cools rapidly to the extent that the Jeans mass decreases by a factor of 10 6 , resulting in a cluster of 10 6 one solar mass stars. The process in our case requires that the factor of 10 6 is replaced investigates the accretion process. Section 6 dis� cusses how accretion can affect the final stellar ...

Mass accretion onto (proto-)stars at high accretion rates \\dot{M}_* > 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 ...

Timescale for runaway growth is shortened if accretion is seeded by a large body in a swarm of small planetesimals. Bodies of mass ~ 10^25 g scattered from the Jupiter-Saturn region are effective in nucleating formation of Uranus and Neptune.

Accretion is a pervasive process in astrophysics. Through accretion, protostars gather enough mass to initiate fusion, and small bodies gather enough mass to become planets. Accretion onto black holes is the second most efficient physical process presently known, allowing ...

In the ``competitive accretion" picture, the high-mass end of the stellar initial mass function (IMF) results from continuing accretion onto the central star from a gas reservoir. Most simulations to date have been conducted assuming formation in a cluster gravitational potential. In this contribution we report a ...

We have investigated how envelope pollution by icy planetesimals affects the critical core mass for gas giant formation and the gas accretion time-scales. In the core-accretion model, runaway gas accretion is triggered after a core reaches a critical core mass. All the previous studies on the ...

We have investigated how envelope pollution by icy planetesimals affects the critical core mass for gas giant formation and the gas accretion time-scales. In the core-accretion model, runaway gas accretion is triggered after a core reaches a critical core mass. All the previous studies on the ...

Numerical simulations of accretion of planetary embryos from small planetesimals are described. In the terrestrial region, runaway growth proceeds as a wave propagating outward, producing an 'oligarchy' of embryos. The efficiency of accretion, i.e. the mass loss due to fragmentation, depends on the initial size of ...

A model of a thin polytropic disk accretion onto a polytropic star through a polytropic boundary layer is analyzed and a one-parameter series of models is calculated with the stellar angular velocity varying from small to somewhat supercritical values along the series. It is demonstrated that there is no limit to the amount of mass ...

Mass accretion onto (proto-)stars at high accretion rates M-dot{sub *}> 10{sup -4} M{sub sun} yr{sup -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? " What is the structure of the inner disk? " How do accretion disks drive jets? ... IXO will reveal the physics of disk accretion, and low L_X ...

A classical nova model is evolved through a complete cycle, including accretion leading to outburst, mass loss, accretion again, ending in another outburst. Mass loss is found to occur in three different stages: shock ejection, continuous mass loss through an optically thick wind, and ...

Formation of massive stars by accretion requires a high accretion rate of \\dot{M}_* > 10^{-4} M_{&sun;} yr^{-1} to overcome the radiation pressure barrier of the forming stars. Here, we study evolution of protostars accreting at such high rates by solving the structure of the central star and the inner ...

A one-zone model is developed for analysis of properties of nuclear shell flashes on accreting degenerate dwarfs and neutron stars. The model provides a description of a steady-state nuclear burning and a linear stability analysis with a small number of algebraic equations. Time evolution of the accreted layer is described with two ...

The evolution of the magnetic field of an accreting magnetic white dwarf with an initially dipolar field at the surface has been studied for non-spherical accretion under simplifying assumptions. Accretion on to the polar regions tends to advect the field toward the stellar equator which is then buried. This tendency is countered by ...

The viscous model for accretion disks encounters difficulties both in providing a source of viscosity and in removing angular momentum from the disk outer edges where it is transported by the viscous stress. This paper investigates radiative tides as an alternative mechanism for accretion in disks in mass-transfer binary systems. We ...

A dark-matter-only Horizon Project simulation is used to investigate the environment and redshift dependences of accretion on to both haloes and subhaloes. These objects grow in the simulation via mergers and via accretion of diffuse non-halo material, and we measure the combined signal from these two modes of accretion. It is found ...

Context. Even though turbulent motions are found everywhere in astrophysical systems, the origin of this turbulence is poorly understood. When cosmic structures form, they grow in mass via accretion from their surrounding environment. Aims: We propose that accretion is able to drive internal turbulent motions in a wide range of ...

There are two dominant models of how stars form. Under gravitational collapse, star-forming molecular clumps, of typically hundreds to thousands of solar masses (M(o)), fragment into gaseous cores that subsequently collapse to make individual stars or small multiple systems. In contrast, competitive accretion theory suggests that at ...

We revisit the problem of low-mass pre-main-sequence stellar evolution and its observational consequences for where stars fall on the Hertzsprung-Russell diagram (HRD). In contrast to most previous work, our models follow stars as they grow from small masses via accretion, and we perform a systematic study of how ...

). The fate of the ejected mass depends on the primary shape and the coefficient of restitution (the ratio, namely w coefficient of restitution, the satellites accrete more than 90% of all of satellite formation declined as the coefficient of restitution was increased, until, above a value of 0

The rotation period of classical T Tauri stars (CTTS) represents a longstanding puzzle. While young low-mass stars show a wide range of rotation periods, many CTTS are slow rotators, spinning at a small fraction of breakup, and their rotation period does not seem to shorten, despite the fact that they are actively accreting and ...

Accreting white dwarfs in cataclysmic variables (CVs) have also recently been shown to exhibit non-radial pulsations similar to their non-interacting counterparts; GW Librae was the first accreting pulsator discovered in 1998 (Warner & van Zyl 1998; van Zyl et al. 2000, 2004). When the orbital period of a CV is close to the evolutionary period minimum, ...

This paper studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing WKB techniques, we obtain the eigen frequencies and eigenfunctions of the modes for different values of the mass and ...

formation of massive stars via accretion. A necessary condition for accre- ... constraints on the accretion rate necessary to produce high mass stars and ...

Galactic nuclei should contain a cluster of stars and compact objects in the vicinity of the central supermassive black hole due to stellar evolution, minor mergers and gravitational dynamical friction. By analogy with protoplanetary migration, nuclear cluster objects (NCOs) can migrate in the accretion discs that power active galactic nuclei (AGN) by exchanging angular ...

Massive stars produce so much light that the spherically averaged radiation pressure exerted on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional adiation-hydrodynamic simulations of the collapse of a massive prestellar core which include ...

We study the growth of galaxy masses, via gas accretion and galaxy mergers. We introduce a toy model that describes (in a single equation) how much baryonic mass is accreted and retained into galaxies as a function of halo mass and redshift. In our model, the evolution of the baryons differs ...

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

From observations collected with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope (CFHT) and with the NARVAL spectropolarimeter at the T�lescope Bernard Lyot (TBL), we report the detection of Zeeman signatures on the prototypical classical T Tauri star AA Tau, both in photospheric lines and accretion-powered emission lines. Using time series of ...

We describe the first three-dimensional simulation of the gravitational collapse of a massive, rotating molecular cloud that includes heating by both non-ionizing and ionizing radiation. These models were performed with the FLASH code, incorporating a hybrid, long characteristic, ray-tracing technique. We find that as the first protostars gain sufficient mass to ionize the ...

We describe the first three-dimensional simulation of the gravitational collapse of a massive, rotating molecular cloud that includes heating by both non-ionizing and ionizing radiation. These models were performed with the FLASH code, incorporating a hybrid, long characteristic, ray-tracing technique. We find that as the first protostars gain sufficient mass to ionize the ...

Massive stars produce so much light that the radiation pressure they exert on the gas and dust around them is stronger than their gravitational attraction, a condition that has long been expected to prevent them from growing by accretion. We present three-dimensional radiation-hydrodynamic simulations of the collapse of a massive prestellar core and find that radiation ...

We have studied the properties of the stellar populations in the field of the NGC 346 cluster in the Small Magellanic Cloud, using a novel self-consistent method that allows us to reliably identify pre-main-sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The method does not require spectroscopy and ...

We report on the current phase of our study of rotating accretion flows onto black holes. We consider hydrodynamical (HD) accretion flows with a spherically symmetric density distribution at the outer boundary but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum. We study ...

The accretion process onto spinning objects in Kerr spacetimes is studied with numerical simulations. Our results show that accretion onto compact objects with Kerr parameter (characterizing the spin) |a|<M and |a|>M is very different. In the superspinning case, for |a| moderately larger than M, the accretion onto the ...

into the properties of the mass accretion history that in uence the #12;nal pro#12;le. Assuming that the velocities of CDM. Using an ensemble of realistic mass accretion histories, we show that the model predictions the density pro#12;les of CDM halos, further moti- vated by the recent #12;nding that the ...

The spin behaviour of a strongly magnetic star accreting from an internally disrupted disc is considered. The torques are calculated due to the interaction of the star with the disc and the accretion flow, together with the modified structure of the disc. The torques depend on the spin period of the star, and a stable equilibrium period can be found at ...

There is a relation between radio and X-ray luminosities and SMBH mass: the Fundamental Plane. If radio and X-ray are proxies for jet and accretion power, a broad link between them can be inferred. The relation may be used as a proxy for M_BH measurement, but the scatter is big (0.75 dex; sample of 18). Without Seyferts it is tighter than the M-sigma ...

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 ...

By analogy to the different accretion states observed in black-hole X-ray binaries (BHXBs), it appears plausible that accretion disks in active galactic nuclei (AGN) undergo a state transition between a radiatively efficient and inefficient accretion flow. If the radiative efficiency changes at some critical ...

Aug 4, 2011 ... Propagating fluctuations in the mass accretion rate of a precessing flow as a power spectral model for black hole binaries ...

This dissertation addresses problems pertaining to the mechanics of accreted planetary bodies. Roughly 4.6 billion years ago, all mass was in form of dust and gas that orbited the sun in a large cloud called the solar nebula. The growth of kilometer-sized objects from sub-micron sized dust grains occurred by the collisional and/or gravitational evolution ...

The UBVR light curves of the massive eclipsing binary RY Sct, obtained at the Maidanak Observatory from 1979 to 1992, have been re-analyzed in order to prove the hypothesis of the presence of an accretion disk in the system. This hypothesis is supported by a new spectroscopic study of Grundstrom et al., and by a specific light-curve shape exhibiting a slight asymmetry around ...

The final composition of giant planets formed as a result of gravitational instability in the disk gas depends on their ability to capture solid material (planetesimals) during their �pre-collapse� stage, when they are extended and cold, and contracting quasi-statically. The duration of the pre-collapse stage is inversely proportional roughly to the square of the planetary ...

The formation and evolution of a circumstellar disk in magnetized cloud cores are investigated from a prestellar core stage until ~104 yr after protostar formation. In the circumstellar disk, fragmentation first occurs due to gravitational instability in a magnetically inactive region, and substellar-mass objects appear. The substellar-mass objects lose ...

3D Magnetohydrodynamic simulations show that when matter accretes onto neutron stars, in particular if the misalignment angle is small, it does not constantly fall at a fixed spot. Instead, the location at which matter reaches the star moves. These moving hot spots can be produced both during stable accretion, where matter falls near ...

Based on the numerical solution of the time-dependent relativistic Euler equations on to a fixed Schwarzschild background space-time, we estimate the accretion rate of radial flow towards the horizon of a test perfect fluid obeying an ideal gas equation of state. We explore the accretion rate in terms of the initial density of the fluid for various values ...

Hot intracluster gas associated with clusters of galaxies can cool and accrete into stationary or slowly moving giant galaxies near the cluster center. Enhanced soft X-ray emission and faint extranucler optical emission lines observed in M87 (in the Virgo cluster) and NGC 1275 (in the Perseus cluster) strongly suggest that this process is occurring. In both cases, however, the ...

We report here the first results of a multi-wavelength campaign focusing on magnetospheric accretion processes within the close binary system V4046 Sgr, hosting two partly convective classical T Tauri stars of masses ?0.9 M&sun; and age ?12 Myr. In this paper, we present time-resolved spectropolarimetric observations collected in 2009 September with ...

New numerical simulations of the evolution and formation of Jupiter were computed. Earlier studies of the core instability model demonstrated that it was possible to form Jupiter with a solid core of 10 to 30 M? within the lifetime of the protoplanetary disk (about 107 years). However, recent interior models of Jupiter suggest a core mass of about 5 M? . We examine the ...

We present the results of axisymmetric, time-dependent magnetohydrodynamic simulations of accretion flows around black holes. The calculations begin from a rotationally supported thick torus which contains a weak poloidal field. Accretion is produced by growth and saturation of the magnetorotational instability (MRI) provided that the wavelength of the ...

Dec 6, 2007 ... "Our computer simulations show that the ridges must have accreted rapidly when Saturn's rings were thin, forming small accretion disks ...

The report deals with the question of whether mass accretion on a star can cause pulsational instability by itself. The principal result is a formula for the effect of accretion on the growth (or decay) rate of a radial pulsation mode. Application of this...

Derivations are made for the mass and the mass-turnover time scale of an accretion disk as a function of the accretion rate, the observed disk radius, the non-viscous disk radius, and two parameters. These parameters depend on the effectiveness of viscosity and tidal angular momentum loss. Application is made to DQ ...

Derivations are made for the mass and the mass-turnover time scale of an accretion disk as a function of the accretion rate, the observed disk radius, the non-viscous disk radius, and two parameters. These parameters depend on the effectiveness of viscosi...

A number of recently discovered extrasolar planet candidates have surprisingly small orbits, which may indicate that considerable orbital migration takes place in protoplanetary systems. A natural consequence of orbital migration is for a series of planets to be accreted, destroyed, and then thoroughly mixed into the convective envelope of the central ...

A star acquires much of its mass by accreting material from a disk. Accretion is probably not continuous but episodic. We have developed a method to include the effects of episodic accretion in simulations of star formation. Episodic accretion results in bursts of radiative feedback, during ...

Photoelectric U, B, V photometry of eclipses in the close binary system IP Peg, obtained mainly with the 6-m telescope, yields brightness profiles along the edgewise accretion disk. The radiation is dominated by a hot spot whose brightness and location in the disk depend on the state of activity in the binary. In the quiescent phase the disk is surrounded by a dark, extended ...

We study the behaviour of multiple radiative-cooling algorithms implemented in seven semi-analytic models (SAMs) of galaxy formation, including a new model we propose in this paper. We use versions of the models without feedback and apply them to dark matter haloes growing in a cosmological context, which have final virial masses that range from 1011 to 1014 M?. First, using ...

We study the behaviour of multiple radiative-cooling algorithms implemented in seven semi-analytic models (SAMs) of galaxy formation, including a new model we propose in this paper. We use versions of the models without feedback and apply them to dark matter haloes growing in a cosmological context, which have final virial masses that range from 1011 to 1014 M?. First, using ...

A unified theory of active galactic nuclei, based on the physics of accretion flows around massive black holes, is described. The accretion modes are dependent on the ratio of accretion rate to the mass of the hole. The spectral characteristics and radiative efficiency for the ion tori, thin disks, and radiation ...

The giant planets' solid cores must have formed prior to the dispersal of the primordial solar nebula, to allow the capture of their massive, gaseous envelopes from the nebula. Recent observations of disks of dust surrounding nearby solar-like stars lead to estimates of nebula lifetimes at 106 to 107 years. Thus, theories of solid particle accretion must explain how the solid ...

In this paper we continue our study on the accretion process onto superspinning Kerr objects with no event horizon (super-spinars). We discuss the counterpart of the Bondi accretion onto black holes. We first report the results of our numerical simulations. We found a quasi-steady-state configuration for any choice of the parameters of our model. The most ...

Using Spitzer Space Telescope photometric observations of the eclipsing, interacting binary WZ Sge, we have discovered that the accretion disk is far more complex than previously believed. Our 4.5 and 8 ?m time series observations reveal that the well-known gaseous accretion disk is surrounded by an asymmetric disk of dusty material with a radius ...

It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albar�de paints a different picture, proposing that the Earth first formed as a dry planet which, like the Moon, was devoid of volatile constituents. He suggests that the Earth's complement of volatile elements was only established later, by the addition of a ...

We consider the effects of ``photon bubble'' shock trains on the vertical structure of radiation pressure-dominated accretion disks. These density inhomogeneities are expected to develop spontaneously in radiation-dominated accretion disks where magnetic pressure exceeds gas pressure, even in the presence of magnetorotational instability (MRI). They ...

The final fate of accreting C + O white dwarfs is either thermonuclear explosion or collapse, if the white dwarf mass grows to the Chandrasekhar mass. We discuss how the fate depends on the initial mass, age, composition of the white dwarf and the mass ac...

We assembled a sample of broad-line and narrow-line active galactic nuclei (AGNs) observed by ASCA, the excess variances of which have been determined. The central black hole masses in this sample can be obtained from the reverberation mapping method and the width of the H? emission line. Using the black hole masses and the bolometric luminosity, the ...

We report on simulations of the formation of the first stars in the universe, where we identify regions of hot atomic gas (f{sub H{sub 2}}< 10{sup -6}) at densities above 10{sup -14} g cm{sup -3}, heated to temperatures ranging between 3000 and 8000 K. Within this temperature range atomic hydrogen is unable to cool effectively. We describe the kinetic and thermal characteristics of these ...

We propose a new method to explore the candidate super-Eddington active galactic nuclei (AGNs). We examine the properties of infrared (IR) emission from the inner edge of the dusty torus in AGNs, which are powered by super- or sub-Eddington accretion flows around black holes, by considering the dependence of the polar angle on the radiation flux of ...

Massive planetary cores (~10 Earth masses) trigger rapid gas accretion to form gas giant planets such as Jupiter and Saturn. We investigate the core growth and the possibilities for cores to reach such a critical core mass. At the late stage, planetary cores grow through collisions with small planetesimals. ...

We consider spherically symmetric accretion of material from an initially homogeneous, uniformly expanding medium onto a Newtonian point mass M. The gas is assumed to evolve adiabatically with a constant adiabatic index F, which we vary over the range ? ? [1, 5/3]. We use a one-dimensional Lagrangian code to follow the spherical infall of material as a ...

ways that can profoundly affect the regional mass distribu- ...... to turbulence in hydrodynamic accretion: Lagrangian analysis of energy growth. Astrophys. ...

Accretion Disk Coronae were first suggested to explain the incomplete eclipses and shallow profiles observed from some Low Mass X-ray Binaries (White and ...

Solution for a stationary spherically symmetric accretion of the relativistic perfect fluid with an equation of state p(rho) onto the Schwarzschild black hole is presented. This solution is a generalization of Michel solution and applicable to the problem of dark energy accretion. It is shown that accretion of phantom energy is ...

Accretion onto and nova events on the surface of a degenerate cold one-solar-mass iron white dwarf are investigated. It is found that, during accretion, hydrogen diffuses deeper into the core than its CNO catalysts, suggesting that the thermonuclear runaway occurs not at the innermost radius achieved by the hydrogen but farther out ...

We explore thermal X-ray iron line emission from the galactic X-ray binary GX 339-4 in the off state, using the models of the advection-dominated accretion flow (ADAF) without or with outflows. The equivalent widths of hydrogen-like and helium-like thermal iron lines are calculated with different model parameters including viscosity parameter ?, mass ...

We consider the spin angular momentum evolution of the accreting components of Algol-type binary stars. In wider Algols the accretion is through a disc so that the accreted material can transfer enough angular momentum to the gainer that material at its equator should be spinning at breakup. We demonstrate that even a ...

We investigate the critical core mass and the envelope growth timescale, assuming grain-free envelopes, to examine how small cores are allowed to form gas giants in the framework of the core-accretion model. This is motivated by a theoretical dilemma concerning Jupiter formation: modelings of Jupiter's interior suggest that it ...

The main physical phenomena which occur on accretion disks, are discussed. A numerical simulation of a three dimensional accretion disk model around a white dwarf in a binary system composed of two equal mass stars is developed. The smoothed particle hydr...

We consider time-dependent accretion onto magnetized white dwarfs. A detailed description of a numerical method of solution to the hydrodynamical equations is given. The postshock flow is cooled by optically thin bremsstrahlung and is thermally unstable. As a result the shock height undergoes periodic oscillations. We consider the properties of this oscillation as a function ...

Moderate exercise is an ineffective strategy to build bone mass. The authors present data demonstrating that allowing bone to rest between each load cycle transforms low- and moderate-magnitude mechanical loading into a signal that potently induces bone accretion. They hypothesize that the osteogenic nature of rest-inserted loading arises by enabling ...

Basic dynamical mechanisms that produce an amplification of the accretion rate of gas clouds into the central regions of barred galaxies, and their subsequent effects on the evolution of barred galaxies, are discussed. Weakly dissipative orbits, representing gas clouds, are computed in a barred galaxy model with a central mass concentration such as a black ...

We present J1145-0033, a candidate for the most distant (z=2.055) lobe-dominated giant radio quasar, with a projected linear size of 1.34 Mpc. This quasar has both FRII-type radio morphology and broad absorption lines in its optical spectrum. Some physical characteristics (e.g., black hole mass, accretion rate, equipartition magnetic field, energy density ...

We deal with phantom energy accretion onto the Schwarzschild de-Sitter black hole. The energy flux conservation, relativistic Bernoulli equation and mass flux conservation equation are formulated to discuss the phantom accretion. We discuss the conditions for critical accretion. It is found that the ...

High resolution infrared spectroscopy is a very valuable tool for the study of the formation of low mass and very low mass stars. At the stage in which these objects are already visible, they still undergo strong mass accretion and mass loss processes. Many of the features observed on these ...

A new computational scheme is developed to study gas accretion from a circumbinary disk. The scheme decomposes the gas velocity into two components one of which denotes the Keplerian rotation and the other of which does the deviation from it. This scheme enables us to solve the centrifugal balance of a gas disk against gravity with better accuracy, since the former inertia ...

A new computational scheme is developed to study gas accretion from a circumbinary disk. The scheme decomposes the gas velocity into two components one of which denotes the Keplerian rotation and the other of which does the deviation from it. This scheme enables us to solve the centrifugal balance of a gas disk against gravity with better accuracy, since the former inertia ...

The far future provides an excellent laboratory for studying cosmological structures. The eventual dominance of the cosmological constant causes the universe to enter a phase of exponential deSitter expansion during which dark matter halos are allowed to relax without interruptions from mass accretion. Using of suite of N-body simulations evolved to scale ...

Kepler has found hundreds of Neptune-size (2-6 R ?) planet candidates within 0.5 AU of their stars. The nature of the vast majority of these planets is not known because their masses have not been measured. Using theoretical models of planet formation, evolution, and structure, we explore the range of minimum plausible masses for low-density exo-Neptunes. ...

We present high-resolution 3D smoothed particle hydrodynamics simulations of the formation and evolution of protostellar discs in a turbulent molecular cloud. Using a piecewise polytropic equation of state, we perform two sets of simulations. In both cases, we find that isolated systems undergo a fundamentally different evolution than members of binary or multiple systems. When formed, isolated ...

Five-color photometric observations of RZ Oph have been obtained from 1981 through 1986, in an effort to deduce the properties of the accretion disk in this long-period Algol. The partial eclipse of the disk by the cool star, and the partial occultation of the cool star by the disk, have both been observed in some detail. A simple gravitationally stratified model of the disk ...

We investigate the formation of terrestrial planets in the late stage of planetary formation using two-planet model. At that time, the protostar has formed for about 3 Myr and the gas disk has dissipated. In the model, the perturbations from Jupiter and Saturn are considered. We also consider variations of the mass of outer planet, and the initial eccentricities and ...

We examine the role of the accretion disk in the steady-burning white dwarf model for supersoft sources. The accretion luminosity of the disk is quite small compared to the nuclear burning luminosity of the central source. Thus, in contrast to standard ac...

Determining the process(es) by which brown dwarfs form is key to understanding their intrinsic nature. If their origins are within circumstellar disks they are akin to giant planets. If, on the other hand, they coalesce from molecular cloud cores, then they share a common lineage with low mass stars. These two mechanisms can be distinguished by investigation of young (< ...

Marsden, Lingenfelter, Rothschild & Higdon have given arguments against the magnetar model for Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs), as forcefully advocated by R. Rothschild at this meeting. We critique these arguments, showing: (1) The claim that SGRs and AXPs are born in unusually dense regions of the ISM is not supported in any compelling way by observations. (2) ...

The effect of rapid accretion on a moderate-mass stellar model is studied. The surface boundary condition is examined for two modes of accretion - disk and direct impact. It is found that the effective temperature must be increased for disk accretion but not for direct-impact accretion. In the ...

T Tauri stars (TTS) are low-mass pre-main-sequence stars that are accreting mass from the surrounding disc. The hotspots detected in some of them are probably heated by the release of gravitational energy in the accretion of the disc material on to the star. In this work we study the UV spectrum of the hotspot ...

Using an accretion-disk model, accretion disk luminosities are calculated for a grid of black hole masses and accretion rates. It is shown that, as the black-hole mass increases with time, the monochromatic luminosity at a given frequency first increases and then decreases rapidly as this ...

We use fractional integrals to generalize the description of hydrodynamic accretion in fractal media. The fractional continuous medium model allows the generalization of the equations of balance of mass density and momentum density. These make it possible to consider the general case of spherical hydrodynamic accretion on to a ...

The highest efficiency of converting rest mass into energy by accreting matter into a Kerr black hole is approximately 31% (Thorne 1974). We propose a new process in which periods of accretion from a thin disk, and the associated spin-up of the black hole, alternate with the periods of no accretion and magnetic ...

Extrasolar debris disks that are bright enough to be observed are dense enough to be collision-dominated; i.e., the small grains that produce their infrared excess have collisional lifetimes shorter than their Poynting-Robertson decay times. This paper describes a numerical code for the modeling of such disks, including accretion and gravitational stirring ...

We have used a model of magnetic accretion to investigate the rotational equilibria of magnetic cataclysmic variables (mCVs). The results of our numerical simulations demonstrate that there is a range of parameter space in the Pspin/Porb versus ?1 plane at which rotational equilibrium occurs. This has allowed us to calculate the theoretical histogram describing the ...

A detailed nonlinear analysis of the internal structure of spherical, charged black holes that are accreting scalar matter is performed in the framework of the Brans-Dicke theory of gravity. We choose the lowest value of the Brans-Dicke parameter that is compatible with observational constraints. First, the homogeneous approximation is used. It indicates that ...

The present study of hypercritical accretion flows onto neutron stars considers steady-state, spherically symmetric flows whose accretion rate range is characterized by the carrying away of gravitational-accretion energy by neutrinos. The models used encompass pair production, radiation diffusion, and general relativistic effects. ...

The collapse of massive molecular clumps can produce high mass stars, but the evolution is not simply a scaled-up version of low mass star formation. Outflows and radiative effects strongly hinder the formation of massive stars via accretion. A necessary condition for accretion growth of a hydrostatic object up to ...

A small planet is not necessarily a terrestrial planet. Planets that form beyond the snow line with too little mass to seed rapid gas accretion (<~10 M?) should be rich in volatile ices like H2O and NH3. Some of these planets should migrate inward by interacting with a circumstellar disk or with other planets. Such objects can ...

Spectroscopic and photometric observations of LMC X-2 reveal the source to be an X-ray binary with a relatively long orbital period, probably 12.5 days. It appears to be a partially eclipsing system. It is one of a small subclass of low-mass X-ray binaries with longer orbital periods and higher X-ray luminosity than average, which contain a compact object ...

The evolution of the primordial solar nebula under the action of convectively driven turbulent viscous stress is studied numerically under various physical conditions. The mass of the nebula is assumed to be small compared with that of the central Sun, and the nebula is assumed to be in pressure equilibrium in the vertical direction. It is found that the ...

The outer parts of standard steady-state accretion discs around quasi-stellar objects (QSOs) are prone to self-gravity, and they might be expected to fragment into stars rather than feed the central black hole. Possible solutions to this well-known problem are examined with an emphasis on general dynamic constraints. Irradiation by the QSO is insufficient for stability even if ...

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 ...

When preplanetary bodies reach proportions of �1 km or larger in size, their accretion rate is enhanced due to gravitational focusing (GF). We have developed a new numerical model to calculate the collisional evolution of the gravitationally-enhanced growth stage. The numerical model is novel as it attempts to preserve the individual particle nature of the bodies (like ...

The influence of gas accretion in determining the shape of the initial stellar mass function is investigated. Stars are assumed to form from protostellar condensations which, having fragmented out of a gas cloud, begin to accrete material from their environment. Within protocluster clouds this accretion phase is ...

Spectral line and continuum observations of the ionized and molecular gas in G20.08-0.14 N explore the dynamics of accretion over a range of spatial scales in this massive star-forming region. Very Large Array (VLA) observations of NH{sub 3} at 4'' angular resolution show a large-scale (0.5 pc) molecular accretion flow around and into a ...

Massive stars influence the surrounding universe far out of proportion to their numbers through ionizing radiation, supernova explosions, and heavy element production. Their formation requires the collapse of massive interstellar gas clouds with very high accretion rates. We discuss results from the first three-dimensional simulations of the gravitational collapse of a ...

The core accretion mechanism for gas giant planet formation requires multiple-Earth-mass protoplanets to form in the presence of the disk gas, i.e., prior to the dispersal of the protoplanetary disk. Such protoplanets are subject to rapid orbital migration through their gravitational interactions with the disk. Studies of the interactions of ...

Spherical (nonrotating) accretion flows with small-scale magnetic fields have been investigated using three-dimensional, time-dependent MHD simulations. These simulations have been designed to model high-resolution (quasi-) steady accretion flows in a wedge computational domain that represents a small fraction of ...

Terrestrial planets, with silicate mantles and metallic cores, are likely to obtain water and carbon compounds during accretion. Here I examine the conditions that allow early formation of a surface water ocean (simultaneous with cooling to clement surface conditions), and the timeline of degassing the planetary interior into the atmosphere. The greatest fraction of a planet's ...

A hydrogen atmosphere of rocky planets is a natural by-product of planet formation in the context of the core accretion model. While such a hydrogen atmosphere makes up only a small fraction of the planet's total mass compared to gas giants' hydrogen envelopes, it has a great influence on planetary surface environments: It is capable ...

The implications and limitations of program ACRETE were tested. The program is a scheme based on Newtonian physics and accretion with unit sticking efficiency, devised to simulate the origin of the planets. The dependence of the results on a variety of radial and vertical density distribution laws, the ratio of gas to dust in the solar nebula, the total nebular ...

We explore the effects of small-scale structure on the formation and equilibrium of dark matter halos in a universe dominated by vacuum energy. We present the results of a suite of four N-body simulations, two with a ?CDM initial power spectrum and two with WDM-like spectra that suppress the early formation of small structures. All simulations are run into ...

The mass accretion rate of transonic spherical accretion flow onto compact objects such as black holes is known as the Bondi accretion rate, which is determined only by the density and the temperature of gas at the outer boundary. A rotating accretion flow has angular momentum, which modifies ...

The kinematics of gas seen at larger galactic latitudes is reviewed. The systematics of the radial motions are presented along with simple experiments, and from this combination one can conclude that overall the halo gas is falling onto the Milky Way (MW). Gas in the halo of the MW has various possible origins. These included thus far the ``galactic fountain'' and the ...

In the favoured core-accretion model of formation of planetary systems, solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars (the most common stars in our Galaxy), this model favours the formation of Earth-mass (M(o)) to ...

During mass exchange in binary systems the star which is receiving mass increases its volume. This effect is estimated by evolutionary calculations for different (constant) accretion rates. It is shown the chance that the two stars do not come in contact ...

We compare observed and simulated images of the accretion flows associated with high mass star formation in the regions G10.6--0.4 and DR21. We describe, as a result of the comparison, the temperature, density, and velocity fields. Our results indicate th...

The regular satellites of the giant planets are believed to have finished their accretion concurrent with the planets, about 4.5 Gyr ago. A population of Saturn's small moons orbiting just outside the main rings are dynamically young (less than 10(7) yr old), which is inconsistent with the formation timescale for the regular satellites. They are also ...

Our adaptive optics observations of nearby AGN at spatial resolutions as small as 0.?085 show strong evidence for recent, but no longer active, nuclear star formation. We begin by describing observations that highlight two contrasting methods by which gas can flow into the central tens of parsecs. Gas accumulation in this region will inevitably lead to a starburst, and we ...

A small fraction of AGN emit powerful radio waves from component structures (cores jets lobes) with sizes ranging from pc to Mpc. The ultimate power source of these radio-loud AGNs (RLAGNs) is most probably the gravitational accretion of galactic stars dust and gas onto central massive black holes (MBHs) imprisoning 106 to 1010 solar ...

The explosion of a supernova forms because of the collapse to a neutron star. In addition an explosion requires that a region of relatively high entropy be in contact with the neutron star and persisting for a relatively protracted period of time. The high entropy region ensures that the maximum temperature in contact with the neutron star and in hydrostatic equilibrium is less than some maximum. ...

We present initial results of a Spitzer Infrared Spectrograph (IRS) low resolution, spectroscopic survey of 29 infrared excess members of the 5 Myr old Upper Scorpius OB Association. The excess stars were identified by the Spitzer 4.5, 8.0, and 16.0 ?m photometric survey of Carpenter et al., which examined more than 200 suspected members of the association. The more massive stars (B and A spectral ...

The final stage in the formation of terrestrial planets consists of the accumulation of �1000-km �planetary embryos� and a swarm of billions of 1 10 km �planetesimals.� During this process, water-rich material is accreted by the terrestrial planets via impacts of water-rich bodies from beyond roughly 2.5 AU. We present results from five high-resolution dynamical ...

We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion rate corresponds to ...

We present the first results of a global axisymmetric simulation of accretion on to rotating magnetized stars from a turbulent accretion disc, where the turbulence is driven by the magnetorotational instability (MRI). We observed that the angular momentum is transported outwards by the magnetic stress and accretion rate corresponds to ...

... If the mass transfer rate is below a certain limit, the accretion disk is unstable and undergoes brightness cycles (outbursts), and if above the limit, the ...

A numerical study of the time variation of the angular momentum and mass capture rates for a central

It is shown here that the behavior of nuclear burning on accreting neutron stars depends on the mass

Sep 4, 1998 ... This is because highly stripped ions are transparent to the ... that when the source is in the high luminosity state the accretion is ...

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 ...

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 ...

We examine whether the accretion of dark matter onto neutron stars could ever have any visible external effects. Captured dark matter which subsequently annihilates will heat the neutron stars, although it seems the effect will be too small to heat close neutron stars at an observable rate while those at the galactic center are obscured by dust. ...

Superspinars are ultracompact objects whose mass M and angular momentum J violate the Kerr bound (cJ/GM^2>1). Recent studies analyzed the observable consequences of gravitational lensing and accretion around superspinars in astrophysical scenarios. In this paper we investigate the dynamical stability of superspinars to gravitational perturbations, ...

Superspinars are ultracompact objects whose mass M and angular momentum J violate the Kerr bound (cJ/GM2>1). Recent studies analyzed the observable consequences of gravitational lensing and accretion around superspinars in astrophysical scenarios. In this paper we investigate the dynamical stability of superspinars to gravitational perturbations, ...

We have carried out detailed numerical calculations of high-harmonic cyclotron emission from a hot plasma, and from these calculations we have developed at self-consistent, quantitative model of X-ray and UV emission from accreting magnetic degenerate dwards. We find that such stars are typically strong UV sources, and emit only at small fraction of their ...

Superspinars are ultracompact objects whose mass M and angular momentum J violate the Kerr bound (cJ/GM{sup 2}>1). Recent studies analyzed the observable consequences of gravitational lensing and accretion around superspinars in astrophysical scenarios. In this paper we investigate the dynamical stability of superspinars to gravitational ...

Planets are believed to have formed through the accumulation of a large number of small bodies. In the case of the gas-giant planets Jupiter and Saturn, they accreted a significant amount of gas directly from the protosolar nebula after accumulating solid cores of about 5-15 Earth masses. Such models, however, have been unable to ...

Narrow Line Seyfert 1 galaxies are among the most interesting class of active nuclei. Their observational properties suggest that they are high accretion rate systems probably associated with relatively small back hole masses with compared to classical Seyfert 1 galaxies. However, the number of known NLSy1 detected above 10 keV is ...

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 ...

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 ...

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 ...

On the origin of the Trojan asteroids: Effects of Jupiter's mass accretion and radial migration illustrate the effects of Jupiter's accretion of nebular gas and the planet's radial migration on its Trojan that Jupiter's thirty�fold growth from a 10M \\Phi core to its present mass causes the libration amplitudes

High spectral resolution observations of the massive young star cluster in the filaments of NGC 1275 reveal that the associated H-alpha emission line has a velocity width comparable to, or somewhat less than, that of H-alpha in typical giant H II regions of similar emission-line luminosity. If this linewidth reflects virial motions of matter within the star-forming region, as has been suggested ...

We describe an updated version of our hybrid N-body-coagulation code for planet formation. In addition to the features of our 2006-2008 code, our treatment now includes algorithms for the one-dimensional evolution of the viscous disk, the accretion of small particles in planetary atmospheres, gas accretion onto massive cores, and the ...

We present the first study of early stellar evolution with ``cloud'' initial conditions utilizing a system of equations that comprises a solar model solution. All previous studies of protostellar collapse either make numerous assumptions specifically tailored for different parts of the flow and different parts of the evolution or they do not reach the pre-main sequence phase. We calculate the ...

We present a Chandra observation of the only low redshift, z = 0.299, galaxy cluster to contain a highly luminous radio-quiet quasar, H1821+643. By simulating the quasar point spread function, we subtract the quasar contribution from the cluster core and determine the physical properties of the cluster gas down to 3arcsec (15kpc) from the point source. The temperature of the cluster gas decreases ...

We present the first detailed phase-resolved spectral analysis of a joint Chandra High-Energy Transmission Grating Spectrometer and Rossi X-ray Timing Explorer observation of the ? variability class in the microquasar GRS 1915+105. The ? cycle displays a high-amplitude, double-peaked flare that recurs roughly every 50 s and is sometimes referred to as the "heartbeat" oscillation. The spectral and ...

The relationships between the effective surface (T_eff_) and internal temperatures of neutron stars (NSs) with and without accreted envelopes are calculated for T_eff_>5x10^4^K using new data on the equation of state and opacities in the outer NS layers. We examine various models of accreted layers (H, He, C, O shells produced by nuclear transformations ...

This article summarizes the processes of high-energy emission in young stellar objects. Stars of spectral type A and B are called Herbig Ae/Be (HAeBe) stars in this stage, all later spectral types are termed classical T Tauri stars (CTTS). Both types are studied by high-resolution X-ray and UV spectroscopy and modeling. Three mechanisms contribute to the high-energy emission from CTTS: 1) CTTS ...

Dec 6, 2007 ... "Our computer simulations show that the ridges must have accreted rapidly when Saturn's rings were thin, forming small accretion disks ...

We have constructed numerically global solutions of advective accretion disks around black holes that describe a continuous transition between the effectively optically thick outer and optically thin inner disk regions. We have concentrated on models of accretion flows with large mass accretion rates, and we have ...

I present H- and K-band light curves of the dwarf nova cataclysmic variable (CV), IP Peg, and the novalike CV, RW Tri, and an H-band light curve of the novalike CV, SW Sex. All three systems showed contributions from the late-type secondary star and the accretion disk, including a primary eclipse of the accretion disk by the secondary star and a secondary ...

We use a semi-analytic model for disk galaxies to explore the origin of the time evolution and small scatter of the galaxy SFR sequence --- the tight correlation between star-formation rate (SFR) and stellar mass (M*). The steep decline of SFR from z 2 to the present, at fixed M*, is a consequence of the following: First, disk galaxies are in a steady ...

Star formation is such a complex process that accurate numerical tools are needed to quantitatively examine the mass distribution and accretion of fragments in collapsing, turbulent, magnetized gas clouds. To enable a numerical treatment of this regime, we implemented sink particles in the adaptive mesh refinement (AMR) hydrodynamics code FLASH. Sink ...

The Mira AB system is a nearby ( 107 pc) example of a wind accreting binary star system. In this class of system, the wind from a mass-losing red giant star is accreted onto a companion, as indicated by emission related to an accretion shock at ultraviolet and X-ray wavelengths. Imaging the companion is difficult ...

Using a version of the ZEUS code, we carry out two-dimensional simulations of self-gravitating shearing sheets, with application to QSO accretion disks at a few thousand Schwarzschild radii, corresponding to a few hundredths of a parsec for a 100-million-solar-mass black hole. Radiation pressure and optically thick radiative cooling are implemented via ...

Muscle mass is critical for athletic performance and, perhaps more importantly for most, health and survival. The metabolic basis for a change in muscle mass is an increase in net muscle protein balance (termed NBAL). NBAL is the difference between MPS (muscle protein synthesis) and MPB (muscle protein breakdown). Thus an increase in MPS and/or a decrease ...

The favoured theoretical explanation for planetary systems formation is the core-accretion model in which solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars, the most common stars of our Galaxy, this model favours the formation of Earth- to ...

Recent work using virial mass estimates and the quasar mass-luminosity plane has yielded several new puzzles regarding quasar accretion, including a sub-Eddington boundary (SEB) on most quasar accretion, near-independence of the accretion rate from properties of the host galaxy, and a cosmic ...

The photoionization of extended narrow-line regions in Seyfert galaxies by the radiation produced in a thick accretion disk is studied. The emission-line spectrum is calculated for a range of black hole masses, varying the values of the ionization parameter and the disk size. It is found that models with a million solar masses fit ...

Star clusters/ULX associations? Survival or dispersion of protoclusters. What is the mass of the accreting BHs in ULXs? From: luminosity. X-ray colors ...

L is the maximal distance, which the proton can cover during the cluster age. Cluster mass. Mass of gas shell between. accretion shock and virial radius: ...

Gamma ray modulation in AGN by interaction with X�rays from an accretion disk hot spot W. Bednarek interactions with X�rays emitted by a hot spot in the inner part of the accretion disk. As the hot spot orbits originates in a relatively small hot spot rotating on the surface of the inner accretion disk. If the ...

Galactic black holes (GBH) are a class of astrophysical sources with X-ray emission that is powered by accretion from a companion star. An important goal of GBH research is to understand the accretion structure and the nature of the variability of these systems. The GBHs sometimes show significant changes in the X-ray emission properties, and these changes ...