Accretion from an inhomogeneous medium

International Nuclear Information System (INIS)

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

1986-01-01

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

Time-dependent Models of Magnetospheric Accretion onto Young Stars

Energy Technology Data Exchange (ETDEWEB)

Robinson, C. E.; Espaillat, C. C. [Department of Astronomy, Boston University, 725 Commonwealth Avenue, Boston, MA 02215 (United States); Owen, J. E. [Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540 (United States); Adams, F. C., E-mail: connorr@bu.edu [Physics Department, University of Michigan, Ann Arbor, MI 48109 (United States)

2017-04-01

Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star’s surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner, and the source of this variability remains an open problem, particularly on the shortest (∼day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ∼day-long timescales (e.g., due to turbulence), this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star, the observed time dependence will be a rapid increase in accretion luminosity, followed by a slower decline, and could be an explanation for some of the short-period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated multi-dimensional simulations and observations.

Time-dependent Models of Magnetospheric Accretion onto Young Stars

International Nuclear Information System (INIS)

Robinson, C. E.; Espaillat, C. C.; Owen, J. E.; Adams, F. C.

2017-01-01

Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star’s surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner, and the source of this variability remains an open problem, particularly on the shortest (∼day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ∼day-long timescales (e.g., due to turbulence), this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star, the observed time dependence will be a rapid increase in accretion luminosity, followed by a slower decline, and could be an explanation for some of the short-period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated multi-dimensional simulations and observations.

Cyclotron Lines in Accreting Neutron Star Spectra

Science.gov (United States)

Wilms, Jörn; Schönherr, Gabriele; Schmid, Julia; Dauser, Thomas; Kreykenbohm, Ingo

2009-05-01

Cyclotron lines are formed through transitions of electrons between discrete Landau levels in the accretion columns of accreting neutron stars with strong (1012 G) magnetic fields. We summarize recent results on the formation of the spectral continuum of such systems, describe recent advances in the modeling of the lines based on a modification of the commonly used Monte Carlo approach, and discuss new results on the dependence of the measured cyclotron line energy from the luminosity of transient neutron star systems. Finally, we show that Simbol-X will be ideally suited to build and improve the observational database of accreting and strongly magnetized neutron stars.

Disk accretion onto magnetic T Tauri stars

International Nuclear Information System (INIS)

Koenigl, A.

1991-01-01

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

He stars and He-accreting CO white dwarfs

International Nuclear Information System (INIS)

Limongi, M.; Tornambe, A.

1991-01-01

He star models in the mass range 0.4-1.0 solar mass have been evolved until the red giant phase or, depending on their mass, until crystallization on the white-dwarf cooling sequence. Some of the degenerate structures obtained in these computations have been successively accreted at various He accretion rates in order to better define the fate of the accreting dwarf versus its mass and accretion rate for a fixed degeneracy level of the accreting dwarf. He stars have been further induced to transfer mass to a degenerate companion through Roche lobe overflow, in conditions of large gravitational wave radiation by the system. CO dwarfs in binary systems with He stars are found to experience a thermal behavior whose effects are such to locate the structure on the verge of obtaining a strong SN-like explosive event. 22 refs

Observational diagnostics of accretion on young stars and brown dwarfs

Science.gov (United States)

Stelzer, Beate; Argiroffi, Costanza

I present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of the emission, new insight obtained from XMM-Newton and Chandra observations has unveiled contributions from accretion and outflow processes to the X-ray emission from young stars. Their smaller siblings, the brown dwarfs, have been shown to undergo a T Tauri phase on the basis of optical/IR observations of disks and measurements of accretion rates. Most re-cently, first evidence was found for X-rays produced by accretion in a young brown dwarf, complementing the suspected analogy between stars and substellar objects.

On the Maximum Mass of Accreting Primordial Supermassive Stars

Energy Technology Data Exchange (ETDEWEB)

Woods, T. E.; Heger, Alexander [Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800 (Australia); Whalen, Daniel J. [Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Portsmouth PO1 3FX (United Kingdom); Haemmerlé, Lionel; Klessen, Ralf S. [Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische. Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg (Germany)

2017-06-10

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 {sub ⊙} yr{sup −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 {sub ⊙} for accretion rates of 0.1–10 M {sub ⊙} yr{sup −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.

The formation of stars by gravitational collapse rather than competitive accretion

Science.gov (United States)

Krumholz, Mark R.; McKee, Christopher F.; Klein, Richard I.

2005-11-01

There are two dominant models of how stars form. Under gravitational collapse, star-forming molecular clumps, of typically hundreds to thousands of solar masses (Msolar), fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems. In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (~0.5Msolar), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump. Competitive accretion models interpret brown dwarfs and free-floating planets as protostars ejected from star-forming clumps before they have accreted much mass; key predictions of this model are that such objects should lack disks, have high velocity dispersions, form more frequently in denser clumps, and that the mean stellar mass should vary within the Galaxy. Here we derive the rate of competitive accretion as a function of the star-forming environment, based partly on simulation, and determine in what types of environments competitive accretion can occur. We show that no observed star-forming region can undergo significant competitive accretion, and that the simulations that show competitive accretion do so because the assumed properties differ from those determined by observation. Our result shows that stars form by gravitational collapse, and explains why observations have failed to confirm predictions of the competitive accretion model.

Accretion Processes in Star Formation

DEFF Research Database (Denmark)

Küffmeier, Michael

for short-lived radionuclides that enrich the cloud as a result of supernova explosions of the massive stars allows us to analyze the distribution of the short-lived radionuclides around young forming stars. In contradiction to results from highly-idealized models, we find that the discrepancy in 26 Al...... that the accretion process of stars is heterogeneous in space, time and among different protostars. In some cases, disks form a few thousand years after stellar birth, whereas in other cases disk formation is suppressed due to efficient removal of angular momentum. Angular momentum is mainly transported outward...... with potentially observable fluctuations in the luminosity profile that are induced by variations in the accretion rate. Considering that gas inside protoplanetary disks is not fully ionized, I implemented a solver that accounts for nonideal MHD effects into a newly developed code framework called dispatch...

MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

Directory of Open Access Journals (Sweden)

Lii Patrick

2014-01-01

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

Compact objects and accretion disks

NARCIS (Netherlands)

Blandford, Roger; Agol, Eric; Broderick, Avery; Heyl, Jeremy; Koopmans, Leon; Lee, Hee-Won

2002-01-01

Recent developments in the spectropolarimetric study of compact objects, specifically black holes (stellar and massive) and neutron stars are reviewed. The lectures are organized around five topics: disks, jets, outflows, neutron stars and black holes. They emphasize physical mechanisms and are

Accretion from a clumpy massive-star wind in supergiant X-ray binaries

Science.gov (United States)

El Mellah, I.; Sundqvist, J. O.; Keppens, R.

2018-04-01

Supergiant X-ray binaries (SgXB) host a compact object, often a neutron star (NS), orbiting an evolved O/B star. Mass transfer proceeds through the intense line-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the NS. The subsequent accretion process on to the NS is responsible for the abundant X-ray emission from SgXB. They also display peak-to-peak variability of the X-ray flux by a factor of a few 10-100, along with changes in the hardness ratios possibly due to varying absorption along the line of sight. We use recent radiation-hydrodynamic simulations of inhomogeneities (a.k.a. clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. For this, we run 3D hydrodynamic simulations of the wind in the vicinity of the accretor to investigate the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the NS magnetosphere. In particular, we show that the impact of the wind clumps on the time variability of the intrinsic mass accretion rate is severely tempered by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line of sight and estimate the final effective variability of the column density and mass accretion rate for different orbital separations. Finally, we compare our results to the most recent analysis of the X-ray flux and the hardness ratio in Vela X-1.

Formation of primordial supermassive stars by rapid mass accretion

Energy Technology Data Exchange (ETDEWEB)

Hosokawa, Takashi; Yoshida, Naoki [Department of Physics and Research Center for the Early Universe, The University of Tokyo, Tokyo 113-0033 (Japan); Yorke, Harold W. [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Inayoshi, Kohei; Omukai, Kazuyuki, E-mail: takashi.hosokawa@phys.s.u-tokyo.ac.jp, E-mail: hosokwtk@gmail.com [Department of Physics, Kyoto University, Kyoto 606-8502 (Japan)

2013-12-01

Supermassive stars (SMSs) forming via very rapid mass accretion ( M-dot {sub ∗}≳0.1 M{sub ⊙} yr{sup −1}) could be precursors of supermassive black holes observed beyond a redshift of about six. Extending our previous work, here we study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10{sup 4–5} M {sub ☉}. Our stellar evolution calculations show that a star becomes supermassive while passing through the 'supergiant protostar' stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass until ≅ 100 AU for M {sub *} ≳ 10{sup 4} M {sub ☉}, after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 10{sup 4} K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M {sub *} ≳ 10{sup 5} M {sub ☉} can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10{sup 5} M {sub ☉}. An extremely massive black hole should form after the collapse of the inner core.

MN Lup: X-RAYS FROM A WEAKLY ACCRETING T TAURI STAR

International Nuclear Information System (INIS)

Günther, H. M.; Wolk, S. J.; Wolter, U.; Robrade, J.

2013-01-01

Young T Tauri stars (TTS) are surrounded by an accretion disk, which over time disperses due to photoevaporation, accretion, and possibly planet formation. The accretion shock on the central star produces an UV/optical veiling continuum, line emission, and X-ray signatures. As the accretion rate decreases, the impact on the central star must change. In this article we study MN Lup, a young star where no indications of a disk are seen in IR observations. We present XMM-Newton and VLT/UVES observations, some of them taken simultaneously. The X-ray data show that MN Lup is an active star with L X /L bol close to the saturation limit. However, we find high densities (n e > 3 × 10 10 cm –3 ) in the X-ray grating spectrum. This can be well fitted using an accretion shock model with an accretion rate of 2 × 10 –11 M ☉ yr –1 . Despite the simple Hα line profile which has a broad component, but no absorption signatures as typically seen on accreting TTS, we find rotational modulation in Ca II K and in photospheric absorption lines. These line profile modulations do not clearly indicate the presence of a localized hot accretion spot on the star. In the Hα line we see a prominence in absorption about 2R * above the stellar surface—the first of its kind on a TTS. MN Lup is also the only TTS where accretion is seen, but no dust disk is detected that could fuel it. We suggest that MN Lup presents a unique and short-lived state in the disk evolution. It may have lost its dust disk only recently and is now accreting the remaining gas at a very low rate.

Formation and pre-MS Evolution of Massive Stars with Growing Accretion

Science.gov (United States)

Maeder, A.; Behrend, R.

2002-10-01

We briefly describe the three existing scenarios for forming massive stars and emphasize that the arguments often used to reject the accretion scenario for massive stars are misleading. It is usually not accounted for the fact that the turbulent pressure associated to large turbulent velocities in clouds necessarily imply relatively high accretion rates for massive stars. We show the basic difference between the formation of low and high mass stars based on the values of the free fall time and of the Kelvin-Helmholtz timescale, and define the concept of birthline for massive stars. Due to D-burning, the radius and location of the birthline in the HR diagram, as well as the lifetimes are very sensitive to the accretion rate dM/dt(accr). If a form dM/dt(accr) propto A(M/Msun)phi is adopted, the observations in the HR diagram and the lifetimes support a value of A approx 10-5 Msun/yr and a value of phi > 1. Remarkably, such a law is consistent with the relation found by Churchwell and Henning et al. between the outflow rates and the luminosities of ultracompact HII regions, if we assume that a fraction 0.15 to 0.3 of the global inflow is accreted. The above relation implies high dM/dt(accr) approx 10-3 Msun/yr for the most massive stars. The physical possibility of such high dM/dt(accr) is supported by current numerical models. Finally, we give simple analytical arguments in favour of the growth of dM/dt(accr) with the already accreted mass. We also suggest that due to Bondi-Hoyle accretion, the formation of binary stars is largely favoured among massive stars in the accretion scenario.

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

Science.gov (United States)

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

2017-06-01

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

Limiting Accretion onto Massive Stars by Fragmentation-Induced Starvation

Energy Technology Data Exchange (ETDEWEB)

Peters, Thomas; /ZAH, Heidelberg; Klessen, Ralf S.; /ZAH, Heidelberg /KIPAC, Menlo Park; Mac Low, Mordecai-Mark; /Amer. Museum Natural Hist.; Banerjee, Robi; /ZAH, Heidelberg

2010-08-25

Massive stars influence their surroundings through radiation, winds, and supernova explosions far out of proportion to their small numbers. However, the physical processes that initiate and govern the birth of massive stars remain poorly understood. Two widely discussed models are monolithic collapse of molecular cloud cores and competitive accretion. To learn more about massive star formation, we perform simulations of the collapse of rotating, massive, cloud cores including radiative heating by both non-ionizing and ionizing radiation using the FLASH adaptive mesh refinement code. These simulations show fragmentation from gravitational instability in the enormously dense accretion flows required to build up massive stars. Secondary stars form rapidly in these flows and accrete mass that would have otherwise been consumed by the massive star in the center, in a process that we term fragmentation-induced starvation. This explains why massive stars are usually found as members of high-order stellar systems that themselves belong to large clusters containing stars of all masses. The radiative heating does not prevent fragmentation, but does lead to a higher Jeans mass, resulting in fewer and more massive stars than would form without the heating. This mechanism reproduces the observed relation between the total stellar mass in the cluster and the mass of the largest star. It predicts strong clumping and filamentary structure in the center of collapsing cores, as has recently been observed. We speculate that a similar mechanism will act during primordial star formation.

Neutron star accretion and the neutrino fireball

International Nuclear Information System (INIS)

Colgate, S.A.; Herant, M.E.; Benz, W.

1991-01-01

The mixing necessary to explain the ''Fe'' line widths and possibly the observed red shifts of 1987A is explained in terms of large scale, entropy conserving, up and down flows (calculated with a smooth particle 2-D code) taking place between the neutron star and the explosion shock wave due to the gravity and neutrino deposition. Depending upon conditions of entropy and mass flux further accretion takes place in single events, similar to relaxation oscillator, fed by the downward flows of low entropy matter. The shock, in turn, is driven by the upflow of the buoyant high entropy bubbles. Some accretion events will reach a temperature high enough to create a neutrino ''fireball,'' a region hot enough, 11 Mev, so as to be partially opaque to its own (neutrino) radiation. The continuing neutrino deposition drives the explosion shock until the entropy of matter flowing downwards onto the neutron star is high enough to prevent further accretion. This process should result in a robust supernova explosion

Accreting Millisecond Pulsars: Neutron Star Masses and Radii

Science.gov (United States)

Strohmayer, Tod

2004-01-01

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

Testing the deep-crustal heating model using quiescent neutron-star very-faint X-ray transients and the possibility of partially accreted crusts in accreting neutron stars

Science.gov (United States)

Wijnands, R.; Degenaar, N.; Page, D.

2013-07-01

It is assumed that accreting neutron stars in low-mass X-ray binaries are heated due to the compression of the existing crust by the freshly accreted matter which gives rise to a variety of nuclear reactions in the crust. It has been shown that most of the energy is released deep in the crust by pycnonuclear reactions involving low-Z elements (the deep-crustal heating scenario). In this paper we discuss if neutron stars in the so-called very-faint X-ray transients (VFXTs; those transients have outburst peak 2-10 keV X-ray luminosities short-term (less than a few tens of thousands of years) and the one throughout their lifetime. The latter is particularly important because it can be so low that the neutron stars might not have accreted enough matter to become massive enough that enhanced core cooling processes become active. Therefore, they could be relatively warm compared to other systems for which such enhanced cooling processes have been inferred. However, the amount of matter can also not be too low because then the crust might not have been replaced significantly by accreted matter and thus a hybrid crust of partly accreted and partly original, albeit further compressed matter, might be present. This would inhibit the full range of pycnonuclear reactions to occur and therefore possibly decrease the amount of heat deposited in the crust. More detailed calculations of the heating and cooling properties of such hybrid crusts have to be performed to be conclusive. Furthermore, better understanding is needed about how a hybrid crust affects other properties such as the thermal conductivity. A potentially interesting way to observe the effects of a hybrid crust on the heating and cooling of an accreting neutron star is to observe the crust cooling of such a neutron star after a prolonged (years to decades) accretion episode and compare the results with similar studies performed for neutron stars with a fully accreted crust. We also show that some individual neutron-star

Studies of accreting and non-accreting neutron stars

International Nuclear Information System (INIS)

Stollman, G.M.

1987-01-01

This thesis is divided into three parts. Part A is devoted to the statistical study of radio pulsars, in which the observations of nearly all known pulsars are used to study their properties such as magnetic field strengths, rotation periods, space velocities as well as their evolution in time. Part B is devoted to the modelling and understanding of quasi-periodic oscillations (QPO) in low-mass X-ray binaries. But, this study is mainly concerned with the accretion process in these sources, and one may hope to learn more about the neutron stars in these systems when the understanding of QPO is improved. In Part C the problem of 'super-Eddington luminosities' in X-ray burst sources is treated. The idea is that a good understanding of the burst process, which takes place directly at the surface of the neutron star, will eventually improve our understanding of the neutron stars themselves. (Auth.)

Generalised model for anisotropic compact stars

Energy Technology Data Exchange (ETDEWEB)

Maurya, S.K. [University of Nizwa, Department of Mathematical and Physical Sciences College of Arts and Science, Nizwa (Oman); Gupta, Y.K. [Raj Kumar Goel Institute of Technology, Department of Mathematics, Ghaziabad, Uttar Pradesh (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India); Deb, Debabrata [Indian Institute of Engineering Science and Technology, Shibpur, Department of Physics, Howrah, West Bengal (India)

2016-12-15

In the present investigation an exact generalised model for anisotropic compact stars of embedding class 1 is sought with a general relativistic background. The generic solutions are verified by exploring different physical aspects, viz. energy conditions, mass-radius relation, stability of the models, in connection to their validity. It is observed that the model presented here for compact stars is compatible with all these physical tests and thus physically acceptable as far as the compact star candidates RXJ 1856-37, SAX J 1808.4-3658 (SS1) and SAX J 1808.4-3658 (SS2) are concerned. (orig.)

Symbiotic stars - a binary model with super-critical accretion

Energy Technology Data Exchange (ETDEWEB)

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

1977-01-01

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

Are some of the luminous high-latitude stars accretion-powered runaways?

International Nuclear Information System (INIS)

Leonard, P.J.T.; Hills, J.G.; Dewey, R.J.

1992-01-01

It is well known that (1) runaway stars can be produced via supernova explosions in close binary systems, (2) most of such runaways should possess neutron star companions, and (3) neutron stars receive randomly oriented kicks of ≅ 100 to 200 km s -1 at birth. We find that this kick sometimes has the right amplitude and direction to make the neutron star fall into the runaway. Accretion onto a neutron star is a source of energy that is roughly an order of magnitude more mass efficient than nuclear burning. Thus, runaways containing neutron stars may live much longer than would normally be expected, which would allow them to travel great distances from their birthplaces during their lifetimes. Some of the early B-type stars far from the Galactic plane and the high-latitude F and G-type supergiants may be accretion-powered runaway stars

Compact Stars with Sequential QCD Phase Transitions

Science.gov (United States)

Alford, Mark; Sedrakian, Armen

2017-10-01

Compact stars may contain quark matter in their interiors at densities exceeding several times the nuclear saturation density. We explore models of such compact stars where there are two first-order phase transitions: the first from nuclear matter to a quark-matter phase, followed at a higher density by another first-order transition to a different quark-matter phase [e.g., from the two-flavor color-superconducting (2SC) to the color-flavor-locked (CFL) phase]. We show that this can give rise to two separate branches of hybrid stars, separated from each other and from the nuclear branch by instability regions, and, therefore, to a new family of compact stars, denser than the ordinary hybrid stars. In a range of parameters, one may obtain twin hybrid stars (hybrid stars with the same masses but different radii) and even triplets where three stars, with inner cores of nuclear matter, 2SC matter, and CFL matter, respectively, all have the same mass but different radii.

SUPERNOVA LIGHT CURVES POWERED BY FALLBACK ACCRETION

Energy Technology Data Exchange (ETDEWEB)

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

2013-07-20

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

CONSTRAINTS ON THE NEUTRON STAR AND INNER ACCRETION FLOW IN SERPENS X-1 USING NuSTAR

Energy Technology Data Exchange (ETDEWEB)

Miller, J. M. [Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1046 (United States); Parker, M. L.; Fabian, A. C. [Institute of Astronomy, The University of Cambridge, Madingley Road, Cambridge CB3 OHA (United Kingdom); Fuerst, F.; Grefenstette, B. W.; Tendulkar, S.; Harrison, F. A.; Rana, V. [Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States); Bachetti, M.; Barret, D. [Universite de Toulouse, UPS-OMP, Toulouse (France); Boggs, S. E.; Craig, W. W.; Tomsick, J. A. [Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States); Chakrabarty, D. [Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Cambridge, MA 02139 (United States); Christensen, F. E. [Danish Technical University, Lyngby (Denmark); Hailey, C. J.; Paerels, F. [Columbia Astrophysics Laboratory and Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027 (United States); Natalucci, L. [Istituto di Astrofisica e Planetologia Spaziali (INAF), Via Fosso del Cavaliere 100, Roma I-00133 (Italy); Stern, D. K. [Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States); Zhang, W. W., E-mail: jonmm@umich.edu [NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)

2013-12-10

We report on an observation of the neutron star low-mass X-ray binary Serpens X-1, made with NuSTAR. The extraordinary sensitivity afforded by NuSTAR facilitated the detection of a clear, robust, relativistic Fe K emission line from the inner disk. A relativistic profile is required over a single Gaussian line from any charge state of Fe at the 5σ level of confidence, and any two Gaussians of equal width at the same confidence. The Compton back-scattering ''hump'' peaking in the 10-20 keV band is detected for the first time in a neutron star X-ray binary. Fits with relativistically blurred disk reflection models suggest that the disk likely extends close to the innermost stable circular orbit (ISCO) or stellar surface. The best-fit blurred reflection models constrain the gravitational redshift from the stellar surface to be z {sub NS} ≥ 0.16. The data are broadly compatible with the disk extending to the ISCO; in that case, z {sub NS} ≥ 0.22 and R {sub NS} ≤ 12.6 km (assuming M {sub NS} = 1.4 M {sub ☉} and a = 0, where a = cJ/GM {sup 2}). If the star is as large or larger than its ISCO, or if the effective reflecting disk leaks across the ISCO to the surface, the redshift constraints become measurements. We discuss our results in the context of efforts to measure fundamental properties of neutron stars, and models for accretion onto compact objects.

Cooling of Accretion-Heated Neutron Stars

Indian Academy of Sciences (India)

Rudy Wijnands

2017-09-12

Sep 12, 2017 ... the magnetic field might play an important role in the heating and cooling of the neutron stars. .... Source near Sgr A ..... marked the start of the research field that uses the cool- ... This curve is just to guide the eye for the individual sources and it is clear ..... Not all accretion-induced nuclear reactions might.

Magnetohydrodynamical processes near compact objects

International Nuclear Information System (INIS)

Bisnovatyi Kogan, G.S.

1979-01-01

Magnetohydrodynamical processes near compact objects are reviewed in this paper. First the accretion of the magnetized matter into a single black hole and spectra of radiation are considered. Then the magnetic-field phenomena in the disk accretion, when the black hole is in a pair are discussed. Furthermore, the magnetohydrodynamics phenomena during supernova explosion are considered. Finally the magnetohydrodynamics in the accretion of a neutron star is considered in connection With x-ray sources

Study of the high energy emission of accreting compact objects with SPI/INTEGRAL

International Nuclear Information System (INIS)

Droulans, R.

2011-01-01

The study of the high energy emission is essential for understanding the radiative processes inherent to accretion flows onto compact objects (black holes and neutron stars). The X/γ-ray continuum of these systems is successfully interpreted in terms of two components. The first component corresponds to blackbody emission from a geometrically thin optically thick accretion disk while the second component is generally associated to Compton scattering of the thermal disk flux off hot electrons. Despite considerable advances throughout the years, the heating mechanisms as well as the structure of the hot Comptonizing plasma remain poorly understood. In order to shed light on the physical processes that govern the innermost regions of the accretion flow, we take advantage of the data archive accumulated by the SPI instrument, a high energy spectrometer (20 keV - 8 MeV) developed at the CESR (now IRAP, Toulouse, France) for the INTEGRAL mission. Above 150 keV, SPI combines a unique spectral resolution with unequalled sensitivity, being thus an ideal tool to study the high energy emission of accreting compact objects. The thesis manuscript reports on the results of timing and spectral studies of three particular systems. First, I address the high energy emission of the enigmatic micro-quasar GRS 1915+105, a source characterized by colossal luminosity and strong chaotic variability in X-rays. On a timescale of about one day, the photon index of the 20 - 200 keV spectrum varies between 2.7 and 3.5; at higher energies (≥150 keV), SPI unveils the systematic presence of an additional emission component, extending without folding energy up to ∼ 500 keV. Second, I study the high energy emission of GX 339-4, a source whose spectral properties are representative of black hole transients. The spectrum of the luminous hard state of this system shows a variable high energy tail (≥150 keV), with significant flux changes on a short timescale (several hours). I explain the

Revisiting Field Burial by Accretion onto Neutron Stars

Indian Academy of Sciences (India)

Dipanjan Mukherjee

2017-09-12

Sep 12, 2017 ... review the recent work on magnetic confinement of accreted matter on neutron stars poles. We present ..... hours to days, see Brown & Bildsten 1998) where the ...... Radhakrishnan, V., Srinivasan, G. 1984, in: Second Asian-.

On hard X-ray spectra of accreting neutron stars

International Nuclear Information System (INIS)

Zheleznyakov, V.V.

1982-01-01

Formation of the spectra of X-ray pulsars and gamma bursters is investigated. Interpretation of a hard X-ray spectrum of pulsars containing cyclotron lines is feasible on the basis of an isothermal model of a polar spot heated due to acccretion to a neutron star. It has been ascertained that in the regions responsible for the formation of continuum radiation and lines the mode polarization is determined by a magnetized vacuum rather than by a plasma. Bearing this in mind, the influence of the magnetic field of a star on the wide wings of the cyclotron line and on its depth is discussed. The part played by the accreting column in the case of strong accretion (approx. equal to 10 19 el cm -3 ) needed for long sustaining of the high level of X-rays from a neutron star-pulsar is studied. There occur the gaps in spectrum at frequencies close to the electron gyro-frequency and its harmonics due to the screening of the hot spot by the opaque gyro-resonant layer located within the accreting column. These gaps ensure the formation of cyclotron lines in absorption irrespective of the presence of such lines in the X-ray spectrum of a polar hot spot. (orig./WL)

Self-similar Hot Accretion Flow onto a Neutron Star

Science.gov (United States)

Medvedev, Mikhail V.; Narayan, Ramesh

2001-06-01

We consider hot, two-temperature, viscous accretion onto a rotating, unmagnetized neutron star. We assume Coulomb coupling between the protons and electrons, as well as free-free cooling from the electrons. We show that the accretion flow has an extended settling region that can be described by means of two analytical self-similar solutions: a two-temperature solution that is valid in an inner zone, r~102.5. In both zones the density varies as ρ~r-2 and the angular velocity as Ω~r-3/2. We solve the flow equations numerically and confirm that the analytical solutions are accurate. Except for the radial velocity, all gas properties in the self-similar settling zone, such as density, angular velocity, temperature, luminosity, and angular momentum flux, are independent of the mass accretion rate; these quantities do depend sensitively on the spin of the neutron star. The angular momentum flux is outward under most conditions; therefore, the central star is nearly always spun down. The luminosity of the settling zone arises from the rotational energy that is released as the star is braked by viscosity, and the contribution from gravity is small; hence, the radiative efficiency, η=Lacc/Mc2, is arbitrarily large at low M. For reasonable values of the gas adiabatic index γ, the Bernoulli parameter is negative; therefore, in the absence of dynamically important magnetic fields, a strong outflow or wind is not expected. The flow is also convectively stable but may be thermally unstable. The described solution is not advection dominated; however, when the spin of the star is small enough, the flow transforms smoothly to an advection-dominated branch of solution.

Modeling of charged anisotropic compact stars in general relativity

Energy Technology Data Exchange (ETDEWEB)

Dayanandan, Baiju; Maurya, S.K.; T, Smitha T. [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman)

2017-06-15

A charged compact star model has been determined for anisotropic fluid distribution. We have solved the Einstein-Maxwell field equations to construct the charged compact star model by using the radial pressure, the metric function e{sup λ} and the electric charge function. The generic charged anisotropic solution is verified by exploring different physical conditions like causality condition, mass-radius relation and stability of the solution (via the adiabatic index, TOV equations and the Herrera cracking concept). It is observed that the present charged anisotropic compact star model is compatible with the star PSR 1937+21. Moreover, we also presented the EOS ρ = f(p) for the present charged compact star model. (orig.)

General Relativity and Compact Stars

International Nuclear Information System (INIS)

Glendenning, Norman K.

2005-01-01

Compact stars--broadly grouped as neutron stars and white dwarfs--are the ashes of luminous stars. One or the other is the fate that awaits the cores of most stars after a lifetime of tens to thousands of millions of years. Whichever of these objects is formed at the end of the life of a particular luminous star, the compact object will live in many respects unchanged from the state in which it was formed. Neutron stars themselves can take several forms--hyperon, hybrid, or strange quark star. Likewise white dwarfs take different forms though only in the dominant nuclear species. A black hole is probably the fate of the most massive stars, an inaccessible region of spacetime into which the entire star, ashes and all, falls at the end of the luminous phase. Neutron stars are the smallest, densest stars known. Like all stars, neutron stars rotate--some as many as a few hundred times a second. A star rotating at such a rate will experience an enormous centrifugal force that must be balanced by gravity or else it will be ripped apart. The balance of the two forces informs us of the lower limit on the stellar density. Neutron stars are 10 14 times denser than Earth. Some neutron stars are in binary orbit with a companion. Application of orbital mechanics allows an assessment of masses in some cases. The mass of a neutron star is typically 1.5 solar masses. They can therefore infer their radii: about ten kilometers. Into such a small object, the entire mass of our sun and more, is compressed

An X-ray outburst from the rapidly accreting young star that illuminates McNeil's nebula.

Science.gov (United States)

Kastner, J H; Richmond, M; Grosso, N; Weintraub, D A; Simon, T; Frank, A; Hamaguchi, K; Ozawa, H; Henden, A

2004-07-22

Young, low-mass stars are luminous X-ray sources whose powerful X-ray flares may exert a profound influence over the process of planet formation. The origin of the X-ray emission is uncertain. Although many (or perhaps most) recently formed, low-mass stars emit X-rays as a consequence of solar-like coronal activity, it has also been suggested that X-ray emission may be a direct result of mass accretion onto the forming star. Here we report X-ray imaging spectroscopy observations which reveal a factor approximately 50 increase in the X-ray flux from a young star that is at present undergoing a spectacular optical/infrared outburst (this star illuminates McNeil's nebula). The outburst seems to be due to the sudden onset of a phase of rapid accretion. The coincidence of a surge in X-ray brightness with the optical/infrared eruption demonstrates that strongly enhanced high-energy emission from young stars can occur as a consequence of high accretion rates. We suggest that such accretion-enhanced X-ray emission from erupting young stars may be short-lived, because intense star-disk magnetospheric interactions are quenched rapidly by the subsequent flood of new material onto the star.

Strange matter in compact stars

Directory of Open Access Journals (Sweden)

Klähn Thomas

2018-01-01

Full Text Available We discuss possible scenarios for the existence of strange matter in compact stars. The appearance of hyperons leads to a hyperon puzzle in ab-initio approaches based on effective baryon-baryon potentials but is not a severe problem in relativistic mean field models. In general, the puzzle can be resolved in a natural way if hadronic matter gets stiffened at supersaturation densities, an effect based on the quark Pauli quenching between hadrons. We explain the conflict between the necessity to implement dynamical chiral symmetry breaking into a model description and the conditions for the appearance of absolutely stable strange quark matter that require both, approximately masslessness of quarks and a mechanism of confinement. The role of strangeness in compact stars (hadronic or quark matter realizations remains unsettled. It is not excluded that strangeness plays no role in compact stars at all. To answer the question whether the case of absolutely stable strange quark matter can be excluded on theoretical grounds requires an understanding of dense matter that we have not yet reached.

Strange matter in compact stars

Science.gov (United States)

Klähn, Thomas; Blaschke, David B.

2018-02-01

We discuss possible scenarios for the existence of strange matter in compact stars. The appearance of hyperons leads to a hyperon puzzle in ab-initio approaches based on effective baryon-baryon potentials but is not a severe problem in relativistic mean field models. In general, the puzzle can be resolved in a natural way if hadronic matter gets stiffened at supersaturation densities, an effect based on the quark Pauli quenching between hadrons. We explain the conflict between the necessity to implement dynamical chiral symmetry breaking into a model description and the conditions for the appearance of absolutely stable strange quark matter that require both, approximately masslessness of quarks and a mechanism of confinement. The role of strangeness in compact stars (hadronic or quark matter realizations) remains unsettled. It is not excluded that strangeness plays no role in compact stars at all. To answer the question whether the case of absolutely stable strange quark matter can be excluded on theoretical grounds requires an understanding of dense matter that we have not yet reached.

Wind accretion and formation of disk structures in symbiotic binary systems

Science.gov (United States)

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

2015-05-01

We investigate gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion. We study the mass accretion and formation of an accretion disk around the secondary caused by the strong wind from the primary late-type component using global 2D and 3D hydrodynamic numerical simulations. In particular, the dependence of the mass accretion rate on the mass loss rate, wind temperature and orbital parameters of the system is considered. For a typical slow and massive wind from an evolved star the mass transfer through a focused wind results in rapid infall onto the secondary. A stream flow is created between the stars with accretion rates of a 2--10% percent of the mass loss from the primary. This mechanism could be an important method for explaining periodic modulations in the accretion rates for a broad range of interacting binary systems and fueling of a large population of X-ray binary systems. We test the plausibility of these accretion flows indicated by the simulations by comparing with observations of the symbiotic variable system CH Cyg.

Spectral energy distributions of T Tauri stars - disk flaring and limits on accretion

International Nuclear Information System (INIS)

Kenyon, S.J.; Hartmann, L.

1987-01-01

The Adams et al. (1987) conclusion that much of the IR excess emission in the spectral energy distribution of T Tauri stars arises from reprocessing of stellar radiation by a dusty circumstellar disk is presently supported by analyses conducted in light of various models of these stars' spectra. A low mass reprocessing disk can, however, produce these spectra as well as a massive accretion disk. The detection of possible boundary layer radiation in the optical and near-UV regions poses the strongest limits on accretion rates. Disk accretion in the T Tauri phase does not significantly modify stellar evolution. 85 references

Testing the deep-crustal heating model using quiescent neutron-star very-faint X-ray transients and the possibility of partially accreted crusts in accreting neutron stars

NARCIS (Netherlands)

Wijnands, R.; Degenaar, N.; Page, D.

2013-01-01

It is assumed that accreting neutron stars in low-mass X-ray binaries are heated due to the compression of the existing crust by the freshly accreted matter which gives rise to a variety of nuclear reactions in the crust. It has been shown that most of the energy is released deep in the crust by

Compact stars

Science.gov (United States)

Estevez-Delgado, Gabino; Estevez-Delgado, Joaquin

2018-05-01

An analysis and construction is presented for a stellar model characterized by two parameters (w, n) associated with the compactness ratio and anisotropy, respectively. The reliability range for the parameter w ≤ 1.97981225149 corresponds with a compactness ratio u ≤ 0.2644959374, the density and pressures are positive, regular and monotonic decrescent functions, the radial and tangential speed of sound are lower than the light speed, moreover, than the plausible stability. The behavior of the speeds of sound are determinate for the anisotropy parameter n, admitting a subinterval where the speeds are monotonic crescent functions and other where we have monotonic decrescent functions for the same speeds, both cases describing a compact object that is also potentially stable. In the bigger value for the observational mass M = 2.05 M⊙ and radii R = 12.957 Km for the star PSR J0348+0432, the model indicates that the maximum central density ρc = 1.283820319 × 1018 Kg/m3 corresponds to the maximum value of the anisotropy parameter and the radial and tangential speed of the sound are monotonic decrescent functions.

Fallback accretion onto magnetized neutron stars and the hidden magnetic field model

International Nuclear Information System (INIS)

Torres, A; Cerdá-Durán, P; Font, J A

2015-01-01

The observation of several neutron stars with relatively low values of the surface magnetic field found in supernova remnants has led in recent years to controversial interpretations. A possible explanation is the slow rotation of the proto-neutron star at birth which is unable to amplify its magnetic field to typical pulsar levels. An alternative possibility, the hidden magnetic field scenario, seems to be favoured over the previous one due to the observation of three low magnetic field magnetars. This scenario considers the accretion of the fallback of the supernova debris onto the neutron star as the responsible for the observed low magnetic field. In this work, we have studied under which conditions the magnetic field of a neutron star can be buried into the crust due to an accreting fluid. We have considered a simplified toy model in general relativity to estimate the balance between the incoming accretion flow an the magnetosphere. We conclude that the burial is possible for values of the surface magnetic field below 10 13 G. The preliminary results reported in this paper for simplified polytropic models should be confirmed using a more realistic thermodynamical setup. (paper)

Study of magnetized accretion flow with variable Γ equation of state

Science.gov (United States)

Singh, Kuldeep; Chattopadhyay, Indranil

2018-05-01

We present here the solutions of magnetized accretion flow on to a compact object with hard surface such as neutron stars. The magnetic field of the central star is assumed dipolar and the magnetic axis is assumed to be aligned with the rotation axis of the star. We have used an equation of state for the accreting fluid in which the adiabatic index is dependent on temperature and composition of the flow. We have also included cooling processes like bremsstrahlung and cyclotron processes in the accretion flow. We found all possible accretion solutions. All accretion solutions terminate with a shock very near to the star surface and the height of this primary shock does not vary much with either the spin period or the Bernoulli parameter of the flow, although the strength of the shock may vary with the period. For moderately rotating central star, there is possible formation of multiple sonic points in the flow and therefore, a second shock far away from the star surface may also form. However, the second shock is much weaker than the primary one near the surface. We found that if rotation period is below a certain value (P*), then multiple critical points or multiple shocks are not possible and P* depends upon the composition of the flow. We also found that cooling effect dominates after the shock and that the cyclotron and the bremsstrahlung cooling processes should be considered to obtain a consistent accretion solution.

Numerical Simulations of Wind Accretion in Symbiotic Binaries

Science.gov (United States)

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

2009-08-01

the mass loss from the AGB star. Our simulations of gravitationally focused wind accretion in symbiotic binaries show the formation of stream flows and enhanced accretion rates onto the compact component. We conclude that mass transfer through a focused wind is an important mechanism in wind accreting interacting binaries and can have a significant impact on the evolution of the binary itself and the individual components.

NUMERICAL SIMULATIONS OF WIND ACCRETION IN SYMBIOTIC BINARIES

International Nuclear Information System (INIS)

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

2009-01-01

on the mass loss from the AGB star. Our simulations of gravitationally focused wind accretion in symbiotic binaries show the formation of stream flows and enhanced accretion rates onto the compact component. We conclude that mass transfer through a focused wind is an important mechanism in wind accreting interacting binaries and can have a significant impact on the evolution of the binary itself and the individual components.

Modelling of anisotropic compact stars of embedding class one

Energy Technology Data Exchange (ETDEWEB)

Bhar, Piyali [Government General Degree College, Department of Mathematics, Singur, Hooghly, West Bengal (India); Maurya, S.K. [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman); Gupta, Y.K. [Raj Kumar Goel Institute of Technology, Department of Mathematics, Ghaziabad, U.P. (India); Manna, Tuhina [St. Xavier' s College, Department of Commerce (Evening), Kolkata, West Bengal (India)

2016-10-15

In the present article, we have constructed static anisotropic compact star models of Einstein field equations for the spherical symmetric metric of embedding class one. By assuming the particular form of the metric function ν, we have solved the Einstein field equations for anisotropic matter distribution. The anisotropic models represent the realistic compact objects such as SAX J 1808.4-3658 (SS1), Her X-1, Vela X-12, PSR J1614-2230 and Cen X-3. We have reported our results in details for the compact star Her X-1 on the ground of physical properties such as pressure, density, velocity of sound, energy conditions, TOV equation and red-shift etc. Along with these, we have also discussed about the stability of the compact star models. Finally we made a comparison between our anisotropic stars with the realistic objects on the key aspects as central density, central pressure, compactness and surface red-shift. (orig.)

FORMING AN O STAR VIA DISK ACCRETION?

International Nuclear Information System (INIS)

Qiu Keping; Zhang Qizhou; Beuther, Henrik; Fallscheer, Cassandra

2012-01-01

We present a study of outflow, infall, and rotation in a ∼10 5 L ☉ star-forming region, IRAS 18360-0537, with Submillimeter Array and IRAM 30 m observations. The 1.3 mm continuum map shows a 0.5 pc dust ridge, of which the central compact part has a mass of ∼80 M ☉ and harbors two condensations, MM1 and MM2. The CO (2-1) and SiO (5-4) maps reveal a biconical outflow centered at MM1, which is a hot molecular core (HMC) with a gas temperature of 320 ± 50 K and a mass of ∼13 M ☉ . The outflow has a gas mass of 54 M ☉ and a dynamical timescale of 8 × 10 3 yr. The kinematics of the HMC are probed by high-excitation CH 3 OH and CH 3 CN lines, which are detected at subarcsecond resolution and unveil a velocity gradient perpendicular to the outflow axis, suggesting a disk-like rotation of the HMC. An infalling envelope around the HMC is evidenced by CN lines exhibiting a profound inverse P Cygni profile, and the estimated mass infall rate, 1.5 × 10 –3 M ☉ yr –1 , is well comparable to that inferred from the mass outflow rate. A more detailed investigation of the kinematics of the dense gas around the HMC is obtained from the 13 CO and C 18 O (2-1) lines; the position-velocity diagrams of the two lines are consistent with the model of a free-falling and Keplerian-like rotating envelope. The observations suggest that the protostar of a current mass ∼10 M ☉ embedded within MM1 will develop into an O star via disk accretion and envelope infall.

Accretion discs around neutron stars

International Nuclear Information System (INIS)

Pringle, J.E.

1982-01-01

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

General Relativistic Radiation MHD Simulations of Supercritical Accretion onto a Magnetized Neutron Star: Modeling of Ultraluminous X-Ray Pulsars

Energy Technology Data Exchange (ETDEWEB)

Takahashi, Hiroyuki R. [Center for Computational Astrophysics, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Mitaka, Tokyo 181-8588 (Japan); Ohsuga, Ken, E-mail: takahashi@cfca.jp, E-mail: ken.ohsuga@nao.ac.jp [Division of Theoretical Astronomy, National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Mitaka, Tokyo 181-8588 (Japan)

2017-08-10

By performing 2.5-dimensional general relativistic radiation magnetohydrodynamic simulations, we demonstrate supercritical accretion onto a non-rotating, magnetized neutron star, where the magnetic field strength of dipole fields is 10{sup 10} G on the star surface. We found the supercritical accretion flow consists of two parts: the accretion columns and the truncated accretion disk. The supercritical accretion disk, which appears far from the neutron star, is truncated at around ≃3 R {sub *} ( R {sub *} = 10{sup 6} cm is the neutron star radius), where the magnetic pressure via the dipole magnetic fields balances with the radiation pressure of the disks. The angular momentum of the disk around the truncation radius is effectively transported inward through magnetic torque by dipole fields, inducing the spin up of a neutron star. The evaluated spin-up rate, ∼−10{sup −11} s s{sup −1}, is consistent with the recent observations of the ultraluminous X-ray pulsars. Within the truncation radius, the gas falls onto a neutron star along the dipole fields, which results in a formation of accretion columns onto the northern and southern hemispheres. The net accretion rate and the luminosity of the column are ≃66 L {sub Edd}/ c {sup 2} and ≲10 L {sub Edd}, where L {sub Edd} is the Eddington luminosity and c is the light speed. Our simulations support a hypothesis whereby the ultraluminous X-ray pulsars are powered by the supercritical accretion onto the magnetized neutron stars.

Gravitational radiation and gamma-ray bursts from accreting neutron stars

International Nuclear Information System (INIS)

Mosquera Cuesta, H.J.; Araujo, J.C.N. de; Aguiar, O.D.; Horvath, J.E.

2000-01-01

It is well known that hydrodynamic instabilities can be induced in rapidly rotating low magnetic field neutron stars, which accrete mass from a companion in both high and low mass X-ray binaries. (author)

Mass-Radius diagram for compact stars

International Nuclear Information System (INIS)

Carvalho, G A; Jr, R M Marinho; Malheiro, M

2015-01-01

The compact stars represent the final stage in the evolution of ordinary stars, they are formed when a star ceases its nuclear fuel, in this point the process that sustain its stability will stop. After this, the internal pressure can no longer stand the gravitational force and the star colapses [2]. In this work we investigate the structure of these stars which are described by the equations of Tolman-Openheimer-Volkof (TOV) [1]. These equations show us how the pressure varies with the mass and radius of the star. We consider the TOV equations for both relativistic and non-relativistic cases. In the case of compact stars (white dwarfs and neutron stars) the internal pressure that balances the gravitational pressure is essentialy the pressure coming from the degeneracy of fermions. To have solved the TOV equations we need a equation of state that shows how this internal pressure is related to the energy density or mass density. Instead of using politropic equations of state we have solved the equations numericaly using the exact relativistic energy equation for the model of fermion gas at zero temperature. We obtain results for the mass-radius relation for white dwarfs and we compared with the results obtained using the politropic equations of state. In addition we discussed a good fit for the mass-radius relation. (paper)

Emerging anisotropic compact stars in f(G,T) gravity

Energy Technology Data Exchange (ETDEWEB)

Shamir, M.F.; Ahmad, Mushtaq [National University of Computer and Emerging Sciences, Lahore (Pakistan)

2017-10-15

The possible emergence of compact stars has been investigated in the recently introduced modified Gauss-Bonnet f(G,T) gravity, where G is the Gauss-Bonnet term and T is the trace of the energy-momentum tensor (Sharif and Ikram, Eur Phys J C 76:640, 2016). Specifically, for this modified f(G,T) theory, the analytic solutions of Krori and Barua have been applied to an anisotropic matter distribution. To determine the unknown constants appearing in the Krori and Barua metric, the well-known three models of the compact stars, namely 4U1820-30, Her X-I, and SAX J 1808.4-3658 have been used. The analysis of the physical behaviour of the compact stars has been presented and the physical features like energy density and pressure, energy conditions, static equilibrium, stability, measure of anisotropy, and regularity of the compact stars, have been discussed. (orig.)

Quark degrees of freedom in compact stars

Energy Technology Data Exchange (ETDEWEB)

Marranghello, G.F.; Vasconcellos, C.A.Z. [Rio Grande do Sul Univ., Porto Alegre, RS (Brazil). Inst. de Fisica. Dept. de Fisica; Hadjimichef, D. [Pelotas Univ., RS (Brazil). Inst. de Fisica e Matematica. Dept. de Fisica

2001-07-01

Nuclear matter may show a phase transition at high densities, where quarks and gluons are set free, forming a so called quark-gluon plasma. At the same range of densities, neutron stars are formed. In this work we have grouped both ideas in the study of the quark-gluon plasma formation inside compact stars, here treated as pure neutron star, hybrid star and pure quark matter star. (author)

Quark degrees of freedom in compact stars

International Nuclear Information System (INIS)

Marranghello, G.F.; Vasconcellos, C.A.Z.; Hadjimichef, D.

2001-01-01

Nuclear matter may show a phase transition at high densities, where quarks and gluons are set free, forming a so called quark-gluon plasma. At the same range of densities, neutron stars are formed. In this work we have grouped both ideas in the study of the quark-gluon plasma formation inside compact stars, here treated as pure neutron star, hybrid star and pure quark matter star. (author)

Accretion dynamics and polarized x-ray emission of magnetized neutron stars

International Nuclear Information System (INIS)

Arons, J.

1991-01-01

The basic ideas of accretion onto magnetized neutron stars are outlined. These are applied to a simple model of the structure of the plasma mound sitting at the magnetic poles of such as star, in which upward diffusion of photons is balanced by their downward advection. This steady flow model of the plasma's dynamical state is used to compute the emission of polarized X-rays from the optically thick, birefringent medium. The linear polarization of the continuum radiation emerging from the quasi-static mound is found to be as much as 40% at some rotation phases, but is insensitive to the geometry of the accretion flow. The role of the accretion shock, whose detailed polarimetric and spectral characteristics have yet to be calculated, is emphasized as the final determinant of the properties of the emerging X-rays. Some results describing the fully time dependent dynamics of the flow are also presented. In particular, steady flow onto a neutron star is shown to exhibit formation of ''photon bubbles,'' regions of greatly reduced plasma density filled with radiation which form and rise on millisecond time scales. The possible role of these complex structures in the flow for the formation of the emergent spectrum is briefly outlined

Accretion dynamics and polarized X-ray emission of magnetized neutron stars

Science.gov (United States)

Arons, Jonathan

1991-01-01

The basic ideas of accretion onto magnetized neutron stars are outlined. These are applied to a simple model of the structure of the plasma mound sitting at the magnetic poles of such a star, in which upward diffusion of photons is balanced by their downward advection. This steady flow model of the plasma's dynamical state is used to compute the emission of polarized X-raysfrom the optically thick, birefringent medium. The linear polarization of the continuum radiation emerging from the quasi-static mound is found to be as much as 40 percent at some rotation phases, but is insensitive to the geometry of the accretion flow. The role of the accretion shock, whose detailed polarimetric and spectral characteristics have yet to be calculated, is emphasized as the final determinant of the properties of the emerging X-rays. Some results describing the fully time dependent dynamics of the flow are also presented. In particular, steady flow onto a neutron star is shown to exhibit formation of 'photon bubbles', regions of greatly reduced plasma density filled with radiation which form and rise on millisecond time scale. The possible role of these complex structures in the flow for the formation of the emergent spectrum is briefly outlined.

Social stars: Modeling the interactive lives of stars in dense clusters and binary systems in the era of time domain astronomy

Science.gov (United States)

MacLeod, Morgan Elowe

This thesis uses computational modeling to study of phases of dramatic interaction that intersperse stellar lifetimes. In galactic centers stars trace dangerously wandering orbits dictated by the combined gravitational force of a central, supermassive black hole and all of the surrounding stars. In binary systems, stars' evolution -- which causes their radii to increase substantially -- can bring initially non-interacting systems into contact. Moments of strong stellar interaction transform stars, their subsequent evolution, and the stellar environments they inhabit. In tidal disruption events, a star is partially or completely destroyed as tidal forces from a supermassive black hole overwhelm the star's self gravity. A portion of the stellar debris falls back to the black hole powering a luminous flare as it accretes. This thesis studies the relative event rates and properties of tidal disruption events for stars across the stellar evolutionary spectrum. Tidal disruptions of giant stars occur with high specific frequency; these objects' extended envelopes make them vulnerable to disruption. More-compact white dwarf stars are tidally disrupted relatively rarely. Their transients are also of very different duration and luminosity. Giant star disruptions power accretion flares with timescales of tens to hundreds of years; white dwarf disruption flares take hours to days. White dwarf tidal interactions can additionally trigger thermonuclear burning and lead to transients with signatures similar to type I supernovae. In binary star systems, a phase of hydrodynamic interaction called a common envelope episode occurs when one star evolves to swallow its companion. Dragged by the surrounding gas, the companion star spirals through the envelope to tighter orbits. This thesis studies accretion and flow morphologies during this phase. Density gradients across the gravitationally-focussed material lead to a strong angular momentum barrier to accretion during common envelope

Compact stars in f(R, T) gravity

Energy Technology Data Exchange (ETDEWEB)

Das, Amit; Guha, B.K. [Indian Institute of Engineering Science and Technology, Department of Physics, Howrah, West Bengal (India); Rahaman, Farook [Jadavpur University, Department of Mathematics, Kolkata, West Bengal (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India)

2016-12-15

In the present paper we generate a set of solutions describing the interior of a compact star under f(R, T) theory of gravity which admits conformal motion. An extension of general relativity, the f(R, T) gravity is associated to Ricci scalar R and the trace of the energy-momentum tensor T. To handle the Einstein field equations in the form of differential equations of second order, first of all we adopt the Lie algebra with conformal Killing vectors (CKV) which enable one to get a solvable form of such equations and second we consider the equation of state (EOS) p = ωρ with 0 < ω < 1 for the fluid distribution consisting of normal matter, ω being the EOS parameter. We therefore analytically explore several physical aspects of the model to represent behavior of the compact stars such as - energy conditions, TOV equation, stability of the system, Buchdahl condition, compactness and redshift. It is checked that the physical validity and the acceptability of the present model within the specified observational constraint in connection to a dozen of the compact star candidates are quite satisfactory. (orig.)

Spherical conformal models for compact stars

Energy Technology Data Exchange (ETDEWEB)

Takisa, P.M.; Maharaj, S.D.; Manjonjo, A.M.; Moopanar, S. [University of KwaZulu-Natal, Astrophysics and Cosmology Research Unit, School of Mathematics, Statistics and Computer Science, Durban (South Africa)

2017-10-15

We consider spherical exact models for compact stars with anisotropic pressures and a conformal symmetry. The conformal symmetry condition generates an integral relationship between the gravitational potentials. We solve this condition to find a new anisotropic solution to the Einstein field equations. We demonstrate that the exact solution produces a relativistic model of a compact star. The model generates stellar radii and masses consistent with PSR J1614-2230, Vela X1, PSR J1903+327 and Cen X-3. A detailed physical examination shows that the model is regular, well behaved and stable. The mass-radius limit and the surface red shift are consistent with observational constraints. (orig.)

Accretion Disks around Young Stars: An Observational Perspective

Science.gov (United States)

Ménard, F.; Bertout, C.

Accretion disks are pivotal elements in the formation and early evolution of solar-like stars. On top of supplying the raw material, their internal conditions also regulate the formation of planets. Their study therefore holds the key to solve this long standing mystery: how did our Solar System form? This chapter focuses on observational studies of the circumstellar environment, and in particular of circumstellar disks, associated with pre-main sequence solar-like stars. The direct measurement of disk parameters poses an obvious challenge: at the distance of the typical star forming regions ( e.g. 140 pc for Taurus), a planetary system like ours (with diameter simeq50 AU out to Pluto, but excluding the Kuiper belt which could extend much farther out) subtends only 0.35''. Yet its surface brightness is low in comparison to the bright central star and high angular and high contrast imaging techniques are required if one hopes to resolve and measure these protoplanetary disks. Fortunately, capable instruments providing 0.1'' resolution or better and high contrast have been available for just about 10 years now. They are covering a large part of the electromagnetic spectrum, from the UV/Optical with HST and the near-infrared from ground-based adaptive optics systems, to the millimetric range with long-baseline radio interferometers. It is therefore not surprising that our knowledge of the structure of the disks surrounding low-mass stars has made a gigantic leap forward in the last decade. In the following pages we will attempt to describe, in a historical perpective, the road that led to the idea that most solar-like stars are surrounded by an accretion disk at one point in their early life and how, nowadays, their structural and physical parameters can be estimated from direct observations. We will follow by a short discussion of a few of the constraints available regarding the evolution and dissipation of these disks. This last topic is particularly relevant today

Compact stars in alternative theories of gravity: Einstein-Dilaton-Gauss-Bonnet gravity

International Nuclear Information System (INIS)

Pani, Paolo; Berti, Emanuele; Cardoso, Vitor; Read, Jocelyn

2011-01-01

We develop a theoretical framework to study slowly rotating compact stars in a rather general class of alternative theories of gravity, with the ultimate goal of investigating constraints on alternative theories from electromagnetic and gravitational-wave observations of compact stars. Our Lagrangian includes as special cases scalar-tensor theories (and indirectly f(R) theories) as well as models with a scalar field coupled to quadratic curvature invariants. As a first application of the formalism, we discuss (for the first time in the literature) compact stars in Einstein-Dilaton-Gauss-Bonnet gravity. We show that compact objects with central densities typical of neutron stars cannot exist for certain values of the coupling constants of the theory. In fact, the existence and stability of compact stars sets more stringent constraints on the theory than the existence of black hole solutions. This work is a first step in a program to systematically rule out (possibly using Bayesian model selection) theories that are incompatible with astrophysical observations of compact stars.

Thermal evolution of compact stars

International Nuclear Information System (INIS)

Schaab, C.; Glendenning, N.K.

1996-01-01

A collection of modern, field-theoretical equations of state is applied to the investigation of cooling properties of compact stars. These comprise neutron stars as well as hypothetical strange-matter stars, made up of absolutely stable 3-flavor strange-quark matter. Various uncertainties in the behavior of matter at supernuclear densities, e.g., hyperonic degrees of freedom, behavior of coupling strengths in matter, pion and meson condensation, superfluidity, transition to quark matter, absolute stability of strange-quark matter, and last but not least the many-body technique itself are tested against the body of observed cooling data. (orig.)

Accretion of matter onto highly magnetized neutron stars: Final report, July 1-September 30, 1985

International Nuclear Information System (INIS)

Hernquist, L.

1986-06-01

A final report is given of two research projects dealing with magnetic fields of neutron stars. These are the modulation of thermal x-rays from cooling neutron stars and plasma instabilities in neutron star accretion columns

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

Science.gov (United States)

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

2018-05-01

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

Supercritical accretion in the evolution of neutron star binaries and its implications

Energy Technology Data Exchange (ETDEWEB)

Lee, Chang-Hwan, E-mail: clee@pusan.ac.kr; Cho, Hee-Suk

2014-08-15

Recently ∼2M{sub ⊙} neutron stars PSR J1614-2230 and PSR J0348+0432 have been observed in neutron star-white dwarf binaries. These observations ruled out many neutron star equations of states with which the maximum neutron star mass becomes less than 2M{sub ⊙}. On the other hand, all well-measured neutron star masses in double neutron star binaries are still less than 1.5M{sub ⊙}. In this article we suggest that 2M{sub ⊙} neutron stars in neutron star-white dwarf binaries are the result of the supercritical accretion onto the first-born neutron star during the evolution of the binary progenitors.

Compact Stars in Eddington-inspired Born-Infeld Gravity and General Relativity

Science.gov (United States)

Sham, Yu Hin

In this thesis we apply the Eddington inspired Born-Infeld (EiBI) gravity to study the structure and the properties of compact stars. The hydrostatic equilibrium structure of compact stars characterized by different equations of state (EOSs) is considered and it is found that EiBI gravity can lead to different new features that are not found in standard general relativity (GR). A unified framework to study radial perturbations and the stability of compact stars in this theory is also developed. As in the GR case, the frequency- square of the fundamental oscillation mode vanishes for the maximum mass stellar configuration. Also, the oscillation modes depend on the parameter kappa introduced in EiBI gravity and the dependence is stronger for higher-order modes. We also discover that EiBI gravity imposes certain constraints on the EOSs that allow physical stable equilibrium states of compact stars to exist. However, such constraints are unphysical as the validity of an EOS should be independent of the theory of gravity, hinting that EiBI gravity needs to be modified. On the other hand, we demonstrate that two universal relations of compact stars, namely the I-Love-Q relation, which relates the moment of intertia, the tidal Love number and the quadrupole moment of compact stars, and the f-I relation, which links the f-mode oscillation frequency and the moment of inertia of compact stars together, still hold in EiBI gravity within the observational bounds of kappa. The origin of the two universal relations is then studied and it is found that a stiff EOS at the core of the compact star guarantees the universality. The two universal relations are further extended and universal relations relating the multipolar f-mode oscillation frequency and the corresponding multipolar tidal Love number, which can be derived analytically in the Newtonian limit for stars with sufficiently stiff EOSs, are found.

LONG-ORBITAL-PERIOD PREPOLARS CONTAINING EARLY K-TYPE DONOR STARS. BOTTLENECK ACCRETION MECHANISM IN ACTION

International Nuclear Information System (INIS)

Tovmassian, G.; González–Buitrago, D.; 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 cataclysmic variables (CVs) with periods exceeding the natural boundary for Roche-lobe-filling zero-age main sequence (ZAMS) secondary stars. We present observational results for V1082 Sgr with a 20.82 hr orbital period, an object that shows a low luminosity state when its flux is totally dominated by a chromospherically active K star with no signs of ongoing accretion. Frequent accretion shutoffs, 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 hr binary system. They both have early K-type stars as donor stars. We argue that, similar to the shorter-period prepolars containing M dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf’s magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white+red dwarf systems

LONG-ORBITAL-PERIOD PREPOLARS CONTAINING EARLY K-TYPE DONOR STARS. BOTTLENECK ACCRETION MECHANISM IN ACTION

Energy Technology Data Exchange (ETDEWEB)

Tovmassian, G.; González–Buitrago, D.; Zharikov, S. [Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 877, Ensenada, Baja California, 22800 México (Mexico); Reichart, D. E.; Haislip, J. B.; Ivarsen, K. M.; LaCluyze, A. P.; Moore, J. P. [Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill, NC 27599 (United States); Miroshnichenko, A. S., E-mail: gag@astro.unam.mx, E-mail: dgonzalez@astro.unam.mx, E-mail: zhar@astro.unam.mx [Department of Physics and Astronomy, University of North Carolina at Greensboro, Greensboro, NC 27402-6170 (United States)

2016-03-01

We studied two objects identified as cataclysmic variables (CVs) with periods exceeding the natural boundary for Roche-lobe-filling zero-age main sequence (ZAMS) secondary stars. We present observational results for V1082 Sgr with a 20.82 hr orbital period, an object that shows a low luminosity state when its flux is totally dominated by a chromospherically active K star with no signs of ongoing accretion. Frequent accretion shutoffs, 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 hr binary system. They both have early K-type stars as donor stars. We argue that, similar to the shorter-period prepolars containing M dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf’s magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white+red dwarf systems.

THE EATING HABITS OF MILKY WAY-MASS HALOS: DESTROYED DWARF SATELLITES AND THE METALLICITY DISTRIBUTION OF ACCRETED STARS

International Nuclear Information System (INIS)

Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

2016-01-01

We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M vir  ∼ 10 12.1 M ⊙ ) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M star  ∼ 10 8 –10 10 M ⊙ . Halos with more quiescent accretion histories tend to have lower mass progenitors (10 8 –10 9 M ⊙ ), and lower overall accreted stellar masses. Ultra-faint mass (M star  < 10 5 M ⊙ ) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < −2. Dwarfs with masses 10 5  < M star /M ⊙  < 10 8 provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with M star  > 10 8 M ⊙ can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil”; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo

Massive Star Formation: Accreting from Companion X. Chen1 ...

Indian Academy of Sciences (India)

Abstract. We report the possible accretion from companion in the mas- sive 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 ...

Accretion by rotating magnetic neutron stars. III. Accretion torques and period changes in pulsating X-ray sources

International Nuclear Information System (INIS)

Ghosh, P.; Lamb, F.K.

1979-01-01

We use the solutions of the two-dimensional hydromagnetic equations obtained previously to calculate the torque on a magnetic neutron star accreting from a Keplerian disk. We find that the magnetic coupling between the star and the plasma outside the inner edge of the disk is appreciable. As a result of this coupling the spin-up torque on fast rotators is substantially less than that on slow rotators; for sufficiently high stellar angular velocities or sufficiently low accretion rates this coupling dominates that de to the plasma and the magnetic field at the inner edge of the disk, braking the star's rotation even while accretion, and hence X-ray emission, continues.We apply these results to pulsating X-ray sources, and show that the observed secular spin-up rates of all the sources in which this rate has been measured can be accounted for quantitatively if one assumes that these sources are accreting from Keplerian disks and have magnetic moments approx.10 29 --10 32 gauss cm 3 . The reduction of the torque on fast rotators provides a natural explanation of the spin-up rate of Her X-1, which is much below that expected for slow rotators. We show further that a simple relation between the secular spin-up rate : P and the quantity PL/sup 3/7/ adequately represents almost all the observational data, P and L being the pulse period and the luminosity of the source, respectively. This ''universal'' relation enables one to estimate any one of the parameters P, P, and L for a given source if the other two are known. We show that the short-term period fluctuations observed in Her X-1, Cen X-3, Vela X-1, and X Per can be accounted for quite naturally as consequences of torque variations caused by fluctuations in the mass transfer rate. We also indicate how the spin-down torque at low luminosities found here may account for the paradoxical existence of a large number of long-period sources with short spin-up time scales

High-energy X-ray production in a boundary layer of an accreting neutron star

International Nuclear Information System (INIS)

Hanawa, Tomoyuki

1991-01-01

It is shown by Monte Carlo simulation that high-energy X-rays are produced through Compton scattering in a boundary layer of an accreting neutron star. The following is the mechanism for the high-energy X-ray production. An accreting neutron star has a boundary layer rotating rapidly on the surface. X-rays radiated from the star's surface are scattered in part in the boundary layer. Since the boundary layer rotates at a semirelativistic speed, the scattered X-ray energy is changed by the Compton effect. Some X-rays are scattered repeatedly between the neutron star and the boundary layer and become high-energy X-rays. This mechanism is a photon analog of the second-order Fermi acceleration of cosmic rays. When the boundary layer is semitransparent, high-energy X-rays are produced efficiently. 17 refs

Quasiequilibrium models for triaxially deformed rotating compact stars

International Nuclear Information System (INIS)

Huang Xing; Markakis, Charalampos; Sugiyama, Noriyuki; Uryu, Koji

2008-01-01

Quasiequilibrium models of rapidly rotating triaxially deformed stars are computed in general relativistic gravity, assuming a conformally flat spatial geometry (Isenberg-Wilson-Mathews formulation) and a polytropic equation of state. Highly deformed solutions are calculated on the initial slice covered by spherical coordinate grids, centered at the source, in all angular directions up to a large truncation radius. Constant rest mass sequences are calculated from nearly axisymmetric to maximally deformed triaxial configurations. Selected parameters are to model (proto-) neutron stars; the compactness is M/R=0.001, 0.1, 0.14, and 0.2 for polytropic index n=0.3 and M/R=0.001, 0.1, 0.12, and 0.14 for n=0.5, where M/R refers to that of a nonrotating spherical star having the same rest mass. We confirmed that the triaxial solutions exist for these parameters as in the case of Newtonian polytropes. However, it is also found that the triaxial sequences become shorter for higher compactness, and those disappear at a certain large compactness for the n=0.5 case. In the scenario of the contraction of proto-neutron stars being subject to strong viscosity and rapid cooling, it is plausible that, once the viscosity driven secular instability sets in during the contraction, the proto-neutron stars are always maximally deformed triaxial configurations, as long as the compactness and the equation of state parameters allow such triaxial sequences. Detection of gravitational waves from such sources may be used as another probe for the nuclear equation of state.

I-Love-Q relations: from compact stars to black holes

International Nuclear Information System (INIS)

Yagi, Kent; Yunes, Nicolás

2016-01-01

The relations between most observables associated with a compact star, such as the mass and radius of a neutron star or a quark star, typically depend strongly on their unknown internal structure. The recently discovered I-Love-Q relations (between the moment of inertia, the tidal deformability and the quadrupole moment) are however approximately insensitive to this structure. These relations become exact for stationary black holes (BHs) in General Relativity as shown by the no-hair theorems, mainly because BHs are vacuum solutions with event horizons. In this paper, we take the first steps toward studying how the approximate I-Love-Q relations become exact in the limit as compact stars become BHs. To do so, we consider a toy model for compact stars, i.e. incompressible stars with anisotropic pressure, which allows us to model an equilibrium sequence of stars with ever increasing compactness that approaches the BH limit arbitrarily closely. We numerically construct such a sequence in the slow-rotation and in the small-tide approximations by extending the Hartle–Thorne formalism, and then extract the I-Love-Q trio from the asymptotic behavior of the metric tensor at spatial infinity. We find that the I-Love-Q relations approach the BH limit in a nontrivial way, with the quadrupole moment and the tidal deformability changing sign as the compactness and the amount of anisotropy are increased. Through a generalization of Maclaurin spheroids to anisotropic stars, we show that the multipole moments also change sign in the Newtonian limit as the amount of anisotropy is increased because the star becomes prolate. We also prove analytically that the stellar moment of inertia reaches the BH limit as the compactness reaches a critical BH value in the strongly anisotropic limit. Modeling the BH limit through a sequence of anisotropic stars, however, can fail when considering other theories of gravity. We calculate the scalar dipole charge and the moment of inertia in a

CSI 2264: characterizing accretion-burst dominated light curves for young stars in NGC 2264

International Nuclear Information System (INIS)

Stauffer, John; Cody, Ann Marie; Rebull, Luisa; Carey, Sean; Baglin, Annie; Alencar, Silvia; Hillenbrand, Lynne A.; Carpenter, John; Findeisen, Krzysztof; Venuti, Laura; Bouvier, Jerome; Turner, Neal J.; Plavchan, Peter; Terebey, Susan; Morales-Calderón, María; Micela, Giusi; Flaccomio, Ettore; Song, Inseok; Gutermuth, Rob; Hartmann, Lee

2014-01-01

Based on more than four weeks of continuous high-cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high-quality, multi-wavelength light curves for young stellar objects whose optical variability is dominated by short-duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief—several hours to one day—brightenings at optical and near-infrared wavelengths with amplitudes generally in the range of 5%-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a 30 day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u – g versus g – r color-color diagram with the largest UV excesses. These stars also have large Hα equivalent widths, and either centrally peaked, lumpy Hα emission profiles or profiles with blueshifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Among the stars with the largest UV excesses or largest Hα equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts.

The Effect of Quantum Fluctuations in Compact Star Observables

Science.gov (United States)

Pósfay, P.; Barnaföldi, G. G.; Jakovác, A.

2018-05-01

Astrophysical measurements regarding compact stars are just ahead of a big evolution jump, since the NICER experiment deployed on ISS on 2017 June 14. This will provide soon data that would enable the determination of compact star radius with less than 10% error. This can be further constrained by the new observation of gravitational waves originated from merging neutron stars, GW170817. This poses new challenges to nuclear models aiming to explain the structure of super dense nuclear matter found in neutron stars. Detailed studies of the QCD phase diagram show the importance of bosonic quantum fluctuations in the cold dense matter equation of state. Here we used a demonstrative model with one bosonic and one fermionic degree of freedom coupled by Yukawa coupling, we show the effect of bosonic quantum fluctuations on compact star observables such as mass, radius, and compactness. We have also calculated the difference in the value of compressibility which is caused by quantum fluctuations. The above-mentioned quantities are calculated in the mean field, one-loop, and in high order many loop approximation. The results show that the magnitude of these effects is in the range of 4-5%, which place it into the region where modern measurements may detect it. This forms a base for further investigations that how these results carry over to more complicated models.

NATO Advanced Research Workshiop on Superdense QCD Matter and Compact Stars

CERN Document Server

Blaschke, David

2006-01-01

This volume covers the main topics in the theory of superdense QCD matter and its application to the astrophysics of compact stars in a comprehensive and yet accessible way. The material is presented as a combination of extensive introductory lectures and more topical contributions. The book is centered around the question whether hypothetical new states of dense matter in the compact star interior could give clues to the explanation of puzzling phenomena such as gamma-ray bursts, pulsar glitches, compact star cooling and gravitational waves.

Compact objects for everyone: I. White dwarf stars

Energy Technology Data Exchange (ETDEWEB)

Jackson, C B; Taruna, J; Pouliot, S L; Ellison, B W; Lee, D D; Piekarewicz, J [Department of Physics, Florida State University, Tallahassee, FL 32306 (United States)

2005-09-01

Based upon previous discussions on the structure of compact stars geared towards undergraduate physics students, a real experiment involving two upper-level undergraduate physics students, a beginning physics graduate and two advanced graduate students was conducted. A recent addition to the physics curriculum at Florida State University, The Physics of Stars, sparked quite a few students' interests in the subject matter involving stellar structure. This, coupled with Stars and statistical physics by Balian and Blaizot (1999 Am. J. Phys. 67 1189) and Neutron stars for undergraduates by Silbar and Reddy (2004 Am. J. Phys. 72 892), is the cornerstone of this small research group who tackled solving the structure equations for compact objects in the summer of 2004. Through the use of a simple finite-difference algorithm coupled to Microsoft Excel and Maple, solutions to the equations for stellar structure are presented in the Newtonian regime appropriate to the physics of white dwarf stars.

Compact objects for everyone: I. White dwarf stars

International Nuclear Information System (INIS)

Jackson, C B; Taruna, J; Pouliot, S L; Ellison, B W; Lee, D D; Piekarewicz, J

2005-01-01

Based upon previous discussions on the structure of compact stars geared towards undergraduate physics students, a real experiment involving two upper-level undergraduate physics students, a beginning physics graduate and two advanced graduate students was conducted. A recent addition to the physics curriculum at Florida State University, The Physics of Stars, sparked quite a few students' interests in the subject matter involving stellar structure. This, coupled with Stars and statistical physics by Balian and Blaizot (1999 Am. J. Phys. 67 1189) and Neutron stars for undergraduates by Silbar and Reddy (2004 Am. J. Phys. 72 892), is the cornerstone of this small research group who tackled solving the structure equations for compact objects in the summer of 2004. Through the use of a simple finite-difference algorithm coupled to Microsoft Excel and Maple, solutions to the equations for stellar structure are presented in the Newtonian regime appropriate to the physics of white dwarf stars

Enigmatic sub-luminous accreting neutron stars in our Galaxy

NARCIS (Netherlands)

Wijnands, R.

2008-01-01

During the last few years a class of enigmatic sub-luminous accreting neutron stars has been found in our Galaxy. They have peak X-ray luminosities (2-10 keV) of a few times 10(34) erg s(−1) to a few times 10(35) erg s(−1), and both persistent and transient sources have been found. I present a short

Veiling and Accretion Around the Young Binary Stars S and VV Corona Australis

Science.gov (United States)

Sullivan, Kendall; Prato, Lisa; Avilez, Ian

2018-01-01

S CrA and VV CrA are two young binary star systems with separations of 170 AU and 250 AU, respectively, in the southern star-forming region Corona Australis. The spectral types of the four stars in these two systems are similar, approximately K7 to M1, hence the stellar masses are also similar. The study of young stars just emerging from their natal cloud cores at the very limits of observability allows us to probe the extreme environments in which planet formation begins to occur. Stars in this early evolutionary stage can have circumstellar or circumbinary disks, and sometimes remnants of the envelopes which surrounded them during the protostellar stage. Envelopes accrete onto disks and disks in turn accrete onto the central stars, triggering elevated continuum emission, line emission, outflows, and stellar winds. This violent stage marks the onset of the epoch of planet formation. Using high-resolution near-infrared, H-band spectroscopy from the Keck II telescope using the NIRSPEC instrument over 4-6 epochs, we are probing the chaotic environment surrounding the four stars in these systems. We determine the spectral types for VV CrA A and B for the first time, and examine the variable veiling and emission occurring around each of these stars. This research was supported in part by NSF grants AST-1461200 and AST-1313399.

Observing Compact Stars with AstroSat

Indian Academy of Sciences (India)

Dipankar Bhattacharya

2017-09-12

Sep 12, 2017 ... based observatory for compact star research. An account is given of ... unprecedented capability to study such rapid variability simultaneously at all ..... Physical Research Laboratory, University of Leicester and the Canadian ...

Optical veiling, disk accretion, and the evolution of T Tauri stars

International Nuclear Information System (INIS)

Hartmann, L.W.; Kenyon, S.J.

1990-01-01

High-resolution spectra of 31 K7-M1 T Tauri stars (TTs) in the Taurus-Auriga molecular cloud demonstrate that most of these objects exhibit substantial excess emission at 5200 A. Extrapolations of these data consistent with low-resolution spectrophotometry indicate that the extra emission is comparable to the stellar luminosity in many cases. If this continuum emission arises in the boundary layers of accreting disks, more than about 30 percent of all TTs may be accreting material at a rate which is sufficiently rapid to alter their evolution from standard Hayashi tracks. It is estimated that roughly 10 percent of the final stellar mass is accreted in the TT phase. This amount of material is comparable to the minimum gravitationally unstable disk mass estimated by Larson and it is speculated that the TT phase represents the final stages of disk accretion driven by gravitational instabilities. 40 refs

Quark phases in neutron stars and a third family of compact stars as signature for phase transitions

International Nuclear Information System (INIS)

Schertler, K.; Greiner, C.; Schaffner-Bielich, J.; Thoma, M.H.

2000-01-01

The appearance of quark phases in the dense interior of neutron stars provides one possibility to soften the equation of state (EOS) of neutron star matter at high densities. This softening leads to more compact equilibrium configurations of neutron stars compared to pure hadronic stars of the same mass. We investigate the question to which amount the compactness of a neutron star can be attributed to the presence of a quark phase. For this purpose we employ several hadronic EOS in the framework of the relativistic mean-field (RMF) model and an extended MIT bag model to describe the quark phase. We find that -- almost independent of the model parameters -- the radius of a pure hadronic neutron star gets typically reduced by 20-30% if a pure quark phase in the center of the star does exist. For some EOS we furthermore find the possibility of a third family of compact stars which may exist besides the two known families of white dwarfs and neutron stars. We show how an experimental proof of the existence of a third family by mass and radius measurements may provide a unique signature for a phase transition inside neutron stars

THE EATING HABITS OF MILKY WAY-MASS HALOS: DESTROYED DWARF SATELLITES AND THE METALLICITY DISTRIBUTION OF ACCRETED STARS

Energy Technology Data Exchange (ETDEWEB)

Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H., E-mail: adeason@stanford.edu [Kavli Institute for Particle Astrophysics and Cosmology and Physics Department, Stanford University, Stanford, CA 94305 (United States)

2016-04-10

We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (M{sub vir} ∼ 10{sup 12.1} M{sub ⊙}) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with M{sub star} ∼ 10{sup 8}–10{sup 10}M{sub ⊙}. Halos with more quiescent accretion histories tend to have lower mass progenitors (10{sup 8}–10{sup 9} M{sub ⊙}), and lower overall accreted stellar masses. Ultra-faint mass (M{sub star} < 10{sup 5} M{sub ⊙}) dwarfs contribute a negligible amount (≪1%) to the accreted stellar mass and, despite having low average metallicities, supply a small fraction (∼2%–5%) of the very metal-poor stars with [Fe/H] < −2. Dwarfs with masses 10{sup 5} < M{sub star}/M{sub ⊙} < 10{sup 8} provide a substantial amount of the very metal-poor stellar material (∼40%–80%), and even relatively metal-rich dwarfs with M{sub star} > 10{sup 8} M{sub ⊙} can contribute a considerable fraction (∼20%–60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil”; a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

Strange exotic states and compact stars

International Nuclear Information System (INIS)

Sagert, Irina; Wietoska, Mirjam; Schaffner-Bielich, Juergen

2006-01-01

We discuss the possible appearance of strange exotic multi-quark states in the interiors of neutron stars and signals for the existence of strange quark matter in the cores of compact stars. We show how the in-medium properties of possible pentaquark states are constrained by pulsar mass measurements. The possibility of generating the observed large pulsar kick velocities by asymmetric emission of neutrinos from strange quark matter in magnetic fields is outlined

Near-ultraviolet Excess in Slowly Accreting T Tauri Stars: Limits Imposed by Chromospheric Emission

Science.gov (United States)

Ingleby, Laura; Calvet, Nuria; Bergin, Edwin; Herczeg, Gregory; Brown, Alexander; Alexander, Richard; Edwards, Suzan; Espaillat, Catherine; France, Kevin; Gregory, Scott G.; Hillenbrand, Lynne; Roueff, Evelyne; Valenti, Jeff; Walter, Frederick; Johns-Krull, Christopher; Brown, Joanna; Linsky, Jeffrey; McClure, Melissa; Ardila, David; Abgrall, Hervé; Bethell, Thomas; Hussain, Gaitee; Yang, Hao

2011-12-01

Young stars surrounded by disks with very low mass accretion rates are likely in the final stages of inner disk evolution and therefore particularly interesting to study. We present ultraviolet (UV) observations of the ~5-9 Myr old stars RECX-1 and RECX-11, obtained with the Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph on the Hubble Space Telescope, as well as optical and near-infrared spectroscopic observations. The two stars have similar levels of near-UV emission, although spectroscopic evidence indicates that RECX-11 is accreting and RECX-1 is not. The line profiles of Hα and He I λ10830 in RECX-11 show both broad and narrow redshifted absorption components that vary with time, revealing the complexity of the accretion flows. We show that accretion indicators commonly used to measure mass accretion rates, e.g., U-band excess luminosity or the Ca II triplet line luminosity, are unreliable for low accretors, at least in the middle K spectral range. Using RECX-1 as a template for the intrinsic level of photospheric and chromospheric emission, we determine an upper limit of 3 × 10-10 M ⊙ yr-1 for RECX-11. At this low accretion rate, recent photoevaporation models predict that an inner hole should have developed in the disk. However, the spectral energy distribution of RECX-11 shows fluxes comparable to the median of Taurus in the near-infrared, indicating that substantial dust remains. Fluorescent H2 emission lines formed in the innermost disk are observed in RECX-11, showing that gas is present in the inner disk, along with the dust. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

Relativistic modeling of compact stars for anisotropic matter distribution

Energy Technology Data Exchange (ETDEWEB)

Maurya, S.K. [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman)

2017-05-15

In this paper we have solved Einstein's field equations of spherically symmetric spacetime for anisotropic matter distribution by assuming physically valid expressions of the metric function e{sup λ} and radial pressure (p{sub r}). Next we have discussed the physical properties of the model in details by taking the radial pressure p{sub r} equal to zero at the boundary of the star. The physical analysis of the star indicates that its model parameters such as density, redshift, radial pressure, transverse pressure and anisotropy are well behaved. Also we have obtained the mass and radius of our compact star which are 2.29M {sub CircleDot} and 11.02 km, respectively. It is observed that the model obtained here for compact stars is compatible with the mass and radius of the strange star PSR 1937 +21. (orig.)

X-ray luminosity by matter accretion on a neutron star

Energy Technology Data Exchange (ETDEWEB)

Baroni, L [Bologna Univ. (Italy). Ist. di Fisica; Fortini, P L [Instituto di Astronomia, Bologna (Italy); Gualdi, C; Callegari, G [Ferrara Univ. (Italy). Ist. di Fisica

1980-11-20

When the accretion rate on a non magnetic neutron star is determined by stellar wind and not by overflowing the Roche lobe, it is shown that X-ray luminosity cannot exceed 10sup(36)-10sup(37) erg/sec. This very low limit is essentially set by radiation pressure which causes an effective braking on the falling matter.

Focused Wind Mass Accretion in Mira AB

Science.gov (United States)

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

2011-05-01

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

Symbiotic star CI Cygni: S-process episode or accretion event

Energy Technology Data Exchange (ETDEWEB)

Kenyon, S J; Webbink, R F; Gallagher, J S; Truran, J W

1982-02-01

Evidence that the symbiotic star C I Cygni is an eclipsing binary is reviewed. It is shown that the s-process episode described by Audouze et al. (1981) during its 1975 outburst arises from superposition of normal gM4 absorption features on the continuum of the hot component during eclipse ingress, and not to sudden enhancements of rare earth elements. The peculiar velocity displacements of absorption lines with different excitation potentials during this episode are identified as signatures of an optically thick accretion disk, which dominates the visible spectrum during outburst. The data presented by Audouze et al., and the shape of the light curve thus provide evidence that the outburst is accretion-powered.

Gravitational effects of condensate dark matter on compact stellar objects

International Nuclear Information System (INIS)

Li, X.Y.; Wang, F.Y.; Cheng, K.S.

2012-01-01

We study the gravitational effect of non-self-annihilating dark matter on compact stellar objects. The self-interaction of condensate dark matter can give high accretion rate of dark matter onto stars. Phase transition to condensation state takes place when the dark matter density exceeds the critical value. A compact degenerate dark matter core is developed and alter the structure and stability of the stellar objects. Condensate dark matter admixed neutron stars is studied through the two-fluid TOV equation. The existence of condensate dark matter deforms the mass-radius relation of neutron stars and lower their maximum baryonic masses and radii. The possible effects on the Gamma-ray Burst rate in high redshift are discussed

Relating follicly-challenged compact stars to bald black holes: A link between two no-hair properties

Science.gov (United States)

Yagi, Kent; Yunes, Nicolás

2015-05-01

Compact stars satisfy certain no-hair relations through which their multipole moments are given by their mass, spin and quadrupole moment. These relations are approximately independent of their equation of state, relating pressure to density. Such relations are similar to the black hole no-hair theorems, but these possess event horizons inside which information that led to their formation can hide. Compact stars do not possess horizons, so whether their no-hair relations are related to the black hole ones is unclear. We investigate how the two relations are related by studying relations among multipole moments for compact stars with anisotropic pressure as a toy model, which allows such stars to be more compact than those with isotropic pressure. We here show numerically that the compact star no-hair relations approach the black hole ones as the compactness approaches that of a black hole. We also prove analytically that the current dipole moment exactly reaches the black hole limit quadratically in compactness as strongly anisotropic stars approach the black hole limit. We moreover show that compact stars become progressively oblate in this limit, even if prolate at low compactness due to strong anisotropies.

Many faces of compact objects: distance, optical extinction and multi-wavelength behaviour

International Nuclear Information System (INIS)

Corbel, Stephane

1999-01-01

This thesis is devoted to a multi-wavelength study of accretion-ejection phenomena around compact stars (black holes and neutron stars). The first part of this manuscript describes problems related to the determination of the distance and the optical extinction to compact objects - fundamental parameters for the evaluation of the energy budget of these systems. To this end, the structure and the dynamics of the Galaxy are studied by observations of the atomic and molecular gas along the line of sight to compact stars. This method leads to the first evaluation of the distance to two Soft Gamma Repeaters: SGR 1806-20 and SGR 1627-41. We then draw some conclusions on the nature of these sources of recurrent gamma-ray bursts. The above method is then applied to two X-ray binaries: Cir X-1 and GX 339-4. In the second part of this thesis, we present a multi-wavelength study of the Galactic black hole candidate GX 339-4. We first discuss the characteristics of the radio emission from GX 339-4. In 1998, GX 339-4 underwent a transition to a soft-high X-ray state and observations in three wavelength regimes (radio, soft and hard X-rays) revealed new patterns of behaviour. This allowed us to constrain the region of origin of the radio emission (a compact jet) in GX 339-4 and allowed a better understanding of the physical coupling between accretion and ejection in GX 339-4. An analogy with the black hole candidate Cyg X-1 is then presented. Finally, these results are discussed in the context of micro-quasars and active galactic nuclei in order to gain a deeper insight into the accretion-ejection coupling around compact objects. (author) [fr

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

Science.gov (United States)

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

2018-02-01

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

Three-dimensional hydrodynamical models of wind and outburst-related accretion in symbiotic systems

Science.gov (United States)

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

2017-07-01

Gravitationally focused wind accretion in binary systems consisting of an evolved star with a gaseous envelope and a compact accreting companion is a possible mechanism to explain mass transfer in symbiotic binaries. We study the mass accretion around the secondary caused by the strong wind from the primary late-type component using global three-dimensional hydrodynamic numerical simulations during quiescence and outburst stages. In particular, the dependence of the mass accretion rate on the mass-loss rate, wind parameters and phases of wind outburst development is considered. For a typical wind from an asymptotic giant branch star with a mass-loss rate of 10-6 M⊙ yr-1 and wind speeds of 20-50 km s-1, the mass transfer through a focused wind results in efficient infall on to the secondary. Accretion rates on to the secondary of 5-20 per cent of the mass-loss from the primary are obtained during quiescence and outburst periods where the wind velocity and mass-loss rates are varied, about 20-50 per cent larger than in the standard Bondi-Hoyle-Lyttleton approximation. This mechanism could be an important method for explaining observed accretion luminosities and periodic modulations in the accretion rates for a broad range of interacting binary systems.

Experimental measurements of the 15O(alpha,gamma)19Ne reaction rate and the stability of thermonuclear burning on accreting neutron stars

International Nuclear Information System (INIS)

Fisker, J; Tan, W; Goerres, J; Wiescher, M; Cooper, R

2007-01-01

Neutron stars in close binary star systems often accrete matter from their companion stars. Thermonuclear ignition of the accreted material in the atmosphere of the neutron star leads to a thermonuclear explosion which is observed as an X-ray burst occurring periodically between hours and days depending on the accretion rate. The ignition conditions are characterized by a sensitive interplay between the accretion rate of the fuel supply and its depletion rate by nuclear burning in the hot CNO cycle and the rp-process. For accretion rates close to stable burning the burst ignition therefore depends critically on the hot CNO breakout reaction 15 O(α, γ) 19 Ne that regulates the flow between the hot CNO cycle and the rapid proton capture process. Until recently, the 15 O(α, γ) 19 Ne reaction rate was not known experimentally and the theoretical estimates carried significant uncertainties. In this paper we perform a parameter study of the uncertainty of this reaction rate and determine the astrophysical consequences of the first measurement of this reaction rate. Our results corroborate earlier predictions and show that theoretically burning remains unstable up to accretion rates near the Eddington limit, in contrast to astronomical observations

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

Science.gov (United States)

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

2018-06-01

We summarize panchromatic Extragalactic Background Light data to place upper limits on the integrated near-IR surface brightness (SB) that may come from Population III stars and possible accretion disks around their stellar-mass black holes (BHs) in the epoch of First Light, broadly taken from z=7-17.We outline the physical properties of zero-metallicity Population III stars from MESA stellar evolution models through helium depletion and of BH accretion disks at z>7. We assume that second-generation non-zero-metallicity stars can form at higher multiplicity, so that BH accretion disks may be fed by Roche-lobe overflow from lower-mass companions.We use these near-infrared SB constraints to calculate the number of caustic transits behind lensing clusters that the James Webb Space Telescope and the next-generation ground-based telescopes may observe for both Population III stars and their BH accretion disks. Typical caustic magnifications can be 10^4-10^5x, with rise times of hours and decline times of z~Economia y Competitividad of Spain Consolider Project CSD2010-00064.

Accreting neutron stars by QFT

Science.gov (United States)

Chen, Shao-Guang

the negative charge from ionosphere electrons again rotate, thereby come into being the solar basal magnetic field. The solar surface plasma with additional electrons get the dynamic balance between the upwards force of stable positive charge distribution in the solar upside gas and the downwards force of the vacuum net nuν _{0} flux pressure (solar gravity). When the Jupiter enter into the connecting line of the Sun and the center of the Galaxy, the pressure (solar gravity) observed from earth will weaken because of the Jupiter stop (shield) the net nuν _{0} flux which shoot to Sun from the center of Galaxy. The dynamic balance of forces on the solar surface plasma at once is broken and the plasma will upwards eject as the solar wind with redundant negative charge. At the same time, the solar surface remain a cavity as a sunspot whorl with the positive electric potential relative to around plasma. The whorl is caused by the reaction of plasma eject front and upwards with the different velocity at different latitude of solar rotation, it leads to the cavity around in the downwards and backwards helix movement. The solar rotation more slow, when the cavity is filled by around plasma in the reverse turn direction and return to carry-over negative charge, the Jupiter at front had been produced a new cavity carry-over positive charge, so we had observe the sunspot pair with different whorl directions and different magnetic polarity. Jupiter possess half mass of all planets in solar system, its action to stop net nuν _{0} flux is primary, so that Jupiter’s period of 11.8 sidereal years accord basically with the period of sunspot eruptions. In my paper ‘Nonlinear superposition of strong gravitational field of compact stars’(E15-0039-08), according to QFT it is deduced that: let q is a positive shielding coefficient, 1- q show the gravity weaken degree, the earth (104 km) as a obstructing layer q = 4.6*10 (-10) . A spherical shell of neutron star as obstructing

The progenitors of the compact early-type galaxies at high redshift

International Nuclear Information System (INIS)

Williams, Christina C.; Giavalisco, Mauro; Lee, Bomee; Cassata, Paolo; Tundo, Elena; Conselice, Christopher J.; Wiklind, Tommy; Guo, Yicheng; Barro, Guillermo; Faber, Sandra M.; Koo, David C.; Wuyts, Stijn; Bell, Eric F.; Dekel, Avishai; Ferguson, Henry C.; Grogin, Norman; Koekemoer, Anton; Hathi, Nimish; Huang, Kuang-Han; Kocevski, Dale

2014-01-01

We use GOODS and CANDELS images to identify progenitors of massive (M > 10 10 M ☉ ) compact early-type galaxies (ETGs) at z ∼ 1.6. Because merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z ∼ 3 on the basis of their mass, star-formation rate (SFR), and central stellar density, and we find that these account for a large fraction of, and possibly all, compact ETGs at z ∼ 1.6. We find that the average far-UV spectral energy distribution (SED) of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration and consistent with more evolved (aging) star formation. This is in line with other evidence suggesting that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended 'halos' surrounding the compact 'core,' both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the Hubble Space Telescope images, with their stellar mass assembling in situ, and that they have not experienced any major merging until the epoch of observations at z ∼ 1.6.

The progenitors of the compact early-type galaxies at high redshift

Energy Technology Data Exchange (ETDEWEB)

Williams, Christina C.; Giavalisco, Mauro; Lee, Bomee [Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003 (United States); Cassata, Paolo [Aix Marseille Université, CNRS, LAM (Laboratoire d' Astrophysique de Marseille) UMR 7326, F-13388 Marseille (France); Tundo, Elena; Conselice, Christopher J. [The School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD (United Kingdom); Wiklind, Tommy [Joint ALMA Observatory, ESO, Santiago (Chile); Guo, Yicheng; Barro, Guillermo; Faber, Sandra M.; Koo, David C. [UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); Wuyts, Stijn [Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, D-85741 Garching (Germany); Bell, Eric F. [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States); Dekel, Avishai [Racah Institute of Physics, The Hebrew University, Jerusalem 91904 (Israel); Ferguson, Henry C.; Grogin, Norman; Koekemoer, Anton [Space Telescope Science Institute, 3700 San Martin Boulevard, Baltimore, MD 21218 (United States); Hathi, Nimish [Carnegie Observatories, Pasadena, CA 91101 (United States); Huang, Kuang-Han [Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States); Kocevski, Dale, E-mail: ccwillia@astro.umass.edu [Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506 (United States); and others

2014-01-01

We use GOODS and CANDELS images to identify progenitors of massive (M > 10{sup 10} M {sub ☉}) compact early-type galaxies (ETGs) at z ∼ 1.6. Because merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z ∼ 3 on the basis of their mass, star-formation rate (SFR), and central stellar density, and we find that these account for a large fraction of, and possibly all, compact ETGs at z ∼ 1.6. We find that the average far-UV spectral energy distribution (SED) of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration and consistent with more evolved (aging) star formation. This is in line with other evidence suggesting that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended 'halos' surrounding the compact 'core,' both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the Hubble Space Telescope images, with their stellar mass assembling in situ, and that they have not experienced any major merging until the epoch of observations at z ∼ 1.6.

Mixing by shear instabilities in differentially rotating inhomogeneous stars with application to accreting white dwarf models for novae

Energy Technology Data Exchange (ETDEWEB)

MacDonald, J.

1983-10-01

The problem of how shear instabilities redistribute matter and angular momentum accreted by a star from a disk is considered. Necessary conditions for stability of the star to nonaxisymmetric perturbations are derived by use of the short wavelength approximation. By considering growth rates, it is shown that freshly accreted material rapidly takes up a quasi-spherical distribution due to dynamical instabilities. However, mixing inward toward the stellar interior occurs on a thermal time scale or longer.

Period variations in pulsating X-ray sources. I. Accretion flow parameters and neutron star structure from timing observations

International Nuclear Information System (INIS)

Lamb, F.K.; Pines, D.; Shaham, J.

1978-01-01

We show that valuable information about both accretion flows and neutron star structure can be obtained from X-ray timing observations of period variations in pulsating sources. Such variations can result from variations in the accretion flow, or from internal torque variations, associated with oscillations of the fluid core or the unpinning of vortices in the inner crust. We develop a statistical description of torque variations in terms of noise processes, indicate how the applicability of such a description may be tested observationally, and show how it may be used to determine from observation both the properties of accretion flows and the internal structure of neutron stars, including the relative inertial moments of the crust and superfluid neutron core, the crust-core coupling time, and the frequencies of any low-frequency internal collective modes. Particular attention is paid to the physical origin of spin-down episodes; it is shown that usyc episodes may result either from external torque reversals or from internal torque variations.With the aid of the statistical description, the response of the star to torque fluctuations is calculated for three stellar models: (i) a completely rigid star; (ii) a two-component star; and (iii) a two-component star with a finite-frequency internal mode, such as the Tkachenko mode of a rotating neutron superfluid. Our calculations show that fluctuating torques could account for the period the period variations and spin-down episodes observed in Her X-1 and Cen X-3, including the large spin-down event observed in the latter source during 1972 September-October. The torque noise strengths inferred from current timing observations using the simple two-component models are shown to be consistent with those to be expected from fluctuations in accretion flows onto magnetic neutron stars

Cooling of hypernuclear compact stars

Science.gov (United States)

Raduta, Adriana R.; Sedrakian, Armen; Weber, Fridolin

2018-04-01

We study the thermal evolution of hypernuclear compact stars constructed from covariant density functional theory of hypernuclear matter and parametrizations which produce sequences of stars containing two-solar-mass objects. For the input in the simulations, we solve the Bardeen-Cooper-Schrieffer gap equations in the hyperonic sector and obtain the gaps in the spectra of Λ, Ξ0, and Ξ- hyperons. For the models with masses M/M⊙ ≥ 1.5 the neutrino cooling is dominated by hyperonic direct Urca processes in general. In the low-mass stars the (Λp) plus leptons channel is the dominant direct Urca process, whereas for more massive stars the purely hyperonic channels (Σ-Λ) and (Ξ-Λ) are dominant. Hyperonic pairing strongly suppresses the processes on Ξ-s and to a lesser degree on Λs. We find that intermediate-mass 1.5 ≤ M/M⊙ ≤ 1.8 models have surface temperatures which lie within the range inferred from thermally emitting neutron stars, if the hyperonic pairing is taken into account. Most massive models with M/M⊙ ≃ 2 may cool very fast via the direct Urca process through the (Λp) channel because they develop inner cores where the S-wave pairing of Λs and proton is absent.

Interaction between bosonic dark matter and stars

Science.gov (United States)

Brito, Richard; Cardoso, Vitor; Macedo, Caio F. B.; Okawa, Hirotada; Palenzuela, Carlos

2016-02-01

We provide a detailed analysis of how bosonic dark matter "condensates" interact with compact stars, extending significantly the results of a recent Letter [1]. We focus on bosonic fields with mass mB , such as axions, axion-like candidates and hidden photons. Self-gravitating bosonic fields generically form "breathing" configurations, where both the spacetime geometry and the field oscillate, and can interact and cluster at the center of stars. We construct stellar configurations formed by a perfect fluid and a bosonic condensate, and which may describe the late stages of dark matter accretion onto stars, in dark-matter-rich environments. These composite stars oscillate at a frequency which is a multiple of f =2.5 ×1014(mBc2/eV ) Hz . Using perturbative analysis and numerical relativity techniques, we show that these stars are generically stable, and we provide criteria for instability. Our results also indicate that the growth of the dark matter core is halted close to the Chandrasekhar limit. We thus dispel a myth concerning dark matter accretion by stars: dark matter accretion does not necessarily lead to the destruction of the star, nor to collapse to a black hole. Finally, we argue that stars with long-lived bosonic cores may also develop in other theories with effective mass couplings, such as (massless) scalar-tensor theories.

The symbiotic star CI Cygni: S-process episode or accretion event

International Nuclear Information System (INIS)

Kenyon, S.J.; Webbink, R.F.; Gallagher, J.S.; Truran, J.W.

1982-01-01

Evidence that the symbiotic star C I Cygni is an eclipsing binary is reviewed. It is shown that the 's-process episode' described by Audouze et al. (1981) during its 1975 outburst arises from superposition of normal gM4 absorption features on the continuum of the hot component during eclipse ingress, and not to sudden enhancements of rare earth elements. The peculiar velocity displacements of absorption lines with different excitation potentials during this episode are identified as signatures of an optically thick accretion disk, which dominates the visible spectrum during outburst. The data presented by Audouze et al., and the shape of the light curve thus provide evidence that the outburst is accretion-powered. (orig.)

The Merger of Two Compact Stars: A Tool for Dense Matter Nuclear Physics

Directory of Open Access Journals (Sweden)

Alessandro Drago

2018-03-01

Full Text Available We discuss the different signals, in gravitational and electromagnetic waves, emitted during the merger of two compact stars. We will focus in particular on the possible contraints that those signals can provide on the equation of state of dense matter. Indeed, the stiffness of the equation of state and the particle composition of the merging compact stars strongly affect, e.g., the life time of the post-merger remnant and its gravitational wave signal, the emission of the short gamma-ray-burst, the amount of ejected mass and the related kilonova. The first detection of gravitational waves from the merger of two compact stars in August 2017, GW170817, and the subsequent detections of its electromagnetic counterparts, GRB170817A and AT2017gfo, is the first example of the era of “multi-messenger astronomy”: we discuss what we have learned from this detection on the equation of state of compact stars and we provide a tentative interpretation of this event, within the two families scenario, as being due to the merger of a hadronic star with a quark star.

arXiv Gravitational Wave Signatures of Highly Compact Boson Star Binaries

CERN Document Server

Palenzuela, Carlos; Bezares, Miguel; Cardoso, Vitor; Lehner, Luis; Liebling, Steven

2017-11-30

Solitonic boson stars are stable objects made of a complex scalar field with a compactness that can reach values comparable to that of neutron stars. A recent study of the collision of identical boson stars produced only nonrotating boson stars or black holes, suggesting that rotating boson stars may not form from binary mergers. Here we extend this study to include an analysis of the gravitational waves radiated during the coalescence of such a binary, which is crucial to distinguish these events from other binaries with LIGO and Virgo observations. Our studies reveal that the remnant’s gravitational wave signature is mainly governed by its fundamental frequency as it settles down to a nonrotating boson star, emitting significant gravitational radiation during this post-merger state. We calculate how the waveforms and their post-merger frequencies depend on the compactness of the initial boson stars and estimate analytically the amount of energy radiated after the merger.

Gravitational radiation emitted when a mass falls onto a compact star.

Science.gov (United States)

Borelli, A.

1997-03-01

The authors study the energy spectrum related to the axial perturbations of a compact star when a particle falls spiralling onto it. They find that both slowly-damped quasi-normal modes and strongly damped w-modes are excited, and that a part of the energy in the process is associated to these w-modes. A substantial difference between the energy spectra of compact stars and black holes is shown.

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

International Nuclear Information System (INIS)

Perets, Hagai B.; Kenyon, Scott J.

2013-01-01

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

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

Science.gov (United States)

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

2016-12-01

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

NEAR-INFRARED SPECTROSCOPY OF LOW-MASS X-RAY BINARIES: ACCRETION DISK CONTAMINATION AND COMPACT OBJECT MASS DETERMINATION IN V404 Cyg AND Cen X-4

International Nuclear Information System (INIS)

Khargharia, Juthika; Froning, Cynthia S.; Robinson, Edward L.

2010-01-01

We present near-infrared (NIR) broadband (0.80-2.42 μm) spectroscopy of two low-mass X-ray binaries: V404 Cyg and Cen X-4. One important parameter required in the determination of the mass of the compact objects in these systems is the binary inclination. We can determine the inclination by modeling the ellipsoidal modulations of the Roche-lobe filling donor star, but the contamination of the donor star light from other components of the binary, particularly the accretion disk, must be taken into account. To this end, we determined the donor star contribution to the infrared flux by comparing the spectra of V404 Cyg and Cen X-4 to those of various field K-stars of known spectral type. For V404 Cyg, we determined that the donor star has a spectral type of K3 III. We determined the fractional donor contribution to the NIR flux in the H and K bands as 0.98 ± 0.05 and 0.97 ± 0.09, respectively. We remodeled the H-band light curve from Sanwal et al. after correcting for the donor star contribution to obtain a new value for the binary inclination. From this, we determined the mass of the black hole in V404 Cyg to be M BH = 9.0 +0.2 -0.6 M sun . We performed the same spectral analysis for Cen X-4 and found the spectral type of the donor star to be in the range K5-M1 V. The donor star contribution in Cen X-4 is 0.94 ± 0.14 in the H band while in the K band, the accretion disk can contribute up to 10% of the infrared flux. We remodeled the H-band light curve from Shahbaz et al., again correcting for the fractional contribution of the donor star to obtain the inclination. From this, we determined the mass of the neutron star as M NS = 1.5 +0.1 -0.4 M sun . However, the masses obtained for both systems should be viewed with some caution since contemporaneous light curve and spectral data are required to obtain definitive masses.

Stationary bound-state massive scalar field configurations supported by spherically symmetric compact reflecting stars

Energy Technology Data Exchange (ETDEWEB)

Hod, Shahar [The Ruppin Academic Center, Emeq Hefer (Israel); The Hadassah Academic College, Jerusalem (Israel)

2017-12-15

It has recently been demonstrated that asymptotically flat neutral reflecting stars are characterized by an intriguing no-hair property. In particular, it has been proved that these horizonless compact objects cannot support spatially regular static matter configurations made of scalar (spin-0) fields, vector (spin-1) fields and tensor (spin-2) fields. In the present paper we shall explicitly prove that spherically symmetric compact reflecting stars can support stationary (rather than static) bound-state massive scalar fields in their exterior spacetime regions. To this end, we solve analytically the Klein-Gordon wave equation for a linearized scalar field of mass μ and proper frequency ω in the curved background of a spherically symmetric compact reflecting star of mass M and radius R{sub s}. It is proved that the regime of existence of these stationary composed star-field configurations is characterized by the simple inequalities 1 - 2M/R{sub s} < (ω/μ){sup 2} < 1. Interestingly, in the regime M/R{sub s} << 1 of weakly self-gravitating stars we derive a remarkably compact analytical equation for the discrete spectrum {ω(M,R_s, μ)}{sup n=∞}{sub n=1} of resonant oscillation frequencies which characterize the stationary composed compact-reflecting-star-linearized-massive-scalar-field configurations. Finally, we verify the accuracy of the analytically derived resonance formula of the composed star-field configurations with direct numerical computations. (orig.)

Radio outburst from a massive (proto)star. When accretion turns into ejection

Science.gov (United States)

Cesaroni, R.; Moscadelli, L.; Neri, R.; Sanna, A.; Caratti o Garatti, A.; Eisloffel, J.; Stecklum, B.; Ray, T.; Walmsley, C. M.

2018-05-01

Context. Recent observations of the massive young stellar object S255 NIRS 3 have revealed a large increase in both methanol maser flux density and IR emission, which have been interpreted as the result of an accretion outburst, possibly due to instabilities in a circumstellar disk. This indicates that this type of accretion event could be common in young/forming early-type stars and in their lower mass siblings, and supports the idea that accretion onto the star may occur in a non-continuous way. Aims: As accretion and ejection are believed to be tightly associated phenomena, we wanted to confirm the accretion interpretation of the outburst in S255 NIRS 3 by detecting the corresponding burst of the associated thermal jet. Methods: We monitored the radio continuum emission from S255 NIRS 3 at four bands using the Karl G. Jansky Very Large Array. The millimetre continuum emission was also observed with both the Northern Extended Millimeter Array of IRAM and the Atacama Large Millimeter/Submillimeter Array. Results: We have detected an exponential increase in the radio flux density from 6 to 45 GHz starting right after July 10, 2016, namely 13 months after the estimated onset of the IR outburst. This is the first ever detection of a radio burst associated with an IR accretion outburst from a young stellar object. The flux density at all observed centimetre bands can be reproduced with a simple expanding jet model. At millimetre wavelengths we infer a marginal flux increase with respect to the literature values and we show this is due to free-free emission from the radio jet. Conclusions: Our model fits indicate a significant increase in the jet opening angle and ionized mass loss rate with time. For the first time, we can estimate the ionization fraction in the jet and conclude that this must be low (memory of MalcolmWalmsley, who passed away before the present study could be completed. Without his insights and enlightened advice this work would have been impossible

The Physics of Wind-Fed Accretion

International Nuclear Information System (INIS)

Mauche, Christopher W.; Liedahl, Duane A.; Akiyama, Shizuka; Plewa, Tomasz

2008-01-01

We provide a brief review of the physical processes behind the radiative driving of the winds of OB stars and the Bondi-Hoyle-Lyttleton capture and accretion of a fraction of the stellar wind by a compact object, typically a neutron star, in detached high-mass X-ray binaries (HMXBs). In addition, we describe a program to develop global models of the radiatively-driven photoionized winds and accretion flows of HMXBs, with particular attention to the prototypical system Vela X-l. The models combine XSTAR photoionization calculations, HULLAC emission models appropriate to X-ray photoionized plasmas, improved models of the radiative driving of photoionized winds, FLASH time-dependent adaptive-mesh hydrodynamics calculations, and Monte Carlo radiation transport. We present two- and three-dimensional maps of the density, temperature, velocity, ionization parameter, and emissivity distributions of representative X-ray emission lines, as well as synthetic global Monte Carlo X-ray spectra. Such models help to better constrain the properties of the winds of HMXBs, which bear on such fundamental questions as the long-term evolution of these binaries and the chemical enrichment of the interstellar medium.

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.

Possibility of higher-dimensional anisotropic compact star

International Nuclear Information System (INIS)

Bhar, Piyali; Rahaman, Farook; Ray, Saibal; Chatterjee, Vikram

2015-01-01

We provide a new class of interior solutions for anisotropic stars admitting conformal motion in higher-dimensional noncommutative spacetime. The Einstein field equations are solved by choosing a particular density distribution function of Lorentzian type as provided by Nazari and Mehdipour [1, 2] under a noncommutative geometry. Several cases with 4 and higher dimensions, e.g. 5, 6, and 11 dimensions, are discussed separately. An overall observation is that the model parameters, such as density, radial pressure, transverse pressure, and anisotropy, all are well behaved and represent a compact star with mass 2.27 M s un and radius 4.17 km. However, emphasis is put on the acceptability of the model from a physical point of view. As a consequence it is observed that higher dimensions, i.e. beyond 4D spacetime, exhibit several interesting yet bizarre features, which are not at all untenable for a compact stellar model of strange quark type; thus this dictates the possibility of its extra-dimensional existence. (orig.)

Possibility of higher-dimensional anisotropic compact star

Energy Technology Data Exchange (ETDEWEB)

Bhar, Piyali; Rahaman, Farook [Jadavpur University, Department of Mathematics, Kolkata, West Bengal (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India); Chatterjee, Vikram [Central Footwear Training Centre, Department of Physics, Parganas, West Bengal (India)

2015-05-15

We provide a new class of interior solutions for anisotropic stars admitting conformal motion in higher-dimensional noncommutative spacetime. The Einstein field equations are solved by choosing a particular density distribution function of Lorentzian type as provided by Nazari and Mehdipour [1, 2] under a noncommutative geometry. Several cases with 4 and higher dimensions, e.g. 5, 6, and 11 dimensions, are discussed separately. An overall observation is that the model parameters, such as density, radial pressure, transverse pressure, and anisotropy, all are well behaved and represent a compact star with mass 2.27 M{sub s}un and radius 4.17 km. However, emphasis is put on the acceptability of the model from a physical point of view. As a consequence it is observed that higher dimensions, i.e. beyond 4D spacetime, exhibit several interesting yet bizarre features, which are not at all untenable for a compact stellar model of strange quark type; thus this dictates the possibility of its extra-dimensional existence. (orig.)

Accretion-Ejection Instability in magnetized accretion disk around compact objects

International Nuclear Information System (INIS)

Varniere, Peggy

2002-01-01

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

Cooling compact stars and phase transitions in dense QCD

Energy Technology Data Exchange (ETDEWEB)

Sedrakian, Armen [J.W. Goethe University, Institute for Theoretical Physics, Frankfurt am Main (Germany)

2016-03-15

We report new simulations of cooling of compact stars containing quark cores and updated fits to the Cas A fast cooling data. Our model is built on the assumption that the transient behaviour of the star in Cas A is due to a phase transition within the dense QCD matter in the core of the star. Specifically, the fast cooling is attributed to an enhancement in the neutrino emission triggered by a transition from a fully gapped, two-flavor, red-green color-superconducting quark condensate to a superconducting crystalline or an alternative gapless, color-superconducting phase. The blue-colored condensate is modeled as a Bardeen-Cooper-Schrieffer (BCS)-type color superconductor with spin-one pairing order parameter. We study the sensitivity of the fits to the phase transition temperature, the pairing gap of blue quarks and the timescale characterizing the phase transition (the latter modelled in terms of a width parameter). Relative variations in these parameter around their best-fit values larger than 10{sup -3} spoil the fit to the data. We confirm the previous finding that the cooling curves show significant variations as a function of compact star mass, which allows one to account for dispersion in the data on the surface temperatures of thermally emitting neutron stars. (orig.)

Local Swift-BAT active galactic nuclei prefer circumnuclear star formation

Science.gov (United States)

Lutz, D.; Shimizu, T.; Davies, R. I.; Herrera-Camus, R.; Sturm, E.; Tacconi, L. J.; Veilleux, S.

2018-01-01

We use Herschel data to analyze the size of the far-infrared 70 μm emission for z BAT selected active galactic nuclei (AGN), and 515 comparison galaxies that are not detected by BAT. For modest far-infrared luminosities 8.5 BAT hosts that is only half that of comparison galaxies of same far-infrared luminosity. The result mostly reflects a more compact distribution of star formation (and hence gas) in the AGN hosts, but compact AGN heated dust may contribute in some extremely AGN dominated systems. Our findings are in support of an AGN-host coevolution where accretion onto the central black hole and star formation are fed from the same gas reservoir, with more efficient black hole feeding if that reservoir is more concentrated. The significant scatter in the far-infrared sizes emphasizes that we are mostly probing spatial scales much larger than those of actual accretion, and that rapid accretion variations can smear the distinction between the AGN and comparison categories. Large samples are hence needed to detect structural differences that favor feeding of the black hole. No size difference between AGN host and comparison galaxies is observed at higher far-infrared luminosities log(LFIR [L⊙]) > 10.5 (star formation rates ≳6 M⊙ yr-1), possibly because these are typically reached in more compact regions. Full Table A.1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/609/A9

Compact Objects in Astrophysics White Dwarfs, Neutron Stars and Black Holes

CERN Document Server

Camenzind, Max

2007-01-01

Compact objects are an important class of astronomical objects in current research. Supermassive black holes play an important role in the understanding of the formation of galaxies in the early Universe. Old white dwarfs are nowadays used to calibrate the age of the Universe. Mergers of neutron stars and black holes are the sources of intense gravitational waves which will be measured in the next ten years by gravitational wave detectors. Camenzind's Compact Objects in Astrophysics gives a comprehensive introduction and up-to-date overview about the physical processes behind these objects, covering the field from the beginning to most recent results, including all relevant observations. After a presentation of the taxonomy of compact objects, the basic principles of general relativity are given. The author then discusses in detail the physics and observations of white dwarfs and neutron stars (including the most recent equations of state for neutron star matter), the gravitational field of rapidly rotating c...

Radial modes of slowly rotating compact stars in the presence of magnetic field

Energy Technology Data Exchange (ETDEWEB)

Panda, N.R. [Institute of Physics, Bhubaneswar (India); Siksha ' O' Anusandhan University, Bhubaneswar (India); Mohanta, K.K. [Rairangpur College, Rairangpur, Odisha (India); Sahu, P.K. [Institute of Physics, Bhubaneswar (India)

2016-09-15

Compact stars are composed of very high-density hadron matter. When the matter is above nuclear matter density, then there is a chance of different phases of matter such as hadron matter to quark matter. There is a possible phase which, having the quark core surrounded by a mixed phase followed by hadronic matter, may be considered as a hybrid phase inside the stars called hybrid star (HS). The star which consists of only u, d and s quarks is called quark star (QS) and the star which has only hadronic matter is called neutron star (NS). For the equation of state (EOS) of hadronic matter, we have considered the Relativistic Mean Field (RMF) theory and we incorporated the effect of strong magnetic fields. For the EOS of the quark phase we use the simple MIT bag model. We have assumed Gaussian parametrization to make the density dependent for both bag pressure in quark matter and magnetic field. We have constructed the intermediate mixed phase by using the Glendenning conjecture. Eigenfrequencies of radial pulsations of slowly rotating magnetized compact stars (NS, QS, HS) are calculated in a general relativistic formalism given by Chandrasekhar and Friedman. We have studied the effect of central density on the square of the frequencies of the compact stars in the presence of zero and strong magnetic field. (orig.)

Low-Metallicity Blue Compact Dwarfs as Templates for Primordial Star Formation

OpenAIRE

Hunt, L. K.; Hirashita, H.; Thuan, T. X.; Izotov, Y. I.; Vanzi, L.

2003-01-01

Understanding how galaxies formed their first stars is a vital cosmological question, but the study of high-redshift objects, caught in the act of forming their first stars, is difficult. Here we argue that two extremely low-metallicity Blue Compact Dwarf galaxies (BCDs), IZw18 and SBS0335-052, could be local templates for primordial star formation, since both lack evolved ($> $1 Gyr) stellar populations; but they form stars differently.

Formation of Compact Ellipticals in the merging star cluster scenario

Science.gov (United States)

Urrutia Zapata, Fernanda Cecilia; Theory and star formation group

2018-01-01

In the last years, extended old stellar clusters have been observed. They are like globular clusters (GCs) but with larger sizes(a limit of Re=10 pc is currently seen as reasonable). These extended objects (EOs) cover a huge range of mass. Objects at the low mass end with masses comparable to normal globular clusters are called extended clusters or faint fuzzies Larsen & Brodie (2000) and objects at the high-mass end are called ultra compact dwarf galaxies (UCDs). Ultra compact dwarf galaxies are compact object with luminositys above the brigtest known GCs. UCDs are more compact than typical dwarf galaxies but with comparable luminosities. Usually, a lower mass limit of 2 × 10^6 Solar masses is applied.Fellhauer & Kroupa (2002a,b) demostrated that object like ECs, FFs and UCDs can be the remnants of the merger of star clusters complexes, this scenario is called the Merging Star Cluster Scenario. Amore concise study was performed by Bruens et al. (2009, 2011).Our work tries to explain the formation of compact elliptical(cE). These objects are a comparatively rare class of spheroidal galaxies, possessing very small Re and high central surface brightnesses (Faber 1973). cEs have the same parameters as extended objects but they are slightly larger than 100 pc and the luminosities are in the range of -11 to -12 Mag.The standard formation sceanrio of these systems proposes a galaxy origin. CEs are the result of tidal stripping and truncation of nucleated larger systems. Or they could be a natural extension of the class of elliptical galaxies to lower luminosities and smaller sizes.We want to propose a completely new formation scenario for cEs. In our project we try to model cEs in a similar way that UCDs using the merging star cluster scenario extended to much higher masses and sizes. We think that in the early Universe we might have produced sufficiently strong star bursts to form cluster complexes which merge into cEs. So far it is observationally unknown if cEs are

Gamma-Rays from Galactic Compact Sources

Science.gov (United States)

Kaaret, Philip

2007-04-01

Recent discoveries have revealed many sources of TeV photons in our Mikly Way galaxy powered by compact objects, either neutron stars or black holes. These objects must be powerful particle accelerators, some with peak energies of at least 100 TeV, and may be neutrino, as well as photon, sources. Future TeV observations will enable us to address key questions concerning particle acceleration by compact objects including the fraction of energy which accreting black holes channel into relativstic jet production, whether the compact object jets are leptonic or hadronic, and the mechanism by which pulsar winds accelerate relativistic particles. We report on work done related to compact Galactic objects in preparation of a White Paper on the status and future of ground-based gamma-ray astronomy requested by the Division of Astrophysics of the American Physical Society.

Anisotropic models for compact stars

Energy Technology Data Exchange (ETDEWEB)

Maurya, S.K.; Dayanandan, Baiju [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman); Gupta, Y.K. [Jaypee Institute of Information Technology University, Department of Mathematics, Noida, Uttar Pradesh (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India)

2015-05-15

In the present paper we obtain an anisotropic analog of the Durgapal and Fuloria (Gen Relativ Gravit 17:671, 1985) perfect fluid solution. The methodology consists of contraction of the anisotropic factor Δ with the help of both metric potentials e{sup ν} and e{sup λ}. Here we consider e{sup λ} the same as Durgapal and Fuloria (Gen Relativ Gravit 17:671, 1985) did, whereas e{sup ν} is as given by Lake (Phys Rev D 67:104015, 2003). The field equations are solved by the change of dependent variable method. The solutions set mathematically thus obtained are compared with the physical properties of some of the compact stars, strange star as well as white dwarf. It is observed that all the expected physical features are available related to the stellar fluid distribution, which clearly indicates the validity of the model. (orig.)

Hydrodynamic ejection of bipolar flows from objects undergoing disk accretion: T Tauri stars, massive pre-main-sequence objects, and cataclysmic variables

International Nuclear Information System (INIS)

Torbett, M.V.

1984-01-01

A general mechanism is presented for generating pressure-driven winds that are intrinsically bipolar from objects undergoing disk accretion. The energy librated in a boundary layer shock as the disk matter impacts the central object is shown to be sufficient to eject a fraction βapprox.10 -2 to 10 -3 of the accreted mass. These winds are driven by a mechanism that accelerates the flow perpendicular to the plane of the disk and can therefore account for the bipolar geometry of the mass loss observed near young stars. The mass loss contained in these winds is comparable to that inferred for young stars. Thus, disk accretion-driven winds may constitute the T Tauri phase of stellar evolution. This mechanism is generally applicable, and thus massive pre-main-sequence objects as well as cataclysmic variables at times of enhanced accretion are predicted to eject bipolar outflows as well. Unmagnetized accreting neutron stas are also expected to eject bipolar flows. Since this mechanism requires stellar surfaces, however, it will not operate in disk accretion onto black holes

A new model for spherically symmetric anisotropic compact star

Energy Technology Data Exchange (ETDEWEB)

Maurya, S.K.; Dayanandan, Baiju [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman); Gupta, Y.K. [Raj Kumar Goel Institute of Technology, Department of Mathematics, Ghaziabad, UP (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India)

2016-05-15

In this article we obtain a new anisotropic solution for Einstein's field equations of embedding class one metric. The solution represents realistic objects such as Her X-1 and RXJ 1856-37. We perform a detailed investigation of both objects by solving numerically the Einstein field equations with anisotropic pressure. The physical features of the parameters depend on the anisotropic factor i.e. if the anisotropy is zero everywhere inside the star then the density and pressures will become zero and the metric turns out to be flat. We report our results and compare with the above mentioned two compact objects as regards a number of key aspects: the central density, the surface density onset and the critical scaling behaviour, the effective mass and radius ratio, the anisotropization with isotropic initial conditions, adiabatic index and red shift. Along with this we have also made a comparison between the classical limit and theoretical model treatment of the compact objects. Finally we discuss the implications of our findings for the stability condition in a relativistic compact star. (orig.)

NuSTAR Discovery of a Cyclotron Line in the Accreting X-Ray Pulsar IGR J16393-4643

Science.gov (United States)

Bodaghee, Arash; Tomsick, John A.; Fornasini, Francesca M.; Krivonos, Roman; Stern, Daniel; Mori, Kaya; Rahoui, Farid; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.;

Formation of new stellar populations from gas accreted by massive young star clusters.

Science.gov (United States)

Li, Chengyuan; de Grijs, Richard; Deng, Licai; Geller, Aaron M; Xin, Yu; Hu, Yi; Faucher-Giguère, Claude-André

2016-01-28

Stars in clusters are thought to form in a single burst from a common progenitor cloud of molecular gas. However, massive, old 'globular' clusters--those with ages greater than ten billion years and masses several hundred thousand times that of the Sun--often harbour multiple stellar populations, indicating that more than one star-forming event occurred during their lifetimes. Colliding stellar winds from late-stage, asymptotic-giant-branch stars are often suggested to be triggers of second-generation star formation. For this to occur, the initial cluster masses need to be greater than a few million solar masses. Here we report observations of three massive relatively young star clusters (1-2 billion years old) in the Magellanic Clouds that show clear evidence of burst-like star formation that occurred a few hundred million years after their initial formation era. We show that such clusters could have accreted sufficient gas to form new stars if they had orbited in their host galaxies' gaseous disks throughout the period between their initial formation and the more recent bursts of star formation. This process may eventually give rise to the ubiquitous multiple stellar populations in globular clusters.

Constraints on the braneworld from compact stars

Energy Technology Data Exchange (ETDEWEB)

Felipe, R.G. [Instituto Politecnico de Lisboa, ISEL, Instituto Superior de Engenharia de Lisboa, Lisboa (Portugal); Instituto Superior Tecnico, Universidade de Lisboa, Departamento de Fisica, Centro de Fisica Teorica de Particulas, CFTP, Lisboa (Portugal); Paret, D.M. [Universidad de la Habana, Departamento de Fisica General, Facultad de Fisica, La Habana (Cuba); Martinez, A.P. [Instituto de Cibernetica, Matematica y Fisica (ICIMAF), La Habana (Cuba); Universidad Nacional Autonoma de Mexico, Instituto de Ciencias Nucleares, Mexico, Distrito Federal (Mexico)

2016-06-15

According to the braneworld idea, ordinary matter is confined on a three-dimensional space (brane) that is embedded in a higher-dimensional space-time where gravity propagates. In this work, after reviewing the limits coming from general relativity, finiteness of pressure and causality on the brane, we derive observational constraints on the braneworld parameters from the existence of stable compact stars. The analysis is carried out by solving numerically the brane-modified Tolman-Oppenheimer-Volkoff equations, using different representative equations of state to describe matter in the star interior. The cases of normal dense matter, pure quark matter and hybrid matter are considered. (orig.)

Evolution of compact stars and dark dynamical variables

Energy Technology Data Exchange (ETDEWEB)

Bhatti, M.Z.; Yousaf, Z. [University of the Punjab, Department of Mathematics, Lahore (Pakistan); Ilyas, M. [University of the Punjab, Centre for High Energy Physics, Lahore (Pakistan)

2017-10-15

This work aims to explore the dark dynamical effects of the f(R, T) modified gravity theory on the dynamics of a compact celestial star. We have taken the interior geometry of a spherical star which is filled with an imperfect fluid distribution. The modified field equations are explored by taking a particular form of the f(R, T) model, i.e. f(R, T) = f{sub 1}(R) + f{sub 2}(R)f{sub 3}(T). These equations are utilized to formulate the well-known structure scalars under the dark dynamical effects of this higher-order gravity theory. Also, with the help of these scalar variables, the evolution equations for expansion and shear are formulated. The whole analysis is made under the condition of a constant R and T. We found a crucial significance of dark source terms and dynamical variables on the evolution and density inhomogeneity of compact objects. (orig.)

A model for neutrino emission from nuclear accretion disks

Science.gov (United States)

Deaton, Michael

2015-04-01

Compact object mergers involving at least one neutron star can produce short-lived black hole accretion engines. Over tens to hundreds of milliseconds such an engine consumes a disk of hot, nuclear-density fluid, and drives changes to its surrounding environment through luminous emission of neutrinos. The neutrino emission may drive an ultrarelativistic jet, may peel off the disk's outer layers as a wind, may irradiate those winds or other forms of ejecta and thereby change their composition, may change the composition and thermodynamic state of the disk itself, and may oscillate in its flavor content. We present the full spatial-, angular-, and energy-dependence of the neutrino distribution function around a realistic model of a nuclear accretion disk, to inform future explorations of these types of behaviors. Spectral Einstein Code (SpEC).

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

Science.gov (United States)

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

2018-02-01

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

On the nature of the symbiotic star BF Cygni

International Nuclear Information System (INIS)

Mikolajewska, J.; Mikolajewski, M.; Kenyon, S.J.

1989-01-01

Optical and ultraviolet spectroscopy of the symbiotic binary BF Cyg obtained during 1979-1988 is discussed. This system consists of a low-mass M5 giant filling about 50 percent of its tidal volume and a hot, luminous compact object similar to the central star of a planetary nebula. The binary is embedded in an asymmetric nebula which includes a small, high-density region and an extended region of lower density. The larger nebula is formed by a slow wind ejected by the cool component and ionized by the hot star, while the more compact nebula is material expelled by the hot component in the form of a bipolar wind. The analysis indicates that disk accretion is essential to maintain the nuclear burning shell of the hot star. 84 refs

Effect of non-stationary accretion on spectral state transitions: An example of a persistent neutron star LMXB 4U1636–536

Science.gov (United States)

Zhang, Hui; Yu, Wen-Fei

2018-03-01

Observations of black hole and neutron star X-ray binaries show that the luminosity of the hard-to-soft state transition is usually higher than that of the soft-to-hard state transition, indicating additional parameters other than mass accretion rate are required to interpret spectral state transitions. It has been found in some individual black hole or neutron star soft X-ray transients that the luminosity corresponding to the hard-to-soft state transition is positively correlated with the peak luminosity of the following soft state. In this work, we report the discovery of the same correlation in the single persistent neutron star low mass X-ray binary (LMXB) 4U 1636–536 based on data from the All Sky Monitor (ASM) on board RXTE, the Gas Slit Camera (GSC) on board MAXI and the Burst Alert Telescope (BAT) on board Swift. We also found such a positive correlation holds in this persistent neutron star LMXB in a luminosity range spanning about a factor of four. Our results indicate that non-stationary accretion also plays an important role in driving X-ray spectral state transitions in persistent accreting systems with small accretion flares, which is much less dramatic compared with the bright outbursts seen in many Galactic LMXB transients.

Reaction rate and composition dependence of the stability of thermonuclear burning on accreting neutron stars

International Nuclear Information System (INIS)

Keek, L.; Cyburt, R. H.; Heger, A.

2014-01-01

The stability of thermonuclear burning of hydrogen and helium accreted onto neutron stars is strongly dependent on the mass accretion rate. The burning behavior is observed to change from Type I X-ray bursts to stable burning, with oscillatory burning occurring at the transition. Simulations predict the transition at a 10 times higher mass accretion rate than observed. Using numerical models we investigate how the transition depends on the hydrogen, helium, and CNO mass fractions of the accreted material, as well as on the nuclear reaction rates of 3α and the hot-CNO breakout reactions 15 O(α, γ) 19 Ne and 18 Ne(α, p) 21 Na. For a lower hydrogen content the transition is at higher accretion rates. Furthermore, most experimentally allowed reaction rate variations change the transition accretion rate by at most 10%. A factor 10 decrease of the 15 O(α, γ) 19 Ne rate, however, produces an increase of the transition accretion rate of 35%. None of our models reproduce the transition at the observed rate, and depending on the true 15 O(α, γ) 19 Ne reaction rate, the actual discrepancy may be substantially larger. We find that the width of the interval of accretion rates with marginally stable burning depends strongly on both composition and reaction rates. Furthermore, close to the stability transition, our models predict that X-ray bursts have extended tails where freshly accreted fuel prolongs nuclear burning.

Rotation and Accretion Powered Pulsars

Energy Technology Data Exchange (ETDEWEB)

Kaspi, V M [Department of Physics, McGill University, 3600 University St, Montreal, QC H3A 2T8 (Canada)

2008-03-07

Pulsar astrophysics has come a long way in the 40 years since the discovery of the first pulsar by Bell and Hewish. From humble beginnings as bits of 'scruff' on the Cambridge University group's chart recorder paper, the field of pulsars has blossomed into a major area of mainstream astrophysics, with an unparalleled diversity of astrophysical applications. These range from Nobel-celebrated testing of general relativity in the strong-field regime to constraining the equation-of-state of ultradense matter; from probing the winds of massive stars to globular cluster evolution. Previous notable books on the subject of pulsars have tended to focus on some particular topic in the field. The classic text Pulsars by Manchester and Taylor (1977 San Francisco, CA: Freeman) targeted almost exclusively rotation-powered radio pulsars, while the Meszaros book High-Energy Radiation from Magnetized Neutron Stars (1992 Chicago, IL: University of Chicago Press) considered both rotation- and accretion-powered neutron stars, but focused on their radiation at x-ray energies and above. The recent book Neutron Stars 1 by Haensel et al (2007 Berlin: Springer) considers only the equation of state and neutron-star structure. Into this context appears Rotation and Accretion Powered Pulsars, by Pranab Ghosh. In contrast to other books, here the author takes an encyclopedic approach and attempts to synthesize practically all of the major aspects of the two main types of neutron star. This is ambitious. The only comparable undertaking is the useful but more elementary Lyne and Graham-Smith text Pulsar Astronomy (1998 Cambridge: Cambridge University Press), or Compact Stellar X-ray Sources (eds Lewin and van der Klis, 2006 Cambridge: Cambridge University Press), an anthology of technical review articles that also includes black hole topics. Rotation and Accretion Powered Pulsars thus fills a clear void in the field, providing a readable, graduate-level book that covers nearly

Rotation and Accretion Powered Pulsars

International Nuclear Information System (INIS)

Kaspi, V M

2008-01-01

Pulsar astrophysics has come a long way in the 40 years since the discovery of the first pulsar by Bell and Hewish. From humble beginnings as bits of 'scruff' on the Cambridge University group's chart recorder paper, the field of pulsars has blossomed into a major area of mainstream astrophysics, with an unparalleled diversity of astrophysical applications. These range from Nobel-celebrated testing of general relativity in the strong-field regime to constraining the equation-of-state of ultradense matter; from probing the winds of massive stars to globular cluster evolution. Previous notable books on the subject of pulsars have tended to focus on some particular topic in the field. The classic text Pulsars by Manchester and Taylor (1977 San Francisco, CA: Freeman) targeted almost exclusively rotation-powered radio pulsars, while the Meszaros book High-Energy Radiation from Magnetized Neutron Stars (1992 Chicago, IL: University of Chicago Press) considered both rotation- and accretion-powered neutron stars, but focused on their radiation at x-ray energies and above. The recent book Neutron Stars 1 by Haensel et al (2007 Berlin: Springer) considers only the equation of state and neutron-star structure. Into this context appears Rotation and Accretion Powered Pulsars, by Pranab Ghosh. In contrast to other books, here the author takes an encyclopedic approach and attempts to synthesize practically all of the major aspects of the two main types of neutron star. This is ambitious. The only comparable undertaking is the useful but more elementary Lyne and Graham-Smith text Pulsar Astronomy (1998 Cambridge: Cambridge University Press), or Compact Stellar X-ray Sources (eds Lewin and van der Klis, 2006 Cambridge: Cambridge University Press), an anthology of technical review articles that also includes black hole topics. Rotation and Accretion Powered Pulsars thus fills a clear void in the field, providing a readable, graduate-level book that covers nearly everything you

An Accretion Model for Anomalous X-Ray Pulsars

Science.gov (United States)

Chatterjee, Pinaki; Hernquist, Lars; Narayan, Ramesh

2000-05-01

We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star toward a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength, and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to the situation in models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.

Dark matter, neutron stars, and strange quark matter.

Science.gov (United States)

Perez-Garcia, M Angeles; Silk, Joseph; Stone, Jirina R

2010-10-01

We show that self-annihilating weakly interacting massive particle (WIMP) dark matter accreted onto neutron stars may provide a mechanism to seed compact objects with long-lived lumps of strange quark matter, or strangelets, for WIMP masses above a few GeV. This effect may trigger a conversion of most of the star into a strange star. We use an energy estimate for the long-lived strangelet based on the Fermi-gas model combined with the MIT bag model to set a new limit on the possible values of the WIMP mass that can be especially relevant for subdominant species of massive neutralinos.

A comparison between star products on regular orbits of compact Lie groups

International Nuclear Information System (INIS)

Fioresi, R.; Lledo, M.A.

2002-01-01

In this paper, an algebraic and a differential star product defined on a regular coadjoint orbit of a compact semisimple group are compared. It has been proved that there is an injective algebra homomorphism between the algebra of polynomials with the algebraic star product and the algebra of differential functions with the differential star product structure. (author)

EVOLUTION OF MASSIVE PROTOSTARS VIA DISK ACCRETION

International Nuclear Information System (INIS)

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

2010-01-01

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

Helium-burning flashes on accreting neutron stars: effects of stellar mass, radius, and magnetic field

International Nuclear Information System (INIS)

Joss, P.C.; Li, F.K.

1980-01-01

We have computed the evolution of the helium-burning shell in an accreting neutron star for various values of the stellar mass (M), radius (R), and surface magnetic fields strength (B). As shown in previous work, the helium-burning shell is often unstable and undergoes thermonuclear flashes that result in the emission of X-ray bursts from the neutron-star surface. The dependence of the properties of these bursts upon the values of M and R can be described by simple scaling relations. A strong magnetic field decreases the radiative and conductive opacities and inhibits convection in the neutron-star surface layers. For B 12 gauss, these effects are unimportant; for B> or approx. =10 13 gauss, the enhancement of the electron thermal conductivity is sufficiently large to stabilize the helium-burning shell against thermonuclear flashes. For intermediate values of B, the reduced opacities increase the recurrence intervals between bursts and the energy released per burst, while the inhibition of convection increases the burst rise times to about a few seconds. If the magnetic field funnels the accreting matter onto the magnetic polar caps, the instability of the helium-burning shell will be very strongly suppressed. These results suggest that it may eventually be possible to extract information on the macroscopic properties of neutron stars from the observed features of X-ray burst sources

Unsteady Plasma Ejections from Hollow Accretion Columns of Galactic Neutron Stars as a Trigger for Gamma-Ray Bursts

Science.gov (United States)

Gvaramadze, V. V.

1995-09-01

We propose a model of gamma-ray bursts (GRBs) based on close Galactic neutron stars with accretion disks. We outline a simple mechanism of unsteady plasma ejections during episodic accretion events. The relative kinetic energy of ejected blobs can be converted into gamma-rays by internal shocks. The beaming of gamma-ray emission can be responsible for the observed isotropic angular distribution of GRBs.

Three-dimensional simulations of the interaction between the nova ejecta, accretion disk, and companion star

Science.gov (United States)

Figueira, Joana; José, Jordi; García-Berro, Enrique; Campbell, Simon W.; García-Senz, Domingo; Mohamed, Shazrene

2018-05-01

Context. Classical novae are thermonuclear explosions hosted by accreting white dwarfs in stellar binary systems. Material piles up on top of the white dwarf star under mildly degenerate conditions, driving a thermonuclear runaway. The energy released by the suite of nuclear processes operating at the envelope, mostly proton-capture reactions and β+-decays, heats the material up to peak temperatures ranging from 100 to 400 MK. In these events, about 10-3-10-7 M⊙, enriched in CNO and, sometimes, other intermediate-mass elements (e.g., Ne, Na, Mg, and Al) are ejected into the interstellar medium. Aims: To date, most of the efforts undertaken in the modeling of classical nova outbursts have focused on the early stages of the explosion and ejection, ignoring the interaction of the ejecta, first with the accretion disk orbiting the white dwarf and ultimately with the secondary star. Methods: A suite of 3D, smoothed-particle hydrodynamics (SPH) simulations of the interaction between the nova ejecta, accretion disk, and stellar companion were performed to fill this gap; these simulations were aimed at testing the influence of the model parameters—that is, the mass and velocity of the ejecta, mass and the geometry of the accretion disk—on the dynamical and chemical properties of the system. Results: We discuss the conditions that lead to the disruption of the accretion disk and to mass loss from the binary system. In addition, we discuss the likelihood of chemical contamination of the stellar secondary induced by the impact with the nova ejecta and its potential effect on the next nova cycle. Movies showing the full evolution of several models are available online at http://https://www.aanda.org and at http://www.fen.upc.edu/users/jjose/Downloads.html

NICER Eyes on Bursting Stars

Science.gov (United States)

Kohler, Susanna

2018-03-01

What happens to a neutron stars accretion disk when its surface briefly explodes? A new instrument recently deployed at the International Space Station (ISS) is now watching bursts from neutron stars and reporting back.Deploying a New X-Ray MissionLaunch of NICER aboard a Falcon 9 rocket in June 2017. [NASA/Tony Gray]In early June of 2017, a SpaceX Dragon capsule on a Falcon 9 rocket launched on a resupply mission to the ISS. The pressurized interior of the Dragon contained the usual manifest of crew supplies, spacewalk equipment, and vehicle hardware. But the unpressurized trunk of the capsule held something a little different: the Neutron star Interior Composition Explorer (NICER).In the two weeks following launch, NICER was extracted from the SpaceX Dragon capsule and installed on the ISS. And by the end of the month, the instrument was already collecting its first data set: observations of a bright X-ray burst from Aql X-1, a neutron star accreting matter from a low-mass binary companion.Impact of BurstsNICERs goal is to provide a new view of neutron-star physics at X-ray energies of 0.212 keV a window that allows us to explore bursts of energy that neutron stars sometimes emit from their surfaces.Artists impression of an X-ray binary, in which a compact object accretes material from a companion star. [ESA/NASA/Felix Mirabel]In X-ray burster systems, hydrogen- and helium-rich material from a low-mass companion star piles up in an accretion disk around the neutron star. This material slowly funnels onto the neutron stars surface, forming a layer that gravitationally compresses and eventually becomes so dense and hot that runaway nuclear fusion ignites.Within seconds, the layer of material is burned up, producing a burst of emission from the neutron star that outshines even the inner regions of the hot accretion disk. Then more material funnels onto the neutron star and the process begins again.Though we have a good picture of the physics that causes these bursts

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

Science.gov (United States)

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

2017-12-13

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

GRB 130603B: No Compelling Evidence for Neutron Star Merger

Directory of Open Access Journals (Sweden)

Shlomo Dado

2015-01-01

Full Text Available The near infrared (NIR flare/rebrightening in the afterglow of the short hard gamma ray burst (SHB 130603B measured with the Hubble Space Telescope (HST and an alleged late-time X-ray excess were interpreted as possible evidence of a neutron star merger origin of SHBs. However, the X-ray afterglow that was measured with the Swift XRT and Newton XMM has the canonical behaviour of a synchrotron afterglow produced by a highly relativistic jet. The H-band flux observed with HST 9.41 days after burst is that expected from the measured late-time X-ray afterglow. The late-time flare/rebrightening of the NIR-optical afterglow of SHB 130603B could have been produced also by jet collision with an interstellar density bump. Moreover, SHB plus a kilonova can be produced also by the collapse of a compact star (neutron star, strange star, or quark star to a more compact object due to cooling, loss of angular momentum, or mass accretion.

Evolution of the accretion structure of the compact object in the symbiotic binary BF Cygni during outburst in 2009-2014

Science.gov (United States)

Tomov, N. A.; Tomova, M. T.; Bisikalo, D. V.

2017-12-01

The eclipsing symbiotic binary BF Cyg has had five orbital minima during its last optical outburst after 2006. The second minimum is much shallower than the first one and after that the minimum get deeper again. We determined the parameters of the accretion structure surrounding the compact object in two minima and traced its evolution until 2014. Moreover, we analysed the continuum of the system in the region of the UBVRCIC photometric bands to derive the parameters of its components at two times orbital maximum and calculated the mass-loss rate of the compact object. The results obtained allow us to conclude about the mechanism of fading of the optical light of the system until 2014. These results show that the optical flux of the outbursted compact object decreases because of "contraction" of its observed photosphere (pseudophotosphere) which, on its side, is due to increase of the velocity of its stellar wind, and the optical flux of the circumbinary nebula decreases mainly because of reduction of its mean density, which, on its side, is due to destruction of the accretion structure.

The scenario of two families of compact stars. Pt. 1. Equations of state, mass-radius relations and binary systems

Energy Technology Data Exchange (ETDEWEB)

Drago, Alessandro; Pagliara, Giuseppe [Ferrara Univ. (Italy). Dipt. di Fisica e Scienze della Terra; INFN, Ferrara (Italy); Lavagno, Andrea; Pigato, Daniele [Politecnico di Torino (Italy). Dept. of Applied Science and Technology; INFN, Torino (Italy)

2016-02-15

We present several arguments which favor the scenario of two coexisting families of compact stars: hadronic stars and quark stars. Besides the well-known hyperon puzzle of the physics of compact stars, a similar puzzle exists also when considering delta resonances. We show that these particles appear at densities close to twice saturation density and must be therefore included in the calculations of the hadronic equation of state. Such an early appearance is strictly related to the value of the L parameter of the symmetry energy that has been found, in recent phenomenological studies, to lie in the range 40 < L < 62 MeV. We discuss also the threshold for the formation of deltas and hyperons for hot and lepton-rich hadronic matter. Similarly to the case of hyperons, also delta resonances cause a softening of the equation of state, which makes it difficult to obtain massive hadronic stars. Quark stars, on the other hand, can reach masses up to 2.75M {sub CircleDot} as predicted by perturbative QCD calculations. We then discuss the observational constraints on the masses and the radii of compact stars. The tension between the precise measurements of high masses and the indications of the existence of very compact stellar objects (with radii of the order of 10 km) is relieved when assuming that very massive compact stars are quark stars and very compact stars are hadronic stars. Finally, we discuss recent interesting measurements of the eccentricities of the orbits of millisecond pulsars in low mass X-ray binaries. The high values of the eccentricities found in some cases could be explained by assuming that the hadronic star, initially present in the binary system, converts to a quark star due to the increase of its central density. (orig.)

Thermodynamics of compact-star matter within an Ising approach

International Nuclear Information System (INIS)

Chomaz, P.; Ducoin, C.; Gulminelli, F.; Hasnaoui, K.; Napolitani, P.

2007-01-01

In the formation and evolution of compact stars, nuclear matter explores high thermal excursions and is the site of intense neutrino emission. Neutrino transport as well as structural properties of this matter depend on the presence of inhomogeneous phases (named 'pasta' phases), which are the result of Coulomb frustration of the Liquid-Gas phase transition. We take into account charge fluctuations by employing a frustrated lattice-gas model to which we impose a neutrality constraint by the addition of an homogeneous background of charge, representing delocalized electrons. Within this schematic model we highlight a generic feature of the phase-transition phenomenology: the temperature interval where pasta phases are formed is enhanced by Coulomb-frustration effects. This result is at variance with the behaviour of frustrated ferromagnetic systems as well as hot nuclei and mean-field approaches. Moreover, the region of phase coexistence is not found to end upon a critical point, indicating that no critical opalescence can occur in compact-star matter

Thermodynamics of compact-star matter within an Ising approach

Energy Technology Data Exchange (ETDEWEB)

Chomaz, P. [Ganil (DSM-CEA/IN2P3-CNRS), Blvd. H. Becquerel, BP 55027, F-14076 Caen cedex 5 (France); Ducoin, C. [Ganil (DSM-CEA/IN2P3-CNRS), Blvd. H. Becquerel, BP 55027, F-14076 Caen cedex 5 (France); LPC - IN2P3-CNRS/Ensicaen et Universite, F-14076 Caen cedex (France); Gulminelli, F. [LPC - IN2P3-CNRS/Ensicaen et Universite, F-14076 Caen cedex (France); Hasnaoui, K. [Ganil (DSM-CEA/IN2P3-CNRS), Blvd. H. Becquerel, BP 55027, F-14076 Caen cedex 5 (France); Napolitani, P. [Ganil (DSM-CEA/IN2P3-CNRS), Blvd. H. Becquerel, BP 55027, F-14076 Caen cedex 5 (France); LPC - IN2P3-CNRS/Ensicaen et Universite, F-14076 Caen cedex (France)

2007-05-01

In the formation and evolution of compact stars, nuclear matter explores high thermal excursions and is the site of intense neutrino emission. Neutrino transport as well as structural properties of this matter depend on the presence of inhomogeneous phases (named 'pasta' phases), which are the result of Coulomb frustration of the Liquid-Gas phase transition. We take into account charge fluctuations by employing a frustrated lattice-gas model to which we impose a neutrality constraint by the addition of an homogeneous background of charge, representing delocalized electrons. Within this schematic model we highlight a generic feature of the phase-transition phenomenology: the temperature interval where pasta phases are formed is enhanced by Coulomb-frustration effects. This result is at variance with the behaviour of frustrated ferromagnetic systems as well as hot nuclei and mean-field approaches. Moreover, the region of phase coexistence is not found to end upon a critical point, indicating that no critical opalescence can occur in compact-star matter.

Thermodynamics of compact-star matter within an Ising approach

Science.gov (United States)

Chomaz, P.; Ducoin, C.; Gulminelli, F.; Hasnaoui, K.; Napolitani, P.

2007-05-01

In the formation and evolution of compact stars, nuclear matter explores high thermal excursions and is the site of intense neutrino emission. Neutrino transport as well as structural properties of this matter depend on the presence of inhomogeneous phases (named "pasta" phases), which are the result of Coulomb frustration of the Liquid-Gas phase transition. We take into account charge fluctuations by employing a frustrated lattice-gas model to which we impose a neutrality constraint by the addition of an homogeneous background of charge, representing delocalised electrons. Within this schematic model we highlight a generaic feature of the phase-transition phenomenology: the temperature interval where pasta phases are formed is enhanced by Coulomb-frustration effects. This result is at variance with the behaviour of frustrated ferromagnetic systems as well as hot nuclei and mean-field approaches. Moreover, the region of phase coexistence is not found to end upon a critical point, indicating that no critical opalescence can occur in compact-star matter.

ON THE ACCRETION-FED GROWTH OF NEUTRON STARS DURING 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)

2015-01-01

This paper models the orbital inspiral of a neutron star (NS) through the envelope of its giant-branch companion during a common envelope (CE) episode. These CE episodes are necessary to produce close pairs of NSs that can inspiral and merge due to gravitational wave losses in less than a Hubble time. Because cooling by neutrinos can be very efficient, NSs have been predicted to accumulate significant mass during CE events, perhaps enough to lead them to collapse to black holes. We revisit this conclusion with the additional consideration of CE structure, particularly density gradients across the embedded NS's accretion radius. This work is informed by our recent numerical simulations that find that the presence of a density gradient strongly limits accretion by imposing a net angular momentum to the flow around the NS. Our calculations suggest that NSs should survive CE encounters. They accrete only modest amounts of envelope material, ≲ 0.1 M {sub ☉}, which is broadly consistent with mass determinations of double NS binaries. With less mass gain, NSs must spiral deeper to eject their CE, leading to a potential increase in mergers. The survival of NSs in CE events has implications for the formation mechanism of observed double NS binaries, as well as for predicted rates of NS binary gravitational wave inspirals and their electromagnetic counterparts.

Accretion onto CO White Dwarfs using MESA

Science.gov (United States)

Feng, Wanda; Starrfield, Sumner

2018-06-01

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

Accreting Compact Object at the Center of the Supernova Remnant RCW 103.

Science.gov (United States)

Sanwal, D.; Garmire, G. P.; Garmire, A.; Pavlov, G. G.; Mignani, R.

2002-05-01

We observed the radio-quiet central compact object of the supernova remnant RCW 103 with the Chandra ACIS during 13.8 hours on 2002 March 3, when the source was in high state, with a time-averaged flux of 8*E-12 erg cm-2 s-1 in the 0.5--8.0 keV band. The complex light curve of the source shows a period of about 6.4 hours and two partial eclipses or dips per period, separated by 180o in phase. The variability of the source proves that it is powered by accretion, likely from a low-mass companion in a binary system. Deep near-IR observations of the source with VLT suggest a potential counterpart of the compact object about 2'' from the nominal Chandra position. The magnitudes of the potential counterpart are J ≈ 22.3, H ≈ 19.6, and Ks ≈ 18.5, with an uncertainty of about 0.5 mag. We will discuss possible interpretations of the observational results. This work was partially supported by NASA grants NAS8-01128 and NAG5-10865.

The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star.

Science.gov (United States)

Campana, S; Lodato, G; D'Avanzo, P; Panagia, N; Rossi, E M; Della Valle, M; Tagliaferri, G; Antonelli, L A; Covino, S; Ghirlanda, G; Ghisellini, G; Melandri, A; Pian, E; Salvaterra, R; Cusumano, G; D'Elia, V; Fugazza, D; Palazzi, E; Sbarufatti, B; Vergani, S D

2011-11-30

The tidal disruption of a solar-mass star around a supermassive black hole has been extensively studied analytically and numerically. In these events, the star develops into an elongated banana-shaped structure. After completing an eccentric orbit, the bound debris falls into the black hole, forming an accretion disk and emitting radiation. The same process may occur on planetary scales if a minor body passes too close to its star. In the Solar System, comets fall directly into our Sun or onto planets. If the star is a compact object, the minor body can become tidally disrupted. Indeed, one of the first mechanisms invoked to produce strong gamma-ray emission involved accretion of comets onto neutron stars in our Galaxy. Here we report that the peculiarities of the 'Christmas' gamma-ray burst (GRB 101225A) can be explained by a tidal disruption event of a minor body around an isolated Galactic neutron star. This would indicate either that minor bodies can be captured by compact stellar remnants more frequently than occurs in the Solar System or that minor-body formation is relatively easy around millisecond radio pulsars. A peculiar supernova associated with a gamma-ray burst provides an alternative explanation.

ULX spectra revisited: Accreting, highly magnetized neutron stars as the engines of ultraluminous X-ray sources

Science.gov (United States)

Koliopanos, Filippos; Vasilopoulos, Georgios; Godet, Olivier; Bachetti, Matteo; Webb, Natalie A.; Barret, Didier

2017-12-01

Aims: In light of recent discoveries of pulsating ultraluminous X-ray sources (ULXs) and recently introduced theoretical schemes that propose neutron stars (NSs) as the central engines of ULXs, we revisit the spectra of eighteen well known ULXs, in search of indications that favour this newly emerging hypothesis. Methods: We examine the spectra from high-quality XMM-Newton and NuSTAR observations. We use a combination of elementary black body and multicolour disk black body (MCD) models, to diagnose the predictions of classic and novel theoretical models of accretion onto NSs. We re-interpret the well established spectral characteristics of ULXs in terms of accretion onto lowly or highly magnetised NSs, and explore the resulting parameter space for consistency. Results: We confirm the previously noted presence of the low-energy (≲6 keV) spectral rollover and argue that it could be interpreted as due to thermal emission. The spectra are well described by a double thermal model consisting of a "hot" (≳1 keV) and a "cool" (≲0.7 keV) multicolour black body (MCB). Under the assumption that the "cool" MCD emission originates in a disk truncated at the neutron star magnetosphere, we find that all ULXs in our sample are consistent with accretion onto a highly magnetised (B ≳ 1012 G) neutron star. We note a strong correlation between the strength of the magnetic field, the temperature of the "hot" thermal component and the total unabsorbed luminosity. Examination of the NuSTAR data supports this interpretation and also confirms the presence of a weak, high-energy (≳15 keV) tail, most likely the result of modification of the MCB emission by inverse Compton scattering. We also note that the apparent high-energy tail, may simply be the result of mismodelling of MCB emission with an atypical temperature (T) versus radius (r) gradient, using a standard MCD model with a fixed gradient of T r-0.75. Conclusions: We have offered a new and robust physical interpretation for

Hot accreting white dwarfs in the quasi-static approximation

International Nuclear Information System (INIS)

Iben, I. Jr.

1982-01-01

Properties of white dwarfs which are accreting hydrogen-rich matter at rates in the range 1.5 x 10 -9 to 2.5 x 10 -7 M/sub sun/ yr -1 are investigated in several approximations. Steady-burning models, in which matter is processed through nuclear-burning shells as rapidly as it is accreted, provide a framework for understanding the properties of models in which thermal pulses induced by hydrogen burning and helium burning are allowed to occur. In these latter models, the underlying carbon-oxygen core is chosen to be in a cycle-averaged steady state with regard to compressional heating and neutrino losses. Several of these models are evolved in the quasi-static approximation. Combining results obtained in the steady-burning approximation with those obtained in the quasi-static approximation, expressions are obtained for estimating, as functions of accretion rate and white dwarf mass, the thermal pulse recurrence period and the duration of hydrogen-burning phases. The time spent by an accreting model burning hydrogen as a large star of giant dimensions versus time spent burning hydrogen as a hot dwarf is also estimated as a function of model mass and accretion rate. Finally, suggestions for detecting observational counterparts of the theoretical models and suggestions for further theoretical investigations are offered. Subject headings: stars: accretion: stars: interiors: stars: novae: stars: symbiotic: stars: white dwarfs

Symbiotic stars

Science.gov (United States)

Kafatos, M.; Michalitsianos, A. G.

1984-01-01

The physical characteristics of symbiotic star systems are discussed, based on a review of recent observational data. A model of a symbiotic star system is presented which illustrates how a cool red-giant star is embedded in a nebula whose atoms are ionized by the energetic radiation from its hot compact companion. UV outbursts from symbiotic systems are explained by two principal models: an accretion-disk-outburst model which describes how material expelled from the tenuous envelope of the red giant forms an inwardly-spiralling disk around the hot companion, and a thermonuclear-outburst model in which the companion is specifically a white dwarf which superheats the material expelled from the red giant to the point where thermonuclear reactions occur and radiation is emitted. It is suspected that the evolutionary course of binary systems is predetermined by the initial mass and angular momentum of the gas cloud within which binary stars are born. Since red giants and Mira variables are thought to be stars with a mass of one or two solar mass, it is believed that the original cloud from which a symbiotic system is formed can consist of no more than a few solar masses of gas.

STAR FORMATION IN DENSE CLUSTERS

International Nuclear Information System (INIS)

Myers, Philip C.

2011-01-01

A model of core-clump accretion with equally likely stopping describes star formation in the dense parts of clusters, where models of isolated collapsing cores may not apply. Each core accretes at a constant rate onto its protostar, while the surrounding clump gas accretes as a power of protostar mass. Short accretion flows resemble Shu accretion and make low-mass stars. Long flows resemble reduced Bondi accretion and make massive stars. Accretion stops due to environmental processes of dynamical ejection, gravitational competition, and gas dispersal by stellar feedback, independent of initial core structure. The model matches the field star initial mass function (IMF) from 0.01 to more than 10 solar masses. The core accretion rate and the mean accretion duration set the peak of the IMF, independent of the local Jeans mass. Massive protostars require the longest accretion durations, up to 0.5 Myr. The maximum protostar luminosity in a cluster indicates the mass and age of its oldest protostar. The distribution of protostar luminosities matches those in active star-forming regions if protostars have a constant birthrate but not if their births are coeval. For constant birthrate, the ratio of young stellar objects to protostars indicates the star-forming age of a cluster, typically ∼1 Myr. The protostar accretion luminosity is typically less than its steady spherical value by a factor of ∼2, consistent with models of episodic disk accretion.

Evolution of the magnetized, neutrino-cooled accretion disk in the aftermath of a black hole-neutron star binary merger

Science.gov (United States)

Hossein Nouri, Fatemeh; Duez, Matthew D.; Foucart, Francois; Deaton, M. Brett; Haas, Roland; Haddadi, Milad; Kidder, Lawrence E.; Ott, Christian D.; Pfeiffer, Harald P.; Scheel, Mark A.; Szilagyi, Bela

2018-04-01

Black hole-torus systems from compact binary mergers are possible engines for gamma-ray bursts (GRBs). During the early evolution of the postmerger remnant, the state of the torus is determined by a combination of neutrino cooling and magnetically driven heating processes, so realistic models must include both effects. In this paper, we study the postmerger evolution of a magnetized black hole-neutron star binary system using the Spectral Einstein Code (SpEC) from an initial postmerger state provided by previous numerical relativity simulations. We use a finite-temperature nuclear equation of state and incorporate neutrino effects in a leakage approximation. To achieve the needed accuracy, we introduce improvements to SpEC's implementation of general-relativistic magnetohydrodynamics (MHD), including the use of cubed-sphere multipatch grids and an improved method for dealing with supersonic accretion flows where primitive variable recovery is difficult. We find that a seed magnetic field triggers a sustained source of heating, but its thermal effects are largely cancelled by the accretion and spreading of the torus from MHD-related angular momentum transport. The neutrino luminosity peaks at the start of the simulation, and then drops significantly over the first 20 ms but in roughly the same way for magnetized and nonmagnetized disks. The heating rate and disk's luminosity decrease much more slowly thereafter. These features of the evolution are insensitive to grid structure and resolution, formulation of the MHD equations, and seed field strength, although turbulent effects are not fully converged.

THE BIMODALITY OF ACCRETION IN T TAURI STARS AND BROWN DWARFS

International Nuclear Information System (INIS)

Vorobyov, E. I.; Basu, Shantanu

2009-01-01

We present numerical solutions of the collapse of prestellar cores that lead to the formation and evolution of circumstellar disks. The disk evolution is then followed for up to three million years. A variety of models of different initial masses and rotation rates allow us to study disk accretion around brown dwarfs and low-mass T Tauri stars (TTSs), with central object mass M * sun , as well as intermediate- and upper-mass TTSs (0.2 M sun * sun ). Our models include self-gravity and allow for nonaxisymmetric motions. In addition to the self-consistently generated gravitational torques, we introduce an effective turbulent α-viscosity with α = 0.01, which allows us particularly to model accretion in the low-mass regime where disk self-gravity is diminishing. A range of models with observationally motivated values of the initial ratio of rotational-to-gravitational energy yield a correlation between mass accretion rate M-dot and M * that is relatively steep, as observed. Additionally, our modeling reveals evidence for a bimodality in the M-dot - M * correlation, with a steeper slope at lower masses and a shallower slope at intermediate and upper masses, as also implied by observations. Furthermore, we show that the neglect of disk self-gravity leads to a much steeper M-dot - M * relation for intermediate- and upper-mass TTSs. This demonstrates that an accurate treatment of global self-gravity is essential to understanding observations of circumstellar disks.

A GMOS-N IFU study of the central H II region in the blue compact dwarf galaxy NGC 4449: kinematics, nebular metallicity and star formation

Science.gov (United States)

Kumari, Nimisha; James, Bethan L.; Irwin, Mike J.

2017-10-01

We use integral field spectroscopic (IFS) observations from the Gemini Multi-Object Spectrograph North (GMOS-N) to study the central H II region in a nearby blue compact dwarf (BCD) galaxy NGC 4449. The IFS data enable us to explore the variation of physical and chemical conditions of the star-forming region and the surrounding gas on spatial scales as small as 5.5 pc. Our kinematical analysis shows possible signatures of shock ionization and shell structures in the surroundings of the star-forming region. The metallicity maps of the region, created using direct Te and indirect strong line methods (R23, O3N2 and N2), do not show any chemical variation. From the integrated spectrum of the central H II region, we find a metallicity of 12 + log(O/H) = 7.88 ± 0.14 ({˜ }0.15^{+0.06}_{-0.04} Z⊙) using the direct method. Comparing the central H II region metallicity derived here with those of H II regions throughout this galaxy from previous studies, we find evidence of increasing metallicity with distance from the central nucleus. Such chemical inhomogeneities can be due to several mechanisms, including gas loss via supernova blowout, galactic winds or metal-poor gas accretion. However, we find that the localized area of decreased metallicity aligns spatially with the peak of star-forming activity in the galaxy, suggesting that gas accretion may be at play here. Spatially resolved IFS data for the entire galaxy are required to confirm the metallicity inhomogeneity found in this study and determine its possible cause.

SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS

International Nuclear Information System (INIS)

Reiners, A.; Basri, G.; Christensen, U. R.

2009-01-01

We have measured the surface magnetic flux on four accreting young brown dwarfs and one nonaccreting young very low mass (VLM) star utilizing high-resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1-2 kG had been proposed for one of the brown dwarfs, Two Micron All Sky Survey (2MASS) J1207334-393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilogauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3σ upper limit for the magnetic flux in 2MASS J1207334-393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of 5 or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It is interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently has an easy physical explanation.

An extended star formation history in an ultra-compact dwarf

Science.gov (United States)

Norris, Mark A.; Escudero, Carlos G.; Faifer, Favio R.; Kannappan, Sheila J.; Forte, Juan Carlos; van den Bosch, Remco C. E.

2015-08-01

There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra-compact dwarfs (UCDs), with suggestions that UCDs are simply the high-mass extension of the globular cluster population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus-type UCDs being known. In this paper, we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped-nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1-2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.

An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.

Science.gov (United States)

Gilfanov, Marat; Bogdán, Akos

2010-02-18

There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.

The primary role of the SW Sextantis stars in the evolution of cataclysmic variables

Science.gov (United States)

Torres, Manuel; Gaensicke, Boris; Rodriguez-Gil, Pablo; Long, Knox; Marsh, Tom; Steeghs, Danny; Munoz-Darias, Teodoro; Shahbaz, Tariq; Schmidtobreick, Linda; Schreiber, Matthias

2009-02-01

SW Sextantis stars are a relatively large group of cataclysmic variables (CVs) which plays a fundamental role in our understanding of CV structure and evolution. Very little is known about the properties of their accreting white dwarfs and their donor stars, as the stellar components are usually outshone by an extremely bright accretion flow. Consequently, a proper assesment of their evolutionary state is illusionary. We are monitoring the brightness of a number of SW Sex stars and request here Gemini/GMOS-N ToO time to obtain orbital phase-resolved spectroscopy if one of them enters a low state, since this is the only opportunity for studying the stellar components individually. These data will be used to accurately measure the binary parameters, white dwarf temperature, and distance to the system for a SW Sex star for the first time. The measured stellar masses and radii will especially be a precious input to the theory of compact binary evolution as a whole.

A generalized model for compact stars

Energy Technology Data Exchange (ETDEWEB)

Aziz, Abdul [Bodai High School (H.S.), Department of Physics, Kolkata, West Bengal (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India); Rahaman, Farook [Jadavpur University, Department of Mathematics, Kolkata, West Bengal (India)

2016-05-15

By virtue of the maximum entropy principle, we get an Euler-Lagrange equation which is a highly nonlinear differential equation containing the mass function and its derivatives. Solving the equation by a homotopy perturbation method we derive a generalized expression for the mass which is a polynomial function of the radial distance. Using the mass function we find a partially stable configuration and its characteristics. We show that different physical features of the known compact stars, viz. Her X-1, RX J 1856-37, SAX J (SS1), SAX J (SS2), and PSR J 1614-2230, can be explained by the present model. (orig.)

The Eating Habits of Milky Way-mass Halos: Destroyed Dwarf Satellites and the Metallicity Distribution of Accreted Stars

Science.gov (United States)

Deason, Alis J.; Mao, Yao-Yuan; Wechsler, Risa H.

2016-04-01

We study the mass spectrum of destroyed dwarfs that contribute to the accreted stellar mass of Milky Way (MW)-mass (Mvir ˜ 1012.1 M⊙) halos using a suite of 45 zoom-in dissipationless simulations. Empirical models are employed to relate (peak) subhalo mass to dwarf stellar mass, and we use constraints from z = 0 observations and hydrodynamical simulations to estimate the metallicity distribution of the accreted stellar material. The dominant contributors to the accreted stellar mass are relatively massive dwarfs with Mstar ˜ 108-1010M⊙. Halos with more quiescent accretion histories tend to have lower mass progenitors (108-109 M⊙), and lower overall accreted stellar masses. Ultra-faint mass (Mstar 108 M⊙ can contribute a considerable fraction (˜20%-60%) of metal-poor stars if their metallicity distributions have significant metal-poor tails. Finally, we find that the generic assumption of a quiescent assembly history for the MW halo seems to be in tension with the mass spectrum of its surviving dwarfs. We suggest that the MW could be a “transient fossil” a quiescent halo with a recent accretion event(s) that disguises the preceding formation history of the halo.

FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

International Nuclear Information System (INIS)

Kirk, Helen; Myers, Philip C.; Bourke, Tyler L.; Gutermuth, Robert A.; Wilson, Grant W.; Hedden, Abigail

2013-01-01

One puzzle in understanding how stars form in clusters is the source of mass—is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of ∼30 M ☉ Myr –1 (inferred from the N 2 H + velocity gradient along the filament), and radially contracting onto the filament at ∼130 M ☉ Myr –1 (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

Nuclear Statistical Equilibrium for compact stars: modelling the nuclear energy functional

International Nuclear Information System (INIS)

Aymard, Francois

2015-01-01

The core collapse supernova is one of the most powerful known phenomena in the universe. It results from the explosion of very massive stars after they have burnt all their fuel. The hot compact remnant, the so-called proto-neutron star, cools down to become an inert catalyzed neutron star. The dynamics and structure of compact stars, that is core collapse supernovae, proto-neutron stars and neutron stars, are still not fully understood and are currently under active research, in association with astrophysical observations and nuclear experiments. One of the key components for modelling compact stars concerns the Equation of State. The task of computing a complete realistic consistent Equation of State for all such stars is challenging because a wide range of densities, proton fractions and temperatures is spanned. This thesis deals with the microscopic modelling of the structure and internal composition of baryonic matter with nucleonic degrees of freedom in compact stars, in order to obtain a realistic unified Equation of State. In particular, we are interested in a formalism which can be applied both at sub-saturation and super-saturation densities, and which gives in the zero temperature limit results compatible with the microscopic Hartree-Fock-Bogoliubov theory with modern realistic effective interactions constrained on experimental nuclear data. For this purpose, we present, for sub-saturated matter, a Nuclear Statistical Equilibrium model which corresponds to a statistical superposition of finite configurations, the so-called Wigner-Seitz cells. Each cell contains a nucleus, or cluster, embedded in a homogeneous electron gas as well as a homogeneous neutron and proton gas. Within each cell, we investigate the different components of the nuclear energy of clusters in interaction with gases. The use of the nuclear mean-field theory for the description of both the clusters and the nucleon gas allows a theoretical consistency with the treatment at saturation

NUSTAR and Suzaku x-ray spectroscopy of NGC 4151: Evidence for reflection from the inner accretion disk

Energy Technology Data Exchange (ETDEWEB)

Keck, M. L.; Brenneman, L. W.; Ballantyne, D. R.; Bauer, F.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Dauser, T.; Elvis, M.; Fabian, A. C.; Fuerst, F.; García, J.; Grefenstette, B. W.; Hailey, C. J.; Harrison, F. A.; Madejski, G.; Marinucci, A.; Matt, G.; Reynolds, C. S.; Stern, D.; Walton, D. J.; Zoghbi, A.

2015-06-15

We present X-ray timing and spectral analyses of simultaneous 150 ks Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku X-ray observations of the Seyfert 1.5 galaxy NGC 4151. We disentangle the continuum emission, absorption, and reflection properties of the active galactic nucleus (AGN) by applying inner accretion disk reflection and absorption-dominated models. With a time-averaged spectral analysis, we find strong evidence for relativistic reflection from the inner accretion disk. We find that relativistic emission arises from a highly ionized inner accretion disk with a steep emissivity profile, which suggests an intense, compact illuminating source. We find a preliminary, near-maximal black hole spin $a\\gt 0.9$ accounting for statistical and systematic modeling errors. We find a relatively moderate reflection fraction with respect to predictions for the lamp post geometry, in which the illuminating corona is modeled as a point source. Through a time-resolved spectral analysis, we find that modest coronal and inner disk reflection (IDR) flux variation drives the spectral variability during the observations. We discuss various physical scenarios for the IDR model and we find that a compact corona is consistent with the observed features.

EPISODIC ACCRETION AT EARLY STAGES OF EVOLUTION OF LOW-MASS STARS AND BROWN DWARFS: A SOLUTION FOR THE OBSERVED LUMINOSITY SPREAD IN H-R DIAGRAMS?

International Nuclear Information System (INIS)

Baraffe, I.; Chabrier, G.; Gallardo, J.

2009-01-01

We present evolutionary models for young low-mass stars and brown dwarfs taking into account episodic phases of accretion at early stages of the evolution, a scenario supported by recent large surveys of embedded protostars. An evolution including short episodes of vigorous accretion followed by longer quiescent phases can explain the observed luminosity spread in H-R diagrams of star-forming regions at ages of a few Myr, for objects ranging from a few Jupiter masses to a few tenths of a solar mass. The gravitational contraction of these accreting objects strongly departs from the standard Hayashi track at constant T eff . The best agreement with the observed luminosity scatter is obtained if most of the accretion shock energy is radiated away. The obtained luminosity spread at 1 Myr in the H-R diagram is equivalent to what can be misinterpreted as an ∼10 Myr age spread for non-accreting objects. We also predict a significant spread in radius at a given T eff , as suggested by recent observations. These calculations bear important consequences for our understanding of star formation and early stages of evolution and on the determination of the initial mass function for young (≤ a few Myr) clusters. Our results also show that the concept of a stellar birthline for low-mass objects has no valid support.

Magnetic field strength of a neutron-star-powered ultraluminous X-ray source

Science.gov (United States)

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

2018-04-01

Ultraluminous X-ray sources (ULXs) are bright X-ray sources in nearby galaxies not associated with the central supermassive black hole. Their luminosities imply they are powered by either an extreme accretion rate onto a compact stellar remnant, or an intermediate mass ( 100-105M⊙) black hole1. Recently detected coherent pulsations coming from three bright ULXs2-5 demonstrate that some of these sources are powered by accretion onto a neutron star, implying accretion rates significantly in excess of the Eddington limit, a high degree of geometric beaming, or both. The physical challenges associated with the high implied accretion rates can be mitigated if the neutron star surface field is very high (1014 G)6, since this suppresses the electron scattering cross-section, reducing the radiation pressure that chokes off accretion for high luminosities. Surface magnetic field strengths can be determined through cyclotron resonance scattering features7,8 produced by the transition of charged particles between quantized Landau levels. Here, we present the detection at a significance of 3.8σ of an absorption line at 4.5 keV in the Chandra spectrum of a ULX in M51. This feature is likely to be a cyclotron resonance scattering feature produced by the strong magnetic field of a neutron star. Assuming scattering off electrons, the magnetic field strength is implied to be 1011 G, while protons would imply a magnetic field of B 1015 G.

The Ultraviolet and Infrared Star Formation Rates of Compact Group Galaxies: An Expanded Sample

Science.gov (United States)

Lenkic, Laura; Tzanavaris, Panayiotis; Gallagher, Sarah C.; Desjardins, Tyler D.; Walker, Lisa May; Johnson, Kelsey E.; Fedotov, Konstantin; Charlton, Jane; Cardiff, Ann H.; Durell, Pat R.

2016-01-01

Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended time-scales. We expand the census of star formation in compact group galaxies by Tzanavaris et al. (2010) and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 m photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFRUV) using the UVOT uvw2 photometry. Similarly, we use the MIPS 24 m photometry to estimate the component of the SFR that is obscured by dust (SFRIR). We find that galaxies which are MIR-active (MIR-red), also have bluer UV colours, higher specific SFRs, and tend to lie in Hi-rich groups, while galaxies that are MIR-inactive (MIR-blue) have redder UV colours, lower specific SFRs, and tend to lie in Hi-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M yr1, indicating this might be the most common value in compact groups. In contrast, the specific SFR distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific SFR is the best tracer of gas depletion and galaxy evolution in compact groups.

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.

Keck-I MOSFIRE spectroscopy of compact star-forming galaxies at z ≳ 2: high velocity dispersions in progenitors of compact quiescent galaxies

International Nuclear Information System (INIS)

Barro, Guillermo; Koo, David C.; Faber, Sandra M.; Guo, Yicheng; Toloba, Elisa; Fang, Jerome J.; Trump, Jonathan R.; Dekel, Avishai; Kassin, Susan A.; Koekemoer, Anton M.; Kocevski, Dale D.; Van der Wel, Arjen; Pérez-González, Pablo G.; Pacifici, Camilla; Simons, Raymond; Campbell, Randy D.; Goodrich, Bob; Kassis, Marc; Ceverino, Daniel; Finkelstein, Steven L.

2014-01-01

We present Keck-I MOSFIRE near-infrared spectroscopy for a sample of 13 compact star-forming galaxies (SFGs) at redshift 2 ≤ z ≤ 2.5 with star formation rates of SFR ∼ 100 M ☉ yr –1 and masses of log(M/M ☉ ) ∼10.8. Their high integrated gas velocity dispersions of σ int =230 −30 +40 km s –1 , as measured from emission lines of Hα and [O III], and the resultant M * -σ int relation and M * -M dyn all match well to those of compact quiescent galaxies at z ∼ 2, as measured from stellar absorption lines. Since log(M * /M dyn ) =–0.06 ± 0.2 dex, these compact SFGs appear to be dynamically relaxed and evolved, i.e., depleted in gas and dark matter (<13 −13 +17 %), and present larger σ int than their non-compact SFG counterparts at the same epoch. Without infusion of external gas, depletion timescales are short, less than ∼300 Myr. This discovery adds another link to our new dynamical chain of evidence that compact SFGs at z ≳ 2 are already losing gas to become the immediate progenitors of compact quiescent galaxies by z ∼ 2.

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

International Nuclear Information System (INIS)

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

1982-01-01

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

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

Science.gov (United States)

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

2018-04-01

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

Energy transport in radially accreting white dwarf stars

Energy Technology Data Exchange (ETDEWEB)

Thompson, A.M.

1986-10-01

Some of the non-thermal energy transport processes which may be present in a white dwarf accretion column are examined and it is determined whether these could in any way contribute to a resolution of the soft X-ray puzzle. The first two Chapters of this Thesis constitute a review of the observations and proposed models for white dwarf accretion columns. In Chapter 3 we show that in Kuijpers and Pringle's original bombardment model of white dwarf accretion columns, in which the energy of the accreting material is deposited uniformly into a static atmosphere which then radiates the energy away as optically thin bremsstrahlung/line radiation, an incorrect Coulomb collisional timescale was used. In Chapter 4 we extend the calculations of Chapter 3 to include the effect of cyclotron radiation. It is concluded that a cyclotron cooled bombardment solution for a white dwarf accretion column may exist. We extend this calculation to derive a simple piecewise uniform temperature structure for such an accretion column, incorporating the effect of thermal conduction. In Chaper 5 we examine two of the non thermal emission mechanisms that might be present in white dwarf accretion columns:- non thermal Lyman-{alpha} emission and non thermal inverse bremsstrahlung emission. It is shown that neither would actually be sufficiently large to be detectable. In Chapter 6 some possible extensions to the work presented are suggested. (author).

Profile disparity of Raman-scattered O VI in symbiotic stars

International Nuclear Information System (INIS)

Lee, Hee-Won

2016-01-01

Symbiotic stars are wide binary systems consisting of a hot compact star (usually a white dwarf) and a mass losing giant. Symbiotic activities are believed to occur through gravitational capture of a fraction of the slow stellar wind from the giant. Raman scattered features of O VI resonance doublet 1032 and 1038 appearing at around 6825 Å and 7082 Å are a unique spectroscopic diagnostic tool to probe the mass transfer process in symbiotic stars. The Raman O VI features often exhibit multiple peak structures and in many cases the blue peak of 7082 features is relatively more suppressed than that of 6825 features. We propose that the disparity of the two profiles is attributed to the local variation of optical depths of O VI, implying that the accretion flow is convergent in the red emission region and divergent in the blue emission region. It is argued in this presentation that Raman scattering by atomic hydrogen is a natural mirror to provide an edge-on view of the accretion disk and a lateral view of the bipolar outflow in symbiotic stars. We discuss the spectropolarimetric implications of this interpretation. (paper)

FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

Energy Technology Data Exchange (ETDEWEB)

Kirk, Helen; Myers, Philip C.; Bourke, Tyler L. [Radio and Geoastronomy Division, Harvard Smithsonian Center for Astrophysics, MS-42, Cambridge, MA, 02138 (United States); Gutermuth, Robert A.; Wilson, Grant W. [Department of Astronomy, University of Massachusetts Amherst, Amherst, MA 01003 (United States); Hedden, Abigail, E-mail: kirkh@mcmaster.ca [Army Research Labs, Adelphi, MD 20783 (United States)

2013-04-01

One puzzle in understanding how stars form in clusters is the source of mass-is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of {approx}30 M{sub Sun} Myr{sup -1} (inferred from the N{sub 2}H{sup +} velocity gradient along the filament), and radially contracting onto the filament at {approx}130 M{sub Sun} Myr{sup -1} (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

THE EFFECT OF TRANSIENT ACCRETION ON THE SPIN-UP OF MILLISECOND PULSARS

Energy Technology Data Exchange (ETDEWEB)

Bhattacharyya, Sudip; Chakrabarty, Deepto, E-mail: sudip@tifr.res.in [Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Colaba, Mumbai 400005 (India)

2017-01-20

A millisecond pulsar is a neutron star that has been substantially spun up by accretion from a binary companion. A previously unrecognized factor governing the spin evolution of such pulsars is the crucial effect of nonsteady or transient accretion. We numerically compute the evolution of accreting neutron stars through a series of outburst and quiescent phases, considering the drastic variation of the accretion rate and the standard disk–magnetosphere interaction. We find that, for the same long-term average accretion rate, X-ray transients can spin up pulsars to rates several times higher than can persistent accretors, even when the spin-down due to electromagnetic radiation during quiescence is included. We also compute an analytical expression for the equilibrium spin frequency in transients, by taking spin equilibrium to mean that no net angular momentum is transferred to the neutron star in each outburst cycle. We find that the equilibrium spin rate for transients, which depends on the peak accretion rate during outbursts, can be much higher than that for persistent sources. This explains our numerical finding. This finding implies that any meaningful study of neutron star spin and magnetic field distributions requires the inclusion of the transient accretion effect, since most accreting neutron star sources are transients. Our finding also implies the existence of a submillisecond pulsar population, which is not observed. This may point to the need for a competing spin-down mechanism for the fastest-rotating accreting pulsars, such as gravitational radiation.

Hydrodynamic simulations of accretion disks in cataclysmic variables

International Nuclear Information System (INIS)

Hirose, Masahito; Osaki, Yoji

1990-01-01

The tidal effects of secondary stars on accretion disks in cataclysmic variables are studied by two-dimensional hydrodynamical simulations. The time evolution of an accretion disk under a constant mass supply rate from the secondary is followed until it reaches a quasi-steady state. We have examined various cases of different mass ratios of binary systems. It is found that the accretion disk settles into a steady state of an elongated disk fixed in the rotating frame of the binary in a binary system with comparable masses of component stars. On the other hand, in the case of a low-mass secondary, the accretion disk develops a non-axisymmetric (eccentric) structure and finally settles into a periodically oscillating state in which a non-axisymmetric eccentric disk rotates in the opposite direction to the orbital motion of the binary in the rotating frame of the binary. The period of oscillation is a few percent longer than the orbital period of the binary, and it offers a natural explanation for the ''superhump'' periodicity of SU UMa stars. Our results thus confirm basically those of Whitehurst (1988, AAA 45.064.032) who discovered the tidal instability of an accretion disk in the case of a low-mass secondary. We then discuss the cause of the tidal instability. It is shown that the tidal instability of accretion disks is caused by a parametric resonance between particle orbits and an orbiting secondary star with a 1:3 period ratio. (author)

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

Science.gov (United States)

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

2013-09-26

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

Hyper-Eddington accretion in GRB

International Nuclear Information System (INIS)

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

2005-01-01

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

Keck-I MOSFIRE spectroscopy of compact star-forming galaxies at z ≳ 2: high velocity dispersions in progenitors of compact quiescent galaxies

Energy Technology Data Exchange (ETDEWEB)

Barro, Guillermo; Koo, David C.; Faber, Sandra M.; Guo, Yicheng; Toloba, Elisa; Fang, Jerome J. [University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 (United States); Trump, Jonathan R. [Pennsylvania State University, University Park, State College, PA 16802 (United States); Dekel, Avishai [Racah Institute of Physics, The Hebrew University, Jerusalem 91904 (Israel); Kassin, Susan A.; Koekemoer, Anton M. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Kocevski, Dale D. [University of Kentucky, Lexington, KY 40506 (United States); Van der Wel, Arjen [Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany); Pérez-González, Pablo G. [Universidad Complutense de Madrid, Avda. de Sneca, 2 Ciudad Universitaria, E-28040 Madrid (Spain); Pacifici, Camilla [Yonsei University Observatory, Yonsei University 50, Yonsei-ro, Seodaemun-gu, Seoul 120-749 (Korea, Republic of); Simons, Raymond [Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218-2683 (United States); Campbell, Randy D.; Goodrich, Bob; Kassis, Marc [W. M. Keck Observatory, California Association for Research in Astronomy, 65-1120 Mamalahoa Highway, Kamuela, HI 96743 (United States); Ceverino, Daniel [Universidad Autonoma de Madrid, Ciudad Universitaria de Cantoblanco, E-28049 Madrid (Spain); Finkelstein, Steven L. [The University of Texas at Austin, Austin, TX 78712 (United States); and others

2014-11-10

We present Keck-I MOSFIRE near-infrared spectroscopy for a sample of 13 compact star-forming galaxies (SFGs) at redshift 2 ≤ z ≤ 2.5 with star formation rates of SFR ∼ 100 M {sub ☉} yr{sup –1} and masses of log(M/M {sub ☉}) ∼10.8. Their high integrated gas velocity dispersions of σ{sub int} =230{sub −30}{sup +40} km s{sup –1}, as measured from emission lines of Hα and [O III], and the resultant M {sub *}-σ{sub int} relation and M {sub *}-M {sub dyn} all match well to those of compact quiescent galaxies at z ∼ 2, as measured from stellar absorption lines. Since log(M {sub *}/M {sub dyn}) =–0.06 ± 0.2 dex, these compact SFGs appear to be dynamically relaxed and evolved, i.e., depleted in gas and dark matter (<13{sub −13}{sup +17}%), and present larger σ{sub int} than their non-compact SFG counterparts at the same epoch. Without infusion of external gas, depletion timescales are short, less than ∼300 Myr. This discovery adds another link to our new dynamical chain of evidence that compact SFGs at z ≳ 2 are already losing gas to become the immediate progenitors of compact quiescent galaxies by z ∼ 2.

Physical attributes of anisotropic compact stars in f(R, G) gravity

Energy Technology Data Exchange (ETDEWEB)

Shamir, M.F.; Zia, Saeeda [National University of Computer and Emerging Sciences, Department of Sciences and Humanities, Lahore (Pakistan)

2017-07-15

Modified gravity is one of the potential candidates to explain the accelerated expansion of the universe. Current study highlights the materialization of anisotropic compact stars in the context of f(R, G) theory of gravity. In particular, to gain insight in the physical behavior of three stars namely, Her X1, SAX J 1808-3658 and 4U 1820-30, energy density, and radial and tangential pressures are calculated. The f(R, G) gravity model is split into a Starobinsky like f(R) model and a power law f(G) model. The main feature of the work is a 3-dimensional graphical analysis in which, anisotropic measurements, energy conditions and stability attributes of these stars are discussed. It is shown that all three stars behave as usual for positive values of the f(G) model parameter n. (orig.)

Simulating the X-ray luminosity of Be X-ray binaries: the case for black holes versus neutron stars

Science.gov (United States)

Brown, R. O.; Ho, W. C. G.; Coe, M. J.; Okazaki, A. T.

2018-04-01

There are over 100 Be stars that are known to have neutron star companions but only one such system with a black hole. Previous theoretical work suggests this is not due to their formation but due to differences in X-ray luminosity. It has also been proposed that the truncation of the Be star's circumstellar disc is dependent on the mass of the compact object. Hence, Be star discs in black hole binaries are smaller. Since accretion onto the compact object from the Be star's disc is what powers the X-ray luminosity, a smaller disc in black hole systems leads to a lower luminosity. In this paper, simulations are performed with a range of eccentricities and compact object mass. The disc's size and density are shown to be dependent on both quantities. Mass capture and, in turn, X-ray luminosity are heavily dependent on the size and density of the disc. Be/black hole binaries are expected to be up to ˜10 times fainter than Be/neutron star binaries when both systems have the same eccentricity and can be 100 times fainter when comparing systems with different eccentricity.

Migration of accreting giant planets

Science.gov (United States)

Robert, C.; Crida, A.; Lega, E.; Méheut, H.

2017-09-01

Giant planets forming in protoplanetary disks migrate relative to their host star. By repelling the gas in their vicinity, they form gaps in the disk's structure. If they are effectively locked in their gap, it follows that their migration rate is governed by the accretion of the disk itself onto the star, in a so-called type II fashion. Recent results showed however that a locking mechanism was still lacking, and was required to understand how giant planets may survive their disk. We propose that planetary accretion may play this part, and help reach this slow migration regime.

News on the X-ray emission from hot subdwarf stars

Directory of Open Access Journals (Sweden)

Palombara Nicola La

2017-12-01

Full Text Available In latest years, the high sensitivity of the instruments on-board the XMM-Newton and Chandra satellites allowed us to explore the properties of the X-ray emission from hot subdwarf stars. The small but growing sample of X-ray detected hot subdwarfs includes binary systems, in which the X-ray emission is due to wind accretion onto a compact companion (white dwarf or neutron star, as well as isolated sdO stars, in which X-rays are probably due to shock instabilities in the wind. X-ray observations of these low-mass stars provide information which can be useful for our understanding of the weak winds of this type of stars and can lead to the discovery of particularly interesting binary systems. Here we report the most recent results we have recently obtained in this research area.

Buchdahl compactness limit for a pure Lovelock static fluid star

Science.gov (United States)

Dadhich, Naresh; Chakraborty, Sumanta

2017-03-01

We obtain the Buchdahl compactness limit for a pure Lovelock static fluid star and verify that the limit following from the uniform-density Schwarzschild's interior solution, which is universal irrespective of the gravitational theory (Einstein or Lovelock), is true in general. In terms of surface potential Φ (r ) , it means at the surface of the star r =r0, Φ (r0)Lovelock order, respectively. For a given N , Φ (r0) is maximum for d =2 N +2 , while it is always 4 /9 , Buchdahl's limit, for d =3 N +1 . It is also remarkable that for N =1 Einstein gravity, or for pure Lovelock in d =3 N +1 , Buchdahl's limit is equivalent to the criterion that gravitational field energy exterior to the star must be less than half its gravitational mass, having no reference to the interior at all.

Compact stars in vector-tensor-Horndeski theory of gravity

Energy Technology Data Exchange (ETDEWEB)

Momeni, Davood; Myrzakulov, Kairat; Myrzakulov, Ratbay [Eurasian National University, Department of General and Theoretical Physics, Eurasian International Center for Theoretical Physics, Astana (Kazakhstan); Faizal, Mir [University of British Columbia-Okanagan, Irving K. Barber School of Arts and Sciences, Kelowna, BC (Canada); University of Lethbridge, Department of Physics and Astronomy, Lethbridge, AB (Canada)

2017-01-15

In this paper, we will analyze a theory of modified gravity, in which the field content of general relativity will be increased to include a vector field. We will use the Horndeski formalism to non-minimally couple this vector field to the metric. As we will be using the Horndeski formalism, this theory will not contain Ostrogradsky ghost degree of freedom. We will analyze compact stars using this vector-tensor-Horndeski theory. (orig.)

Probing the clumpy winds of giant stars with high mass X-ray binaries

Science.gov (United States)

Grinberg, Victoria; Hell, Natalie; Hirsch, Maria; Garcia, Javier; Huenemoerder, David; Leutenegger, Maurice A.; Nowak, Michael; Pottschmidt, Katja; Schulz, Norbert S.; Sundqvists, Jon O.; Townsend, Richard D.; Wilms, Joern

2016-04-01

Line-driven winds from early type stars are structured, with small, overdense clumps embedded in tenuous hot gas. High mass X-ray binaries (HMXBs), systems where a neutron star or a black hole accretes from the line-driven stellar wind of an O/B-type companion, are ideal for studying such winds: the wind drives the accretion onto the compact object and thus the X-ray production. The radiation from close to the compact object is quasi-pointlike and effectively X-rays the wind.We used RXTE and Chandra-HETG observations of two of the brightest HMXBs, Cyg X-1 and Vela X-1, to decipher their wind structure. In Cyg X-1, we show that the orbital variability of absorption can be only explained by a clumpy wind model and constrain the porosity of the wind as well as the onion-like structure of the clumps. In Vela X-1 we show, using the newest reference energies for low ionization Si-lines obtained with LLNL’s EBIT-I, that the ionized phase of the circumstellar medium and the cold clumps have different velocities.

NuSTAR reveals the extreme properties of the super-Eddington accreting supermassive black hole in PG 1247+267

DEFF Research Database (Denmark)

Lanzuisi, G.; Perna, M.; Comastri, A.

2016-01-01

PG1247+267 is one of the most luminous known quasars at z similar to 2 and is a strongly super-Eddington accreting supermassive black hole (SMBH) candidate. We obtained NuSTAR data of this intriguing source in December 2014 with the aim of studying its high-energy emission, leveraging the broad...

HOT HIGH-MASS ACCRETION DISK CANDIDATES

International Nuclear Information System (INIS)

Beuther, H.; Walsh, A. J.; Longmore, S. N.

2009-01-01

To better understand the physical properties of accretion disks in high-mass star formation, we present a study of a dozen high-mass accretion disk candidates observed at high spatial resolution with the Australia Telescope Compact Array (ATCA) in the high-excitation (4,4) and (5,5) lines of NH 3 . All of our originally selected sources were detected in both NH 3 transitions, directly associated with CH 3 OH Class II maser emission and implying that high-excitation NH 3 lines are good tracers of the dense gas components in hot-core-type targets. Only the one source that did not satisfy the initial selection criteria remained undetected. From the 11 mapped sources, six show clear signatures of rotation and/or infall motions. These signatures vary from velocity gradients perpendicular to the outflows, to infall signatures in absorption against ultracompact H II regions, to more spherical infall signatures in emission. Although our spatial resolution is ∼1000 AU, we do not find clear Keplerian signatures in any of the sources. Furthermore, we also do not find flattened structures. In contrast to this, in several of the sources with rotational signatures, the spatial structure is approximately spherical with sizes exceeding 10 4 AU, showing considerable clumpy sub-structure at even smaller scales. This implies that on average typical Keplerian accretion disks-if they exist as expected-should be confined to regions usually smaller than 1000 AU. It is likely that these disks are fed by the larger-scale rotating envelope structure we observe here. Furthermore, we do detect 1.25 cm continuum emission in most fields of view. While in some cases weak cm continuum emission is associated with our targets, more typically larger-scale H II regions are seen offset more than 10'' from our sources. While these H II regions are unlikely to be directly related to the target regions, this spatial association nevertheless additionally stresses that high-mass star formation rarely

Relativistic, accreting disks

International Nuclear Information System (INIS)

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

1978-01-01

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

Physics of white dwarf stars

Energy Technology Data Exchange (ETDEWEB)

Koester, D.; Chanmugam, G. (Louisiana State Univ., New Orleans, LA (USA))

1990-07-01

White dwarf stars, compact objects with extremely high interior densities, are the most common end product in the evolution of stars. In this paper we review the history of their discovery, and of the realisation that their structure is determined by the physics of the degenerate electron gas. Spectral types and surface chemical composition show a complicated pattern dominated by diffusion processes and their interaction with accretion, convection and mass loss. While this interaction is not completely understood in all its detail at present, the study may ultimately lead to important constraints on the theory of stellar evolution in general. Variability, caused by non-radial oscillations of the star, is a common phenomenon and is shown to be a powerful probe of the structure of deeper layers that are not directly accessible to observation. Very strong magnetic fields detected in a small fraction of white dwarfs offer a unique opportunity to study the behaviour of atoms under conditions that cannot be simulated in terrestrial laboratories. (author).

Encounters between degenerate stars and extrasolar comet clouds

International Nuclear Information System (INIS)

Pineault, S.; Poisson, E.

1989-01-01

Under the assumption that the presence of comet clouds around otherwise normal stars is a common occurrence in the Galaxy, the observational consequences of random penetration encounters between the general Galactic population of degenerate stars and these comet clouds is considered. The only case considered is where the compact stars is a single star. For this scenario, encounters involving neutron stars (NSs) result in impact rates 1000-10,000 times slower than in the model of Tremaine and Zytkow (1986). The rate for white dwarfs (WDs) is larger than the one for NSs by a factor of about 30 times the ratio of the degenerate star number densities. The mean impact rate is significantly increased if the number of comets in a cloud is nearly independent of the mass of the central star. It is concluded that some of the observed gamma-ray bursts may be caused by accretion of comets onto NSs and that this scenario, but with a WD as the accretor, probably contributes to the optical flash background rate. 38 refs

Spiral shocks on a Roche lobe overflow in a semi-detached binary system

International Nuclear Information System (INIS)

Sawada, K.; Matsuda, T.; Hachisu, I.

1986-01-01

Two-dimensional hydrodynamic calculations of a Roche lobe overflow in a semi-detached binary system with a mass ratio of unity are carried out. The region of the computation covers both a mass-losing star filling its critical Roche lobe and a mass-accreting compact star. Gas ejected from the mass-losing star with specified energy flows through the L1 point to form an elephant trunk and an accretion ring. It is found that spiral-shaped shocks are formed on the accretion ring. It is suggested that the gas in the accretion ring loses angular momentum at the shocks and spirals in towards the compact star even without viscosity. (author)

Theory of quasi-spherical accretion in X-ray pulsars

Science.gov (United States)

Shakura, N.; Postnov, K.; Kochetkova, A.; Hjalmarsdotter, L.

2012-02-01

A theoretical model for quasi-spherical subsonic accretion on to slowly rotating magnetized neutron stars is constructed. In this model, the accreting matter subsonically settles down on to the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere. The settling regime of accretion can be realized for moderate accretion rates ? g s-1. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of the spin-up/spin-down rates (the angular rotation frequency derivative ?, and ? near the torque reversal) of X-ray pulsars with known orbital periods, it is possible to determine the main dimensionless parameters of the model, as well as to estimate the magnetic field of the neutron star. We illustrate the model by determining these parameters for three wind-fed X-ray pulsars GX 301-2, Vela X-1 and GX 1+4. The model explains both the spin-up/spin-down of the pulsar frequency on large time-scales and the irregular short-term frequency fluctuations, which can correlate or anticorrelate with the X-ray flux fluctuations in different systems. It is shown that in real pulsars an almost iso-angular-momentum rotation law with ω˜ 1/R2, due to strongly anisotropic radial turbulent motions sustained by large-scale convection, is preferred.

Role of local absorption on the X-ray emission from MHD accretion shocks in classical T Tauri stars

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Bonito

2014-01-01

Full Text Available Accretion processes onto classical T Tauri stars (CTTSs are believed to generate shocks at the stellar surface due to the impact of supersonic downflowing plasma. Although current models of accretion streams provide a plausible global picture of this process, several aspects are still unclear. For example, the observed X-ray luminosity in accretion shocks is, in general, well below the predicted value. A possible explanation discussed in the literature is in terms of significant absorption of the emission due to the thick surrounding medium. Here we consider a 2D MHD model describing an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere. The model includes all the relevant physics, namely the gravity, the thermal conduction, and the radiative cooling, and a realistic description of the unperturbed stellar atmosphere (from the chromosphere to the corona. From the model results, we synthesize the X-ray emission emerging from the hot slab produced by the accretion shock, exploring different configurations and strengths of the stellar magnetic field. The synthesis includes the local absorption by the thick surrounding medium and the Doppler shift of lines due to the component of plasma velocity along the line-of-sight. We explore the effects of absorption on the emerging X-ray spectrum, considering different inclinations of the accretion stream with respect to the observer. Finally we compare our results with the observations.

Relativistic, accreting disks

Energy Technology Data Exchange (ETDEWEB)

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

1978-02-01

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

Evolving ONe WD+He star systems to intermediate-mass binary pulsars

Science.gov (United States)

Liu, D.; Wang, B.; Chen, W.; Zuo, Z.; Han, Z.

2018-06-01

It has been suggested that accretion-induced collapse (AIC) is a non-negligible path for the formation of the observed neutron stars (NSs). An ONe white dwarf (WD) that accretes material from a He star may experience AIC process and eventually produce intermediate-mass binary pulsars (IMBPs), named as the ONe WD+He star scenario. Note that previous studies can only account for part of the observed IMBPs with short orbital periods. In this work, we investigate the evolution of about 900 ONe WD+He star binaries to explore the distribution of IMBPs. We found that the ONe WD+He star scenario could form IMBPs including pulsars with 5-340 ms spin periods and 0.75-1.38 M_{⊙} WD companions, in which the orbital periods range from 0.04 to 900 d. Compared with the 20 observed IMBPs, this scenario can cover the parameters of 13 sources in the final orbital period-WD mass plane and the Corbet diagram, most of which have short orbital periods. We found that the ONe WD+He star scenario can explain almost all the observed IMBPs with short orbital periods. This work can well match the observed parameters of PSR J1802-2124 (one of the two precisely observed IMBPs), providing a possible evolutional path for its formation. We also speculate that the compact companion of HD 49798 (a hydrogen depleted sdO6 star) may be not a NS based on this work.

Transitional millisecond pulsars in the low-level accretion state

Science.gov (United States)

Jaodard, Amruta D.; Hessels, Jason W. T.; Archibald, Anne; Bogdanov, Slavko; Deller, Adam; Hernandez Santisteban, Juan; Patruno, Alessandro; D'Angelo, Caroline; Bassa, Cees; Amruta Jaodand

2018-01-01

In the canonical pulsar recycling scenario, a slowly spinning neutron star can be rejuvenated to rapid spin rates by the transfer of angular momentum and mass from a binary companion star. Over the last decade, the discovery of three transitional millisecond pulsars (tMSPs) has allowed us to study recycling in detail. These systems transition between accretion-powered (X-ray) and rotation-powered (radio) pulsar states within just a few days, raising questions such as: what triggers the state transition, when does the recycling process truly end, and what will the radio pulsar’s final spin rate be? Systematic multi-wavelength campaigns over the last decade have provided critical insights: multi-year-long, low-level accretion states showing coherent X-ray pulsations; extremely stable, bi-modal X-ray light curves; outflows probed by radio continuum emission; a surprising gamma-ray brightening during accretion, etc. In my thesis I am trying to bring these clues together to understand the low-level accretion process that recycles a pulsar. For example, recently we timed PSR J1023+0038 in the accretion state and found it to be spinning down ~26% faster compared to the non-accreting radio pulsar state. We are currently conducting simultaneous multi-wavelength campaigns (XMM, HST, Kepler and VLA) to understand the global variability of the accretion flow, as well as high-energy Fermi-LAT observations to probe the gamma-ray emission mechanism. I will highlight these recent developments, while also presenting a broad overview of tMSPs as exciting new laboratories to test low-level accretion onto magnetized neutron stars.

Compact stars with a small electric charge: the limiting radius to mass relation and the maximum mass for incompressible matter

Energy Technology Data Exchange (ETDEWEB)

Lemos, Jose P.S.; Lopes, Francisco J.; Quinta, Goncalo [Universidade de Lisboa, UL, Departamento de Fisica, Centro Multidisciplinar de Astrofisica, CENTRA, Instituto Superior Tecnico, IST, Lisbon (Portugal); Zanchin, Vilson T. [Universidade Federal do ABC, Centro de Ciencias Naturais e Humanas, Santo Andre, SP (Brazil)

2015-02-01

One of the stiffest equations of state for matter in a compact star is constant energy density and this generates the interior Schwarzschild radius to mass relation and the Misner maximum mass for relativistic compact stars. If dark matter populates the interior of stars, and this matter is supersymmetric or of some other type, some of it possessing a tiny electric charge, there is the possibility that highly compact stars can trap a small but non-negligible electric charge. In this case the radius to mass relation for such compact stars should get modifications. We use an analytical scheme to investigate the limiting radius to mass relation and the maximum mass of relativistic stars made of an incompressible fluid with a small electric charge. The investigation is carried out by using the hydrostatic equilibrium equation, i.e., the Tolman-Oppenheimer-Volkoff (TOV) equation, together with the other equations of structure, with the further hypothesis that the charge distribution is proportional to the energy density. The approach relies on Volkoff and Misner's method to solve the TOV equation. For zero charge one gets the interior Schwarzschild limit, and supposing incompressible boson or fermion matter with constituents with masses of the order of the neutron mass one finds that the maximum mass is the Misner mass. For a small electric charge, our analytical approximating scheme, valid in first order in the star's electric charge, shows that the maximum mass increases relatively to the uncharged case, whereas the minimum possible radius decreases, an expected effect since the new field is repulsive, aiding the pressure to sustain the star against gravitational collapse. (orig.)

Photometric Determination of the Mass Accretion Rates of Pre-main-sequence Stars. V. Recent Star Formation in the 30 Dor Nebula

Science.gov (United States)

De Marchi, Guido; Panagia, Nino; Beccari, Giacomo

2017-09-01

We report on the properties of the low-mass stars that recently formed in the central ˜ 2\\buildrel{ \\prime}\\over{.} 7× 2\\buildrel{ \\prime}\\over{.} 7 of 30 Dor, including the R136 cluster. Using the photometric catalog of De Marchi et al., based on observations with the Hubble Space Telescope, and the most recent extinction law for this field, we identify 1035 bona fide pre-main-sequence (PMS) stars showing {{H}}α excess emission at the 4σ level with an {{H}}α equivalent width of 20 Å or more. We find a wide spread in age spanning the range ˜ 0.1{--}50 {Myr}. We also find that the older PMS objects are placed in front of the R136 cluster and are separated from it by a conspicuous amount of absorbing material, indicating that star formation has proceeded from the periphery into the interior of the region. We derive physical parameters for all PMS stars, including masses m, ages t, and mass accretion rates {\\dot{M}}{acc}. To identify reliable correlations between these parameters, which are intertwined, we use a multivariate linear regression fit of the type {log}{\\dot{M}}{acc}=a× {log}t+b× {log}m+c. The values of a and b for 30 Dor are compatible with those found in NGC 346 and NGC 602. We extend the fit to a uniform sample of 1307 PMS stars with 0.5contract NAS5-26555.

Spontaneous Scalarization of Black Holes and Compact Stars from a Gauss-Bonnet Coupling.

Science.gov (United States)

Silva, Hector O; Sakstein, Jeremy; Gualtieri, Leonardo; Sotiriou, Thomas P; Berti, Emanuele

2018-03-30

We identify a class of scalar-tensor theories with coupling between the scalar and the Gauss-Bonnet invariant that exhibit spontaneous scalarization for both black holes and compact stars. In particular, these theories formally admit all of the stationary solutions of general relativity, but these are not dynamically preferred if certain conditions are satisfied. Remarkably, black holes exhibit scalarization if their mass lies within one of many narrow bands. We find evidence that scalarization can occur in neutron stars as well.

Contribution of High-Mass Black Holes to Mergers of Compact Binaries

International Nuclear Information System (INIS)

Bethe, H.A.; Brown, G.E.

1999-01-01

become a black hole and the less massive star B is a giant reaching out to A. We then have a common envelope, and we expect hypercritical accretion to star A. Star A will accept a small fraction of the mass of the envelope of star B, but it will plunge deep into star B while expelling the envelope of star B. We expect that star B can at least be in the mass range 15∼35 M circle-dot , while the black hole A has a mass of 10 M circle-dot . About 20% of the binaries of this type are found to end up in a range of orbital radii favorable for merging; i.e., outside of the relevant Roche lobes, but close enough so that these final binaries of compact objects will merge in Hubble time. The narrow black hole O star orbits do not seem to be found in population syntheses, because in them mergers happen almost completely as a result of kick velocities. In the exception (case H of Portegies Zwart ampersand Yungelson, which includes hypercritical accretion), common envelope evolution is more effective and we are in agreement with their results. We find that the high-mass black hole neutron star systems contribute substantially to the predicted observational frequency of gravitational waves. We discuss how our high-mass black hole formation can be reconciled with the requirements of nucleosynthesis, and we indicate that a bimodal distribution of masses of black holes in single stars can account, at least qualitatively, for the many transient sources that contain high-mass black holes. copyright copyright 1999. The American Astronomical Society

Contrasting Behaviour from Two Be/X-ray Binary Pulsars: Insights into Differing Neutron Star Accretion Modes

Science.gov (United States)

Townsend, L. J.; Drave, S. P.; Hill, A. B.; Coe, M. J.; Corbet, R. H. D.; Bird, A. J.

2013-01-01

In this paper we present the identification of two periodic X-ray signals coming from the direction of the Small Magellanic Cloud (SMC). On detection with the Rossi X-ray Timing Explorer (RXTE), the 175.4 s and 85.4 s pulsations were considered to originate from new Be/X-ray binary (BeXRB) pulsars with unknown locations. Using rapid follow-up INTEGRAL and XMM-Newton observations, we show the first pulsar (designated SXP175) to be coincident with a candidate high-mass X-ray binary (HMXB) in the northern bar region of the SMC undergoing a small Type II outburst. The orbital period (87d) and spectral class (B0-B0.5IIIe) of this system are determined and presented here for the first time. The second pulsar is shown not to be new at all, but is consistent with being SXP91.1 - a pulsar discovered at the very beginning of the 13 year long RXTE key monitoring programme of the SMC. Whilst it is theoretically possible for accreting neutron stars to change spin period so dramatically over such a short time, the X-ray and optical data available for this source suggest this spin-up is continuous during long phases of X-ray quiescence, where accretion driven spin-up of the neutron star should be minimal.

Constraining the number of compact remnants near Sgr A*

Science.gov (United States)

Deegan, Patrick; Nayakshin, Sergei

2007-05-01

Due to dynamical friction stellar mass black holes and neutron stars are expected to form high-density cusps in the inner parsec of our Galaxy. These compact remnants, expected to number around 20000, may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. Here we build a simple but detailed time-dependent model of such emission. The possibility that these accretion flows are radiatively inefficient is taken into account and brings in some uncertainty in the conclusions. Despite this uncertainty, we find that at least several X-ray sources of this nature should be detectable with Chandra at any one time. Turning this issue around, we also ask a question of what current observational constraints might be telling us about the total number of compact remnants. In our `best guess' model, a cusp of ~40000 remnants overpredicts the number of discrete sources and the total X-ray luminosity of the inner parsec, and is hence ruled out. In the most radiatively inefficient scenario that we consider, the radiative efficiency is set to be as small as ɛ = 10-5. In this rather unlikely scenario, a cusp of ~40000 black holes would be allowed by the data, but several individual sources should still be visible. Future observations of the distribution and orbits of the cold ionized gas in the inner parsec of our Galaxy will put tighter constraints on the cusp of compact remnants.

Searching for X-ray Pulsations from Neutron Stars Using NICER

Science.gov (United States)

Ray, Paul S.; Arzoumanian, Zaven; Gendreau, Keith C.; Bogdanov, Slavko; Bult, Peter; Chakrabarty, Deepto; Chakrabarty, Deepto; Guillot, Sebastien; Harding, Alice; Ho, Wynn C. G.; Lamb, Frederick; Mahmoodifar, Simin; Miller, Cole; Strohmayer, Tod; Wilson-Hodge, Colleen; Wolff, Michael T.; NICER Science Team Working Group on Pulsation Searches and Multiwavelength Coordination

2018-01-01

The Neutron Star Interior Composition Explorer (NICER) presents an exciting new capability for discovering new modulation properties of X-ray emitting neutron stars, including large area, low background, extremely precise absolute time stamps, superb low-energy response and flexible scheduling. The Pulsation Searches and Multiwavelength Coordination working group has designed a 2.5 Ms observing program to search for pulsations and characterize the modulation properties of about 30 known or suspected neutron star sources across a number of source categories. A key early goal will be to search for pulsations from millisecond pulsars that might exhibit thermal pulsations from the surface suitable for pulse profile modeling to constrain the neutron star equation of state. In addition, we will search for pulsations from transitional millisecond pulsars, isolated neutron stars, LMXBs, accretion-powered millisecond pulsars, central compact objects and other sources. We present our science plan and initial results from the first months of the NICER mission.

The Great Wall: Urca Cooling Layers in the Accreted NS Crust

Directory of Open Access Journals (Sweden)

Meisel Zach

2018-01-01

Full Text Available Accreting neutron stars host a number of astronomical observables which can be used to infer the properties of the underlying dense matter. These observables are sensitive to the heating and cooling processes taking place in the accreted neutron star (NS crust. Within the past few years it has become apparent that electron-capture/beta-decay (urca cycles can operate within the NS crust at high temperatures. Layers of nuclei undergoing urca cycling can create a thermal barrier, or Great Wall, between heating occurring deep in the crust and the regions above the urca layers. This paper briefly reviews the urca process and the implications for observables from accreting neutron stars.

MEASURING TINY MASS ACCRETION RATES ONTO YOUNG BROWN DWARFS

International Nuclear Information System (INIS)

Herczeg, Gregory J.; Cruz, Kelle L.; Hillenbrand, Lynne A.

2009-01-01

We present low-resolution Keck I/LRIS spectra spanning from 3200 to 9000 A of nine young brown dwarfs and three low-mass stars in the TW Hya Association and in Upper Sco. The optical spectral types of the brown dwarfs range from M5.5 to M8.75, though two have near-IR spectral types of early L dwarfs. We report new accretion rates derived from excess Balmer continuum emission for the low-mass stars TW Hya and Hen 3-600A and the brown dwarfs 2MASS J12073347-3932540, UScoCTIO 128, SSSPM J1102-3431, USco J160606.29-233513.3, DENIS-P J160603.9-205644, and Oph J162225-240515B, and upper limits on accretion for the low-mass star Hen 3-600B and the brown dwarfs UScoCTIO 112, Oph J162225-240515A, and USco J160723.82-221102.0. For the six brown dwarfs in our sample that are faintest at short wavelengths, the accretion luminosity or upper limit is measurable only when the image is binned over large wavelength intervals. This method extends our sensitivity to accretion rate down to ∼10 -13 M sun yr -1 for brown dwarfs. Since the ability to measure an accretion rate from excess Balmer continuum emission depends on the contrast between excess continuum emission and the underlying photosphere, for objects with earlier spectral types the upper limit on accretion rate is much higher. Absolute uncertainties in our accretion rate measurements of ∼3-5 include uncertainty in accretion models, brown dwarf masses, and distance. The accretion rate of 2 x 10 -12 M sun yr -1 onto 2MASS J12073347-3932540 is within 15% of two previous measurements, despite large changes in the Hα flux.

Accretion Disks and Coronae in the X-Ray Flashlight

Science.gov (United States)

Degenaar, Nathalie; Ballantyne, David R.; Belloni, Tomaso; Chakraborty, Manoneeta; Chen, Yu-Peng; Ji, Long; Kretschmar, Peter; Kuulkers, Erik; Li, Jian; Maccarone, Thomas J.; Malzac, Julien; Zhang, Shu; Zhang, Shuang-Nan

2018-02-01

Plasma accreted onto the surface of a neutron star can ignite due to unstable thermonuclear burning and produce a bright flash of X-ray emission called a Type-I X-ray burst. Such events are very common; thousands have been observed to date from over a hundred accreting neutron stars. The intense, often Eddington-limited, radiation generated in these thermonuclear explosions can have a discernible effect on the surrounding accretion flow that consists of an accretion disk and a hot electron corona. Type-I X-ray bursts can therefore serve as direct, repeating probes of the internal dynamics of the accretion process. In this work we review and interpret the observational evidence for the impact that Type-I X-ray bursts have on accretion disks and coronae. We also provide an outlook of how to make further progress in this research field with prospective experiments and analysis techniques, and by exploiting the technical capabilities of the new and concept X-ray missions ASTROSAT, NICER, Insight-HXMT, eXTP, and STROBE-X.

A 4D spacetime embedded in a 5D pseudo-Euclidean space describing interior of compact stars

Energy Technology Data Exchange (ETDEWEB)

Singh, K.N. [National Defence Academy, Department of Physics, Khadakwasla (India); Murad, Mohammad Hassan [BRAC University, Department of Mathematics and Natural Sciences, Dhaka (Bangladesh); Pant, Neeraj [National Defence Academy, Department of Mathematics, Khadakwasla (India)

2017-02-15

The present paper provides a new model of compact stars satisfying the Karmarkar condition. The model is obtained by assuming a new type of metric potential for g{sub rr} from the condition of embedding class I. The model parameters are obtained accordingly by employing the metric potentials to Einstein's field equations. Our model is free from geometric singularity and satisfies all the physical conditions. The obtained mass and radius of the compact stars Cen X-3, EXO 1785-248 and SAX 1808.4-3658 obtained from the model are consistent with the observational data of T. Gangopadhyay et al. Detailed analyses of these neutron stars (Cen X-3, EXO 1785-248 and SAX 1808.4-3658) are also given with the help of graphical representations. (orig.)

Diskoseismology: Probing accretion disks. I - Trapped adiabatic oscillations

Science.gov (United States)

Nowak, Michael A.; Wagoner, Robert V.

1991-01-01

The normal modes of acoustic oscillations within thin accretion disks which are terminated by an innermost stable orbit around a slowly rotating black hole or weakly magnetized compact neutron star are analyzed. The dominant relativistic effects which allow modes to be trapped within the inner region of the disk are approximated via a modified Newtonian potential. A general formalism is developed for investigating the adiabatic oscillations of arbitrary unperturbed disk models. The generic behavior is explored by way of an expansion of the Lagrangian displacement about the plane of symmetry and by assuming separable solutions with the same radial wavelength for the horizontal and vertical perturbations. The lowest eigenfrequencies and eigenfunctions of a particular set of radial and quadrupole modes which have minimum motion normal for the plane are obtained. These modes correspond to the standard dispersion relation of disk theory.

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.

THE COMPACT STAR-FORMING COMPLEX AT THE HEART OF NGC 253

Energy Technology Data Exchange (ETDEWEB)

Davidge, T. J., E-mail: tim.davidge@nrc.ca [Dominion Astrophysical Observatory, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7 (Canada)

2016-02-20

We discuss integral field spectra of the compact star-forming complex that is the brightest near-infrared (NIR) source in the central regions of the starburst galaxy NGC 253. The spectra cover the H and K passbands and were recorded with the Gemini NIR Spectrograph during subarcsecond seeing conditions. Absorption features in the spectrum of the star-forming complex are weaker than in the surroundings. An absorption feature is found near 1.78 μm that coincides with the location of a C{sub 2} bandhead. If this feature is due to C{sub 2} then the star-forming complex has been in place for at least a few hundred Myr. Emission lines of Brγ, [Fe ii], and He i 2.06 μm do not track the NIR continuum light. Pockets of star-forming activity that do not have associated concentrations of red supergiants, and so likely have ages <8 Myr, are found along the western edge of the complex, and there is evidence that one such pocket contains a rich population of Wolf–Rayet stars. Unless the star-forming complex is significantly more metal-poor than the surroundings, then a significant fraction of its total mass is in stars with ages <8 Myr. If the present-day star formation rate is maintained then the timescale to double its stellar mass ranges from a few Myr to a few tens of Myr, depending on the contribution made by stars older than ∼8 Myr. If—as suggested by some studies—the star-forming complex is centered on the galaxy’s nucleus, which presumably contains a large population of old and intermediate-age stars, then the nucleus of NGC 253 is currently experiencing a phase of rapid growth in its stellar mass.

Formation and Evolution of Neutron Star Binaries: Masses of Neutron Stars

Directory of Open Access Journals (Sweden)

Lee Chang-Hwan

2012-02-01

Full Text Available Neutron star (NS is one of the most interesting astrophysical compact objects for hardronic physics. It is believed that the central density of NS can reach several times the normal nuclear matter density (ρ0. Hence, the inner part of NS is the ultimate testing place for the physics of dense matter. Recently, the mass of NS in a NS-white dwarf (WD binary PSR J1614-2230 has been estimated to be 1.97 ± 0.04M๏ [1]. Since this estimate is based on the observed Shapiro delay, it can give the lower limit of the maximum NS mass and rules out many soft equations of state. On the other hand, all the well-measured NS masses in NS-NS binaries are smaller than 1.5M๏. In this work, by introducing the supercritical accretion during the binary evolution, we propose a possibility of forming higher mass NS in NS-WD binaries. In this scenario, the lifetimes of NS and WD progenitors are significantly different, and NS in NS-WD binary can accrete > 0.5M๏ after NS formation during the giant phase of the progenitor of WD. On the other hand, for the binary system with NS and heavier (> 8M๏ giants, the first-born NS will accrete more from the companion and can collapse into black hole. The only way to avoid the supercritical accretion is that the initial masses of progenitors of NS binary should be very close so that they evolve almost at the same time and don’t have time to accrete after NS formation.

An Active Black Hole in a Compact Dwarf

Science.gov (United States)

Kohler, Susanna

2016-05-01

A new type of galaxy has just been added to the galaxy zoo: a small, compact, and old elliptical galaxy that shows signs of a monster black hole actively accreting material in its center. What can this unusual discovery tell us about how compact elliptical galaxies form?A New Galactic BeastCompact elliptical galaxies are an extremely rare early-type dwarf galaxy. Consistent with their name, compact ellipticals are small, very compact collections of ancient stars; these galaxies exhibit a high surface brightness and arent actively forming stars.Optical view of the ancient compact elliptical galaxy SDSS J085431.18+173730.5 (center of image) in an SDSS color composite image. [Adapted from Paudel et al. 2016]Most compact ellipticals are found in dense environments, particularly around massive galaxies. This has led astronomers to believe that compact ellipticals might form via the tidal stripping of a once-large galaxy in interactions with another, massive galaxy. In this model, once the original galaxys outer layers are stripped away, the compact inner bulge component would be left behind as a compact elliptical galaxy. Recent discoveries of a few isolated compact ellipticals, however, have strained this model.Now a new galaxy has been found to confuse our classification schemes: the first-ever compact elliptical to also display signs of an active galactic nucleus. Led by Sanjaya Paudel (Korea Astronomy and Space Science Institute), a team of scientists discovered SDSS J085431.18+173730.5 serendipitously in Sloan Digital Sky Survey data. The team used SDSS images and spectroscopy in combination with data from the Canada-France-Hawaii Telescope to learn more about this unique galaxy.Puzzling CharacteristicsSDSS J085431.18+173730.5 presents an interesting conundrum. Ancient compact ellipticals are supposed to be devoid of gas, with no fuel left to trigger nuclear activity. Yet SDSS J085431.18+173730.5 clearly shows the emission lines that indicate active accretion onto

Compact Starburst Galaxies with Fast Outflows: Spatially Resolved Stellar Mass Profiles

Science.gov (United States)

Gottlieb, Sophia; Diamond-Stanic, Aleksandar; Lipscomb, Charles; Ohene, Senyo; Rines, Josh; Moustakas, John; Sell, Paul; Tremonti, Christy; Coil, Alison; Rudnick, Gregory; Hickox, Ryan C.; Geach, James; Kepley, Amanda

2018-01-01

Powerful galactic winds driven by stellar feedback and black hole accretion are thought to play an important role in regulating star formation in galaxies. In particular, strong stellar feedback from supernovae, stellar winds, radiation pressure, and cosmic rays is required by simulations of star-forming galaxies to prevent the vast majority of baryons from cooling and collapsing to form stars. However, it remains unclear whether these stellar processes play a significant role in expelling gas and shutting down star formation in massive progenitors of quiescent galaxies. What are the limits of stellar feedback? We present multi-band photometry with HST/WFC3 (F475W, F814W, F160W) for a dozen compact starburst galaxies at z~0.6 with half-light radii that suggest incredibly large central escape velocities. These massive galaxies are driving fast (>1000 km/s) outflows that have been previously attributed to stellar feedback associated with the compact (r~100 pc) starburst. But how compact is the stellar mass? In the context of the stellar feedback hypothesis, it is unclear whether these fast outflows are being driven at velocities comparable to the escape velocity of an incredibly dense stellar system (as predicted by some models of radiation-pressure winds) or at velocities that exceed the central escape velocity by large factor. Our spatially resolved measurements with HST show that the stellar mass is more extended than the light, and this requires that the physical mechanism responsible for driving the winds must be able to launch gas at velocities that are factors of 5-10 beyond the central escape velocity.

The INTEGRAL long monitoring of persistent ultra compact X-ray bursters

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Fiocchi, M.; Bazzano, A.; Ubertini, P.; Bird, A. J.; Natalucci, L.; Sguera, V.

2008-12-01

Context: The combination of compact objects, short period variability and peculiar chemical composition of the ultra compact X-ray binaries make up a very interesting laboratory to study accretion processes and thermonuclear burning on the neutron star surface. Improved large optical telescopes and more sensitive X-ray satellites have increased the number of known ultra compact X-ray binaries allowing their study with unprecedented detail. Aims: We analyze the average properties common to all ultra compact bursters observed by INTEGRAL from 0.2 keV to 150 keV. Methods: We have performed a systematic analysis of the INTEGRAL public data and Key-Program proprietary observations of a sample of the ultra compact X-ray binaries. In order to study their average properties in a very broad energy band, we combined INTEGRAL with BeppoSAX and SWIFT data whenever possible. For sources not showing any significant flux variations along the INTEGRAL monitoring, we build the average spectrum by combining all available data; in the case of variable fluxes, we use simultaneous INTEGRAL and SWIFT observations when available. Otherwise we compared IBIS and PDS data to check the variability and combine BeppoSAX with INTEGRAL /IBIS data. Results: All spectra are well represented by a two component model consisting of a disk-blackbody and Comptonised emission. The majority of these compact sources spend most of the time in a canonical low/hard state, with a dominating Comptonised component and accretion rate dot {M} lower than 10-9 {M⊙}/yr, not depending on the model used to fit the data. INTEGRAL is an ESA project with instruments and Science Data Center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA.

X-ray sources in stars formation areas: T Tauri stars and proto-stars in the rho Ophiuchi dark cloud

International Nuclear Information System (INIS)

Grosso, Nicolas

1999-01-01

This thesis studies from large to small scales, X-ray sources in the rho Ophiuchi dark cloud. After some background on the formation of the low-mass young stars (Chapter 1), Chapter 2 takes an interest in the T Tauri star population. Chapter 3 tackles the search of the magnetic activity at the younger stage of protostar, presenting a powerful X-ray emission from an IR protostar, called YLW15, during a flare, and a quasi-periodic flare of the same source; as well as a new detection of another IR protostar in the ROSAT archives. It ends with a review of protostar detections. Some IR protostar flares show a very long increasing phase. Chapter 4 links this behaviour with a modulation by the central star rotation. The standard model of jet emission assumes that the central star rotates at the same speed that the inner edge of its accretion disk. This chapter shows that the observation of the YLW15 quasi-periodic flare suggests rather that the forming star rotates faster than its accretion disk, at the break up limit. The synchronism with the accretion disk, observed on T Tauri stars, must be reach progressively by magnetic breaking during the IR protostar stage, and more or less rapidly depending on the forming star mass. Recent studies have shown that T Tauri star X-ray emission could ionize the circumstellar disk, and play a role in the instability development, as well as stimulate the accretion. The protostar X-ray emission might be higher than the T Tauri star one, Chapter 5 presents a millimetric interferometric observation dedicated to measure this effect on YLW15. Finally, Chapter 6 reassembles conclusions and perspectives of this work. (author) [fr

Bondi-Hoyle-Lyttleton Accretion onto Binaries

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

2018-01-01

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

MOLECULAR CLOUD EVOLUTION. III. ACCRETION VERSUS STELLAR FEEDBACK

International Nuclear Information System (INIS)

Vazquez-Semadeni, Enrique; ColIn, Pedro; Gomez, Gilberto C.; Ballesteros-Paredes, Javier; Watson, Alan W.

2010-01-01

We numerically investigate the effect of feedback from the ionization heating from massive stars on the evolution of giant molecular clouds (GMCs) and their star formation efficiency (SFE), which we treat as an instantaneous, time-dependent quantity. We follow the GMCs' evolution from their formation to advanced star-forming stages. After an initial period of contraction, the collapsing clouds begin forming stars, whose feedback evaporates part of the clouds' mass, opposing the continuing accretion from the infalling gas. Our results are as follows: (1) in the presence of feedback, the clouds attain levels of the SFE that are consistent at all times with observational determinations for regions of comparable star formation rates. (2) However, the dense gas mass is larger in general in the presence of feedback, while the total mass (dense gas + stars) is nearly insensitive to the presence of feedback, suggesting that it is determined mainly by the accretion, while the feedback inhibits mainly the conversion of dense gas to stars, because it acts directly to reheat and disperse the gas that is directly on its way to forming stars. (3) The factor by which the SFE is reduced upon the inclusion of feedback is a decreasing function of the cloud's mass, for clouds of size ∼10 pc. This naturally explains the larger observed SFEs of massive-star-forming regions. (4) The clouds may attain a pseudo-virialized state, with a value of the virial mass very similar to the actual cloud mass. However, this state differs from true virialization in that the clouds, rather than being equilibrium entities, are the centers of a larger-scale collapse, in which accretion replenishes the mass consumed by star formation. (5) The higher-density regions within the clouds are in a similar situation, accreting gas infalling from the less-dense, more extended regions of the clouds. (6) The density probability density functions of the regions containing the clouds in general exhibit a shape

New Evidence for a Black Hole in the Compact Binary Cygnus X-3

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Shrader, Chris R.; Titarchuk, Lev; Shaposhnikov, Nikolai

2010-01-01

The bright and highly variable X-ray and radio source known as Cygnus X-3 was among the first X-ray sources discovered, yet it remains in many ways an enigma. Its known to consist of a massive. Wolf-Rayet primary in an extremely tight orbit with a compact object. Yet one of the most basic of pa.ranietern the mass of the compact object - is not known. Nor is it even clear whether its is a neutron star or a black hole. In this Paper we present our analysis of the broad-band high-energy continua covering a substantial range in luminosity and spectral morphology. We apply these results to a recently identified scaling relationship which has been demonstrated to provide reliable estimates of the compact object mass in a number of accretion powered binaries. This analysis leads us to conclude that the compact object in Cygnus X-3 has a mass greater than 4.2 solar mass thus clearly indicative of a black hole and as such resolving a longstanding issue. The full range of uncertainty in our analysis and from using a. range of recently published distance estimates constrains the compact object mass to lie between 4.2 solar mass and 14.4 solar mass. Our favored estimate, based on a 9.0 kpc distance estimate is approx. l0 solar mass, with the. error margin of 3.2 solar masses. This result may thus pose challenges to shared-envelope evolutionary models of compact binaries. as well as establishing Cygnus X-3 as the first confirmed accretion-powered galactic gamma: ray source.

PREFACE: 2nd International Symposium on the Modern Physics of Compact Stars and Relativistic Gravity

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Edvard Chubaryan, Professor; Aram Saharian, Professor; Armen Sedrakian, Professor

2014-03-01

The international conference ''The Modern Physics of Compact Stars and Relativistic Gravity'' took place in Yerevan, Armenia, from 18-21 September 2013. This was the second in a series of conferences which aim to bring together people working in astrophysics of compact stars, physics of dense matter, gravitation and cosmology, observations of pulsars and binary neutron stars and related fields. The conference was held on the occasion of 100th birthday of the founder of the Theoretical Physics Chair at the Department of Physics of Yerevan State University and prominent Armenian scientist Academician Gurgen S Sahakyan. The field of compact stars has seen extraordinary development since the discovery of pulsars in 1967. Even before this discovery, pioneering work of a number of theoretical groups had laid the foundation for this development. A pioneer of this effort was Professor G S Sahakyan who, together with Professor Victor Ambartsumyan and a group of young scientists, started in the early sixties their fundamental work on the properties of superdense matter and on the relativistic structure of compact stellar objects. This conference explored the vast diversity of the manifestations of compact stars, including the modern aspects of the equation of state of superdense matter, its magnetic and thermal properties, rotational dynamics, superfluidity and superconductivity, phase transition from hadronic to quark matter, etc. The articles on these subjects collected in this volume are evidence of liveliness of the field and of the continuous feedback between theory and the experiment. A part of this volume is devoted to the cosmology and the theories of gravity — the subfields of astrophysics that are of fundamental importance to our understanding of the universe. The reader will find here articles touching on the most diverse aspects of these fields such as modern problems in Einstein's classical theory of gravity and its alternatives, string theory motivated

POPULATION III GAMMA-RAY BURSTS AND BREAKOUT CRITERIA FOR ACCRETION-POWERED JETS

Energy Technology Data Exchange (ETDEWEB)

Nagakura, Hiroki; Suwa, Yudai [Yukawa Institute for Theoretical Physics, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan); Ioka, Kunihito, E-mail: hiroki@heap.phys.waseda.ac.jp [KEK Theory Center, 1-1 Oho, Tsukuba 305-0801 (Japan)

2012-08-01

We investigate the propagation of accretion-powered jets in various types of massive stars such as Wolf-Rayet stars, light Population III (Pop III) stars, and massive Pop III stars, all of which are the progenitor candidates of gamma-ray bursts (GRBs). We perform two-dimensional axisymmetric simulations of relativistic hydrodynamics, taking into account both the envelope collapse and the jet propagation (i.e., the negative feedback of the jet on the accretion). Based on our hydrodynamic simulations, we show for the first time that the accretion-powered jet can potentially break out relativistically from the outer layers of Pop III progenitors. In our simulations, the accretion rate is estimated by the mass flux going through the inner boundary, and the jet is injected with a fixed accretion-to-jet conversion efficiency {eta}. By varying the efficiency {eta} and opening angle {theta}{sub op} for more than 40 models, we find that the jet can make a relativistic breakout from all types of progenitors for GRBs if a simple condition {eta} {approx}> 10{sup -4}({theta}{sub op}/8 Degree-Sign ){sup 2} is satisfied, which is consistent with analytical estimates. Otherwise no explosion or some failed spherical explosions occur.

ACCRETION AND MAGNETIC RECONNECTION IN THE CLASSICAL T TAURI BINARY DQ TAU

International Nuclear Information System (INIS)

Tofflemire, Benjamin M.; Mathieu, Robert D.; Ardila, David R.; Akeson, Rachel L.; Ciardi, David R.; Johns-Krull, Christopher; Herczeg, Gregory J.; Quijano-Vodniza, Alberto

2017-01-01

The theory of binary star formation predicts that close binaries ( a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory.

ACCRETION AND MAGNETIC RECONNECTION IN THE CLASSICAL T TAURI BINARY DQ TAU

Energy Technology Data Exchange (ETDEWEB)

Tofflemire, Benjamin M.; Mathieu, Robert D. [Department of Astronomy, University of Wisconsin–Madison, 475 North Charter Street, Madison, WI 53706 (United States); Ardila, David R. [The Aerospace Corporation, M2-266, El Segundo, CA 90245 (United States); Akeson, Rachel L.; Ciardi, David R. [NASA Exoplanet Science Institute, IPAC/Caltech, Pasadena, CA 91125 (United States); Johns-Krull, Christopher [Department of Physics and Astronomy, Rice University, Houston, TX 77005 (United States); Herczeg, Gregory J. [The Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871 (China); Quijano-Vodniza, Alberto [University of Nariño Observatory, Pasto, Nariño (Colombia)

2017-01-20

The theory of binary star formation predicts that close binaries ( a < 100 au) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk (CBD), cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (∼daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (flares) during the collision of stellar magnetospheres near periastron could, however, produce the same periodic, broadband behavior when observed at a one-day cadence. To reveal the dominant physical mechanism seen in DQ Tau’s low-cadence observations, we have obtained continuous, moderate-cadence, multiband photometry over 10 orbital periods, supplemented with 27 nights of minute-cadence photometry centered on four separate periastron passages. While both accretion and stellar flares are present, the dominant timescale and morphology of brightening events are characteristic of accretion. On average, the mass accretion rate increases by a factor of five near periastron, in good agreement with recent models. Large variability is observed in the morphology and amplitude of accretion events from orbit to orbit. We argue that this is due to the absence of stable circumstellar disks around each star, compounded by inhomogeneities at the inner edge of the CBD and within the accretion streams themselves. Quasiperiodic apastron accretion events are also observed, which are not predicted by binary accretion theory.

Nearly collisionless spherical accretion

International Nuclear Information System (INIS)

Begelman, M.C.

1977-01-01

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

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

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

2011-05-01

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

A model of two-stream non-radial accretion for binary X-ray pulsars

International Nuclear Information System (INIS)

Lipunov, V.M.

1982-01-01

The general case of non-radial accretion is assumed to occur in real binary systems containing X-ray pulsars. The structure and the stability of the magnetosphere, the interaction between the magnetosphere and accreted matter, as well as evolution of neutron star in close binary system are examined within the framework of the two-stream model of nonradial accretion onto a magnetized neutron star. Observable parameters of X-ray pulsars are explained in terms of the model considered. (orig.)

X-ray emission from hot subdwarfs with compact companions

Directory of Open Access Journals (Sweden)

Esposito P.

2013-03-01

Full Text Available We review the X-ray observations of hot subdwarf stars. While no X-ray emission has been detected yet from binaries containing B-type subdwarfs, interesting results have been obtained in the case of the two luminous O-type subdwarfs HD 49798 and BD + 37° 442. Both of them are members of binary systems in which the X-ray luminosity is powered by accretion onto a compact object: a rapidly spinning (13.2 s and massive (1.28  M⊙ white dwarf in the case of HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD + 37° 442. Their study can shed light on the poorly known processes taking place during common envelope evolutionary phases and on the properties of wind mass loss from hot subdwarfs.

Spherically symmetric charged compact stars

Energy Technology Data Exchange (ETDEWEB)

Maurya, S.K. [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman); Gupta, Y.K. [Jaypee Institute of Information Technology University, Department of Mathematics, Noida, Uttar Pradesh (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India); Chowdhury, Sourav Roy [Seth Anandaram Jaipuria College, Department of Physics, Kolkata, West Bengal (India)

2015-08-15

In this article we consider the static spherically symmetric metric of embedding class 1. When solving the Einstein-Maxwell field equations we take into account the presence of ordinary baryonic matter together with the electric charge. Specific new charged stellar models are obtained where the solutions are entirely dependent on the electromagnetic field, such that the physical parameters, like density, pressure etc. do vanish for the vanishing charge. We systematically analyze altogether the three sets of Solutions I, II, and III of the stellar models for a suitable functional relation of ν(r). However, it is observed that only the Solution I provides a physically valid and well-behaved situation, whereas the Solutions II and III are not well behaved and hence not included in the study. Thereafter it is exclusively shown that the Solution I can pass through several standard physical tests performed by us. To validate the solution set presented here a comparison has also been made with that of the compact stars, like RX J 1856 - 37, Her X - 1, PSR 1937+21, PSRJ 1614-2230, and PSRJ 0348+0432, and we have shown the feasibility of the models. (orig.)

Photoionization Models for the Inner Gaseous Disks of Herbig Be Stars: Evidence against Magnetospheric Accretion?

Energy Technology Data Exchange (ETDEWEB)

Patel, P.; Sigut, T. A. A.; Landstreet, J. D., E-mail: ppatel54@uwo.ca [Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7 (Canada)

2017-02-20

We investigate the physical properties of the inner gaseous disks of three hot Herbig B2e stars, HD 76534, HD 114981, and HD 216629, by modeling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by photospheric radiation from the central star in order to test whether the optical and near-infrared emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of H i (H α and H β ), He i, Ca ii, and Fe ii. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as ∼10{sup −10}( R {sub *}/ R ){sup 3} g cm{sup −3} in the equatorial plane of a 25 R {sub *} (0.78 au) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the H α -emitting portion of the inner gaseous disk of ∼10{sup −9} M {sub *}. We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a ≈1 au, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium.

Photoionization Models for the Inner Gaseous Disks of Herbig Be Stars: Evidence against Magnetospheric Accretion?

International Nuclear Information System (INIS)

Patel, P.; Sigut, T. A. A.; Landstreet, J. D.

2017-01-01

We investigate the physical properties of the inner gaseous disks of three hot Herbig B2e stars, HD 76534, HD 114981, and HD 216629, by modeling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by photospheric radiation from the central star in order to test whether the optical and near-infrared emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of H i (H α and H β ), He i, Ca ii, and Fe ii. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as ∼10 −10 ( R * / R ) 3 g cm −3 in the equatorial plane of a 25 R * (0.78 au) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the H α -emitting portion of the inner gaseous disk of ∼10 −9 M * . We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a ≈1 au, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium.

The diversity of neutron stars: Nearby thermally emitting neutron stars and the compact central objects in supernova remnants

Science.gov (United States)

Kaplan, David L.

Neutron stars are invaluable tools for exploring stellar death, the physics of ultra-dense matter, and the effects of extremely strong magnetic fields. The observed population of neutron stars is dominated by the > 1000 radio pulsars, but there are distinct sub-populations that, while fewer in number, can have significant impact on our understanding of the issues mentioned above. These populations are the nearby isolated neutron stars discovered by ROSAT, and the central compact objects in supernova remnants. The studies of both of these populations have been greatly accelerated in recent years through observations with the Chandra X-ray Observatory and the XMM-Newton telescope. First, we discuss radio, optical, and X-ray observations of the nearby neutron stars aimed at determining their relation to the Galactic neutron star population and at unraveling their complex physical processes by determining the basic astronomical parameters that define the population -- instances, ages, and magnetic fields -- the uncertainties in which limit any attempt to derive basic physical parameters for these objects. We conclude that these sources are 10^6 year-old cooling neutron stars with magnetic fields above 10^13 G. Second, we describe the hollow supernova remnant problem: why many of the supernova remnants in the Galaxy have no indication central neutron stars. We have undertaken an X-ray census of neutron stars in a volume-limited sample of Galactic supernova remnants, and from it conclude that either many supernovae do not produce neutron stars contrary to expectation, or that neutron stars can have a wide range in cooling behavior that makes many sources disappear from the X-ray sky.

Mass-Accretion effects on white dwarf interiors

International Nuclear Information System (INIS)

Canal, R.; Hernanz, M.; Isern, J.; Labay, J.; Mochkovitch, R.

1986-01-01

There is observational evidence of the presence of young neutron stars in old binary systems. A likely explanation is that those neutron stars were produced in the collapse of old C+O white dwarfs. Old white dwarfs being cold and at least partially solid, accretion-induced mass growth should finally lead in a number of cases, to their collapse rather than to their explosion. We show in detail how mass accretion on initially solid white dwarfs can leave central solid cores when dynamical instability sets in. We also study the different effects of the existence of such cores on the outcome of the competition between thermonuclear explosion and gravitational collapse

Starquakes, Heating Anomalies, and Nuclear Reactions in the Neutron Star Crust

Science.gov (United States)

Deibel, Alex Thomas

When the most massive stars perish, their cores may remain intact in the form of extremely dense and compact stars. These stellar remnants, called neutron stars, are on the cusp of becoming black holes and reach mass densities greater than an atomic nucleus in their centers. Although the interiors of neutron stars were difficult to investigate at the time of their discovery, the advent of modern space-based telescopes (e.g., Chandra X-ray Observatory) has pushed our understanding of the neutron star interior into exciting new realms. It has been shown that the neutron star interior spans an enormous range of densities and contains many phases of matter, and further theoretical progress must rely on numerical calculations of neutron star phenomena built with detailed nuclear physics input. To further investigate the properties of the neutron star interior, this dissertation constructs numerical models of neutron stars, applies models to various observations of neutron star high-energy phenomena, and draws new conclusions about the neutron star interior from these analyses. In particular, we model the neutron star's outermost ? 1 km that encompasses the neutron star's envelope, ocean, and crust. The model must implement detailed nuclear physics to properly simulate the hydrostatic and thermal structure of the neutron star. We then apply our model to phenomena that occur in these layers, such as: thermonuclear bursts in the envelope, g-modes in the ocean, torsional oscillations of the crust, and crust cooling of neutron star transients. A comparison of models to observations provides new insights on the properties of dense matter that are often difficult to probe through terrestrial experiments. For example, models of the quiescent cooling of neutron stars, such as the accreting transient MAXI J0556-332, at late times into quiescence probe the thermal transport properties of the deep neutron star crust. This modeling provides independent data from astronomical

Rapid Mergers in a Mixed System of Black Holes and Neutron Stars

Science.gov (United States)

Tagawa, Hiromichi; Umemura, Masayuki

Recently, it has been argued that r-process elements in galaxies primarily originate from the mergers of double neutron stars (NSs) and black hole (BH)-NS. However, there is a momentous problem that the merger timescale is estimated to be much longer than the production timescale of r-process elements inferred from metal poor stars in the Galactic halo. To solve this problem, we propose the rapid merger processes in gas-rich first-generation objects in a high redshift epoch. In such an era, it is expected that the dynamical friction by dense gas effectively promotes the merger of compact objects. To explore the possibility of mergers in a system composed of multiple NSs as well as BHs, we perform post Newtonian N-body simulations, incorporating the gas dynamical friction, the gas accretion, and the gravitational wave emission including the recoil kick. As a result, we find that NS-NS or NS-BH can merge within 10 Myr in first-generation objects. Furthermore, to satisfy the condition of the mass ejection of r-process elements, the gas accretion rate need to be lower than 0.1 Hoyle-Lyttleton accretion rate. These results imply that the mergers in early cosmic epochs may reconcile the conflict on the timescale of NS mergers.

Nonlinear effects in Pulsations of Compact Stars and Gravitational Waves

International Nuclear Information System (INIS)

Passamonti, A

2007-01-01

Nonlinear stellar oscillations can be studied by using a multiparameter perturbative approach, which is appropriate for investigating the low and mild nonlinear dynamical regimes. We present the main properties of our perturbative framework for describing, in the time domain, the nonlinear coupling between the radial and nonradial perturbations of spherically symmetric and perfect fluid compact stars. This particular coupling can be described by gauge invariant quantities that obeys a system of partial differential equations with source terms, which are made up of product of first order radial and nonradial perturbations. We report the results of numerical simulations for both the axial and polar coupling perturbations, that exhibit in the stellar dynamics and in the associated gravitational wave signal some interesting nonlinear effects, such as combination harmonics and resonances. In particular, we concentrate on the axial case, where the linear axial perturbations describe a harmonic component of a differentially rotating neutron star. The gravitational wave signal of this stellar configuration mirrors at second perturbative order the spectral features of the linear radial normal modes. In addition, a signal amplification appears when one of the radial frequencies is close to the axial w-mode frequencies of the star

Three-dimensional GRMHD Simulations of Neutrino-cooled Accretion Disks from Neutron Star Mergers

Science.gov (United States)

Siegel, Daniel M.; Metzger, Brian D.

2018-05-01

Merging binaries consisting of two neutron stars (NSs) or an NS and a stellar-mass black hole typically form a massive accretion torus around the remnant black hole or long-lived NS. Outflows from these neutrino-cooled accretion disks represent an important site for r-process nucleosynthesis and the generation of kilonovae. We present the first three-dimensional, general-relativistic magnetohydrodynamic (GRMHD) simulations including weak interactions and a realistic equation of state of such accretion disks over viscous timescales (380 ms). We witness the emergence of steady-state MHD turbulence, a magnetic dynamo with an ∼20 ms cycle, and the generation of a “hot” disk corona that launches powerful thermal outflows aided by the energy released as free nucleons recombine into α-particles. We identify a self-regulation mechanism that keeps the midplane electron fraction low (Y e ∼ 0.1) over viscous timescales. This neutron-rich reservoir, in turn, feeds outflows that retain a sufficiently low value of Y e ≈ 0.2 to robustly synthesize third-peak r-process elements. The quasi-spherical outflows are projected to unbind 40% of the initial disk mass with typical asymptotic escape velocities of 0.1c and may thus represent the dominant mass ejection mechanism in NS–NS mergers. Including neutrino absorption, our findings agree with previous hydrodynamical α-disk simulations that the entire range of r-process nuclei from the first to the third r-process peak can be synthesized in the outflows, in good agreement with observed solar system abundances. The asymptotic escape velocities and quantity of ejecta, when extrapolated to moderately higher disk masses, are consistent with those needed to explain the red kilonova emission following the NS merger GW170817.

X-rays from neutron stars

International Nuclear Information System (INIS)

Boerner, G.

1979-08-01

The basic theoretical in the models of regularly pulsating X-ray sources are discussed, and put in relation to the observations. The topics covered include physics of the magnetosphere of an accreting neutron star, hydrodynamics of the accretion column, physical processes close to the surface of the neutron star such as proton-electron collisions, photon-electron interactions. (orig.)

WHAT SETS THE INITIAL ROTATION RATES OF MASSIVE STARS?

International Nuclear Information System (INIS)

Rosen, Anna L.; Krumholz, Mark R.; Ramirez-Ruiz, Enrico

2012-01-01

The physical mechanisms that set the initial rotation rates in massive stars are a crucial unknown in current star formation theory. Observations of young, massive stars provide evidence that they form in a similar fashion to their low-mass counterparts. The magnetic coupling between a star and its accretion disk may be sufficient to spin down low-mass pre-main-sequence (PMS) stars to well below breakup at the end stage of their formation when the accretion rate is low. However, we show that these magnetic torques are insufficient to spin down massive PMS stars due to their short formation times and high accretion rates. We develop a model for the angular momentum evolution of stars over a wide range in mass, considering both magnetic and gravitational torques. We find that magnetic torques are unable to spin down either low-mass or high-mass stars during the main accretion phase, and that massive stars cannot be spun down significantly by magnetic torques during the end stage of their formation either. Spin-down occurs only if massive stars' disk lifetimes are substantially longer or their magnetic fields are much stronger than current observations suggest.

Strange Quark Stars in Binaries: Formation Rates, Mergers, and Explosive Phenomena

Science.gov (United States)

Wiktorowicz, G.; Drago, A.; Pagliara, G.; Popov, S. B.

2017-09-01

Recently, the possible coexistence of a first family composed of “normal” neutron stars (NSs) with a second family of strange quark stars (QSs) has been proposed as a solution of problems related to the maximum mass and to the minimal radius of these compact stellar objects. In this paper, we study the mass distribution of compact objects formed in binary systems and the relative fractions of quark and NSs in different subpopulations. We incorporate the strange QS formation model provided by the two-families scenario, and we perform a large-scale population synthesis study in order to obtain the population characteristics. According to our results, the main channel for strange QS formation in binary systems is accretion from a secondary companion on an NS. Therefore, a rather large number of strange QSs form by accretion in low-mass X-ray binaries and this opens the possibility of having explosive GRB-like phenomena not related to supernovae and not due to the merger of two NSs. The number of double strange QS systems is rather small, with only a tiny fraction that merge within a Hubble time. This drastically limits the flux of strangelets produced by the merger, which turns out to be compatible with all limits stemming from Earth and lunar experiments. Moreover, this value of the flux rules out at least one relevant channel for the transformation of all NSs into strange QSs by strangelets’ absorption.

Strange Quark Stars in Binaries: Formation Rates, Mergers, and Explosive Phenomena

International Nuclear Information System (INIS)

Wiktorowicz, G.; Drago, A.; Pagliara, G.; Popov, S. B.

2017-01-01

Recently, the possible coexistence of a first family composed of “normal” neutron stars (NSs) with a second family of strange quark stars (QSs) has been proposed as a solution of problems related to the maximum mass and to the minimal radius of these compact stellar objects. In this paper, we study the mass distribution of compact objects formed in binary systems and the relative fractions of quark and NSs in different subpopulations. We incorporate the strange QS formation model provided by the two-families scenario, and we perform a large-scale population synthesis study in order to obtain the population characteristics. According to our results, the main channel for strange QS formation in binary systems is accretion from a secondary companion on an NS. Therefore, a rather large number of strange QSs form by accretion in low-mass X-ray binaries and this opens the possibility of having explosive GRB-like phenomena not related to supernovae and not due to the merger of two NSs. The number of double strange QS systems is rather small, with only a tiny fraction that merge within a Hubble time. This drastically limits the flux of strangelets produced by the merger, which turns out to be compatible with all limits stemming from Earth and lunar experiments. Moreover, this value of the flux rules out at least one relevant channel for the transformation of all NSs into strange QSs by strangelets’ absorption.

Strange Quark Stars in Binaries: Formation Rates, Mergers, and Explosive Phenomena

Energy Technology Data Exchange (ETDEWEB)

Wiktorowicz, G. [Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw (Poland); Drago, A.; Pagliara, G. [Dipartimento di Fisica e Scienze della Terra dell’Università di Ferrara and INFN Sezione di Ferrara, Via Saragat 1, I-44100 Ferrara (Italy); Popov, S. B., E-mail: gwiktoro@astrouw.edu.pl [Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetsky prospekt 13, 119234, Moscow (Russian Federation)

2017-09-10

Recently, the possible coexistence of a first family composed of “normal” neutron stars (NSs) with a second family of strange quark stars (QSs) has been proposed as a solution of problems related to the maximum mass and to the minimal radius of these compact stellar objects. In this paper, we study the mass distribution of compact objects formed in binary systems and the relative fractions of quark and NSs in different subpopulations. We incorporate the strange QS formation model provided by the two-families scenario, and we perform a large-scale population synthesis study in order to obtain the population characteristics. According to our results, the main channel for strange QS formation in binary systems is accretion from a secondary companion on an NS. Therefore, a rather large number of strange QSs form by accretion in low-mass X-ray binaries and this opens the possibility of having explosive GRB-like phenomena not related to supernovae and not due to the merger of two NSs. The number of double strange QS systems is rather small, with only a tiny fraction that merge within a Hubble time. This drastically limits the flux of strangelets produced by the merger, which turns out to be compatible with all limits stemming from Earth and lunar experiments. Moreover, this value of the flux rules out at least one relevant channel for the transformation of all NSs into strange QSs by strangelets’ absorption.

All spherically symmetric charged anisotropic solutions for compact stars

Energy Technology Data Exchange (ETDEWEB)

Maurya, S.K. [University of Nizwa, Department of Mathematical and Physical Sciences, College of Arts and Science, Nizwa (Oman); Gupta, Y.K. [Raj Kumar Goel Institute of Technology, Department of Mathematics, Ghaziabad, UP (India); Ray, Saibal [Government College of Engineering and Ceramic Technology, Department of Physics, Kolkata, West Bengal (India)

2017-06-15

In the present paper we develop an algorithm for all spherically symmetric anisotropic charged fluid distributions. Considering a new source function ν(r) we find a set of solutions which is physically well behaved and represents compact stellar models. A detailed study specifically shows that the models actually correspond to strange stars in terms of their mass and radius. In this connection we investigate several physical properties like energy conditions, stability, mass-radius ratio, electric charge content, anisotropic nature and surface redshift through graphical plots and mathematical calculations. All the features from these studies are in excellent agreement with the already available evidence in theory as well as observations. (orig.)

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

Science.gov (United States)

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

2015-10-01

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

Insights from simulations of star formation

International Nuclear Information System (INIS)

Larson, Richard B

2007-01-01

Although the basic physics of star formation is classical, numerical simulations have yielded essential insights into how stars form. They show that star formation is a highly nonuniform runaway process characterized by the emergence of nearly singular peaks in density, followed by the accretional growth of embryo stars that form at these density peaks. Circumstellar discs often form from the gas being accreted by the forming stars, and accretion from these discs may be episodic, driven by gravitational instabilities or by protostellar interactions. Star-forming clouds typically develop filamentary structures, which may, along with the thermal physics, play an important role in the origin of stellar masses because of the sensitivity of filament fragmentation to temperature variations. Simulations of the formation of star clusters show that the most massive stars form by continuing accretion in the dense cluster cores, and this again is a runaway process that couples star formation and cluster formation. Star-forming clouds also tend to develop hierarchical structures, and smaller groups of forming objects tend to merge into progressively larger ones, a generic feature of self-gravitating systems that is common to star formation and galaxy formation. Because of the large range of scales and the complex dynamics involved, analytic models cannot adequately describe many aspects of star formation, and detailed numerical simulations are needed to advance our understanding of the subject. 'The purpose of computing is insight, not numbers.' Richard W Hamming, in Numerical Methods for Scientists and Engineers (1962) 'There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.' William Shakespeare, in Hamlet, Prince of Denmark (1604) (key issues review)

Insights from simulations of star formation

Energy Technology Data Exchange (ETDEWEB)

Larson, Richard B [Department of Astronomy, Yale University, Box 208101, New Haven, CT 06520-8101 (United States)

2007-03-15

Although the basic physics of star formation is classical, numerical simulations have yielded essential insights into how stars form. They show that star formation is a highly nonuniform runaway process characterized by the emergence of nearly singular peaks in density, followed by the accretional growth of embryo stars that form at these density peaks. Circumstellar discs often form from the gas being accreted by the forming stars, and accretion from these discs may be episodic, driven by gravitational instabilities or by protostellar interactions. Star-forming clouds typically develop filamentary structures, which may, along with the thermal physics, play an important role in the origin of stellar masses because of the sensitivity of filament fragmentation to temperature variations. Simulations of the formation of star clusters show that the most massive stars form by continuing accretion in the dense cluster cores, and this again is a runaway process that couples star formation and cluster formation. Star-forming clouds also tend to develop hierarchical structures, and smaller groups of forming objects tend to merge into progressively larger ones, a generic feature of self-gravitating systems that is common to star formation and galaxy formation. Because of the large range of scales and the complex dynamics involved, analytic models cannot adequately describe many aspects of star formation, and detailed numerical simulations are needed to advance our understanding of the subject. 'The purpose of computing is insight, not numbers.' Richard W Hamming, in Numerical Methods for Scientists and Engineers (1962) 'There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.' William Shakespeare, in Hamlet, Prince of Denmark (1604) (key issues review)

RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

International Nuclear Information System (INIS)

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

2012-01-01

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

RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

Energy Technology Data Exchange (ETDEWEB)

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

2012-01-10

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

Foundations of Black Hole Accretion Disk Theory

Directory of Open Access Journals (Sweden)

Marek A. Abramowicz

2013-01-01

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

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).

How young the accretion-powered pulsars could be?

Science.gov (United States)

Kostina, M. V.; Ikhsanov, N. R.

2017-12-01

A question about the age of accretion-powered X-ray pulsars has recently been reopened by a discovery of the X-ray pulsar SXP 1062 in the SMC. This High Mass X-ray Binary (HMXB) contains a neutron star rotating with the period of 1062 s and is associated with a supernova remnant of the age ∼ 104 yr. An attempt to explain the origin of this young long-period X-ray pulsar within the traditional scenario of three basic states (ejector, propeller and accretor) encounters difficulties. Even if this pulsar were born as a magnetar the spin-down time during the propeller stage would exceed 104 yr. Here we explore a more circuitous way of the pulsar spin evolution in HMXBs, in which the propeller stage in the evolutionary track is avoided. We find this way to be possible if the stellar wind of the massive companion to the neutron star is magnetized. The geometry of plasma flow captured by the neutron star in this case differs from spherically symmetrical and the magnetospheric radius of the neutron star is smaller than that evaluated in the convention accretion scenarios. We show that the age of an accretion-powered pulsar in this case can be as small as ∼ 104 years without the need of invoking initial magnetic field in excess of 1013 G.

COMPLEX GAS KINEMATICS IN COMPACT, RAPIDLY ASSEMBLING STAR-FORMING GALAXIES

Energy Technology Data Exchange (ETDEWEB)

Amorin, R.; Vilchez, J. M.; Perez-Montero, E. [Instituto de Astrofisica de Andalucia-CSIC, Glorieta de la Astronomia S/N, E-18008 Granada (Spain); Haegele, G. F.; Firpo, V. [Facultad de Ciencias Astronomicas y Geofisicas, Universidad de la Plata, Paseo del Bosque S/N, 1900 La Plata (Argentina); Papaderos, P., E-mail: amorin@iaa.es [Centro de Astrofisica and Faculdade de Ciencias, Universidade do Porto, Rua das Estrelas, 4150-762 Porto (Portugal)

2012-08-01

Deep, high-resolution spectroscopic observations have been obtained for six compact, strongly star-forming galaxies at redshift z {approx} 0.1-0.3, most of them also known as green peas. Remarkably, these galaxies show complex emission-line profiles in the spectral region including H{alpha}, [N II] {lambda}{lambda}6548, 6584, and [S II] {lambda}{lambda}6717, 6731, consisting of the superposition of different kinematical components on a spatial extent of few kiloparsecs: a very broad line emission underlying more than one narrower component. For at least two of the observed galaxies some of these multiple components are resolved spatially in their two-dimensional spectra, whereas for another one a faint detached H{alpha} blob lacking stellar continuum is detected at the same recessional velocity {approx}7 kpc away from the galaxy. The individual narrower H{alpha} components show high intrinsic velocity dispersion ({sigma} {approx} 30-80 km s{sup -1}), suggesting together with unsharped masking Hubble Space Telescope images that star formation proceeds in an ensemble of several compact and turbulent clumps, with relative velocities of up to {approx}500 km s{sup -1}. The broad underlying H{alpha} components indicate in all cases large expansion velocities (full width zero intensity {>=}1000 km s{sup -1}) and very high luminosities (up to {approx}10{sup 42} erg s{sup -1}), probably showing the imprint of energetic outflows from supernovae. These intriguing results underline the importance of green peas for studying the assembly of low-mass galaxies near and far.

Detecting metal-poor gas accretion in the star-forming dwarf galaxies UM 461 and Mrk 600

Science.gov (United States)

Lagos, P.; Scott, T. C.; Nigoche-Netro, A.; Demarco, R.; Humphrey, A.; Papaderos, P.

2018-03-01

Using VIMOS-IFU observations, we study the interstellar medium (ISM) of two star-forming dwarf galaxies, UM 461 and Mrk 600. Our aim was to search for the existence of metallicity inhomogeneities that might arise from infall of nearly pristine gas feeding ongoing localized star-formation. The IFU data allowed us to study the impact of external gas accretion on the chemical evolution as well as the ionised gas kinematics and morphologies of these galaxies. Both systems show signs of morphological distortions, including cometary-like morphologies. We analysed the spatial variation of 12 + log(O/H) abundances within both galaxies using the direct method (Te), the widely applied HII-CHI-mistry code, as well as by employing different standard calibrations. For UM 461 our results show that the ISM is fairly well mixed, at large scales, however we find an off-centre and low-metallicity region with 12 + log(O/H) ISM in our analysed galaxies are consistent with these systems being at different evolutionary stages.

I-Love relations for incompressible stars and realistic stars

Science.gov (United States)

Chan, T. K.; Chan, AtMa P. O.; Leung, P. T.

2015-02-01

In spite of the diversity in the equations of state of nuclear matter, the recently discovered I-Love-Q relations [Yagi and Yunes, Science 341, 365 (2013), 10.1126/science.1236462], which relate the moment of inertia, tidal Love number (deformability), and the spin-induced quadrupole moment of compact stars, hold for various kinds of realistic neutron stars and quark stars. While the physical origin of such universality is still a current issue, the observation that the I-Love-Q relations of incompressible stars can well approximate those of realistic compact stars hints at a new direction to approach the problem. In this paper, by establishing recursive post-Minkowskian expansion for the moment of inertia and the tidal deformability of incompressible stars, we analytically derive the I-Love relation for incompressible stars and show that the so-obtained formula can be used to accurately predict the behavior of realistic compact stars from the Newtonian limit to the maximum mass limit.

Growth of black holes in the interior of rotating neutron stars

DEFF Research Database (Denmark)

Kouvaris, C.; Tinyakov, P.

2014-01-01

Mini-black holes made of dark matter that can potentially form in the interior of neutron stars always have been thought to grow by accreting the matter of the core of the star via a spherical Bondi accretion. However, neutron stars have sometimes significant angular velocities that can...... in principle stall the spherical accretion and potentially change the conclusions derived about the time it takes for black holes to destroy a star. We study the effect of the star rotation on the growth of such black holes and the evolution of the black hole spin. Assuming no mechanisms of angular momentum...... evacuation, we find that even moderate rotation rates can in fact destroy spherical accretion at the early stages of the black hole growth. However, we demonstrate that the viscosity of nuclear matter can alleviate the effect of rotation, making it possible for the black hole to maintain spherical accretion...

Charged Compact Boson Stars in a Theory of Massless Scalar Field

Science.gov (United States)

Kumar, Sanjeev

2018-05-01

In this work we present some new results obtained in a study of the phase diagram of charged compact boson stars in a theory involving a complex scalar field with a conical potential coupled to a U(1) gauge field and gravity. We obtain new bifurcation points in this model. We present a detailed discussion of the various regions of the phase diagram with respect to the bifurcation points. The theory is seen to contain rich physics in a particular domain of the phase diagram.

Symbiotic Stars in X-rays

Science.gov (United States)

Luna, G. J. M.; Sokoloski, J. L.; Mukai, K.; Nelson, T.

2014-01-01

Until recently, symbiotic binary systems in which a white dwarf accretes from a red giant were thought to be mainly a soft X-ray population. Here we describe the detection with the X-ray Telescope (XRT) on the Swift satellite of 9 white dwarf symbiotics that were not previously known to be X-ray sources and one that was previously detected as a supersoft X-ray source. The 9 new X-ray detections were the result of a survey of 41 symbiotic stars, and they increase the number of symbiotic stars known to be X-ray sources by approximately 30%. Swift/XRT detected all of the new X-ray sources at energies greater than 2 keV. Their X-ray spectra are consistent with thermal emission and fall naturally into three distinct groups. The first group contains those sources with a single, highly absorbed hard component, which we identify as probably coming from an accretion-disk boundary layer. The second group is composed of those sources with a single, soft X-ray spectral component, which likely arises in a region where low-velocity shocks produce X-ray emission, i.e. a colliding-wind region. The third group consists of those sources with both hard and soft X-ray spectral components. We also find that unlike in the optical, where rapid, stochastic brightness variations from the accretion disk typically are not seen, detectable UV flickering is a common property of symbiotic stars. Supporting our physical interpretation of the two X-ray spectral components, simultaneous Swift UV photometry shows that symbiotic stars with harder X-ray emission tend to have stronger UV flickering, which is usually associated with accretion through a disk. To place these new observations in the context of previous work on X-ray emission from symbiotic stars, we modified and extended the alpha/beta/gamma classification scheme for symbiotic-star X-ray spectra that was introduced by Muerset et al. based upon observations with the ROSAT satellite, to include a new sigma classification for sources with

MASSIVE BLACK HOLES IN STELLAR SYSTEMS: 'QUIESCENT' ACCRETION AND LUMINOSITY

International Nuclear Information System (INIS)

Volonteri, M.; Campbell, D.; Mateo, M.; Dotti, M.

2011-01-01

Only a small fraction of local galaxies harbor an accreting black hole, classified as an active galactic nucleus. However, many stellar systems are plausibly expected to host black holes, from globular clusters to nuclear star clusters, to massive galaxies. The mere presence of stars in the vicinity of a black hole provides a source of fuel via mass loss of evolved stars. In this paper, we assess the expected luminosities of black holes embedded in stellar systems of different sizes and properties, spanning a large range of masses. We model the distribution of stars and derive the amount of gas available to a central black hole through a geometrical model. We estimate the luminosity of the black holes under simple, but physically grounded, assumptions on the accretion flow. Finally, we discuss the detectability of 'quiescent' black holes in the local universe.

Starbursts in Blue compact dwarf galaxies

International Nuclear Information System (INIS)

Thuan, T.X.

1987-01-01

We summarize all the arguments for a bursting mode of star formation in blue compact dwarf galaxies. We show in particular how spectral synthesis of far ultraviolet spectra of Blue compact dwarf galaxy constitutes a powerful way for studying the star formation history in these galaxies. Blue compact dwarf galaxy luminosity functions show jumps and discontinuities. These jumps act like fossil records of the star-forming bursts, helping us to count and date the bursts

FEEDBACK EFFECTS ON LOW-MASS STAR FORMATION

International Nuclear Information System (INIS)

Hansen, Charles E.; Klein, Richard I.; McKee, Christopher F.; Fisher, Robert T.

2012-01-01

Protostellar feedback, both radiation and bipolar outflows, dramatically affects the fragmentation and mass accretion from star-forming cores. We use ORION, an adaptive mesh refinement gravito-radiation-hydrodynamics code, to simulate low-mass star formation in a turbulent molecular cloud in the presence of protostellar feedback. We present results of the first simulations of a star-forming cluster that include both radiative transfer and protostellar outflows. We run four simulations to isolate the individual effects of radiation feedback and outflow feedback as well as the combination of the two. We find that outflows reduce protostellar masses and accretion rates each by a factor of three and therefore reduce protostellar luminosities by an order of magnitude. This means that, while radiation feedback suppresses fragmentation, outflows render protostellar radiation largely irrelevant for low-mass star formation above a mass scale of 0.05 M ☉ . We find initial fragmentation of our cloud at half the global Jeans length, around 0.1 pc. With insufficient protostellar radiation to stop it, these 0.1 pc cores fragment repeatedly, forming typically 10 stars each. The accretion rate in these stars scales with mass as predicted from core accretion models that include both thermal and turbulent motions; the accretion rate does not appear to be consistent with either competitive accretion or accretion from an isothermal sphere. We find that protostellar outflows do not significantly affect the overall cloud dynamics, in the absence of magnetic fields, due to their small opening angles and poor coupling to the dense gas. The outflows reduce the mass from the cores by 2/3, giving a core to star efficiency, ε core ≅ 1/3. The simulations are also able to reproduce many observation of local star-forming regions. Our simulation with radiation and outflows reproduces the observed protostellar luminosity function. All of the simulations can reproduce observed core mass

Embedded class solutions compatible for physical compact stars in general relativity

Science.gov (United States)

Newton Singh, Ksh.; Pant, Neeraj; Tewari, Neeraj; Aria, Anil K.

2018-05-01

We have explored a family of new solutions satisfying Einstein's field equations and Karmarkar condition. We have assumed an anisotropic stress-tensor with no net electric charge. Interestingly, the new solutions yield zero values of all the physical quantities for all even integer n > 0. However, for all n >0 (n ≠ even numbers) they yield physically possible solutions. We have tuned the solution for neutron star Vela X-1 so that the solutions matches the observed mass and radius. For the same star we have extensively discussed the behavior of the solutions. The solutions yield a stiffer equation of state for larger values of n since the adiabatic index increases and speed of sound approaches the speed of light. It is also found that the solution is physically possible for Vela X-1 if 1.8 ≤ n < 7 (with n≠ 2,4,6). All the solutions for n ≥ 7 violates the causality condition and all the solutions with 0 < n < 1.8 lead to complex values of transverse sound speed vt. The range of well-behaved n depends on the mass and radius of compact stars.

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.

Kilonova/Macronova Emission from Compact Binary Mergers

Directory of Open Access Journals (Sweden)

Masaomi Tanaka

2016-01-01

Full Text Available We review current understanding of kilonova/macronova emission from compact binary mergers (mergers of two neutron stars or a neutron star and a black hole. Kilonova/macronova is emission powered by radioactive decays of r-process nuclei and it is one of the most promising electromagnetic counterparts of gravitational wave sources. Emission from the dynamical ejecta of ~0.01M⊙ is likely to have a luminosity of ~1040–1041 erg s−1 with a characteristic timescale of about 1 week. The spectral peak is located in red optical or near-infrared wavelengths. A subsequent accretion disk wind may provide an additional luminosity or an earlier/bluer emission if it is not absorbed by the precedent dynamical ejecta. The detection of near-infrared excess in short GRB 130603B and possible optical excess in GRB 060614 supports the concept of the kilonova/macronova scenario. At 200 Mpc distance, a typical peak brightness of kilonova/macronova with 0.01M⊙ ejecta is about 22 mag and the emission rapidly fades to >24 mag within ~10 days. Kilonova/macronova candidates can be distinguished from supernovae by (1 the faster time evolution, (2 fainter absolute magnitudes, and (3 redder colors. Since the high expansion velocity (v~0.1–0.2c is a robust outcome of compact binary mergers, the detection of smooth spectra will be the smoking gun to conclusively identify the gravitational wave source.

EVIDENCE FOR ACCRETION IN A NEARBY, YOUNG BROWN DWARF

International Nuclear Information System (INIS)

Reiners, Ansgar

2009-01-01

We report on the discovery of the young, nearby, brown dwarf 2MASS J0041353-562112. The object has a spectral type of M7.5; it shows Li absorption and signatures of accretion, which implies that it still has a disk and suggests an age below 10 Myr. The space motion vector and position on the sky indicate that the brown dwarf is probably a member of the ∼20 Myr old Tuc-Hor association, or that it may be an ejected member of the ∼12 Myr old β Pic association; both would imply that 2MASS J0041353-562112 may in fact be older than 10 Myr. No accreting star or brown dwarf was previously known in these associations. Assuming an age of 10 Myr, the brown dwarf has a mass of about 30 M Jup and is located at 35 pc distance. The newly discovered object is the closest accreting brown dwarf known. Its membership to an association older than 10 Myr implies that either disks in brown dwarfs can survive as long as in more massive stars, perhaps even longer, or that star formation in Tuc-Hor or β Pic occurred more recently than previously thought. The history and evolution of this object can provide new fundamental insight into the formation process of stars, brown dwarfs, and planets.

Ngc7538 Irs1 - A Highly Collimated Ionized Wind Source Powered By Accretion

Science.gov (United States)

Sandell, Goran H. L.; Wright, M.; Goss, W.; Corder, S.

2009-01-01

Recent images show that NGC7538 IRS1 is not a conventional Ultracompact or Hypercompact HII region, but is completely wind-excited (other broad recombination line hypercompact HII regions may be similar to IRS1). NGC 7538 IRS1 is a well studied young high-mass star (L 2 10^5 L_Sun).VLA images at 6 and 2 cm (Cambell 1984; ApJ, 282, L27) showed a compact bipolar core (lobe separation 0.2") with more extended faint lobes. Recombination line observations (Gaume et al. 1995, ApJ, 438, 776) show extremely wide line profiles indicating substantial mass motion of the ionized gas. We re-analyzed high angular resolution VLA archive data from 6 cm to 7 mm, and measured the flux from the compact core and the extended (1.5 - 2") bipolar lobes. We find that the compact core has a spectral index, alpha 0.6, which could be explained by an optically thick hypercompact core with a density gradient. However, the size of the core shrinks with increasing frequency; from 0.24" at 6 cm to 0.1" at 7 mm, consistent with that expected for a collimated jet (Reynolds 1986, ApJ, 304, 713). If we do a crude size correction so that we compare emission from the optically thick inner part of the jet for a set of 2 cm and 7 mm observations we get alpha 1.6, i.e. close to the optically thick value. BIMA and CARMA continuum observations at 3 mm show some dust excess, while. HCO+ J=1-0 observations combined with FCRAO single dish data show a clear inverse P Cygni profile towards IRS1. These observations confirm that IRS1 is heavily accreting with an accretion rate 2 10^-4 M_Sun/year, sufficient to quench the formation of an HII region.

COSMIC probes into compact binary formation and evolution

Science.gov (United States)

Breivik, Katelyn

2018-01-01

The population of compact binaries in the galaxy represents the final state of all binaries that have lived up to the present epoch. Compact binaries present a unique opportunity to probe binary evolution since many of the interactions binaries experience can be imprinted on the compact binary population. By combining binary evolution simulations with catalogs of observable compact binary systems, we can distill the dominant physical processes that govern binary star evolution, as well as predict the abundance and variety of their end products.The next decades herald a previously unseen opportunity to study compact binaries. Multi-messenger observations from telescopes across all wavelengths and gravitational-wave observatories spanning several decades of frequency will give an unprecedented view into the structure of these systems and the composition of their components. Observations will not always be coincident and in some cases may be separated by several years, providing an avenue for simulations to better constrain binary evolution models in preparation for future observations.I will present the results of three population synthesis studies of compact binary populations carried out with the Compact Object Synthesis and Monte Carlo Investigation Code (COSMIC). I will first show how binary-black-hole formation channels can be understood with LISA observations. I will then show how the population of double white dwarfs observed with LISA and Gaia could provide a detailed view of mass transfer and accretion. Finally, I will show that Gaia could discover thousands black holes in the Milky Way through astrometric observations, yielding view into black-hole astrophysics that is complementary to and independent from both X-ray and gravitational-wave astronomy.

Bare AGN: an Unobscured View of the Innermost Accretion Geometry

Science.gov (United States)

Fink, M.; Dauser, T.; Beuchert, T.; Jeffreson, S.; Tawabutr, J.; Wilms, J.; Garcia, J.; Walton, D.

2017-10-01

In a systematic study of the relativistic reflection spectra and coronal properties for a sample of bare AGN we analyze high signal-to-noise spectra obtained with the XMM-Newton and NuSTAR observatories utilizing state-of-the-art reflection codes. Features of blurred reflection off an ionized accretion disk are modelled using different flavors of the relativistic ray-tracing code Relxill. We show that the more physically motivated and self-consistent lamp-post geometry is largely consistent with fits of broken power-law emissivity profiles. We provide good constraints on parameters describing the compact reprocessing corona, i.e., the reflection fraction and the lamp-post height. The latter are found to be prevalent within 1-10 r_{g}, while our models generally find close-to-maximal black hole spins. These results are discussed and compared with previous studies by Walton et al. (2013).

TIME-VARYING DYNAMICAL STAR FORMATION RATE

Energy Technology Data Exchange (ETDEWEB)

Lee, Eve J.; Chang, Philip; Murray, Norman, E-mail: evelee@berkeley.edu [Canadian Institute for Theoretical Astrophysics, 60 St. George Street, University of Toronto, Toronto, ON M5S 3H8 (Canada)

2015-02-10

We present numerical evidence of dynamic star formation in which the accreted stellar mass grows superlinearly with time, roughly as t {sup 2}. We perform simulations of star formation in self-gravitating hydrodynamic and magnetohydrodynamic turbulence that is continuously driven. By turning the self-gravity of the gas in the simulations on or off, we demonstrate that self-gravity is the dominant physical effect setting the mass accretion rate at early times before feedback effects take over, contrary to theories of turbulence-regulated star formation. We find that gravitational collapse steepens the density profile around stars, generating the power-law tail on what is otherwise a lognormal density probability distribution function. Furthermore, we find turbulent velocity profiles to flatten inside collapsing regions, altering the size-line width relation. This local flattening reflects enhancements of turbulent velocity on small scales, as verified by changes to the velocity power spectra. Our results indicate that gas self-gravity dynamically alters both density and velocity structures in clouds, giving rise to a time-varying star formation rate. We find that a substantial fraction of the gas that forms stars arrives via low-density flows, as opposed to accreting through high-density filaments.

Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

Science.gov (United States)

Martínez-Núñez, Silvia; Kretschmar, Peter; Bozzo, Enrico; Oskinova, Lidia M.; Puls, Joachim; Sidoli, Lara; Sundqvist, Jon Olof; Blay, Pere; Falanga, Maurizio; Fürst, Felix; Gímenez-García, Angel; Kreykenbohm, Ingo; Kühnel, Matthias; Sander, Andreas; Torrejón, José Miguel; Wilms, Jörn

2017-10-01

-ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations. This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.

Carbon-enhanced metal-poor stars and thermohaline mixing

NARCIS (Netherlands)

Stancliffe, R.J.; Glebbeek, E.; Izzard, R.G.; Pols, O.R.

2007-01-01

One possible scenario for the formation of carbon-enhanced metal-poor stars is the accretion of carbon-rich material from a binary companion which may no longer visible. It is generally assumed that the accreted material remains on the surface of the star and does not mix with the interior until

IUE observations of long period eclipsing binaries: a study of accretion onto non-degenerate stars

International Nuclear Information System (INIS)

Plavec, M.J.

1980-01-01

It has long been thought that β Lyrae is a unique system, by virtue of its UV spectrum and its nature. The author argues that a whole class of interacting long-period binaries exists, similar to β Lyrae. According to IUE observations made in 1978-79 this group comprises: RX Cas, SX Cas, V 367 Cyg, W Cru, β Lyr, and W Ser. AR Pav is a transition case linking them with the symbiotics. The author also suggests that HD 218393 (KX And), HD 72754, and HD 51480 are their non-eclipsing counterparts. The whole group is called the W Serpentis stars. These systems are mass-transfering binaries (case B) in which the mass transfer rate is relatively high, probably on the order 10 -6 to 10 -4 solar masses/year. They display an ultraviolet continuum with a color temperature definitely higher than the one observed in the optical region. Even more characteristical is the presence of strong emission lines of N V, C IV, Si IV, Fe III, Al III, and lower ions of C and Si. The author discusses these phenomena on the assumption that they are due to accretion onto non-degenerate stars. (Auth.)

Mass-accreting white dwarfs and type Ia supernovae

Science.gov (United States)

Wang, Bo

2018-05-01

Type Ia supernovae (SNe Ia) play a prominent role in understanding the evolution of the Universe. They are thought to be thermonuclear explosions of mass-accreting carbon-oxygen white dwarfs (CO WDs) in binaries, although the mass donors of the accreting WDs are still not well determined. In this article, I review recent studies on mass-accreting WDs, including H- and He-accreting WDs. I also review currently most studied progenitor models of SNe Ia, i.e., the single-degenerate model (including the WD+MS channel, the WD+RG channel and the WD+He star channel), the double-degenerate model (including the violent merger scenario) and the sub-Chandrasekhar mass model. Recent progress on these progenitor models is discussed, including the initial parameter space for producing SNe Ia, the binary evolutionary paths to SNe Ia, the progenitor candidates for SNe Ia, the possible surviving companion stars of SNe Ia, some observational constraints, etc. Some other potential progenitor models of SNe Ia are also summarized, including the hybrid CONe WD model, the core-degenerate model, the double WD collision model, the spin-up/spin-down model and the model of WDs near black holes. To date, it seems that two or more progenitor models are needed to explain the observed diversity among SNe Ia.

The evolution of supermassive Population III stars

Science.gov (United States)

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

2018-02-01

Supermassive primordial stars forming in atomically cooled haloes at z ˜ 15-20 are currently thought to be the progenitors of the earliest quasars in the Universe. In this picture, the star evolves under accretion rates of 0.1-1 M⊙ yr-1 until the general relativistic instability triggers its collapse to a black hole at masses of ˜105 M⊙. However, the ability of the accretion flow to sustain such high rates depends crucially on the photospheric properties of the accreting star, because its ionizing radiation could reduce or even halt accretion. Here we present new models of supermassive Population III protostars accreting at rates 0.001-10 M⊙ yr-1, computed with the GENEVA stellar evolution code including general relativistic corrections to the internal structure. We compute for the first time evolutionary tracks in the mass range M > 105 M⊙. We use the polytropic stability criterion to estimate the mass at which the collapse occurs, which has been shown to give a lower limit of the actual mass at collapse in recent hydrodynamic simulations. We find that at accretion rates higher than 0.01 M⊙ yr-1, the stars evolve as red, cool supergiants with surface temperatures below 104 K towards masses >105 M⊙. Moreover, even with the lower rates 0.001 M_{⊙} yr{^{-1}}feedback remains weak, reinforcing the case for direct collapse as the origin of the first quasars. We provide numerical tables for the surface properties of our models.

Determining the properties of dense matter: Superconductivity, bulk viscosity, and light reflection in compact stars

Science.gov (United States)

Good, Gerald J.

In this dissertation, we investigate the properties of matter, denser than nuclei, that exists inside compact stars. First, we examine a mixed superfluid/superconductor system, which likely occurs in neutron star cores. We derive an effective theory of Cooper pair quasiparticles from a microscopic theory of nucleons, and calculate the coupling strengths between quasiparticles. We then calculate the structure of magnetic flux tubes, taking into consideration interactions between neutron and proton Cooper pairs. We find that interactions between the condensates can lead to interesting phenomena and new phases at the border between type-I and type-II behavior. Next, we examine the response of nuclear matter to vibrational modes by calculating the bulk viscosity from purely leptonic processes. We find that for hot neutron stars, the bulk viscosity due to leptons is very small compared to the bulk viscosity due to nucleons, but for cold neutron stars, the leptonic component is dominant. Finally, we derive the reflection and transmission properties of light at boundaries between phases of matter that have two independent U(1) generators, which may exist at the surface of "strange stars" or at boundaries between different phases of matter in a neutron star.

Star formation: Cosmic feast

Science.gov (United States)

Scaringi, Simone

2017-03-01

Low-mass stars form through a process known as disk accretion, eating up material that orbits in a disk around them. It turns out that the same mechanism also describes the formation of more massive stars.

Dynamic effects on cyclotron scattering in pulsar accretion columns

International Nuclear Information System (INIS)

Brainerd, J.J.; Meszaros, P.

1991-01-01

A resonant scattering model for photon reprocessing in a pulsar accretion column is presented. The accretion column is optically thin to Thomson scattering and optically thick to resonant scattering at the cyclotron frequency. Radiation from the neutron star surface propagates freely through the column until the photon energy equals the local cyclotron frequency, at which point the radiation is scattered, much of it back toward the star. The radiation pressure in this regime is insufficient to stop the infall. Some of the scattered radiation heats the stellar surface around the base of the column, which adds a softer component to the spectrum. The partial blocking by the accretion column of X-rays from the surface produces a fan beam emission pattern. X-rays above the surface cyclotron frequency freely escape and are characterized by a pencil beam. Gravitational light bending produces a pencil beam pattern of column-scattered radiation in the antipodal direction, resulting in a strongly angle-dependent cyclotron feature. 31 refs

COMPACT STELLAR BINARY ASSEMBLY IN THE FIRST NUCLEAR STAR CLUSTERS AND r-PROCESS SYNTHESIS IN THE EARLY UNIVERSE

International Nuclear Information System (INIS)

Ramirez-Ruiz, Enrico; MacLeod, Morgan; Trenti, Michele; Roberts, Luke F.; Lee, William H.; Saladino-Rosas, Martha I.

2015-01-01

Investigations of elemental abundances in the ancient and most metal deficient stars are extremely important because they serve as tests of variable nucleosynthesis pathways and can provide critical inferences of the type of stars that lived and died before them. The presence of r-process elements in a handful of carbon-enhanced metal-poor (CEMP-r) stars, which are assumed to be closely connected to the chemical yield from the first stars, is hard to reconcile with standard neutron star mergers. Here we show that the production rate of dynamically assembled compact binaries in high-z nuclear star clusters can attain a sufficient high value to be a potential viable source of heavy r-process material in CEMP-r stars. The predicted frequency of such events in the early Galaxy, much lower than the frequency of Type II supernovae but with significantly higher mass ejected per event, can naturally lead to a high level of scatter of Eu as observed in CEMP-r stars

COMPACT STELLAR BINARY ASSEMBLY IN THE FIRST NUCLEAR STAR CLUSTERS AND r-PROCESS SYNTHESIS IN THE EARLY UNIVERSE

Energy Technology Data Exchange (ETDEWEB)

Ramirez-Ruiz, Enrico; MacLeod, Morgan [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); Trenti, Michele [Kavli Institute for Cosmology and Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA (United Kingdom); Roberts, Luke F. [TAPIR, California Institute of Technology, Pasadena, California 91125 (United States); Lee, William H.; Saladino-Rosas, Martha I. [Instituto de Astronomía, Universidad Nacional Autónoma de México, México DF 04510, México (Mexico)

2015-04-01

Investigations of elemental abundances in the ancient and most metal deficient stars are extremely important because they serve as tests of variable nucleosynthesis pathways and can provide critical inferences of the type of stars that lived and died before them. The presence of r-process elements in a handful of carbon-enhanced metal-poor (CEMP-r) stars, which are assumed to be closely connected to the chemical yield from the first stars, is hard to reconcile with standard neutron star mergers. Here we show that the production rate of dynamically assembled compact binaries in high-z nuclear star clusters can attain a sufficient high value to be a potential viable source of heavy r-process material in CEMP-r stars. The predicted frequency of such events in the early Galaxy, much lower than the frequency of Type II supernovae but with significantly higher mass ejected per event, can naturally lead to a high level of scatter of Eu as observed in CEMP-r stars.

Stellar X-ray sources

International Nuclear Information System (INIS)

Katz, J.I.; Washington Univ., St. Louis, MO

1988-01-01

I Review some of the salient accomplishments of X-rap studies of compact objects. Progress in this field has closely followed the improvement of observational methods, particularly in angular resolution and duration of exposure. Luminous compact X-ray sources are accreting neutron stars or black holes. Accreting neutron stars may have characteristic temporal signatures, but the only way to establish that an X-ray source is a black hole is to measure its mass. A rough phenomenological theory is succesful, but the transport of angular momentum in accretion flows is not onderstood. A number of interesting complications have been observed, including precessing accretion discs, X-ray bursts, and the acceleration of jets in SS433. Many puzzles remain unsolved, including the excitation of disc precession, the nature of the enigmatic A- and gamma-ray source Cyg X-3, the mechanism by which slowly spinning accreting neutron stars lose angular momentum, and the superabundance of X-ray sources in globular clusters. 41 refs.; 5 figs

Wind accretion: Theory and observations

Science.gov (United States)

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

2015-07-01

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

SIMULATING THE FORMATION OF MASSIVE PROTOSTARS. I. RADIATIVE FEEDBACK AND ACCRETION DISKS

Energy Technology Data Exchange (ETDEWEB)

Klassen, Mikhail; Pudritz, Ralph E. [Department of Physics and Astronomy, McMaster University, 1280 Main Street W, Hamilton, ON L8S 4M1 (Canada); Kuiper, Rolf [Institute of Astronomy and Astrophysics, University of Tübingen, Auf der Morgenstelle 10, D-72076 Tübingen (Germany); Peters, Thomas [Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, D-85748 Garching (Germany); Banerjee, Robi, E-mail: klassm@mcmaster.ca [Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg (Germany)

2016-05-20

We present radiation hydrodynamic simulations of collapsing protostellar cores with initial masses of 30, 100, and 200 M {sub ⊙}. We follow their gravitational collapse and the formation of a massive protostar and protostellar accretion disk. We employ a new hybrid radiative feedback method blending raytracing techniques with flux-limited diffusion for a more accurate treatment of the temperature and radiative force. In each case, the disk that forms becomes Toomre-unstable and develops spiral arms. This occurs between 0.35 and 0.55 freefall times and is accompanied by an increase in the accretion rate by a factor of 2–10. Although the disk becomes unstable, no other stars are formed. In the case of our 100 and 200 M {sub ⊙} simulations, the star becomes highly super-Eddington and begins to drive bipolar outflow cavities that expand outwards. These radiatively driven bubbles appear stable, and appear to be channeling gas back onto the protostellar accretion disk. Accretion proceeds strongly through the disk. After 81.4 kyr of evolution, our 30 M {sub ⊙} simulation shows a star with a mass of 5.48 M {sub ⊙} and a disk of mass 3.3 M {sub ⊙}, while our 100 M {sub ⊙} simulation forms a 28.8 M {sub ⊙} mass star with a 15.8 M {sub ⊙} disk over the course of 41.6 kyr, and our 200 M {sub ⊙} simulation forms a 43.7 M {sub ⊙} star with an 18 M {sub ⊙} disk in 21.9 kyr. In the absence of magnetic fields or other forms of feedback, the masses of the stars in our simulation do not appear to be limited by their own luminosities.

THE GLOBAL EVOLUTION OF GIANT MOLECULAR CLOUDS. II. THE ROLE OF ACCRETION

International Nuclear Information System (INIS)

Goldbaum, Nathan J.; Krumholz, Mark R.; Matzner, Christopher D.; McKee, Christopher F.

2011-01-01

We present virial models for the global evolution of giant molecular clouds (GMCs). Focusing on the presence of an accretion flow and accounting for the amount of mass, momentum, and energy supplied by accretion and star formation feedback, we are able to follow the growth, evolution, and dispersal of individual GMCs. Our model clouds reproduce the scaling relations observed in both galactic and extragalactic clouds. We find that accretion and star formation contribute roughly equal amounts of turbulent kinetic energy over the lifetime of the cloud. Clouds attain virial equilibrium and grow in such a way as to maintain roughly constant surface densities, with typical surface densities of order 50-200 M sun pc -2 , in good agreement with observations of GMCs in the Milky Way and nearby external galaxies. We find that as clouds grow, their velocity dispersion and radius must also increase, implying that the linewidth-size relation constitutes an age sequence. Lastly, we compare our models to observations of GMCs and associated young star clusters in the Large Magellanic Cloud and find good agreement between our model clouds and the observed relationship between H II regions, young star clusters, and GMCs.

The structure and spectrum of the accretion shock in the atmospheres of young stars

Science.gov (United States)

Dodin, Alexandr

2018-04-01

The structure and spectrum of the accretion shock have been self-consistently simulated for a wide range of parameters typical for Classical T Tauri Stars (CTTS). Radiative cooling of the shocked gas was calculated, taking into account the self-absorption and non-equilibrium (time-dependent) effects in the level populations. These effects modify the standard cooling curve for an optically thin plasma in coronal equilibrium, however the shape of high-temperature (T > 3 × 105 K) part of the curve remains unchanged. The applied methods allow us to smoothly describe the transition from the cooling flow to the hydrostatic stellar atmosphere. Thanks to this approach, it has been found that the narrow component of He II lines is formed predominantly in the irradiated stationary atmosphere (hotspot), i.e. at velocities of the settling gas law via the full energy flux.

Barium and Tc-poor S stars: Binary masqueraders among carbon stars

OpenAIRE

Jorissen, A.; Van Eck, S.

1997-01-01

The current understanding of the origin of barium and S stars is reviewed, based on new orbital elements and binary frequencies. The following questions are addressed: (i) Is binarity a necessary condition to produce a barium star? (ii) What is the mass transfer mode (wind accretion or RLOF?) responsible for their formation? (iii) Do barium stars form as dwarfs or as giants? (iv) Do barium stars evolve into Tc-poor S stars? (v) What is the relative frequency of Tc-rich and Tc-poor S stars?

Clumpy wind accretion in Supergiant X-ray Binaries

Science.gov (United States)

El Mellah, I.; Sundqvist, J. O.; Keppens, R.

2017-12-01

Supergiant X-ray binaries (\\sgx) contain a neutron star (NS) orbiting a Supergiant O/B star. The fraction of the dense and fast line-driven wind from the stellar companion which is accreted by the NS is responsible for most of the X-ray emission from those system. Classic \\sgx display photometric variability of their hard X-ray emission, typically from a few 10^{35} to a few 10^{37}erg\\cdots^{-1}. Inhomogeneities (\\aka clumps) in the wind from the star are expected to play a role in this time variability. We run 3D hydrodynamical (HD) finite volume simulations to follow the accretion of the inhomogeneous stellar wind by the NS over almost 3 orders of magnitude. To model the unperturbed wind far upstream the NS, we use recent simulations which managed to resolve its micro-structure. We observe the formation of a Bondi-Hoyle-Lyttleton (BHL) like bow shock around the accretor and follow the clumps as they cross it, down to the NS magnetosphere. Compared to previous estimations discarding the HD effects, we measure lower time variability due to both the damping effect of the shock and the necessity to evacuate angular momentum to enable accretion. We also compute the associated time-variable column density and compare it to recent observations in Vela X-1.

Accretion of dark matter by stars.

Science.gov (United States)

Brito, Richard; Cardoso, Vitor; Okawa, Hirotada

2015-09-11

Searches for dark matter imprints are one of the most active areas of current research. We focus here on light fields with mass m_{B}, such as axions and axionlike candidates. Using perturbative techniques and full-blown nonlinear numerical relativity methods, we show the following. (i) Dark matter can pile up in the center of stars, leading to configurations and geometries oscillating with a frequency that is a multiple of f=2.5×10^{14}(m_{B}c^{2}/eV)  Hz. These configurations are stable throughout most of the parameter space, and arise out of credible mechanisms for dark-matter capture. Stars with bosonic cores may also develop in other theories with effective mass couplings, such as (massless) scalar-tensor theories. We also show that (ii) collapse of the host star to a black hole is avoided by efficient gravitational cooling mechanisms.

RECONSTRUCTING THE ACCRETION HISTORY OF THE GALACTIC STELLAR HALO FROM CHEMICAL ABUNDANCE RATIO DISTRIBUTIONS

International Nuclear Information System (INIS)

Lee, Duane M.; Johnston, Kathryn V.; Sen, Bodhisattva; Jessop, Will

2015-01-01

Observational studies of halo stars during the past two decades have placed some limits on the quantity and nature of accreted dwarf galaxy contributions to the Milky Way (MW) stellar halo by typically utilizing stellar phase-space information to identify the most recent halo accretion events. In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from 11 “MW-like” halos to generate satellite template sets (STSs) of 2D CARDs of accreted dwarf satellites, which are composed of accreted dwarfs from various mass regimes and epochs of accretion. Next, we randomly drew samples of ∼10 3–4 mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those 11 halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the STS used and the sample size. For certain STSs used we typically can identify the relative mass contributions of all accreted satellites to within a factor of two. We also find that this method is particularly sensitive to older accretion events involving low-luminosity dwarfs, e.g., ultra-faint dwarfs—precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ∼6–9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us to recover its accretion history—and the luminosity function of infalling dwarf galaxies—across cosmic time

RECONSTRUCTING THE ACCRETION HISTORY OF THE GALACTIC STELLAR HALO FROM CHEMICAL ABUNDANCE RATIO DISTRIBUTIONS

Energy Technology Data Exchange (ETDEWEB)

Lee, Duane M. [Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030 (China); Johnston, Kathryn V. [Department of Astronomy, Columbia University, New York City, NY 10027 (United States); Sen, Bodhisattva; Jessop, Will, E-mail: duane@shao.ac.cn [Department of Statistics, Columbia University, New York City, NY 10027 (United States)

2015-03-20

Observational studies of halo stars during the past two decades have placed some limits on the quantity and nature of accreted dwarf galaxy contributions to the Milky Way (MW) stellar halo by typically utilizing stellar phase-space information to identify the most recent halo accretion events. In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from 11 “MW-like” halos to generate satellite template sets (STSs) of 2D CARDs of accreted dwarf satellites, which are composed of accreted dwarfs from various mass regimes and epochs of accretion. Next, we randomly drew samples of ∼10{sup 3–4} mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those 11 halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the STS used and the sample size. For certain STSs used we typically can identify the relative mass contributions of all accreted satellites to within a factor of two. We also find that this method is particularly sensitive to older accretion events involving low-luminosity dwarfs, e.g., ultra-faint dwarfs—precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ∼6–9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us to recover its accretion history—and the luminosity function of infalling dwarf galaxies—across cosmic time.

Applying a physical continuum model to describe the broadband X-ray spectra of accreting pulsars at high luminosity

Science.gov (United States)

Pottschmidt, Katja; Hemphill, Paul B.; Wolff, Michael T.; Cheatham, Diana M.; Iwakiri, Wataru; Gottlieb, Amy M.; Falkner, Sebastian; Ballhausen, Ralf; Fuerst, Felix; Kuehnel, Matthias; Ferrigno, Carlo; Becker, Peter A.; Wood, Kent S.; Wilms, Joern

2018-01-01

A new window for better understanding the accretion onto strongly magnetized neutron stars in X-ray binaries is opening. In these systems the accreted material follows the magnetic field lines as it approaches the neutron star, forming accretion columns above the magnetic poles. The plasma falls toward the neutron star surface at near-relativistic speeds, losing energy by emitting X-rays. The X-ray spectral continua are commonly described using phenomenological models, i.e., power laws with different types of curved cut-offs at higher energies. Here we consider high luminosity pulsars. In these systems the mass transfer rate is high enough that the accreting plasma is thought to be decelerated in a radiation-dominated radiative shock in the accretion columns. While the theory of the emission from such shocks had already been developed by 2007, a model for direct comparison with X-ray continuum spectra in xspec or isis has only recently become available. Characteristic parameters of this model are the accretion column radius and the plasma temperature, among others. Here we analyze the broadband X-ray spectra of the accreting pulsars Centaurus X-3 and 4U 1626-67 obtained with NuSTAR. We present results from traditional empirical modeling as well as successfully apply the radiation-dominated radiative shock model. We also take the opportunity to compare to similar recent analyses of both sources using these and other observations.

KEY ISSUES REVIEW: Insights from simulations of star formation

Science.gov (United States)

Larson, Richard B.

2007-03-01

Although the basic physics of star formation is classical, numerical simulations have yielded essential insights into how stars form. They show that star formation is a highly nonuniform runaway process characterized by the emergence of nearly singular peaks in density, followed by the accretional growth of embryo stars that form at these density peaks. Circumstellar discs often form from the gas being accreted by the forming stars, and accretion from these discs may be episodic, driven by gravitational instabilities or by protostellar interactions. Star-forming clouds typically develop filamentary structures, which may, along with the thermal physics, play an important role in the origin of stellar masses because of the sensitivity of filament fragmentation to temperature variations. Simulations of the formation of star clusters show that the most massive stars form by continuing accretion in the dense cluster cores, and this again is a runaway process that couples star formation and cluster formation. Star-forming clouds also tend to develop hierarchical structures, and smaller groups of forming objects tend to merge into progressively larger ones, a generic feature of self-gravitating systems that is common to star formation and galaxy formation. Because of the large range of scales and the complex dynamics involved, analytic models cannot adequately describe many aspects of star formation, and detailed numerical simulations are needed to advance our understanding of the subject. 'The purpose of computing is insight, not numbers.' Richard W Hamming, in Numerical Methods for Scientists and Engineers (1962) 'There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.' William Shakespeare, in Hamlet, Prince of Denmark (1604)

On the model of symbiotic stars

International Nuclear Information System (INIS)

Tutukov, A.V.; Yungelson, L.R.

1982-01-01

The authors discuss conditions necessary for appearance and discovery of the symbiotic star phenomenon within the model of a binary consisting of a red (super)giant 3 solar masses not filling the Roche lobe and of an accreting hot degenerate CO-dwarf 0.8 solar masses. Within this model ''classical'' symbiotic stars may exist only within a narrow region of mass accretion rates and separations of components: 10 -7 approximately -7 solar masses/y and 3x10 13 approximately 14 cm. The evolutionary status of symbiotic stars and related objects and the mechanisms of their variability are discussed. (Auth.)

X-RAY DETERMINATION OF THE VARIABLE RATE OF MASS ACCRETION ONTO TW HYDRAE

Energy Technology Data Exchange (ETDEWEB)

Brickhouse, N. S.; Cranmer, S. R.; Dupree, A. K.; Guenther, H. M.; Wolk, S. J. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Luna, G. J. M. [Current address: Instituto de Astronomia y Fisica del Espacio (IAFE), Buenos Aires (Argentina)

2012-12-01

Diagnostics of electron temperature (T{sub e} ), electron density (n{sub e} ), and hydrogen column density (N{sub H}) from the Chandra High Energy Transmission Grating spectrum of He-like Ne IX in TW Hydrae (TW Hya), in conjunction with a classical accretion model, allow us to infer the accretion rate onto the star directly from measurements of the accreting material. The new method introduces the use of the absorption of Ne IX lines as a measure of the column density of the intervening, accreting material. On average, the derived mass accretion rate for TW Hya is 1.5 Multiplication-Sign 10{sup -9} M{sub Sun} yr{sup -1}, for a stellar magnetic field strength of 600 G and a filling factor of 3.5%. Three individual Chandra exposures show statistically significant differences in the Ne IX line ratios, indicating changes in N{sub H}, T{sub e} , and n{sub e} by factors of 0.28, 1.6, and 1.3, respectively. In exposures separated by 2.7 days, the observations reported here suggest a five-fold reduction in the accretion rate. This powerful new technique promises to substantially improve our understanding of the accretion process in young stars.

Tidal interaction, star formation and chemical evolution in blue compact dwarf galaxy Mrk 22

Science.gov (United States)

Paswan, A.; Omar, A.; Jaiswal, S.

2018-02-01

The optical spectroscopic and radio interferometric H I 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission-line features corresponding to high ionization lines of He II λ4686 and C IV λ5808 from young massive stars are detected. The ages of two prominent star-forming regions in the galaxy are estimated as ∼10 and ∼ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young starburst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission-line features. A significant N/O enrichment is seen in the fainter star-forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98±0.07 and 7.46±0.09, respectively. The galaxy has a large diffuse H I envelop. The H I images reveal disturbed gas kinematics and H I clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.

Stellar Populations in Compact Galaxy Groups: a Multi-wavelength Study of HCGs 16, 22, and 42, Their Star Clusters, and Dwarf Galaxies

Science.gov (United States)

Konstantopoulos, I. S.; Maybhate, A.; Charlton, J. C.; Fedotov, K.; Durrell, P. R.; Mulchaey, J. S.; English, J.; Desjardins, T. D.; Gallagher, S. C.; Walker, L. M.;

Effects of livestock species and stocking density on accretion rates in grazed salt marshes

Science.gov (United States)

Nolte, Stefanie; Esselink, Peter; Bakker, Jan P.; Smit, Christian

2015-01-01

Coastal ecosystems, such as salt marshes, are threatened by accelerated sea-level rise (SLR). Salt marshes deliver valuable ecosystem services such as coastal protection and the provision of habitat for a unique flora and fauna. Whether salt marshes in the Wadden Sea area are able to survive accelerated SLR depends on sufficient deposition of sediments which add to vertical marsh accretion. Accretion rate is influenced by a number of factors, and livestock grazing was recently included. Livestock grazing is assumed to reduce accretion rates in two ways: (a) directly by increasing soil compaction through trampling, and (b) indirectly by affecting the vegetation structure, which may lower the sediment deposition. For four years, we studied the impact of two livestock species (horse and cattle) at two stocking densities (0.5 and 1.0 animal ha-1) on accretion in a large-scale grazing experiment using sedimentation plates. We found lower cumulative accretion rates in high stocking densities, probably because more animals cause more compaction and create a lower canopy. Furthermore, a trend towards lower accretion rates in horse-compared to cattle-grazed treatments was found, most likely because (1) horses are more active and thus cause more compaction, and (2) herbage intake by horses is higher than by cattle, which causes a higher biomass removal and shorter canopy. During summer periods, negative accretion rates were found. When the grazing and non-grazing seasons were separated, the impact of grazing differed among years. In summer, we only found an effect of different treatments if soil moisture (precipitation) was relatively low. In winter, a sufficiently high inundation frequency was necessary to create differences between grazing treatments. We conclude that stocking densities, and to a certain extent also livestock species, affect accretion rates in salt marshes. Both stocking densities and livestock species should thus be taken into account in management

Final stages of evolution of cold, mass-accreting white dwarfs

International Nuclear Information System (INIS)

Hernanz, M.; Isern, J.; Canal, R.; Labay, J.; Mochkovitch, R.

1988-01-01

The evolution of solid C + O white dwarf models upon mass accretion is calculated up to the point of either explosive thermonuclear ignition or gravitational collapse. It is shown that both explosions and quiet collapses to a neutron star are possible for each of two different phase diagrams for high-density C + O mixtures. The ranges of initial masses and temperatures and of accretion rates leading to the different outcomes are determined. Problems concerning the chemical composition of the accreted matter and the effects of tidal dissipation are discussed. 68 references

A statistical spectropolarimetric study of Herbig Ae/Be stars

Science.gov (United States)

Ababakr, K. M.; Oudmaijer, R. D.; Vink, J. S.

2017-11-01

We present H α linear spectropolarimetry of a large sample of Herbig Ae/Be stars. Together with newly obtained data for 17 objects, the sample contains 56 objects, the largest such sample to date. A change in linear polarization across the H α line is detected in 42 (75 per cent) objects, which confirms the previous finding that the circumstellar environment around these stars on small spatial scales has an asymmetric structure, which is typically identified with a disc. A second outcome of this research is that we confirm that Herbig Ae stars are similar to T Tauri stars in displaying a line polarization effect, while depolarization is more common among Herbig Be stars. This finding had been suggested previously to indicate that Herbig Ae stars form in the same manner than T Tauri stars through magnetospheric accretion. It appears that the transition between these two differing polarization line effects occurs around the B7-B8 spectral type. This would in turn not only suggest that Herbig Ae stars accrete in a similar fashion as lower mass stars, but also that this accretion mechanism switches to a different type of accretion for Herbig Be stars. We report that the magnitude of the line effect caused by electron scattering close to the stars does not exceed 2 per cent. Only a very weak correlation is found between the magnitude of the line effect and the spectral type or the strength of the H α line. This indicates that the detection of a line effect only relies on the geometry of the line-forming region and the geometry of the scattering electrons.

Variable accretion of stellar winds onto Sgr A*

Science.gov (United States)

Cuadra, Jorge; Nayakshin, Sergei

2006-12-01

We report a 3-dimensional numerical study of the accretion of stellar winds onto Sgr A*, the super-massive black hole at the centre of our Galaxy. Compared with previous investigations, we allow the stars to be on realistic orbits, include the recently discovered slow wind sources, and allow for optically thin radiative cooling. We frst show the strong inflience of the stellar dynamics on the accretion onto the central black hole. We then present more realistic simulations of Sgr A* accretion and frid that the slow winds shock and rapidly cool, forming cold gas clumps and flaments that coexist with the hot X-ray emitting gas. The accretion rate in this case is highly variable on time-scales of tens to hundreds of years. Such variability can in principle lead to a strongly non-linear response through accretion fbw physics not resolved here, making Sgr A* an important energy source for the Galactic centre.

Variable accretion of stellar winds onto Sgr A*

Energy Technology Data Exchange (ETDEWEB)

Cuadra, Jorge [Max-Planck-Institut fuer Astrophysik, D-85741 Garching (Germany); Nayakshin, Sergei [Department of Physics and Astronomy, University of Leicester, LEI 7RH (United Kingdom)

2006-12-15

We report a 3-dimensional numerical study of the accretion of stellar winds onto Sgr A*, the super-massive black hole at the centre of our Galaxy. Compared with previous investigations, we allow the stars to be on realistic orbits, include the recently discovered slow wind sources, and allow for optically thin radiative cooling. We frst show the strong inflience of the stellar dynamics on the accretion onto the central black hole. We then present more realistic simulations of Sgr A* accretion and frid that the slow winds shock and rapidly cool, forming cold gas clumps and flaments that coexist with the hot X-ray emitting gas. The accretion rate in this case is highly variable on time-scales of tens to hundreds of years. Such variability can in principle lead to a strongly non-linear response through accretion fbw physics not resolved here, making Sgr A* an important energy source for the Galactic centre.

Variable accretion of stellar winds onto Sgr A*

International Nuclear Information System (INIS)

Cuadra, Jorge; Nayakshin, Sergei

2006-01-01

We report a 3-dimensional numerical study of the accretion of stellar winds onto Sgr A*, the super-massive black hole at the centre of our Galaxy. Compared with previous investigations, we allow the stars to be on realistic orbits, include the recently discovered slow wind sources, and allow for optically thin radiative cooling. We frst show the strong inflience of the stellar dynamics on the accretion onto the central black hole. We then present more realistic simulations of Sgr A* accretion and frid that the slow winds shock and rapidly cool, forming cold gas clumps and flaments that coexist with the hot X-ray emitting gas. The accretion rate in this case is highly variable on time-scales of tens to hundreds of years. Such variability can in principle lead to a strongly non-linear response through accretion fbw physics not resolved here, making Sgr A* an important energy source for the Galactic centre

Burst Oscillations: A New Spin on Neutron Stars

Science.gov (United States)

Strohmayer, Tod

2007-01-01

Observations with NASA's Rossi X-ray Timing Explorer (RXTE) have shown that the X-ray flux during thermonuclear X-ray bursts fr-om accreting neutron stars is often strongly pulsed at frequencies as high as 620 Hz. We now know that these oscillations are produced by spin modulation of the thermonuclear flux from the neutron star surface. In addition to revealing the spin frequency, they provide new ways to probe the properties and physics of accreting neutron stars. I will briefly review our current observational and theoretical understanding of these oscillations and discuss what they are telling us about neutron stars.

Hot and dense matter in compact stars - from nuclei to quarks

International Nuclear Information System (INIS)

Hempel, Matthias

2010-01-01

This dissertation deals with the equation of state of hot and dense matter in compact stars, with special focus on first order phase transitions. A general classification of first order phase transitions is given and the properties of mixed phases are discussed. Aspects of nucleation and the role of local constraints are investigated. The derived theoretical concepts are applied to matter in neutron stars and supernovae, in the hadron-quark and the liquid-gas phase transition. For the detailed description of the liquid-gas phase transition a new nuclear statistical equilibrium model is developed. It is based on a thermodynamic consistent implementation of relativistic mean-field interactions and excluded volume effects. With this model different equation of state tables are calculated and the composition and thermodynamic properties of supernova matter are analyzed. As a first application numerical simulations of core-collapse supernovae are presented. For the hadron-quark phase transition two possible scenarios are studied in more detail. First the appearance of a new mixed phase in a proto neutron star and the implications on its evolution. In the second scenario the consequences of the hadron-quark transition in corecollapse supernovae are investigated. Simulations show that the appearance of quark matter has clear observable signatures and can even lead to the generation of an explosion. (orig.)

Hot and dense matter in compact stars - from nuclei to quarks

Energy Technology Data Exchange (ETDEWEB)

Hempel, Matthias

2010-10-19

This dissertation deals with the equation of state of hot and dense matter in compact stars, with special focus on first order phase transitions. A general classification of first order phase transitions is given and the properties of mixed phases are discussed. Aspects of nucleation and the role of local constraints are investigated. The derived theoretical concepts are applied to matter in neutron stars and supernovae, in the hadron-quark and the liquid-gas phase transition. For the detailed description of the liquid-gas phase transition a new nuclear statistical equilibrium model is developed. It is based on a thermodynamic consistent implementation of relativistic mean-field interactions and excluded volume effects. With this model different equation of state tables are calculated and the composition and thermodynamic properties of supernova matter are analyzed. As a first application numerical simulations of core-collapse supernovae are presented. For the hadron-quark phase transition two possible scenarios are studied in more detail. First the appearance of a new mixed phase in a proto neutron star and the implications on its evolution. In the second scenario the consequences of the hadron-quark transition in corecollapse supernovae are investigated. Simulations show that the appearance of quark matter has clear observable signatures and can even lead to the generation of an explosion. (orig.)

Simulating X-ray bursts during a transient accretion event

Science.gov (United States)

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

2018-06-01

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

COMPARING THE ACCRETION DISK EVOLUTION OF BLACK HOLE AND NEUTRON STAR X-RAY BINARIES FROM LOW TO SUPER-EDDINGTON LUMINOSITY

International Nuclear Information System (INIS)

Weng Shanshan; Zhang Shuangnan

2011-01-01

Low-mass X-ray binaries (LMXBs) are systems in which a low-mass companion transfers mass via Roche-lobe overflow onto a black hole (BH) or a weakly magnetized neutron star (NS). It is believed that both the solid surface and the magnetic field of an NS can affect the accretion flow and show some observable effects. Using the disk emission dominant data, we compare the disk evolution of the two types of systems from low luminosity to super-Eddington luminosity. As the luminosity decreases the disk in the NS LMXB 4U1608-522 begins to leave the innermost stable circular orbit (ISCO) at much higher luminosity (∼0.1 L Edd ), compared with BH LMXBs at much lower luminosity (∼0.03 L Edd ), due to the interaction between the NS magnetosphere and accretion flow. However, as the luminosity increases above a critical luminosity, the disks in BH and NS LMXBs trace the same evolutionary pattern, because the magnetosphere is restricted inside ISCO, and then both the NS surface emission and (dipole) magnetic field do not significantly affect the secular evolution of the accretion disk, which is driven by the increased radiation pressure in the inner region. We further suggest that the NS surface emission provides additional information about the accretion disk not available in BH systems. Through the observed NS surface emission, we argue that the disk thickness H/R is less than 0.3-0.4, and that the significant outflow from the inner disk edge exists at a luminosity close to Eddington luminosity.

VARIATIONS OF THE 10 μm SILICATE FEATURES IN THE ACTIVELY ACCRETING T TAURI STARS: DG Tau AND XZ Tau

International Nuclear Information System (INIS)

Bary, Jeffrey S.; Leisenring, Jarron M.; Skrutskie, Michael F.

2009-01-01

Using the Infrared Spectrograph aboard the Spitzer Space Telescope, we observed multiple epochs of 11 actively accreting T Tauri stars in the nearby Taurus-Auriga star-forming region. In total, 88 low-resolution mid-infrared spectra were collected over 1.5 years in Cycles 2 and 3. The results of this multi-epoch survey show that the 10 μm silicate complex in the spectra of two sources-DG Tau and XZ Tau-undergoes significant variations with the silicate feature growing both weaker and stronger over month- and year-long timescales. Shorter timescale variations on day- to week-long timescales were not detected within the measured flux errors. The time resolution coverage of this data set is inadequate for determining if the variations are periodic. Pure emission compositional models of the silicate complex in each epoch of the DG Tau and XZ Tau spectra provide poor fits to the observed silicate features. These results agree with those of previous groups that attempted to fit only single-epoch observations of these sources. Simple two-temperature, two-slab models with similar compositions successfully reproduce the observed variations in the silicate features. These models hint at a self-absorption origin of the diminution of the silicate complex instead of a compositional change in the population of emitting dust grains. We discuss several scenarios for producing such variability including disk shadowing, vertical mixing, variations in disk heating, and disk wind events associated with accretion outbursts.

Planet population synthesis driven by pebble accretion in cluster environments

Science.gov (United States)

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

2018-02-01

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

POPULATION SYNTHESIS OF YOUNG ISOLATED NEUTRON STARS: THE EFFECT OF FALLBACK DISK ACCRETION AND MAGNETIC FIELD EVOLUTION

International Nuclear Information System (INIS)

Fu, Lei; Li, Xiang-Dong

2013-01-01

The spin evolution of isolated neutron stars (NSs) is dominated by their magnetic fields. The measured braking indices of young NSs show that the spin-down mechanism due to magnetic dipole radiation with constant magnetic fields is inadequate. Assuming that the NS magnetic field is buried by supernova fallback matter and re-emerges after accretion stops, we carry out a Monte Carlo simulation of the evolution of young NSs, and show that most of the pulsars have braking indices ranging from –1 to 3. The results are compatible with the observational data of NSs associated with supernova remnants. They also suggest that the initial spin periods of NSs might occupy a relatively wide range

Supernovae, compact stars and nuclear physics

International Nuclear Information System (INIS)

Glendenning, N.K.

1989-01-01

We briefly review the current understanding of supernova. We investigate the implications of rapid rotation corresponding to the frequency of the new pulsar reported in the supernovae remnant SN1987A. It places very stringent conditions on the equation of state if the star is assumed to be bound by gravity alone. We find that the central energy density of the star must be greater than 12 times that of nuclear density to be stable against the most optimistic estimate of general relativistic instabilities. This is too high for the matter to plausibly consist of individual hadrons. We conclude that the newly discovered pulsar, if its half-millisecond signals are attributable to rotation, cannot be a neutron star. We show that it can be a strange quark star, and that the entire family of strange stars can sustain high rotation under appropriate conditions. We discuss the conversion of a neutron star to strange star, the possible existence of a crust of heavy ions held in suspension by centrifugal and electric forces, the cooling and other features. 39 refs., 8 figs., 2 tabs

Accretion onto stellar mass black holes

Science.gov (United States)

Deegan, Patrick

2009-12-01

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

Stochastic spin evolution of neutron stars

OpenAIRE

Popov, S. B.; Prokhorov, M. E.; Khoperskov, A. V.; Lipunov, V. M.

2001-01-01

In this paper we present calculations of period distribution for old accreting isolated neutron stars (INSs). At the age about a few billions years low velocity INSs come to the stage of accretion. At that stage their period evolution is governed by magnetic breaking and accreted angular momentum. Due to turbulence of the interstellar medium (ISM) accreted momentum can both accelerate and decelerate rotation of an INS and spin evolution has chaotic character. Calculations show that for consta...

ACCRETION-INHIBITED STAR FORMATION IN THE WARM MOLECULAR DISK OF THE GREEN-VALLEY ELLIPTICAL GALAXY NGC 3226?

International Nuclear Information System (INIS)

Appleton, P. N.; Bitsakis, T.; Alatalo, K.; Mundell, C.; Lacy, M.; Armus, L.; Charmandaris, V.; Duc, P.-A.; Lisenfeld, U.; Ogle, P.

2014-01-01

We present archival Spitzer photometry and spectroscopy and Herschel photometry of the peculiar ''Green Valley'' elliptical galaxy NGC 3226. The galaxy, which contains a low-luminosity active galactic nucleus (AGN), forms a pair with NGC 3227 and is shown to lie in a complex web of stellar and H I filaments. Imaging at 8 and 16 μm reveals a curved plume structure 3 kpc in extent, embedded within the core of the galaxy and coincident with the termination of a 30 kpc long H I tail. In situ star formation associated with the infrared (IR) plume is identified from narrowband Hubble Space Telescope (HST) imaging. The end of the IR plume coincides with a warm molecular hydrogen disk and dusty ring containing 0.7-1.1 × 10 7 M ☉ detected within the central kiloparsec. Sensitive upper limits to the detection of cold molecular gas may indicate that a large fraction of the H 2 is in a warm state. Photometry derived from the ultraviolet (UV) to the far-IR shows evidence for a low star-formation rate of ∼0.04 M ☉ yr –1 averaged over the last 100 Myr. A mid-IR component to the spectral energy distribution (SED) contributes ∼20% of the IR luminosity of the galaxy, and is consistent with emission associated with the AGN. The current measured star formation rate is insufficient to explain NGC 3226's global UV-optical ''green'' colors via the resurgence of star formation in a ''red and dead'' galaxy. This form of ''cold accretion'' from a tidal stream would appear to be an inefficient way to rejuvenate early-type galaxies and may actually inhibit star formation

X-ray observations of symbiotic stars

Energy Technology Data Exchange (ETDEWEB)

Allen, D A [Anglo-Australian Observatory, Epping (Australia)

1981-11-01

Observations of 19 symbiotic stars made with the image proportional counter of the Einstein Observatory are reported. Three were detected as soft X-ray sources. All three have shown slow-nova eruptions in the past 40 years. The data are interpreted as support for a model for slow novae involving thermonuclear events on white dwarfs which accrete from M giant companions. Symbiotic stars in their steady state, not being detected X-ray sources, are presumed to be powered by the accretion process alone.

IUE observations of new A star candidate proto-planetary systems

Science.gov (United States)

Grady, Carol A.

1994-01-01

As a result of the detection of accreting gas in the A5e PMS Herbig Ae star, HR 5999, most of the observations for this IUE program were devoted to Herbig Ae stars rather than to main sequence A stars. Mid-UV emission at optical minimum light was detected for UX Ori (A1e), BF Ori (A5e), and CQ Tau (F2e). The presence of accreting gas in HD 45677 and HD 50138 prompted reclassification of these stars as Herbig Be stars rather than as protoplanetary nebulae. Detailed results are discussed.

A STRANGE STAR SCENARIO FOR THE FORMATION OF ECCENTRIC MILLISECOND PULSAR/HELIUM WHITE DWARF BINARIES

Energy Technology Data Exchange (ETDEWEB)

Jiang, Long; Li, Xiang-Dong [Department of Astronomy, Nanjing University, Nanjing 210046 (China); Dey, Jishnu; Dey, Mira, E-mail: lixd@nju.edu.cn [Department of Physics, Presidency University, 86/1, College Street, Kolkata 700 073 (India)

2015-07-01

According to the recycling scenario, millisecond pulsars (MSPs) have evolved from low-mass X-ray binaries (LMXBs). Their orbits are expected to be circular due to tidal interactions during binary evolution, as observed in most binary MSPs. There are some peculiar systems that do not fit this picture. Three recent examples are the PSRs J2234+06, J1946+3417, and J1950+2414, all of which are MSPs in eccentric orbits but with mass functions compatible with expected He white dwarf (WD) companions. It has been suggested these MSPs may have formed from delayed accretion-induced collapse of massive WDs, or the eccentricity may be induced by dynamical interaction between the binary and a circumbinary disk. Assuming that the core density of accreting neutron stars (NSs) in LMXBs may reach the density of quark deconfinement, which can lead to phase transition from NSs to strange quark stars, we show that the resultant MSPs are likely to have an eccentric orbit, due to the sudden loss of the gravitational mass of the NS during the transition. The eccentricities can be reproduced with a reasonable estimate of the mass loss. This scenario might also account for the formation of the youngest known X-ray binary Cir X–1, which also possesses a low-field compact star in an eccentric orbit.

Symbiotic star UV emission and theoretical models

International Nuclear Information System (INIS)

Kafatos, M.

1982-01-01

Observations of symbiotic stars in the far UV have provided important information on the nature of these objects. The canonical spectrum of a symbiotic star, e.g. RW Hya, Z And, AG Peg, is dominated by strong allowed and semiforbidden lines of a variety of at least twice ionized elements. Weaker emission from neutral and singly ionized species is also present. A continuum may or may not be present in the 1200 - 2000 A range but is generally present in the range 2000 - 3200 A range. The suspected hot subdwarf continuum is seen in some cases in the range 1200 - 2000 A (RW Hya, AG Peg, SY Mus). The presence of an accretion disk is difficult to demonstrate and to this date the best candidate for accretion to a main sequence star remains CI Cyg. A number of equations have been derived by the author that can yield the accretion parameters from the observable quantities. Boundary layer temperatures approximately 10 5 K and accretion rates approximately > 10 -5 solar masses/yr are required for accreting main sequence companions. To this date, though, most of the symbiotics may only require the presence of a approximately 10 5 K hot subdwarf. (Auth.)

Accretion disc origin of the Earth's water.

Science.gov (United States)

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

2013-07-13

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

Grain processes in massive star formation

International Nuclear Information System (INIS)

Wolfire, M.G.; Cassinelli, J.P.

1986-01-01

Observational evidence suggests that stars greater than 100 M(solar) exist in the Galaxy and Large Magellanic Cloud (LMC), however classical star formation theory predicts stellar mass limits of only approx. 60 M(solar). A protostellar accretion flow consists of inflowing gas and dust. Grains are destroyed as they are near the central protostar creating a dust shell or cocoon. Radiation pressure acting on the grain can halt the inflow of material thereby limiting the amount of mass accumulated by the protostar. We first consider rather general constraints on the initial grain to gas ratio and mass accretion rates that permit inflow. We further constrain these results by constructing a numerical model. Radiative deceleration of grains and grain destruction processes are explicitly accounted for in an iterative solution of the radiation-hydrodynamic equations. Findings seem to suggest that star formation by spherical accretion requires rather extreme preconditioning of the grain and gas environment

THE INTERSTELLAR MEDIUM AND FEEDBACK IN THE PROGENITORS OF THE COMPACT PASSIVE GALAXIES AT z ∼ 2

Energy Technology Data Exchange (ETDEWEB)

Williams, Christina C.; Giavalisco, Mauro; Lee, Bomee [Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003 (United States); Tundo, Elena [INAF, Osservatorio Astrofisico di Firenze, Largo Enrico Fermi 5, I-50125, Firenze (Italy); Mobasher, Bahram; Nayyeri, Hooshang [Department of Physics and Astronomy, University of California, Riverside, 900 University Avenue, Riverside, CA 92521 (United States); Ferguson, Henry C.; Koekemoer, Anton; Grogin, Norman [Space Telescope Science Institute, 3700 San Martin Boulevard, Baltimore, MD 21218 (United States); Trump, Jonathan R. [Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 (United States); Cassata, Paolo [Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Valparaíso (Chile); Dekel, Avishai [Racah Institute of Physics, The Hebrew University, Jerusalem 91904 (Israel); Guo, Yicheng [UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); Lee, Kyoung-Soo [Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907 (United States); Pentericci, Laura; Castellano, Marco; Fontana, Adriano; Grazian, Andrea [INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (Italy); Bell, Eric F. [Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States); Finkelstein, Steven L., E-mail: ccwilliams@email.arizona.edu [Department of Astronomy, University of Texas, Austin (United States); and others

2015-02-10

Quenched galaxies at z > 2 are nearly all very compact relative to z ∼ 0, suggesting a physical connection between high stellar density and efficient, rapid cessation of star-formation. We present rest-frame UV spectra of Lyman-break galaxies (LBGs) at z ∼ 3 selected to be candidate progenitors of the quenched galaxies at z ∼ 2 based on their compact rest-frame-optical sizes and high Σ{sub SFR}. We compare their UV properties to those of more extended LBGs of similar mass and star-formation rate (non-candidates). We find that candidate progenitors have faster bulk interstellar medium (ISM) gas velocities and higher equivalent widths of interstellar absorption lines, implying larger velocity spread among absorbing clouds. Candidates deviate from the relationship between equivalent widths of Lyα and interstellar absorption lines in that their Lyα emission remains strong despite high interstellar absorption, possibly indicating that the neutral H I fraction is patchy, such that Lyα photons can escape. We detect stronger C IV P-Cygni features (emission and absorption) and He II emission in candidates, indicative of larger populations of metal-rich Wolf-Rayet stars compared to non-candidates. The faster bulk motions, broader spread of gas velocity, and Lyα properties of candidates are consistent with their ISM being subject to more energetic feedback than non-candidates. Together with their larger metallicity (implying more evolved star-formation activity) this leads us to propose, if speculatively, that they are likely to quench sooner than non-candidates, supporting the validity of selection criteria used to identify them as progenitors of z ∼ 2 passive galaxies. We propose that massive, compact galaxies undergo more rapid growth of their stellar mass content, perhaps because the gas accretion mechanisms are different, and quench sooner than normally sized LBGs at these (early) epochs.

THE INTERSTELLAR MEDIUM AND FEEDBACK IN THE PROGENITORS OF THE COMPACT PASSIVE GALAXIES AT z ∼ 2

International Nuclear Information System (INIS)

Williams, Christina C.; Giavalisco, Mauro; Lee, Bomee; Tundo, Elena; Mobasher, Bahram; Nayyeri, Hooshang; Ferguson, Henry C.; Koekemoer, Anton; Grogin, Norman; Trump, Jonathan R.; Cassata, Paolo; Dekel, Avishai; Guo, Yicheng; Lee, Kyoung-Soo; Pentericci, Laura; Castellano, Marco; Fontana, Adriano; Grazian, Andrea; Bell, Eric F.; Finkelstein, Steven L.

2015-01-01

Quenched galaxies at z > 2 are nearly all very compact relative to z ∼ 0, suggesting a physical connection between high stellar density and efficient, rapid cessation of star-formation. We present rest-frame UV spectra of Lyman-break galaxies (LBGs) at z ∼ 3 selected to be candidate progenitors of the quenched galaxies at z ∼ 2 based on their compact rest-frame-optical sizes and high Σ SFR . We compare their UV properties to those of more extended LBGs of similar mass and star-formation rate (non-candidates). We find that candidate progenitors have faster bulk interstellar medium (ISM) gas velocities and higher equivalent widths of interstellar absorption lines, implying larger velocity spread among absorbing clouds. Candidates deviate from the relationship between equivalent widths of Lyα and interstellar absorption lines in that their Lyα emission remains strong despite high interstellar absorption, possibly indicating that the neutral H I fraction is patchy, such that Lyα photons can escape. We detect stronger C IV P-Cygni features (emission and absorption) and He II emission in candidates, indicative of larger populations of metal-rich Wolf-Rayet stars compared to non-candidates. The faster bulk motions, broader spread of gas velocity, and Lyα properties of candidates are consistent with their ISM being subject to more energetic feedback than non-candidates. Together with their larger metallicity (implying more evolved star-formation activity) this leads us to propose, if speculatively, that they are likely to quench sooner than non-candidates, supporting the validity of selection criteria used to identify them as progenitors of z ∼ 2 passive galaxies. We propose that massive, compact galaxies undergo more rapid growth of their stellar mass content, perhaps because the gas accretion mechanisms are different, and quench sooner than normally sized LBGs at these (early) epochs

Strangeon and Strangeon Star

Science.gov (United States)

Xiaoyu, Lai; Renxin, Xu

2017-06-01

The nature of pulsar-like compact stars is essentially a central question of the fundamental strong interaction (explained in quantum chromo-dynamics) at low energy scale, the solution of which still remains a challenge though tremendous efforts have been tried. This kind of compact objects could actually be strange quark stars if strange quark matter in bulk may constitute the true ground state of the strong-interaction matter rather than 56Fe (the so-called Witten’s conjecture). From astrophysical points of view, however, it is proposed that strange cluster matter could be absolutely stable and thus those compact stars could be strange cluster stars in fact. This proposal could be regarded as a general Witten’s conjecture: strange matter in bulk could be absolutely stable, in which quarks are either free (for strange quark matter) or localized (for strange cluster matter). Strange cluster with three-light-flavor symmetry is renamed strangeon, being coined by combining “strange nucleon” for the sake of simplicity. A strangeon star can then be thought as a 3-flavored gigantic nucleus, and strangeons are its constituent as an analogy of nucleons which are the constituent of a normal (micro) nucleus. The observational consequences of strangeon stars show that different manifestations of pulsarlike compact stars could be understood in the regime of strangeon stars, and we are expecting more evidence for strangeon star by advanced facilities (e.g., FAST, SKA, and eXTP).

The scenario of two families of compact stars. Pt. 2. Transition from hadronic to quark matter and explosive phenomena

Energy Technology Data Exchange (ETDEWEB)

Drago, Alessandro; Pagliara, Giuseppe [Ferrara Univ. (Italy). Dipt. di Fisica e Scienze della Terra; INFN, Ferrara (Italy)

2016-02-15

We will follow the two-families scenario described in the accompanying paper, in which compact stars having a very small radius and masses not exceeding about 1.5M {sub CircleDot} are made of hadrons, while more massive compact stars are quark stars. In the present paper we discuss the dynamics of the transition of a hadronic star into a quark star. We will show that the transition takes place in two phases: a very rapid one, lasting a few milliseconds, during which the central region of the star converts into quark matter and the process of conversion is accelerated by the existence of strong hydrodynamical instabilities, and a second phase, lasting about ten seconds, during which the process of conversion proceeds as far as the surface of the star via production and diffusion of strangeness. We will show that these two steps play a crucial role in the phenomenological implications of the model. We will discuss the possible implications of this scenario both for long and for short Gamma Ray Bursts (GRBs), using the proto-magnetar model as the reference frame of our discussion. We will show that the process of quark deconfinement can be connected to specific observed features of the GRBs. In the case of long GRBs we will discuss the possibility that quark deconfinement is at the origin of the second peak present in quite a large fraction of bursts. Also we will discuss the possibility that long GRBs can take place in binary systems without being associated with a SN explosion. Concerning short GRBs, quark deconfinement can play the crucial role in limiting their duration. Finally we will shortly revisit the possible relevance of quark deconfinement in some specific type of Supernova explosions, in particular in the case of very massive progenitors. (orig.)

Neutron star formation in theoretical supernovae. Low mass stars and white dwarfs

International Nuclear Information System (INIS)

Nomoto, K.

1986-01-01

The presupernova evolution of stars that form semi-degenerate or strongly degenerate O + Ne + Mg cores is discussed. For the 10 to 13 Msub solar stars, behavior of off-center neon flashes is crucial. The 8 to 10 m/sub solar stars do not ignite neon and eventually collapse due to electron captures. Properties of supernova explosions and neutron stars expected from these low mass progenitors are compared with the Crab nebula. The conditions for which neutron stars form from accretion-induced collapse of white dwarfs in clsoe binary systems is also examined

X-ray observations of symbiotic stars

International Nuclear Information System (INIS)

Allen, D.A.

1981-01-01

Observations of 19 symbiotic stars made with the image proportional counter of the Einstein Observatory are reported. Three were detected as soft X-ray sources. All three have shown slow-nova eruptions in the past 40 years. The data are interpreted as support for a model for slow novae involving thermonuclear events on white dwarfs which accrete from M giant companions. Symbiotic stars in their steady state, not being detected X-ray sources, are presumed to be powered by the accretion process alone. (author)

Cold, clumpy accretion onto an active supermassive black hole.

Science.gov (United States)

Tremblay, Grant R; Oonk, J B Raymond; 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; Li, Yuan; Maury, Anaëlle; Russell, Helen R; Quillen, Alice C; Urry, C Megan; Sanders, Jeremy S; Wise, Michael W

2016-06-09

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 produce a rain of cold clouds that fall towards the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing 'shadows' cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

Constraining the Stellar Populations and Star Formation Histories of Blue Compact Dwarf Galaxies with SED Fits

Energy Technology Data Exchange (ETDEWEB)

Janowiecki, Steven [International Center for Radio Astronomy Research, M468, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia, 6009 (Australia); Salzer, John J.; Zee, Liese van [Department of Astronomy, Indiana University, 727 East Third Street, Bloomington, IN 47405 (United States); Rosenberg, Jessica L. [Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030 (United States); Skillman, Evan, E-mail: steven.janowiecki@uwa.edu.au [Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street, SE Minneapolis, MN, 55455 (United States)

2017-02-10

We discuss and test possible evolutionary connections between blue compact dwarf galaxies (BCDs) and other types of dwarf galaxies. BCDs provide ideal laboratories to study intense star formation episodes in low-mass dwarf galaxies, and have sometimes been considered a short-lived evolutionary stage between types of dwarf galaxies. To test these connections, we consider a sample of BCDs as well as a comparison sample of nearby galaxies from the Local Volume Legacy (LVL) survey for context. We fit the multi-wavelength spectral energy distributions (SED, far-ultra-violet to far-infrared) of each galaxy with a grid of theoretical models to determine their stellar masses and star formation properties. We compare our results for BCDs with the LVL galaxies to put BCDs in the context of normal galaxy evolution. The SED fits demonstrate that the star formation events currently underway in BCDs are at the extreme of the continuum of normal dwarf galaxies, both in terms of the relative mass involved and in the relative increase over previous star formation rates. Today’s BCDs are distinctive objects in a state of extreme star formation that is rapidly transforming them. This study also suggests ways to identify former BCDs whose star formation episodes have since faded.

Study of magnetized accretion flow with cooling processes

Indian Academy of Sciences (India)

Kuldeep Singh

2018-02-09

Feb 9, 2018 ... 2University of Delhi, South Campus, Delhi 110 021, India. ∗ ... Abstract. We have studied shock in magnetized accretion flow/funnel flow in case of neutron star with .... where Ap is the area of cross-section of the flux tube.

Nuclear star formation activity and black hole accretion in nearby Seyfert galaxies

Energy Technology Data Exchange (ETDEWEB)

Esquej, P. [Centro de Astrobiología, INTA-CSIC, Villafranca del Castillo, E-28850, Madrid (Spain); Alonso-Herrero, A.; Hernán-Caballero, A. [Instituto de Física de Cantabria, CSIC-Universidad de Cantabria, E-39005 Santander (Spain); González-Martín, O.; Ramos Almeida, C.; Rodríguez Espinosa, J. M. [Instituto de Astrofísica de Canarias (IAC), C/Vía Láctea, E-38205, La Laguna (Spain); Hönig, S. F. [UCSB Department of Physics, Broida Hall 2015H, Santa Barbara, CA (United States); Roche, P. [Department of Physics, University of Oxford, Oxford OX1 3RH (United Kingdom); Mason, R. E. [Gemini Observatory, Northern Operations Center, 670 North A' ohoku, HI 96720 (United States); Díaz-Santos, T. [Spitzer Science Center, 1200 East California Boulevard, Pasadena, CA 91125 (United States); Levenson, N. A. [Gemini Observatory, Casilla 603, La Serena (Chile); Aretxaga, I. [Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Aptdo. Postal 51 y 216, 72000 Puebla (Mexico); Packham, C. [Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249 (United States)

2014-01-01

Recent theoretical and observational works indicate the presence of a correlation between the star-formation rate (SFR) and active galactic nucleus (AGN) luminosity (and, therefore, the black hole accretion rate, M-dot {sub BH}) of Seyfert galaxies. This suggests a physical connection between the gas-forming stars on kpc scales and the gas on sub-pc scales that is feeding the black hole. We compiled the largest sample of Seyfert galaxies to date with high angular resolution (∼0.''4-0.''8) mid-infrared (8-13 μm) spectroscopy. The sample includes 29 Seyfert galaxies drawn from the AGN Revised Shapley-Ames catalog. At a median distance of 33 Mpc, our data allow us to probe nuclear regions on scales of ∼65 pc (median value). We found no general evidence of suppression of the 11.3 μm polycyclic aromatic hydrocarbon (PAH) emission in the vicinity of these AGN, and we used this feature as a proxy for the SFR. We detected the 11.3 μm PAH feature in the nuclear spectra of 45% of our sample. The derived nuclear SFRs are, on average, five times lower than those measured in circumnuclear regions of 600 pc in size (median value). However, the projected nuclear SFR densities (median value of 22 M {sub ☉} yr{sup –1} kpc{sup –2}) are a factor of 20 higher than those measured on circumnuclear scales. This indicates that the SF activity per unit area in the central ∼65 pc region of Seyfert galaxies is much higher than at larger distances from their nuclei. We studied the connection between the nuclear SFR and M-dot {sub BH} and showed that numerical simulations reproduce our observed relation fairly well.

NuSTAR Observation of the Symbiotic System GX 1+4

Science.gov (United States)

Wolff, Michael Thomas; Becker, Peter A.; Enoto, Teruaki; Pottschmidt, Katja; Wood, Kent

2017-08-01

We report on a NuSTAR observation of the symbiotic binary system GX 1+4. GX 1+4 is one of a small number of systems with a red giant mass donor and a magnetic neutron star in orbit around each other. The accreting pulsar in GX 1+4 has a spin period of ~150 seconds with epochs of both spin-up and spin-down. The orbital period that has not been determined. Magnetic accretion theory in such systems suggests that the neutron star has a magnetic field in the range 1013-1014 Gauss although this is not settled because no cyclotron absorption feature has been observed in the X-ray spectrum. The NuSTAR spectrum shows broad Fe-line emission near ~6.5 keV and also shows a broad power law shape detected up to ~60 keV. We analyze and discuss the NuSTAR X-ray data with particular attention to the question of what can the spectrum tell us about the structure of the accretion flow onto the neutron star and the magnetic field strength.

STELLAR POPULATIONS IN COMPACT GALAXY GROUPS: A MULTI-WAVELENGTH STUDY OF HCGs 16, 22, AND 42, THEIR STAR CLUSTERS, AND DWARF GALAXIES

International Nuclear Information System (INIS)

Konstantopoulos, I. S.; Maybhate, A.; Charlton, J. C.; Gronwall, C.; Fedotov, K.; Desjardins, T. D.; Gallagher, S. C.; Durrell, P. R.; Mulchaey, J. S.; English, J.; Walker, L. M.; Johnson, K. E.; Tzanavaris, P.

2013-01-01

We present a multi-wavelength analysis of three compact galaxy groups, Hickson compact groups (HCGs) 16, 22, and 42, which describe a sequence in terms of gas richness, from space- (Swift, Hubble Space Telescope (HST), and Spitzer) and ground-based (Las Campanas Observatory and Cerro Tololo Inter-American Observatory) imaging and spectroscopy. We study various signs of past interactions including a faint, dusty tidal feature about HCG 16A, which we tentatively age-date at <1 Gyr. This represents the possible detection of a tidal feature at the end of its phase of optical observability. Our HST images also resolve what were thought to be double nuclei in HCG 16C and D into multiple, distinct sources, likely to be star clusters. Beyond our phenomenological treatment, we focus primarily on contrasting the stellar populations across these three groups. The star clusters show a remarkable intermediate-age population in HCG 22, and identify the time at which star formation was quenched in HCG 42. We also search for dwarf galaxies at accordant redshifts. The inclusion of 33 members and 27 ''associates'' (possible members) radically changes group dynamical masses, which in turn may affect previous evolutionary classifications. The extended membership paints a picture of relative isolation in HCGs 16 and 22, but shows HCG 42 to be part of a larger structure, following a dichotomy expected from recent studies. We conclude that (1) star cluster populations provide an excellent metric of evolutionary state, as they can age-date the past epochs of star formation; and (2) the extended dwarf galaxy population must be considered in assessing the dynamical state of a compact group.

Accretion onto hot white dwarfs in relation to symbiotic novae

International Nuclear Information System (INIS)

Livio, M.; Prialnik, D.; Regev, O.

1989-01-01

Numerical calculations are used to study the hydrodynamic evolution of a hot white dwarf with 1 solar mass accreting hydrogen-rich matter at rates between 10 to the -8th and 10 to the -6th solar masses/yr. It is found that for accretion at a rate of about 10 to the -8th solar masses/yr, nova-type outbursts of long duration occur at intervals of about 1500 yr. About half of the accreted envelope is ejected during these outbursts. At a rate of about 10 to the -7th solar masses/yr, the star alternates between comparable periods at a high plateau luminosity and giant dimensions and periods at a low luminosity and white dwarf dimension. At 10 to the -6th solar masses/yr, equilibrium is achieved with a typical red giant luminosity supported by steady hydrogen burning. It is concluded that symbiotic novae are more likely to occur in detached systems involving wind accretors. Thus, the contribution of symbiotic stars to the frequency of type I supernovae is severely constrained. 39 refs

Destruction of a Magnetized Star

Science.gov (United States)

Kohler, Susanna

2017-01-01

What happens when a magnetized star is torn apart by the tidal forces of a supermassive black hole, in a violent process known as a tidal disruption event? Two scientists have broken new ground by simulating the disruption of stars with magnetic fields for the first time.The magnetic field configuration during a simulation of the partial disruption of a star. Top left: pre-disruption star. Bottom left: matter begins to re-accrete onto the surviving core after the partial disruption. Right: vortices form in the core as high-angular-momentum debris continues to accrete, winding up and amplifying the field. [Adapted from Guillochon McCourt 2017]What About Magnetic Fields?Magnetic fields are expected to exist in the majority of stars. Though these fields dont dominate the energy budget of a star the magnetic pressure is a million times weaker than the gas pressure in the Suns interior, for example they are the drivers of interesting activity, like the prominences and flares of our Sun.Given this, we can wonder what role stars magnetic fields might play when the stars are torn apart in tidal disruption events. Do the fields change what we observe? Are they dispersed during the disruption, or can they be amplified? Might they even be responsible for launching jets of matter from the black hole after the disruption?Star vs. Black HoleIn a recent study, James Guillochon (Harvard-Smithsonian Center for Astrophysics) and Michael McCourt (Hubble Fellow at UC Santa Barbara) have tackled these questions by performing the first simulations of tidal disruptions of stars that include magnetic fields.In their simulations, Guillochon and McCourt evolve a solar-mass star that passes close to a million-solar-mass black hole. Their simulations explore different magnetic field configurations for the star, and they consider both what happens when the star barely grazes the black hole and is only partially disrupted, as well as what happens when the black hole tears the star apart

PS1-10jh CONTINUES TO FOLLOW THE FALLBACK ACCRETION RATE OF A TIDALLY DISRUPTED STAR

Energy Technology Data Exchange (ETDEWEB)

Gezari, S. [Department of Astronomy, University of Maryland, Stadium Drive, College Park, MD 20742-2421 (United States); Chornock, R. [Astrophysical Institute, Department of Physics and Astronomy, 251B Clippinger Lab, Ohio University Athens, OH 45701 (United States); Lawrence, A. [Institute for Astronomy, University of Edinburgh Scottish Universities Physics Alliance, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom); Rest, A. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Jones, D. O. [Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States); Berger, E.; Challis, P. M. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Narayan, G., E-mail: suvi@astro.umd.edu [NOAO, 950 North Cherry Avenue, Tucson, AZ 85719 (United States)

2015-12-10

We present late-time observations of the tidal disruption event candidate PS1-10jh. UV and optical imaging with Hubble Space Telescope/WFC3 localize the transient to be coincident with the host galaxy nucleus to an accuracy of 0.023 arcsec, corresponding to 66 pc. The UV flux in the F225W filter, measured 3.35 rest-frame years after the peak of the nuclear flare, is consistent with a decline that continues to follow a t{sup −5/3} power-law with no spectral evolution. Late epochs of optical spectroscopy obtained with MMT ∼ 2 and 4 years after the peak, enable a clean subtraction of the host galaxy from the early spectra, revealing broad helium emission lines on top of a hot continuum, and placing stringent upper limits on the presence of hydrogen line emission. We do not measure Balmer Hδ absorption in the host galaxy that is strong enough to be indicative of a rare, post-starburst “E+A” galaxy as reported by Arcavi et al. The light curve of PS1-10jh over a baseline of 3.5 years is best modeled by fallback accretion of a tidally disrupted star. Its strong broad helium emission relative to hydrogen (He iiλ4686/Hα > 5) could be indicative of either the hydrogen-poor chemical composition of the disrupted star, or certain conditions in the tidal debris of a solar-composition star in the presence of an optically thick, extended reprocessing envelope.

Occultations from an Active Accretion Disk in a 72-day Detached Post-Algol System Detected by K2

DEFF Research Database (Denmark)

Zhou, G.; Rappaport, S.; Nelson, L.

2018-01-01

Disks in binary systems can cause exotic eclipsing events. MWC 882 (BD –22 4376, EPIC 225300403) is such a disk-eclipsing system identified from observations during Campaign 11 of the K2 mission. We propose that MWC 882 is a post-Algol system with a B7 donor star of mass in a 72-day orbit around...... an A0 accreting star of mass . The disk around the accreting star occults the donor star once every orbit, inducing 19-day long, 7% deep eclipses identified by K2 and subsequently found in pre-discovery All-Sky Automated Survey and All Sky Automated Survey for Supernovae observations. We coordinated...

Accretion disc boundary layers - geometrically and optically thin case

International Nuclear Information System (INIS)

Regev, Oded; Hougerat, A.A.

1988-01-01

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

The 2014-2017 outburst of the young star ASASSN-13db. A time-resolved picture of a very-low-mass star between EXors and FUors

Science.gov (United States)

Sicilia-Aguilar, A.; Oprandi, A.; Froebrich, D.; Fang, M.; Prieto, J. L.; Stanek, K.; Scholz, A.; Kochanek, C. S.; Henning, Th.; Gredel, R.; Holoien, T. W.-S.; Rabus, M.; Shappee, B. J.; Billington, S. J.; Campbell-White, J.; Zegmott, T. J.

2017-11-01

Context. Accretion outbursts are key elements in star formation. ASASSN-13db is a M5-type star with a protoplanetary disk, the lowest-mass star known to experience accretion outbursts. Since its discovery in 2013, it has experienced two outbursts, the second of which started in November 2014 and lasted until February 2017. Aims: We explore the photometric and spectroscopic behavior of ASASSN-13db during the 2014-2017 outburst. Methods: We use high- and low-resolution spectroscopy and time-resolved photometry from the ASAS-SN survey, the LCOGT and the Beacon Observatory to study the light curve of ASASSN-13db and the dynamical and physical properties of the accretion flow. Results: The 2014-2017 outburst lasted for nearly 800 days. A 4.15 d period in the light curve likely corresponds to rotational modulation of a star with hot spot(s). The spectra show multiple emission lines with variable inverse P-Cygni profiles and a highly variable blue-shifted absorption below the continuum. Line ratios from metallic emission lines (Fe I/Fe II, Ti I/Ti II) suggest temperatures of 5800-6000 K in the accretion flow. Conclusions: Photometrically and spectroscopically, the 2014-2017 event displays an intermediate behavior between EXors and FUors. The accretion rate ([Ṁ]= 1-3 × 10-7 M⊙/yr), about two orders of magnitude higher than the accretion rate in quiescence, is not significantly different from the accretion rate observed in 2013. The absorption features in the spectra suggest that the system is viewed at a high angle and drives a powerful, non-axisymmetric wind, maybe related to magnetic reconnection. The properties of ASASSN-13db suggest that temperatures lower than those for solar-type stars are needed for modeling accretion in very-low-mass systems. Finally, the rotational modulation during the outburst reveals that accretion-related structures settle after the beginning of the outburst and can be relatively stable and long-lived. Our work also demonstrates the power

Magnetohydrodynamic Simulations of Black Hole Accretion

Science.gov (United States)

Avara, Mark J.

Black holes embody one of the few, simple, solutions to the Einstein field equations that describe our modern understanding of gravitation. In isolation they are small, dark, and elusive. However, when a gas cloud or star wanders too close, they light up our universe in a way no other cosmic object can. The processes of magnetohydrodynamics which describe the accretion inflow and outflows of plasma around black holes are highly coupled and nonlinear and so require numerical experiments for elucidation. These processes are at the heart of astrophysics since black holes, once they somehow reach super-massive status, influence the evolution of the largest structures in the universe. It has been my goal, with the body of work comprising this thesis, to explore the ways in which the influence of black holes on their surroundings differs from the predictions of standard accretion models. I have especially focused on how magnetization of the greater black hole environment can impact accretion systems.

Angular momentum transfer in steady disk accretion

International Nuclear Information System (INIS)

Gorbatskij, V.G.

1977-01-01

The conditions of steady disk accretion have been investigated. The disk axisymmetric model is considered. It is shown that the gas is let at the outer boundary of the disk with the azimuthal velocity which is slightly less than the Kepler circular one. Gas possesses the motion quality moment which is transferred from the outer layers of the disk to the surface of the star. The steady state of the disk preserved until the inflow of the moment to the star increases its rotation velocity up to magnitudes close to the critical one

Multiwavelength diagnostics of accretion in an X-ray selected sample of CTTSs

Science.gov (United States)

Curran, R. L.; Argiroffi, C.; Sacco, G. G.; Orlando, S.; Peres, G.; Reale, F.; Maggio, A.

2011-02-01

Context. High resolution X-ray spectroscopy has revealed soft X-rays from high density plasma in classical T Tauri stars (CTTSs), probably arising from the accretion shock region. However, the mass accretion rates derived from the X-ray observations are consistently lower than those derived from UV/optical/NIR studies. Aims: We aim to test the hypothesis that the high density soft X-ray emission originates from accretion by analysing, in a homogeneous manner, optical accretion indicators for an X-ray selected sample of CTTSs. Methods: We analyse optical spectra of the X-ray selected sample of CTTSs and calculate the accretion rates based on measuring the Hα, Hβ, Hγ, He ii 4686 Å, He i 5016 Å, He i 5876 Å, O i 6300 Å, and He i 6678 Å equivalent widths. In addition, we also calculate the accretion rates based on the full width at 10% maximum of the Hα line. The different optical tracers of accretion are compared and discussed. The derived accretion rates are then compared to the accretion rates derived from the X-ray spectroscopy. Results: We find that, for each CTTS in our sample, the different optical tracers predict mass-accretion rates that agree within the errors, albeit with a spread of ≈ 1 order of magnitude. Typically, mass-accretion rates derived from Hα and He i 5876 Å are larger than those derived from Hβ, Hγ, and O i. In addition, the Hα full width at 10%, whilst a good indicator of accretion, may not accurately measure the mass-accretion rate. When the optical mass-accretion rates are compared to the X-ray derived mass-accretion rates, we find that: a) the latter are always lower (but by varying amounts); b) the latter range within a factor of ≈ 2 around 2 × 10-10 M⊙ yr-1, despite the former spanning a range of ≈ 3 orders of magnitude. We suggest that the systematic underestimate of the X-ray derived mass-accretion rates could depend on the density distribution inside the accretion streams, where the densest part of the stream is

BLACK HOLE-NEUTRON STAR MERGERS WITH A HOT NUCLEAR EQUATION OF STATE: OUTFLOW AND NEUTRINO-COOLED DISK FOR A LOW-MASS, HIGH-SPIN CASE

International Nuclear Information System (INIS)

Deaton, M. Brett; Duez, Matthew D.; Foucart, Francois; O'Connor, Evan; Ott, Christian D.; Scheel, Mark A.; Szilagyi, Bela; Kidder, Lawrence E.; Muhlberger, Curran D.

2013-01-01

Neutrino emission significantly affects the evolution of the accretion tori formed in black hole-neutron star mergers. It removes energy from the disk, alters its composition, and provides a potential power source for a gamma-ray burst. To study these effects, simulations in general relativity with a hot microphysical equation of state (EOS) and neutrino feedback are needed. We present the first such simulation, using a neutrino leakage scheme for cooling to capture the most essential effects and considering a moderate mass (1.4 M ☉ neutron star, 5.6 M ☉ black hole), high-spin (black hole J/M 2 = 0.9) system with the K 0 = 220 MeV Lattimer-Swesty EOS. We find that about 0.08 M ☉ of nuclear matter is ejected from the system, while another 0.3 M ☉ forms a hot, compact accretion disk. The primary effects of the escaping neutrinos are (1) to make the disk much denser and more compact, (2) to cause the average electron fraction Y e of the disk to rise to about 0.2 and then gradually decrease again, and (3) to gradually cool the disk. The disk is initially hot (T ∼ 6 MeV) and luminous in neutrinos (L ν ∼ 10 54 erg s –1 ), but the neutrino luminosity decreases by an order of magnitude over 50 ms of post-merger evolution

Rotational mixing in carbon-enhanced metal-poor stars with s-process enrichment

Science.gov (United States)

Matrozis, E.; Stancliffe, R. J.

2017-10-01

Carbon-enhanced metal-poor (CEMP) stars with s-process enrichment (CEMP-s) are believed to be the products of mass transfer from an asymptotic giant branch (AGB) companion, which has long since become a white dwarf. The surface abundances of CEMP-s stars are thus commonly assumed to reflect the nucleosynthesis output of the first AGB stars. We have previously shown that, for this to be the case, some physical mechanism must counter atomic diffusion (gravitational settling and radiative levitation) in these nearly fully radiative stars, which otherwise leads to surface abundance anomalies clearly inconsistent with observations. Here we take into account angular momentum accretion by these stars. We compute in detail the evolution of typical CEMP-s stars from the zero-age main sequence, through the mass accretion, and up the red giant branch for a wide range of specific angular momentum ja of the accreted material, corresponding to surface rotation velocities, vrot, between about 0.3 and 300 kms-1. We find that only for ja ≳ 1017 cm2s-1 (vrot > 20 kms-1, depending on mass accreted) angular momentum accretion directly causes chemical dilution of the accreted material. This could nevertheless be relevant to CEMP-s stars, which are observed to rotate more slowly, if they undergo continuous angular momentum loss akin to solar-like stars. In models with rotation velocities characteristic of CEMP-s stars, rotational mixing primarily serves to inhibit atomic diffusion, such that the maximal surface abundance variations (with respect to the composition of the accreted material) prior to first dredge-up remain within about 0.4 dex without thermohaline mixing or about 0.5-1.5 dex with thermohaline mixing. Even in models with the lowest rotation velocities (vrot ≲ 1 kms-1), rotational mixing is able to severely inhibit atomic diffusion, compared to non-rotating models. We thus conclude that it offers a natural solution to the problem posed by atomic diffusion and cannot be

Nuclear fusion and carbon flashes on neutron stars

Science.gov (United States)

Taam, R. E.; Picklum, R. E.

1978-01-01

This paper reports on detailed calculations of the thermal evolution of the carbon-burning shells in the envelopes of accreting neutron stars for mass-accretion rates of 1 hundred-billionth to 2 billionths of a solar mass per yr and neutron-star masses of 0.56 and 1.41 solar masses. The work of Hansen and Van Horn (1975) is extended to higher densities, and a more detailed treatment of nuclear processing in the hydrogen- and helium-burning regions is included. Results of steady-state calculations are presented, and results of time-dependent computations are examined for accretion rates of 3 ten-billionths and 1 billionth of solar mass per yr. It is found that two evolutionary sequences lead to carbon flashes and that the carbon abundance at the base of the helium shell is a strong function of accretion rate. Upper limits are placed on the accretion rates at which carbon flashes will be important.

AN ACCOUNTING OF THE DUST-OBSCURED STAR FORMATION AND ACCRETION HISTORIES OVER THE LAST ∼11 BILLION YEARS

International Nuclear Information System (INIS)

Murphy, E. J.; Chary, R.-R.; Dickinson, M.; Pope, A.; Frayer, D. T.; Lin, L.

2011-01-01

We report on an accounting of the star-formation- and accretion-driven energetics of 24 μm-detected sources in the Great Observatories Origins Deep Survey-North field. For sources having infrared (IR; 8-1000 μm) luminosities ∼>3 x 10 12 L sun when derived by fitting local spectral energy distributions (SEDs) to 24 μm photometry alone, we find these IR luminosity estimates to be a factor of ∼4 times larger than those estimated when the SED fitting includes additional 16 and 70 μm data (and in some cases mid-IR spectroscopy and 850 μm data). This discrepancy arises from the fact that high-luminosity sources at z >> 0 appear to have far- to mid-IR ratios, as well as aromatic feature equivalent widths, typical of lower luminosity galaxies in the local universe. Using our improved estimates for IR luminosity and active galactic nucleus (AGN) contributions, we investigate the evolution of the IR luminosity density versus redshift arising from star formation and AGN processes alone. We find that, within the uncertainties, the total star-formation-driven IR luminosity density is constant between 1.15 ∼ 2. AGNs appear to account for ∼ 11 L sun ≤ L IR 12 L sun ) appear to dominate the star formation rate density along with normal star-forming galaxies (L IR 11 L sun ) between 0.6 ∼ 2, the contribution from ultraluminous infrared galaxies (L IR ≥ 10 12 L sun ) becomes comparable with that of LIRGs. Using our improved IR luminosity estimates, we find existing calibrations for UV extinction corrections based on measurements of the UV spectral slope typically overcorrect UV luminosities by a factor of ∼2, on average, for our sample of 24 μm-selected sources; accordingly we have derived a new UV extinction correction more appropriate for our sample.

DASCH ON KU Cyg: A ∼ 5 YEAR DUST ACCRETION EVENT IN ∼ 1900

International Nuclear Information System (INIS)

Tang Sumin; Grindlay, Jonathan; Los, Edward; Servillat, Mathieu

2011-01-01

KU Cyg is an eclipsing binary consisting of an F-type star accreting through a large accretion disk from a K5III red giant. Here we present the discovery of a 5 year dip around 1900 found from its 100 year DASCH light curve. It showed a ∼0.5 mag slow fading from 1899 to 1903 and brightened back around 1904 on a relatively shorter timescale. The light curve shape of the 1899-1904 fading-brightening event differs from the dust production and dispersion process observed in R Coronae Borealis stars, which usually has a faster fading and slower recovery, and for KU Cyg is probably related to the accretion disk surrounding the F star. The slow fading in KU Cyg is probably caused by increases in dust extinction in the disk, and the subsequent quick brightening may be due to the evaporation of dust transported inward through the disk. The extinction excess which caused the fading may arise from increased mass transfer rate in the system or from dust clump ejections from the K giant.

Dynamics of Mass Transfer in Wide Symbiotic Systems

Science.gov (United States)

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

2010-01-01

We investigate the formation of accretion disks around the secondary in detached systems consisting of an Asymptotic Giant Branch (AGB) star and a compact accreting companion as a function of mass loss rate and orbital parameters. In particular, we study winds from late-type stars that are gravitationally focused by a companion in a wide binary system using hydrodynamical simulations. For a typical slow and massive wind from an evolved star there is a stream flow between the stars with accretion rates of a few percent of the mass loss from the primary. Mass transfer through a focused wind is an important mechanism for a broad range of interacting binary systems and can explain the formation of Barium stars and other chemically peculiar stars.

The symbiotic star H1-36

International Nuclear Information System (INIS)

Allen, D.A.

1983-01-01

Optical and infrared spectrophotometry is presented of the high-excitation emission-line star H1-36. The presence of a variable M giant is established: H1-36 may therefore be classified as a symbiotic star. The observations are interpreted in terms of the usual binary model for symbiotic stars, namely that an unseen star is heated by accretion of gas from its companion M giant. (author)

The accretion-heated crust of the transiently accreting 11-Hz X-ray pulsar in the globular cluster Terzan 5

NARCIS (Netherlands)

Degenaar, N.; Wijnands, R.

2011-01-01

We report on a Chandra Director’s Discretionary Time observation of the globular cluster Terzan 5, carried out ∼7 weeks after the cessation of the 2010 outburst of the newly discovered transiently accreting 11-Hz X-ray pulsar. We detect a thermal spectrum that can be fitted with a neutron star

Constraining jet physics in weakly accreting black holes

NARCIS (Netherlands)

Markoff, S.

2007-01-01

Outflowing jets are observed in a variety of astronomical objects such as accreting compact objects from X-ray binaries (XRBs) to active galactic nuclei (AGN), as well as at stellar birth and death. Yet we still do not know exactly what they are comprised of, why and how they form, or their exact

Eccentric binaries of compact objects in strong-field gravity

International Nuclear Information System (INIS)

Gold, Roman

2011-01-01

In this thesis we study the dynamics as well as the resulting gravitational radiation from eccentric binaries of compact objects in the non-linear regime of General Relativity. For this purpose we solve Einstein's field equation numerically in a 3+1 decomposition using the moving-puncture technique. We focus our study on very particular orbits, arising as a purely relativistic phenomenon of the two-body problem in General Relativity, which are associated with unstable circular orbits. They are governed by a fast, nearly circular revolution at a short distance followed by a slow, radial motion on a nearly elliptic trajectory. Due to the unique features of their orbital trajectories they are called zoom-whirl orbits. We analyze how the peculiar dynamics manifests itself in the emitted gravitational radiation and to which extent one can infer the orbital properties from observations of the gravitational waves. In the first part, we consider black hole binaries. We perform a comprehensive parameter study by varying the initial eccentricity, computing and characterizing the resulting gravitational waveforms. We address aspects, which can only be obtained from non-perturbative methods, and which are crucial to the astrophysical relevance of these orbits. In particular, our results imply a fairly low amount of fine-tuning necessary to spot zoom-whirl effects. We find whirl orbits for values of the eccentricities, which fall in disjunct intervals extending to rather low values. Furthermore, we show that whirl effects just before merger cause a signal with significant amplitude. In the second part, we investigate neutron star binaries on eccentric orbits in full General Relativity, which has not been studied so far. We explore their phenomenology and study the consequences for the matter after the neutron stars have merged. In these evolutions the merged neutron stars sooner or later collapse to form a black hole. During the collapse most of the matter is accreted on to the

Black Hole Event Horizons and Advection-Dominated Accretion

Science.gov (United States)

McClintock, Jeffrey; Mushotzky, Richard F. (Technical Monitor)

2002-01-01

The work supported in part by this grant is part of a larger program on the detection of black hole event horizons, which is also partially supported by NASA grant GO0-1105A. This work has been carried out primarily in collaboration with Dr. M. Garcia and Prof. R. Narayan at the Harvard-Smithsonian Center for Astrophysics and with D. Barret and J. Hameury at Centre d'Etude Spoliate des Rayonnements, France. Our purpose is to confirm the existence of black-hole event horizons by comparing accreting black holes to secreting neutron stars in quiescent X-ray novae. Such a comparison is feasible because black holes and neutron stars are both present in similar environments in X-ray novae. Our second purpose is to assess the nature of accretion flows onto black holes at very low mass transfer rates. Observations of some XMM targets are still pending, whereas most of the Chandra observations have been completed. We anticipate further publications on this work in the future.

A radio-pulsing white dwarf binary star.

Science.gov (United States)

Marsh, T R; Gänsicke, B T; Hümmerich, S; Hambsch, F-J; Bernhard, K; Lloyd, C; Breedt, E; Stanway, E R; Steeghs, D T; Parsons, S G; Toloza, O; Schreiber, M R; Jonker, P G; van Roestel, J; Kupfer, T; Pala, A F; Dhillon, V S; Hardy, L K; Littlefair, S P; Aungwerojwit, A; Arjyotha, S; Koester, D; Bochinski, J J; Haswell, C A; Frank, P; Wheatley, P J

2016-09-15

White dwarfs are compact stars, similar in size to Earth but approximately 200,000 times more massive. Isolated white dwarfs emit most of their power from ultraviolet to near-infrared wavelengths, but when in close orbits with less dense stars, white dwarfs can strip material from their companions and the resulting mass transfer can generate atomic line and X-ray emission, as well as near- and mid-infrared radiation if the white dwarf is magnetic. However, even in binaries, white dwarfs are rarely detected at far-infrared or radio frequencies. Here we report the discovery of a white dwarf/cool star binary that emits from X-ray to radio wavelengths. The star, AR Scorpii (henceforth AR Sco), was classified in the early 1970s as a δ-Scuti star, a common variety of periodic variable star. Our observations reveal instead a 3.56-hour period close binary, pulsing in brightness on a period of 1.97 minutes. The pulses are so intense that AR Sco's optical flux can increase by a factor of four within 30 seconds, and they are also detectable at radio frequencies. They reflect the spin of a magnetic white dwarf, which we find to be slowing down on a 10 7 -year timescale. The spin-down power is an order of magnitude larger than that seen in electromagnetic radiation, which, together with an absence of obvious signs of accretion, suggests that AR Sco is primarily spin-powered. Although the pulsations are driven by the white dwarf's spin, they mainly originate from the cool star. AR Sco's broadband spectrum is characteristic of synchrotron radiation, requiring relativistic electrons. These must either originate from near the white dwarf or be generated in situ at the M star through direct interaction with the white dwarf's magnetosphere.

Diverse Formation Mechanisms for Compact Galaxies

Science.gov (United States)

Kim, Jin-Ah; Paudel, Sanjaya; Yoon, Suk-Jin

2018-01-01

Compact, quenched galaxies such as M32 are unusual ones located off the mass - size scaling relation defined by normal galaxies. Still, their formation mechanisms remain unsolved. Here we investigate the evolution of ~100 compact, quenched galaxies at z = 0 identified in the Illustris cosmological simulation. We identify three ways for a galaxy to become a compact one and, often, multiple mechanisms operate in a combined manner. First, stripping is responsible for making about a third of compact galaxies. Stripping removes stars from galaxies, usually while keeping their sizes intact. About one third are galaxies that cease their growth early on after entering into more massive, gigantic halos. Finally, about half of compact galaxies, ~ 35 % of which turn out to undergo stripping, experience the compaction due to the highly centrally concentrated star formation. We discuss the evolutionary path of compact galaxies on the mass – size plane for each mechanism in a broader context of dwarf galaxy formation and evolution.

Gas-rich dwarfs and accretion phenomena in early-type galaxies

Science.gov (United States)

Silk, J.; Norman, C.

1979-01-01

An analysis is presented of the combined effects of cloud accretion and galactic winds and coronae. An accretion model is developed wherein gas-rich dwarf galaxies are accreted into galactic halos, which provides an adequate source of H I to account for observations of neutral gas in early-type galaxies. Accretion is found to fuel the wind, thereby regulating the accretion flow and yielding a time-dependent model for star formation, enrichment, and nuclear activity. The permissible parameter range for intergalactic gas clouds and galaxy groups is discussed, along with the frequency of gas-rich dwarfs and their large ratios of gas mass to luminosity. Also considered is the occurrence of gas stripping and the consequent formation of dwarf spheroidal systems that remain in the halo, and gas clouds that dissipate and suffer further infall. A cosmological implication of the model is that, because the characteristic time scale of a gas-rich dwarf galaxy to be accreted and lose its gas is comparable to a Hubble time, there may have been a far more extensive primordial distribution of such systems at earlier epochs.

Undergoing spherically symmetric steady-state accretion stability of white dwarfs

Energy Technology Data Exchange (ETDEWEB)

Sienkiewicz, R [Polska Akademia Nauk, Warsaw. N. Copernicus Astronomical Center

1980-01-01

Thermal and vibrational stabilities of accreting white dwarfs with steady-state nuclear burning were considered, assuming spherically symmetric accretion of the hydrogen-rich matter and using linear stability analysis. Almost all models with masses 0.2 M(sun) - 1.39 M(sun) were found to be unstable in some way. The type of instability expected to dominate is given as a function of the accretion rate. For most accretion rates it is the thermal instability. Oscillation periods are given for the models in which the vibrational instability is the most violent one. These periods are of the order of seconds or minutes. We expect that our stability analysis may suggest the cause of the variabilities of the hot components of some symbiotic stars, for a wide range of the accretion rates. In this case our models may serve as the initial conditions for evolutionary computations. The results predict that short-period oscillations should be observed in some hot nuclei of planetary nebulae.

A Be-type star with a black-hole companion.

Science.gov (United States)

Casares, J; Negueruela, I; Ribó, M; Ribas, I; Paredes, J M; Herrero, A; Simón-Díaz, S

2014-01-16

Stellar-mass black holes have all been discovered through X-ray emission, which arises from the accretion of gas from their binary companions (this gas is either stripped from low-mass stars or supplied as winds from massive ones). Binary evolution models also predict the existence of black holes accreting from the equatorial envelope of rapidly spinning Be-type stars (stars of the Be type are hot blue irregular variables showing characteristic spectral emission lines of hydrogen). Of the approximately 80 Be X-ray binaries known in the Galaxy, however, only pulsating neutron stars have been found as companions. A black hole was formally allowed as a solution for the companion to the Be star MWC 656 (ref. 5; also known as HD 215227), although that conclusion was based on a single radial velocity curve of the Be star, a mistaken spectral classification and rough estimates of the inclination angle. Here we report observations of an accretion disk line mirroring the orbit of MWC 656. This, together with an improved radial velocity curve of the Be star through fitting sharp Fe II profiles from the equatorial disk, and a refined Be classification (to that of a B1.5-B2 III star), indicates that a black hole of 3.8 to 6.9 solar masses orbits MWC 656, the candidate counterpart of the γ-ray source AGL J2241+4454 (refs 5, 6). The black hole is X-ray quiescent and fed by a radiatively inefficient accretion flow giving a luminosity less than 1.6 × 10(-7) times the Eddington luminosity. This implies that Be binaries with black-hole companions are difficult to detect in conventional X-ray surveys.

The low-mass stellar population in the young cluster Tr 37. Disk evolution, accretion, and environment

Science.gov (United States)

Sicilia-Aguilar, Aurora; Kim, Jinyoung Serena; Sobolev, Andrej; Getman, Konstantin; Henning, Thomas; Fang, Min

2013-11-01

Aims: We present a study of accretion and protoplanetary disks around M-type stars in the 4 Myr-old cluster Tr 37. With a well-studied solar-type population, Tr 37 is a benchmark for disk evolution. Methods: We used low-resolution spectroscopy to identify and classify 141 members (78 new ones) and 64 probable members, mostly M-type stars. Hα emission provides information about accretion. Optical, 2MASS, Spitzer, and WISE data are used to trace the spectral energy distributions (SEDs) and search for disks. We construct radiative transfer models to explore the structures of full-disks, pre-transition, transition, and dust-depleted disks. Results: Including the new members and the known solar-type stars, we confirm that a substantial fraction (~2/5) of disks show signs of evolution, either as radial dust evolution (transition/pre-transition disks) or as a more global evolution (with low small-dust masses, dust settling, and weak/absent accretion signatures). Accretion is strongly dependent on the SED type. About half of the transition objects are consistent with no accretion, and dust-depleted disks have weak (or undetectable) accretion signatures, especially among M-type stars. Conclusions: The analysis of accretion and disk structure suggests a parallel evolution of dust and gas. We find several distinct classes of evolved disks, based on SED type and accretion status, pointing to different disk dispersal mechanisms and probably different evolutionary paths. Dust depletion and opening of inner holes appear to be independent processes: most transition disks are not dust-depleted, and most dust-depleted disks do not require inner holes. The differences in disk structure between M-type and solar-type stars in Tr 37 (4 Myr old) are not as remarkable as in the young, sparse, Coronet cluster (1-2 Myr old), suggesting that other factors, like the environment/interactions in each cluster, are likely to play an important role in the disk evolution and dispersal. Finally, we

Magnetic fields of Herbig Ae/Be stars

Directory of Open Access Journals (Sweden)

Hubrig S.

2014-01-01

Full Text Available We report on the status of our spectropolarimetric studies of Herbig Ae/Be stars carried out during the last years. The magnetic field geometries of these stars, investigated with spectropolarimetric time series, can likely be described by centred dipoles with polar magnetic field strengths of several hundred Gauss. A number of Herbig Ae/Be stars with detected magnetic fields have recently been observed with X-shooter in the visible and the near-IR, as well as with the high-resolution near-IR spectrograph CRIRES. These observations are of great importance to understand the relation between the magnetic field topology and the physics of the accretion flow and the accretion disk gas emission.

On relativistic models of strange stars

Indian Academy of Sciences (India)

tractable models of superdense stars in equilibrium. Several aspects of physical relevance of compact star models, based on Vaidya–Tikekar ansatz, have been in- vestigated [7–10] by a number of workers. Mukherjee et al [11–13] indicated the possibility of using this set-up to describe models of the compact star like Her.