server

    Dib, Sami

    2012-01-01

    We show how the mass function of dense cores (CMF) which results from the gravoturbulent fragmentation of a molecular cloud evolves in time under the effect of gas accretion. Accretion onto the cores leads to the formation of larger numbers of massive cores and to a flattening of the CMF. This effect should be visible in the CMF of star forming regions that are massive enough to contain high mass cores and when comparing the CMF of cores in and off dense filaments which have different environmental gas densities.

  2. Self-collimated axial jets from thin accretion disks

    CERN Document Server

    Tirabassi, Giulio; Carlevaro, Nakia; Benini, Riccardo

    2012-01-01

    We show how an appropriate stationary crystalline structure of the magnetic field can induce a partial fragmentation of the accretion disk, generating an axial jet composed of hot rising plasma twisted in a funnel-like structure by the rotation of the system. The most important feature of the obtained jet is its high degree of collimation, naturally arising from the condition for its existence. The presence of non-zero dissipative effects allows the plasma ejection throughout the axial jet and the predicted values of the accretion rate are in agreement with observations.

  3. Transonic properties of the accretion disk around compact objects

    CERN Document Server

    Mukhopadhyay, Banibrata

    2008-01-01

    An accretion flow is necessarily transonic around a black hole. However, around a neutron star it may or may not be transonic, depending on the inner disk boundary conditions influenced by the neutron star. I will discuss various transonic behavior of the disk fluid in general relativistic (or pseudo general relativistic) framework. I will address that there are four types of sonic/critical point possible to form in an accretion disk. It will be shown that how the fluid properties including location of sonic points vary with angular momentum of the compact object which controls the overall disk dynamics and outflows.

  4. A Toy Model for Advection Dominated Accretion Flows

    Institute of Scientific and Technical Information of China (English)

    汪定雄; 雷卫华; 肖看

    2003-01-01

    A toy disc model with advection dominated accretion on to a black hole is presented. The advection dominated accretion flows (ADAFs) are assumed to exist in the inner thick disc with rin < r < rout, and the disc is assumed to be geometrically thin for r > rout. Compared with Paczynski's toy model the thick disc is not limited to be 100% advective. It turns out that the inner radius rin depends not only on the outer radius rout but also on the ADAF parameters f and ε. The effects of the inner thick disc on the radiation efficiency and the temperature profile of the outer thin disc are discussed in details.

  5. The diffusion of contaminant through an accretion disc

    Science.gov (United States)

    Clarke, C. J.; Pringle, J. E.

    1988-01-01

    The manner in which a trace contaminant diffuses through an accretion disk is calculated. For a steady disk in which the ratio of diffusivity to viscosity, zeta, is taken to be constant through the disk and for which the surface density distribution is taken to be a power law, analytic solutions to the time-dependent equations governing contaminant evolution are presented. The amount of contaminant which can diffuse its way outwards against the bulk inward accretion flow is calculated, and it is shown that this is a sensitive function of a and zeta.

  6. Accretion processes for general spherically symmetric compact objects

    Energy Technology Data Exchange (ETDEWEB)

    Bahamonde, Sebastian [University College London, Department of Mathematics, London (United Kingdom); Jamil, Mubasher [National University of Sciences and Technology (NUST), H-12, Department of Mathematics, School of Natural Sciences (SNS), Islamabad (Pakistan)

    2015-10-15

    We investigate the accretion process for different spherically symmetric space-time geometries for a static fluid. We analyze this procedure using the most general black hole metric ansatz. After that, we examine the accretion process for specific spherically symmetric metrics obtaining the velocity of the sound during the process and the critical speed of the flow of the fluid around the black hole. In addition, we study the behavior of the rate of change of the mass for each chosen metric for a barotropic fluid. (orig.)

  7. Accretion Processes for General Spherically Symmetric Compact Objects

    CERN Document Server

    Bahamonde, Sebastian

    2015-01-01

    We investigate the accretion process for different spherically symmetric space-time geometries for a static fluid. We analyse this procedure using the most general black hole metric ansatz. After that, we examine the accretion process for specific spherically symmetric metrics obtaining the velocity of the sound during the process and the critical speed of the flow of the fluid around the black hole. In addition, we study the behaviour of the rate of change of the mass for each chosen metric for a barotropic fluid.

  8. Accretion onto Some Well-Known Regular Black Holes

    CERN Document Server

    Jawad, Abdul

    2016-01-01

    In this work, we discuss the accretion onto static spherical symmetric regular black holes for specific choices of equation of state parameter. The underlying regular black holes are charged regular black hole using Fermi-Dirac Distribution, logistic distribution, nonlinear electrodynamics, respectively and Kehagias-Sftesos asymptotically flat regular black hole. We obtain the critical radius, critical speed and squared sound speed during the accretion process near the regular black holes. We also study the behavior of radial velocity, energy density and rate of change of mass for each regular black holes.

  9. Probing the magnetic field structure in Sgr A* on Black Hole Horizon Scales with Polarized Radiative Transfer Simulations

    CERN Document Server

    Gold, Roman; Johnson, Michael D; Doeleman, Sheperd S

    2016-01-01

    Magnetic fields are believed to drive accretion and relativistic jets in black hole accretion systems, but the magnetic-field structure that controls these phenomena remains uncertain. We perform general relativistic (GR) polarized radiative transfer of time-dependent three-dimensional GR magnetohydrodynamical (MHD) simulations to model thermal synchrotron emission from the Galactic Center source Sagittarius A$^\\ast$ (Sgr A*). We compare our results to new polarimetry measurements by the Event Horizon Telescope (EHT) and show how polarization in the visibility (Fourier) domain distinguishes and constrains accretion flow models with different magnetic field structures. These include models with small-scale fields in disks driven by the magnetorotational instability (MRI) as well as models with large-scale ordered fields in magnetically-arrested disks (MAD). We also consider different electron temperature and jet mass-loading prescriptions that control the brightness of the disk, funnel-wall jet, and Blandford-...

  10. A Translational Polarization Rotator

    CERN Document Server

    Chuss, David T; Pisano, Giampaolo; Ackiss, Sheridan; U-Yen, Kongpop; Ng, Ming wah

    2012-01-01

    We explore a free-space polarization modulator in which a variable phase introduction between right- and left-handed circular polarization components is used to rotate the linear polarization of the outgoing beam relative to that of the incoming beam. In this device, the polarization states are separated by a circular polarizer that consists of a quarter-wave plate in combination with a wire grid. A movable mirror is positioned behind and parallel to the circular polarizer. As the polarizer-mirror distance is separated, an incident linear polarization will be rotated through an angle that is proportional to the introduced phase delay. We demonstrate a prototype device that modulates Stokes Q and U over a 20% bandwidth.

  11. Polarized Light Corridor Demonstrations.

    Science.gov (United States)

    Davies, G. R.

    1990-01-01

    Eleven demonstrations of light polarization are presented. Each includes a brief description of the apparatus and the effect demonstrated. Illustrated are strain patterns, reflection, scattering, the Faraday Effect, interference, double refraction, the polarizing microscope, and optical activity. (CW)

  12. NESDIS VIIRS Polar Winds

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — This dataset contains the Level 3 Polar Winds Northern and Southern Hemisphere datasets. The Level 3 Polar Winds data from VIIRS for the Arctic and Antarctic from 65...

  13. Molecule sublimation as a tracer of protostellar accretion: Evidence for accretion bursts from high angular resolution C18O images

    CERN Document Server

    Jorgensen, Jes K; Williams, Jonathan P; Bergin, Edwin A

    2015-01-01

    The accretion histories of embedded protostars are an integral part of descriptions of their physical and chemical evolution. In particular, are the accretion rates smoothly declining from the earlier toward later stages or in fact characterized by variations such as intermittent bursts? We aim to characterize the impact of possible accretion variations in a sample of embedded protostars by measuring the size of the inner regions of their envelopes where CO is sublimated and relate those to their temperature profiles dictated by their current luminosities. Using observations from the Submillimeter Array we measure the extents of the emission from the C18O isotopologue toward 16 deeply embedded protostars. We compare these measurements to the predicted extent of the emission given the current luminosities of the sources through dust and line radiative transfer calculations. Eight out of sixteen sources show more extended C18O emission than predicted by the models. The modeling shows that the likely culprit for...

  14. Timing of deformation and accretion of the Antimonio terrane, Sonora, from paleomagnetic data

    Science.gov (United States)

    Molina Garza, Roberto S.; Geissman, John W.

    1996-12-01

    At present, few paleomagnetic data exist to test tectonic models that link northern Mexico with the evolution of the southern Cordillera of North America. We present new paleomagnetic data for the Antimonio terrane in northwest Mexico. A moderate-inclination, dual-polarity yet secondary magnetization in Triassic-Jurassic strata of the Antimonio Formation and a primary remanence in Cretaceous igneous rocks (combined result: D = 338.9°, I = 57.4° α95 = 5.6°, k = 34.8; N = 20 sites) are interpreted to indicate stability of the Antimonio terrane with respect to the craton since the Late Cretaceous. A more important result, however, is a shallow-inclination, dual-polarity magnetization in Antimonio Formation strata that fails a fold test (D = 186.7°, I = -28.4°; α95 = 7.6°, k = 28.2; N = 14 sites). We interpret the shallow-inclination magnetization as a secondary remanence acquired during accretion of the Antimonio terrane to the North American margin in the Middle or Late Jurassic, or during deformation within an essentially autochthonous Jurassic continental arc.

  15. Plasma polarization spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Fujimoto, Takashi [Department of Engineering Physics and Mechanics, Graduate School of Engineering, Kyoto University, Kyoto (Japan)

    2000-03-01

    Polarization of radiation emitted from a plasma reflects the anisotropic properties of the plasma, especially the angular anisotropic distribution of electron velocities. Polarization has been observed on impurity ion lines from the WT-3 tokamak and the GAMMA-10 tandem mirror machines. The soft x-ray laser line from the neonlike germanium was also found polarized. (author)

  16. Polarity at Many Levels

    Science.gov (United States)

    Flannery, Maura C.

    2004-01-01

    An attempt is made to find how polarity arises and is maintained, which is a central issue in development. It is a fundamental attribute of living things and cellular polarity is also important in the development of multicellular organisms and controversial new work indicates that polarization in mammals may occur much earlier than previously…

  17. Geographical Income Polarization

    DEFF Research Database (Denmark)

    Azhar, Hussain; Jonassen, Anders Bruun

    In this paper we estimate the degree, composition and development of geographical income polarization based on data at the individual and municipal level in Denmark from 1984 to 2002. Rising income polarization is reconfirmed when applying new polarization measures, the driving force being greater...

  18. Graphing Polar Curves

    Science.gov (United States)

    Lawes, Jonathan F.

    2013-01-01

    Graphing polar curves typically involves a combination of three traditional techniques, all of which can be time-consuming and tedious. However, an alternative method--graphing the polar function on a rectangular plane--simplifies graphing, increases student understanding of the polar coordinate system, and reinforces graphing techniques learned…

  19. The Residual Polar Caps of Mars: Geological Differences and Possible Consequences

    Science.gov (United States)

    Thomas, P. C.; Sullivan, R.; Ingersoll, A. P.; Murray, B. C.; Danielson, G. E.; Herkenhoff, K. E.; Soderblom, L.; Malin, M. C.; Edgett, K. S.; James, P. B.

    2000-01-01

    The Martian polar regions have been known to have thick layered sequences (presumed to consist of silicates and ice), CO2 seasonal frost, and residual frosts that remain through the summer: H2O in the north, largely CO2 in the south. The relationship of the residual frosts to the underlying layered deposits could not be determined from Viking images. The Mars Orbiter Camera on Mars Global Surveyor has provided a 50-fold increase in resolution that shows more differences between the two poles. The north residual cap surface has rough topography of pits, cracks, and knobs, suggestive of ablational forms. This topography is less than a few meters in height, and grades in to surfaces exposing the layers underneath. In contrast, the south residual cap has distinctive collapse and possibly ablational topography emplaced in four or more layers, each approx. two meters thick. The top surface has polygonal depressions suggestive of thermal contraction cracks. The collapse and erosional forms include circular and cycloidal depressions, long sinuous troughs, and nearly parallel sets of troughs. The distinctive topography occurs throughout the residual cap area, but not outside it. Unconformities exposed in polar layers, or other layered materials, do not approximate the topography seen on the south residual cap. The coincidence of a distinct geologic feature, several layers modified by collapse, ablation, and mass movement with the residual cap indicates a distinct composition and/or climate compared to both the remainder of the south polar layered units and those in the north.

  20. The multipolar magnetic fields of accreting pre-main-sequence stars: B at the inner disk, B along the accretion flow, and B at the accretion shock

    CERN Document Server

    Gregory, Scott G; Hussain, Gaitee A J

    2016-01-01

    Zeeman-Doppler imaging studies have revealed the complexity of the large-scale magnetic fields of accreting pre-main-sequence stars. All have multipolar magnetic fields with the octupole component being the dominant field mode for many of the stars studied thusfar. Young accreting stars with fully convective interiors often feature simple axisymmetric magnetic fields with dipole components of order a kilo-Gauss (at least those of mass $\\gtrsim0.5\\,{\\rm M}_\\odot$), while those with substantially radiative interiors host more complex non-axisymmetric magnetic fields with dipole components of order a few 0.1 kilo-Gauss. Here, via several simple examples, we demonstrate that i). in most cases, the dipole component alone can be used to estimate the disk truncation radius (but little else); ii) due the presence of higher order magnetic field components, the field strength in the accretion spots is far in excess of that expected if a pure dipole magnetic field is assumed. (Fields of $\\sim$6$\\,{\\rm kG}$ have been mea...

  1. ISHIKAWA ITERATIVE PROCESS FOR CONSTRUCTING SOLUTIONS OF m-ACCRETIVE OPERATOR EQUATIONS

    Institute of Scientific and Technical Information of China (English)

    曾六川

    2002-01-01

    The convergence of the Ishikawa iteration sequences with errors for constructing solutions of m-accretive operator equations is characterized. Moreover, the error estimates of approximate solutions for locally Lipschitzian and m-accretive operator equations are established.

  2. X-ray Polarization from Black Holes: GEMS Scientific White Paper

    CERN Document Server

    Schnittman, Jeremy; Baring, Matthew; Baumgartner, Wayne; Black, Kevin; Dotson, Jessie; Ghosh, Pranab; Harding, Alice; Hill, Joanne; Jahoda, Keith; Kaaret, Phillip; Kallman, Tim; Krawczynski, Henric; Krolik, Julian; Lai, Dong; Markwardt, Craig; Marshall, Herman; Martoff, Jeffrey; Morris, Robin; Okajima, Takashi; Petre, Robert; Poutanen, Juri; Reynolds, Stephen; Scargle, Jeffrey; Serlemitsos, Peter; Soong, Yang; Strohmayer, Tod; Swank, Jean; Tawara, Yuzuru; Tamagawa, Toru

    2013-01-01

    We present here a summary of the scientific goals behind the Gravity and Extreme Magnetism SMEX (GEMS) X-ray polarimetry mission's black hole (BH) observing program. The primary targets can be divided into two classes: stellar-mass galactic BHs in accreting binaries, and super-massive BHs in the centers of active galactic nuclei (AGN). The stellar-mass BHs can in turn be divided into various X-ray spectral states: thermal-dominant (disk), hard (radio jet), and steep power-law (hot corona). These different spectral states are thought to be generated by different accretion geometries and emission mechanisms. X-ray polarization is an ideal tool for probing the geometry around these BHs and revealing the specific properties of the accreting gas.

  3. Initial sizes of planetesimals and accretion of the asteroids

    Science.gov (United States)

    Weidenschilling, S. J.

    2011-08-01

    The present size frequency distribution (SFD) of bodies in the asteroid belt appears to have preserved some record of the primordial population, with an excess of bodies of diameter D ˜ 100 km relative to a simple power law. The survival of Vesta's basaltic crust also implies that the early SFD had a shallow slope in the range ˜10-100 km. (Morbidelli, A., Bottke, W.F., Nesvorny, D., Levison, H.F. [2009]. Icarus 204, 558-573) were unable to produce these features by accretion from an initial population of km-sized planetesimals. They concluded that bodies with sizes in the range ˜100-1000 km and a SFD similar to the current population were produced directly from solid particles of sub-meter scale, without experiencing accretion through intermediate sizes. We present results of new accretion simulations in the primordial asteroid region. The requisite SFD can be produced from an initial population of planetesimals of sizes ≲0.1 km, smaller than the usual assumption of km-sized bodies. The bump at D ˜ 100 km is produced by a transition from dispersion-dominated runaway growth to a regime dominated by Keplerian shear, before the formation of large protoplanetary embryos. Thus, accretion of the asteroids from an initial population of small (sub-km) planetesimals cannot be ruled out.

  4. Interpreting observations of edge-on gravitationally unstable accretion flows

    CERN Document Server

    Liu, Hauyu Baobab

    2016-01-01

    Gravitational collapse of molecular cloud or cloud core/clump may lead to the formation of geometrically flattened, rotating accretion flow surrounding the new born star or star cluster. Gravitational instability may occur in such accretion flow when the gas to stellar mass ratio is high (e.g. over $\\sim$10\\%). This paper takes the OB cluster-forming region G10.6-0.4 as an example. We introduce the enclosed gas mass around its central ultra compact (UC) H\\textsc{ii} region, addresses the gravitational stability of the accreting gas, and outline the observed potential signatures of gravitational instability. The position-velocity (PV) diagrams of various molecular gas tracers on G10.6-0.4 consistently show asymmetry in the spatial and the velocity domain. We deduce the morphology of the dense gas accretion flow by modeling velocity distribution of the azimuthally asymmetric gas structures, and by directly de-projecting the PV diagrams. We found that within the 0.3 pc radius, an infall velocity of 1-2 km\\,s$^{-...

  5. Accretion and destruction of planetesimals in turbulent disks

    CERN Document Server

    Ida, Shigeru; Morbidelli, Alessandro

    2008-01-01

    We study the conditions for collisions between planetesimals to be accretional or disruptive in turbulent disks, through analytical arguments based on fluid dynamical simulations and orbital integrations. In turbulent disks, the velocity dispersion of planetesimals is pumped up by random gravitational perturbations from density fluctuations of the disk gas. When the velocity dispersion is larger than the planetesimals' surface escape velocity, collisions between planetesimals do not result in accretion, and may even lead to their destruction. In disks with a surface density equal to that of the ``minimum mass solar nebula'' and with nominal MRI turbulence, we find that accretion proceeds only for planetesimals with sizes above $\\sim 300$ km at 1AU and $\\sim 1000$ km at 5AU. We find that accretion is facilitated in disks with smaller masses. However, at 5AU and for nominal turbulence strength, km-sized planetesimals are in a highly erosive regime even for a disk mass as small as a fraction of the mass of Jupit...

  6. Chemical tracers of episodic accretion in low-mass protostars

    CERN Document Server

    Visser, Ruud; Jorgensen, Jes K

    2015-01-01

    Aims: Accretion rates in low-mass protostars can be highly variable in time. Each accretion burst is accompanied by a temporary increase in luminosity, heating up the circumstellar envelope and altering the chemical composition of the gas and dust. This paper aims to study such chemical effects and discusses the feasibility of using molecular spectroscopy as a tracer of episodic accretion rates and timescales. Methods: We simulate a strong accretion burst in a diverse sample of 25 spherical envelope models by increasing the luminosity to 100 times the observed value. Using a comprehensive gas-grain network, we follow the chemical evolution during the burst and for up to 10^5 yr after the system returns to quiescence. The resulting abundance profiles are fed into a line radiative transfer code to simulate rotational spectra of C18O, HCO+, H13CO+, and N2H+ at a series of time steps. We compare these spectra to observations taken from the literature and to previously unpublished data of HCO+ and N2H+ 6-5 from th...

  7. Evolution of Thick Accretion Disks Produced by Tidal Disruption Events

    CERN Document Server

    Ulmer, A

    1997-01-01

    Geometrically thick disks may form after tidal disruption events, and rapid accretion may lead to short flares followed by long-term, lower-level emission. Using a novel accretion disk code which relies primarily on global conservation laws and the assumption that viscosity is everywhere positive, a broad range of physically allowed evolutionary sequences of thick disks is investigated. The main result is that accretion in the thick disk phase can consume only a fraction of the initial disk material before the disk cools and becomes thin. This fraction is ~0.5-0.9 for disruptions around 10^6 to 10^7 M_ødot black holes and is sensitive to the mean angular momentum of the disk. The residual material will accrete in some form of thin disk over a longer period of time. The initial thick disk phase may reduce the dimming timescale of the disk by a factor of ~2 from estimates based on thin disks alone. Assuming an 0.5 M_ødot initial thick disk, even if the thin disks become advection dominated, the black hole mas...

  8. Presupernova evolution of accreting white dwarfs with rotation

    CERN Document Server

    Yoon, S C

    2004-01-01

    We discuss the effects of rotation on the evolution of accreting carbon-oxygen white dwarfs, with the emphasis on possible consequences in Type Ia supernova (SN Ia) progenitors. Starting with a slowly rotating white dwarf, we simulate the accretion of matter and angular momentum from a quasi-Keplerian accretion disk. The role of the various rotationally induced hydrodynamic instabilities for the transport of angular momentum inside the white dwarf is investigated. We find that the dynamical shear instability is the most important one in the highly degenerate core. Our results imply that accreting white dwarfs rotate differentially throughout,with a shear rate close to the threshold value for the onset of the dynamical shear instability. As the latter depends on the temperature of the white dwarf, the thermal evolution of the white dwarf core is found to be relevant for the angular momentum redistribution. As found previously, significant rotation is shown to lead to carbon ignition masses well above 1.4 Msun....

  9. Accretion among preplanetary bodies : The many faces of runaway growth

    NARCIS (Netherlands)

    Ormel, C. W.; Dullemond, C. P.; Spaans, M.

    2010-01-01

    When preplanetary bodies reach proportions of similar to 1 km or larger in size, their accretion rate is enhanced due to gravitational focusing (GF). We have developed a new numerical model to calculate the collisional evolution of the gravitationally-enhanced growth stage. The numerical model is no

  10. Accretion among preplanetary bodies: The many faces of runaway growth

    NARCIS (Netherlands)

    Ormel, C. W.; Dullemond, C. P.; Spaans, M.

    2010-01-01

    When preplanetary bodies reach proportions of ˜1 km or larger in size, their accretion rate is enhanced due to gravitational focusing (GF). We have developed a new numerical model to calculate the collisional evolution of the gravitationally-enhanced growth stage. The numerical model is novel as it

  11. The Radiative Efficiency of Accretion Flows in Individual AGN

    CERN Document Server

    Davis, Shane W

    2010-01-01

    The radiative efficiency of AGN is commonly estimated based on the total mass accreted and the total AGN light emitted per unit volume in the universe integrated over time (the Soltan argument). In individual AGN, thin accretion disk model spectral fits can be used to deduce the absolute accretion rate Mdot, if the black hole mass M is known. The radiative efficiency {\\eta} is then set by the ratio of the bolometric luminosity L_bol to Mdot c^2. We apply this method to determine {\\eta} in a sample of 80 PG quasars with well determined L_bol, where Mdot is set by thin accretion disk model fits to the optical luminosity density, and the M determination based on the bulge stellar velocity dispersion (13 objects) or the broad line region (BLR). For the BLR-based masses, we derive a mean log {\\eta} = -1.05 +/- 0.52 consistent with the Soltan argument based estimates. We find a strong correlation of {\\eta} with M, rising from {\\eta} ~ 0.03 at M = 10^7 M{\\odot} and L/L_Edd ~ 1 to {\\eta} ~ 0.4 at M = 10^9 M{\\odot} an...

  12. Accretion Events in Binary Systems: AZ Cas and VV Cep

    CERN Document Server

    Gałan, C; Tomov, T; Wiȩcek, M; Majcher, A; Wychudzki, P; Świerczyński, E; Kolev, D; Brożek, T; Maciejewski, G; Zoła, S; Kurpińska-Winiarska, M; Winiarski, M; Ogłoza, W; Drożdż, M; Krzesiński, J

    2011-01-01

    The sudden lengthening of orbital period of VV Cep eclipsing binary by about 1% was observed in the last epoch. The mass transfer and/or mass loss are most possible explanations of this event. The photometric behaviour of AZ Cas, the cousin of VV Cep, suggests that the accretion can occur and could be important in this system, too.

  13. Gas Accretion in the M32 Nucleus: Past & Present

    CERN Document Server

    Seth, Anil C

    2010-01-01

    Using adaptive optics assisted Gemini/NIFS data, I study the present and past gas accretion in the central 3" of the M32 nucleus. From changes in the spectral slope and CO line depths near the center, I find evidence for unresolved dust emission resulting from BH accretion. With a luminosity of ~2e38 erg/s, this dust emission appears to be the most luminous tracer of current BH accretion, two orders of magnitude more luminous than previously detected X-ray emission. These observations suggest that using high resolution infrared data to search for dust emission may be an effective way to detect other nearby, low luminosity BHs, such as those in globular clusters. I also examine the fossil evidence of gas accretion contained in the kinematics of the stars in the nucleus. The higher-order moments (h3 and h4) of the line-of-sight velocity distribution show patterns that are remarkably similar to those seen on larger scales in elliptical galaxies and in gas-rich merger simulations. The kinematics suggests the pres...

  14. Constraints on Bygone Nucleosynthesis of Accreting Neutron Stars

    Science.gov (United States)

    Meisel, Zach; Deibel, Alex

    2017-03-01

    Nuclear burning near the surface of an accreting neutron star produces ashes that, when compressed deeper by further accretion, alter the star’s thermal and compositional structure. Bygone nucleosynthesis can be constrained by the impact of compressed ashes on the thermal relaxation of quiescent neutron star transients. In particular, Urca cooling nuclei pairs in nuclear burning ashes that cool the neutron star crust via neutrino emission from {e}--capture/{β }--decay cycles and provide signatures of prior nuclear burning over the ˜century timescales it takes to accrete to the {e}--capture depth of the strongest cooling pairs. Using crust cooling models of the accreting neutron star transient MAXI J0556-332, we show that this source likely lacked Type I X-ray bursts and superbursts ≳120 years ago. Reduced nuclear physics uncertainties in rp-process reaction rates and {e}--capture weak transition strengths for low-lying transitions will improve nucleosynthesis constraints using this technique.

  15. Viscosity Approximation Methods for Two Accretive Operators in Banach Spaces

    Directory of Open Access Journals (Sweden)

    Jun-Min Chen

    2013-01-01

    Full Text Available We introduced a viscosity iterative scheme for approximating the common zero of two accretive operators in a strictly convex Banach space which has a uniformly Gâteaux differentiable norm. Some strong convergence theorems are proved, which improve and extend the results of Ceng et al. (2009 and some others.

  16. Space manufacturing utilizing the directional electrostatic accretion process

    Science.gov (United States)

    Mortensen, A.

    1986-01-01

    The Directional Electrostatic Accretion Process (DEAP) is described with respect to both the physical process and its application to manufacturing in space. This high precision portable manufacturing method will revolutionize current practices in manufacturing and repair of spacecraft and space structures. The cost effectiveness of this process will be invaluable to future space manufacturing projects.

  17. The pattern of accretion flow onto Sgr A*

    CERN Document Server

    Czerny, M M T K D B

    2006-01-01

    The material accreting onto Sgr A* most probably comes from the nearby stars. We analyze the pattern of this flow at distances of a fraction of a parsec and we argue that the net angular momentum of this material is low but non-negligible, and the initially supersonic disk accretion changes into subsonic flow with constant angular momentum. Next we estimate the flow parameters at a distance $R_{BHL}$ from the black hole and we argue that for the plausible parameter range the accretion flow is non-stationary. The inflow becomes supersonic at distance of $\\sim 10^4 R_g$ but the solution does not continue below the horizon and the material piles up forming a torus, or a ring, at a distance of a few up to tens of Schwarzchild radii. Such a torus is known to be unstable and may explain strong variability of the flow in Sgr A*. Our considerations show that the temporary formation of such a torus seems to be unavoidable. Our best fitting model predicts a rather large accretion rate of around $4 \\cdot 10^{-6} M_{\\odo...

  18. Hot Radiative Accretion onto a Spinning Neutron Star

    CERN Document Server

    Medvedev, M V

    2004-01-01

    (Abridged) A new type of self-similar hot viscous radiative accretion flow onto a rapidly spinning neutron star has recently been discovered. This ``hot brake'' flow forms in the two-temperature zone (close to a central object), but at a sufficiently low accretion rate and a high spin it may extend in the radial direction beyond ~300 Schwarzchild radii into a one-temperature zone. When the spin of the star is small enough, the flow transforms smoothly to an advection-dominated accretion flow. All gas parameters (density, angular velocity, temperature, luminosity, angular momentum flux) except for the radial velocity are independent of the mass accretion rate. The radiative efficiency may be arbitrarily large as M-dot -> 0. The gas angular momentum is transported outward under most conditions, hence the central star is nearly always spun-down. The flow is convectively stable. We also find that themal conduction in the flow is strong enough to make the flow thermally stable. The very fact that the density, temp...

  19. On the X-ray heated skin of Accretion Disks

    CERN Document Server

    Nayakshin, S

    1999-01-01

    We present a simple analytical formula for the Thomson depth of the X-rayheated skin of accretion disks valid at any radius and for a broad range ofspectral indices of the incident X-rays, accretion rates and black hole masses.We expect that this formula may find useful applications in studies of geometryof the inner part of accretion flows around compact objects, and in severalother astrophysically important problems, such as the recently observed X-ray``Baldwin'' effect (i.e., monotonic decrease of Fe line's equivalent width withthe X-ray luminosity of AGN), the problem of missing Lyman edge in AGN, andline and continuum variability studies in accretion disks around compactobjects. We compute the reflected X-ray spectra for several representativecases and show that for hard X-ray spectra and large ionizing fluxes the skinrepresents a perfect mirror that does not produce any Fe lines or absorptionfeatures. At the same time, for soft X-ray spectra or small ionizing fluxes,the skin produces very strong ionized...

  20. Detecting gravitational wave emission from the known accreting neutron stars

    NARCIS (Netherlands)

    Watts, A.L.; Krishnan, B.; Bildsten, L.; Schutz, B.F.

    2008-01-01

    Detection of gravitational waves from accreting neutron stars (NSs) in our Galaxy, due to ellipticity or internal oscillation, would be a breakthrough in our understanding of compact objects and explain the absence of NSs rotating near the break-up limit. Direct detection, however, poses a formidabl