A possible accretion model associated with the ionization instability of quasar disks is proposed to

A possible accretion model associated with the ionization instability of quasar disks is proposed to

A possible accretion model associated with the ionization instability of quasar disks is proposed to

With the discovery that Alfven waves can be strongly unstable in a magnetized accretion disk, it can be expected to explain the turbulence necessary to drive accretion in these disks. The instability of non-axisymmetric Alfven waves is studied using analy...

We consider the consequences of magnetic buoyancy and the magnetic shearing instability (MSI) on the

The global nonlinear time-dependent evolution of the inertial-acoustic mode instability in accretion

The influence of accretion disk instabilities on the UV fluxes of Seyfert 1 galaxies was studied. The observed characteristic timescales of variability are consistent with the interpretation that most of the UV radiation originates in an accretion disk, a...

planet :: smoking gun : caught in the actsmoking gun : caught in the act Most simulations of disk-planetDisk Instability vsvs. Core Accretion. Core Accretion Formed planet : forming planet ::Formed planet : formingMost simulations of disk-planet interactions start with fully-formed ...

In the accretion disk around a black hole or a neutron star, the radiative viscosity will much exceed the viscosity of the fully ionized gas. The Reynolds number, depending strongly on the accretion rate, will reach a minimum in the zone where the bulk of the energy is released. Thermal instability will set in, ...

Apr 12, 2011 ... The characteristics of a particular planetary system depend upon the ... planets are formed from dynamic instabilities in accretion disks. ...

The current status of the physics of accretion disks in close binary stars is reviewed, with an emphasis on the hydrodynamic overreflection instability, which is a factor leading to the accretion disk turbulence. The estimated turbulent viscosity coefficients are in good agreement with ...

A linear theory is developed for thermal convective instability in radiating gas dominated by radiation pressure. Analysis demonstrates how the magnetic field and differential rotation will affect the onset of thermal convection in the interior of a black hole accretion disk.

It is shown that an ionized disk in a vertical magnetic field (e.g., the dipole field of the central object for an accretion or planetary disk) is subject to spiral instabilities of low azimuthal wavenumber, driven by differential rotation. The amplification is due to the Swing mechanism already known for the ...

This presentation describes the behavior of matter in environments with extreme magnetic and gravitational fields, explains the instability/stability of accretion disks in certain systems, and discusses how emergent radiation affects accretion flow. Magne...

The stability of magnetic fields within the accretion disks is examined, including the effects of magnetic buoyancy. Rayleigh-Taylor, and Kelvin-Helmholtz instabilities. The purpose of the study is to examine the efficacy of turbulent accretion (..cap alpha..-) disk models which depend upon ...

The effects of radial advection and thermal diffusion were considered in investigating the linear stability of an optically thin, two-temperature accretion disk. If the disk has very little advection, we prove that the thermal instability exists when the disk is geometrically thin. However, it ...

Supercomputer simulations have been used in conjunction with analytic studies to investigate the central issue of astrophysical accretion-disk dynamics: the nature of the angular momentum transport. Simulations provide the means to investigate and experiment with candidate mechanisms, including global hydrodynamic instabilities, spiral shock waves, and ...

The nonlinear evolution and spatial propagation of the thermal instability in accretion disks in galactic nuclei are investigated. Integrations of the vertical structure of the disks are described for different alpha prescriptions, and the thermal stability is examined. Global time-dependent calculations of the ...

The transport of angular momentum in the outward direction is the fundamental requirement for accretion to proceed in an accretion disk. This objective can be achieved if the accretion flow is turbulent. Instabilities are one of the sources for the turbulence. We study a differentially rotating ...

Differentially rotating disks are subject to the axisymmetric instability for perfectly conducting plasma in the presence of poloidal magnetic fields. For nonaxisymmetric perturbations, the authors find localized unstable eigenmodes whose eigenfunction is...

The nonaxisymmetric stability of self-gravitating, geometrically thick accretion disks has been studied for protostellar systems having a wide range of disk-to-central object mass ratios. Global eigenmodes with four distinctly different characters were id...

We derive the evolution equations describing a thin axisymmetric disk of gas and stars with an arbitrary rotation curve that is kept in a state of marginal gravitational instability and energy equilibrium due to the balance between energy released by accretion and energy lost due to decay of turbulence. Rather than adopting a ...

Gas giant planets have been detected in orbit around an increasing number of nearby stars. Two theories have been advanced for the formation of such planets: core accretion and disk instability. Core accretion, the generally accepted mechanism, requires several million years or more to form a gas giant planet in a ...

By analyzing the fourth-order dispersion relation, the influence of coupled radial and azimuthal perturbations on the stability of a radiation-pressure-dominated accretion disk is investigated. If the azimuthal perturbations are considered, we find that the stability properties of disk are different from that in purely radial ...

The evolution of thick accretion tori around black holes was investigated. Using three-dimensional computer models, it is shown that isentropic, pressure supported tori with constant specific angular momentum (Omega = r sup - 2) are unstable. Nonaxisymmet...

accretion disks in the western Gulf region (Texas and Louisiana). Accretion ... protostellar disks are believed to be accretion disks associated with young, ...

One possible mechanism for giant planet formation is disk instability in which the planet is formed as a result of gravitational instability in the protoplanetary disk surrounding the young star. The final composition and core mass of the planet will depend on the planet's mass, environment, and the planetesimal ...

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 stars can be explained by the magnetospheric accretion model, in ...

The thermal equilibrum structure and dynamical behavior of externally irradiated accretion disks are investigated. For radiative disks only the surface layer is heated, while for convective disks the heat penetrates deeply into the disk. For sufficiently strong radiation and given irradiation ...

Accretion describes the process by which matter in an astrophysical disk falls onto a central massive object. Accretion disks are present in many astrophysical situations including binary star systems, young stellar objects, and near black holes at the center of galaxies. Measurements from observations of these ...

We investigate the properties and evolution of accretion tori formed after the coalescence of two compact objects. At these extreme densities and temperatures, the accreting torus is cooled mainly by neutrino emission produced primarily by electron and positron capture on nucleons (?-reactions). We solve for the disk structure and its ...

The evolution and stability of pair-supercritical accretion disks are examined. The microphysics of electron-positron pair processes are incorporated into a self-consistent accretion disk model, using the full set of time-dependent equations for a two-temperature disk cooled by bremsstrahlung ...

Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks

We calculate the structure of an effectively optically thin and geometrically thin accretion disk in

A model for disk accretion by a rotating magnetic neutron star is proposed which includes a detailed

We explore the properties of cold gravitoturbulent accretion disks-non-fragmenting disks hovering on the verge of gravitational instability (GI)-using a realistic prescription for the effective viscosity caused by gravitational torques. This prescription is based on a direct relationship between the angular ...

We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that ...

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 ...

New instabilities in multicomponent cold magnetized accretion disks are found using not the magnetohydrodynamics (MHD) framework but the equations of motion and continuity for each disk component and Maxwell's equations, where the magnetic field perturbations are substituted by the electric field ...

The nonlinear evolution of the Parker instability in accretion disks and galactic gas disks is studied by using a two-dimensional magnetohydrodynamic (MHD) code. The gas layer is assumed to be located at some distance from a point mass which is the origin of gravity. The magnetic fields are assumed to be parallel ...

The growth of global gravitational instabilities in young stellar objects (YSOs) with associated circumstellar disks is studied. The possibility that the accretion ultimately owes its origin to the growth of spiral gravitational instabilities is explored. The results indicate that YSO disks ...

We review current models for the accumulation of the Moon from an impact-generated debris disk. Such a disk is dynamically distinguished by its substantial mass relative to the Earth and a very centrally condensed radial profile, with a mean orbital radius near the classical Roche limit. In the inner protolunar disk, ...

Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary ...

We investigate the launching and stability of extragalactic jets through magnetohydrodynamic simulations of jet evolution. In these simulations, a small scale equilibrium magnetic corona is twisted by a differentially rotating accretion disk. Two-dimensional calculations show the formation of a collimated outflow. This outflow is divided into two regions ...

Gas giant planets undoubtedly form from the orbiting gas and dust disks commonly observed around young stars, and there are two principal mechanisms proposed for how this may occur. The core accretion plus gas capture model argues that a solid core forms first and then accretes gas from the surrounding disk once ...

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 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 ...

The linear equations that describe the thermal stability of the convective and radiative vertical structure of an alpha model accretion disk are derived. For a specific model chosen to be representative of a dwarf nova disk, it is found that the eigenfunction for the temperature shows large excursions in the zone of partial ionization, ...

X-rays from young stellar objects can influence the physical properties of their surroundings. A particularly interesting example is the ionization of the accretion disk by hard X-rays at distances of the order of 1 AU or more. The authors show that X-rays induce a layered ionization structure in which the outer layer is sufficiently ionized to permit the ...

The radial-azimuthal instability of a hot two-temperature accretion disc with advection is analyzed. After obtaining the dispersion equation, we find that advection and viscous force have an influence on the stability of acoustic modes, but not on the stability of thermal and viscous modes. We also find azimuthal perturbations to affect the stability of ...

Active galactic nuclei are believed to be powered by accretion through a disk onto a super-massive black hole at the center of the host galaxy. Luminous quasars then require accretion rates of one to ten solar masses per year. I will show that this requires disk surface densities of order 400 grams per square ...

Bolometric luminosities and black hole (BH) masses are estimated by various methods for a sample of young radio galaxies with known ages. We find that the ages are positively correlated with the bolometric luminosities in these young radio galaxies. This positive correlation is consistent with the theoretical prediction based on the radiation pressure instability of ...

Recent theoretical advances in our understanding of the dynamics of circumstellar accretion disks are reviewed. Of particular importance are developments regarding angular momentum transport processes in disks. It has recently been shown through direct numerical simulation that vertical convection results in inward transport, and thus ...

We aim to illustrate the role of hot protons in enhancing the magnetorotational instability (MRI) via the �hybrid� viscosity, which is due to the redirection of protons interacting with static magnetic field perturbations, and to establish that it is the only relevant mechanism in this situation. It has recently been shown by Balbus [1] and Islam & Balbus [11] using a ...

In two previous publications, we have demonstrated that stationary rotation of magnetized plasma about a compact central object permits an enormous number of different MHD instabilities, with the well-known magneto-rotational instability as just one of them. We here concentrate on the new instabilities found that are driven by ...

The limit-cycle thermal instability model for accretion disks is used to study the soft X-ray transient A0620-00. Thermal instability in geometrically thin, Keplerian alpha-model disks is reviewed. The observational constraints on A0620-00 are presented and the parameters chosen for the model ...

We obtained steady solutions of magnetized black hole accretion disks taking into account the gas pressure, magnetic pressure, and radiation pressure. We assumed the bremsstrahlung cooling in the optically thin limit and the black body cooling in the optically thick limit. W assumed that magnetic fields inside the disk are turbulent ...

High Energy Astrophysics: Accretion Disks II 1/59 Accretion Disks II 1 The dynamical equations Astrophysics: Accretion Disks II 2/59 The simplest accretion disc model to construct is that of the thin disc on the azimuthal angle z #12;High Energy ...

This program has led to a series of papers being written and published in the Astrophysical Journal. Together these papers try to explain major parts of the LINER and low luminosity AGN puzzle. One paper ('Accretion Disk Instabilities, Cold Dark Matter Mo...

The collisional origin of the Moon is considered. The angular momentum of the Earth-Moon system is less than sufficient to spin the Earth to rotational instability. Nevertheless the mass of the body striking a tangential blow to the protoearth, imparting the angular momentum of the Earth-Moon system to the protoearth is examined.

We report experimental observation of an instability in a Couette-Taylor flow of a polymer fluid in a thin gap between two coaxially rotating cylinders in a regime where their angular velocity decreases with the radius while the specific angular momentum increases with the radius. In the considered regime, neither the inertial Rayleigh instability nor the ...

We report experimental observation of an instability in a Couette-Taylor flow of a polymer fluid in a thin gap between two coaxially rotating cylinders in a regime where their angular velocity decreases with the radius while the specific angular momentum increases with the radius. In the considered regime, neither the inertial Rayleigh instability nor the ...

The formation of thick stellar disks in spiral galaxies is studied. Simulations of gas-rich young galaxies show formation of internal clumps by gravitational instabilities, clump coalescence into a bulge, and disk thickening by strong stellar scattering. The bulge and thick disks of modern galaxies may form this ...

It is demonstrated through nonlinear numerical simulations that there are two kinds of magnetic accretion disks. One is a high-beta disk, called a solar-type disk, where magnetic flux escapes from the disk due to magnetic buoyancy (or Parker) instability to produce a ...

We have constructed vertically integrated, steady state convective models of accretion disks to explore the mechanism of instability in dwarf novae. The models and observations of dwarf novae suggest a picture in which transferred matter piles up in an optically thin torus. The torus eventually becomes optically thick and the resulting ...

Vertically integrated, steady state convective models of accretion disks have been constructed to explore the mechanism of instability in dwarf novae. The models and observations of dwarf novae suggest a picture in which transferred matter piles up in an optically thin torus. The torus eventually becomes optically thick, and the ...

We study rapidly accreting, gravitationally unstable disks with a series of idealized global, numerical experiments using the code ORION. Our numerical parameter study focuses on protostellar disks, showing that one can predict disk behavior and the multiplicity of the accreting star system as ...

data yield the accretion disk thickness, diameter, electron density, and magnetic field. Introduction. The structure of AGN accretion disks on sub-parsec ...

The structure of AGN accretion disks on sub-parsec scales can be probed t,hrough ... GHz to map absorption caused by an inner accretion disk. ...

Nov 19, 2004 ... In an accretion disk, material near the outside moves slower than ... Depending on the angle at which we observe the accretion disk, ...

Jan 29, 2010 ... KY Relativistic Accretion Disk models. A set of models from Dovciak et al. (2004 ) for accretion disk spectra in the strong gravity regime. ...

do not have accretion disks. Optical spectra are the only way that we can .... inner accretion disks) confirms the asymmetrical distribution of such stars ...

Sep 27, 2002 ... Accretion Disk Simulation ... Instead, the graphic shows spiral shock waves in a three dimensional simulation of an accretion disk -- material ...

Based on the characteristics of the magnetorotational instability (MRI) and the MRI-driven turbulence, we construct a steady model for a geometrically thin disk using "non-standard" ?-prescription. The efficiency of the angular momentum transport depends on the magnetic Prandtl number, Pm = ?/?, where ? and ? are the microscopic viscous and magnetic ...

High Energy Astrophysics: Accretion Disks I 1/60 Accretion Disks I References: Accretion Power in Astrophysics, J. Frank, A. King and D. Raine. High Energy Astrophysics, Vol. 2, M.S. Longair, Cambridge Astrophysics: Accretion Disks I 2/60 1 Overview ...

We investigate the structure of accretion disks around massive protostar applying steady state models of thin disks. The thin disk equations are solved with proper opacities for dust and gas taking into account the huge temperature variation along the disk. We explore a wide parameter range ...

We examine the stability of geometrically thin and optically thick and thin accretion disks to axisymmetric perturbations. We show that the standard ..cap alpha..-disk models admit not only classical thermal and viscous instabilities, which are generally associated with regions where electron scattering and ...

The stability and evolution of windy accretion disks is investigated in detail. The basic disk evolution equations are briefly recapitulated, and an idealized analytic treatment of the wind and viscosity is used to show that steady disk flow is indeed unstable for sufficiently large ratio of the mass loss rate in ...

A general theory for instabilities of multicomponent warm magnetized accretion disks is developed using Maxwell's equations and the equations of motion and continuity for each disk component. The compressibility, anisotropic thermal pressure, and collisions of the charged species with neutrals are taken ...

A general theory for instabilities of multicomponent warm magnetized accretion disks is developed using Maxwell's equations and the equations of motion and continuity for each disk component. The compressibility, anisotropic thermal pressure, and collisions of the charged species with neutrals are taken into ...

The spectral evolution of accretion disks in X-ray binaries containing black holes is studied, based on the disk instability model. The thermal transition of the outer portions of the disk controls the mass flow rate into the inner portions of the disk, thus modulating the ...

Disks, Accretion. USE. ACCRETION DISKS. Disks, Optical. USE. VIDEO DISKS. Disposal (In Space), Hazardous Material. USE. HAZARDOUS MATERIAL DISPOSAL (IN ...

A disk model with gas pressure viscosity. The spectrum from an accretion disk ... viscosity scales as the gas pressure. From Stella and Rosner 1984, ApJ, ...

Context. Turbulence and angular momentum transport in accretion disks remains a topic of debate. With the realization that dead zones are robust features of protoplanetary disks, the search for hydrodynamical sources of turbulence continues. A possible source is the baroclinic instability (BI), which has been shown ...

A radial-azimuthal stability analysis of a geometrically thin, gas-pressure-dominated accretion disk is presented. In the purely radial perturbation case, the disk is pulsationally unstable to the acoustic modes but stable to the thermal and viscous modes. If the coupling of radial and azimuthal perturbations is considered, the ...

The origin of hydrodynamic turbulence in rotating shear flow is a long standing puzzle. Resolving it is especially important in astrophysics when the flow's angular momentum profile is Keplerian which forms an accretion disk having negligible molecular viscosity. Hence, any viscosity in such systems must be due to turbulence, arguably governed by ...

We describe in detail our two-dimensional, axisymmetric computer code for calculating fully relativistic ideal gas hydrodynamics around a Kerr black hole. This code is being used to study fully dynamic inviscid fluid accretion onto black holes, as well as the evolution and developement of nonlinear instabilities in pressure-supported ...

The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was

Equations for the structure and appearance of supercritical accretion disks and the radiation-driven

An effort is made to observationally constrain accretion disks on the basis of light curves from the

In the study of astrophysical accretion disks, a key problem is the process by which angular momentum is transported outward at a rate high enough to account for the observed luminosities of accretion-powered astronomical sources. Plasma-physics processes in the disk such as the Magneto-Rotational ...

: accretion, accretion disks -- instabilities -- X�rays: stars -- stars: novae, cataclysmic variables 1. Introduction VY Scl stars are a subclass of nova�like, cataclysmic variables which are bright most of the time for VY Scl (see Table 2). 4. Discussion 4.1. Comparison between VY Scl stars and dwarf novae Not ...

A simple model of gas accretion in young galaxy disks suggests that fast turbulent motions can be driven by accretion energy for a time t{sub acc} {approx} 2(epsilon{sup 0.5} GM {sup 2}/xiV {sup 3}){sup 0.5} where epsilon is the fraction of the accretion energy going into disk turbulence, M and ...

For an optical monitoring blazar sample set whose typical minimum variability timescale is about 1 hr, we estimate a mean value of the viscosity parameter in their accretion disk. We assume that optical variability on timescales of hours is caused by local instabilities in the inner accretion ...

The energetics of a ..gamma..-ray burster can be explained by the rapid accretion of > or approx. =10/sup 19/ g of matter onto a neutron star surface. It is proposed that this sudden infall of matter may be the result of a radiation-driven instability which occurs at the boundary between an accretion disk and ...

By analyzing the dispersion relation analytically, I conclude that the radial-azimuthal instability properties of an isothermal accretion disk are different from those in a previous research paper (Cao 1992, A&A 262, 350). In the case without viscosity, I find that the acoustic modes are always stable, whereas the non-axisymmetric ...

The magnetorotational instability (MRI) is the most likely candidate for angular momentum transport and radial flow of mass in protoplanetary disks (PPDs), but demands a minimum ionization fraction for the gas to couple to the magnetic field. It is known that large regions of PPDs are insufficiently ionized for the MRI to act, and are �dead zones.� ...

The formation and evolution of the circumstellar disk in unmagnetized molecular clouds is investigated using three-dimensional hydrodynamic simulations from the prestellar core until the end of the main accretion phase. In collapsing cloud cores, the first (adiabatic) core with a size of gsim3 AU forms prior to the formation of the protostar. At its ...

The formation and evolution of the circumstellar disk in unmagnetized molecular clouds is investigated using three-dimensional hydrodynamic simulations from the prestellar core until the end of the main accretion phase. In collapsing cloud cores, the first (adiabatic) core with a size of {approx}>3 AU forms prior to the formation of the protostar. ...

In almost all of plasma theories for astrophysical objects, we have assumed the charge quasineutrality of unmagnetized plasmas in global scales. This assumption has been justified because if there is a charged plasma, it induces the electric field which attracts the opposite charge, and this opposite charge reduces the charge separation. Here, we report a newly discovered ...

We outline recent progress in understanding the accretion of plasma to rotating magnetized stars obtained from global axisymmetric (2D) and 3D magnetohydrodynamic (MHD) simulations in three main areas: (1.) Formation of jets from disk accretion onto rotating magnetized stars: From simulations where the viscosity and magnetic ...

Nov 6, 2000 ... "accretion" means collecting of additional material. Two major places where astronomers see accretion disks are in binary star systems (two ...

Sep 4, 1998 ... 3. Formation of an Accretion Disk. Bonnet-Bidaud and van der Klis (1979) showed that even if the accretion is from the stellar wind, ...

Recently, Ida et al. made an N-body simulation of lunar accretion from a protolunar disk formed by a giant impact. One of their important conclusions is that the accretion time of the Moon is as short as one month. Such rapid accretion is a necessary consequence of the high surface density of a lunar mass ...

Differential rotation occurs in conducting flows in accretion disks and planetary cores. In such systems, the magnetorotational instability can arise from coupling Lorentz and centrifugal forces to cause large radial angular momentum fluxes. We present the first experimental observation of the magnetorotational ...

This study is devoted to the experimental analysis of the stratorotational instability (SRI). This instability affects the classical cylindrical Couette flow when the fluid is stably stratified in the axial direction. In agreement with recent theoretical and numerical analyses, we describe for the first time in detail the destabilization of the stratified ...

Although the magnetorotational instability (MRI) has been widely accepted as a powerful accretion mechanism in magnetized accretion disks, it has not been realized in the laboratory. The possibility of studying MRI in a rotating liquid-metal annulus (Couette flow) is explored by local and global stability analysis ...

The formation and evolution of an accretion disk formed during the collapse of a rotating cloud core

The aim of our research is to address why millisecond pulsars have relatively weak surface magnetic fields, of about 10^8 G, with a narrow spread. We propose that the accretion of plasma from the companion star fully screens the original neutron star field, but the accretion disk carries additional magnetic flux from the companion ...

A scenario for the central engine of AGN has been developed, consisting of a massive black hole (MBH) onto which gas accretes through an accretion disk. The accretion disk radiates the observed optical and ultraviolet continua. Surrounding the MBH is a no...

Theoretical interest in astrophysical disks with counterrotating components of stars and/or gas has been stimulated by recently discovered counterrotating spiral and S0 galaxies. A variety of physical processes can occur in counterrotating disks. We have shown that a strong two-stream instability can occur for one armed (m = 1) ...

290 Stepinski:On Maonetic Dynamos in Thin Accretion Disks Around Stars pressure is smaller than gas pressure, otherwise the assumption of a thin disk ...

They could see details as fine as the neutron star's accretion disk, a ring of gas swirling around and flowing onto the neutron star, as the disk buckled ...

accretion disk where the viscosity is dominated by radiation pressure. ... viscosity norm. 2. 2cos /i d where i is the inclination of the disk and d is the ...

A model for the central engine of active galactic nuclei (abbreviated as AGN) has been constructed to simultaneously explain their UV bump and X-ray spectra. It is based on an accretion disk model around a supermassive black hole in that a cold optically thick disk is surrounded by a hot 'corona' and that energy dissipation is supposed ...

We present a simple theoretical framework for massive galaxies at high redshift, where the main assembly and star formation occurred, and report on the first cosmological simulations that reveal clumpy disks consistent with our analysis. The evolution is governed by the interplay between smooth and clumpy cold streams, disk ...

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. ...

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...

Too hot accretion disks in ULX and superluminal jet sources ... Standard accretion disk around Kerr black hole can explain the hard disk spectra? ...

We have performed two-dimensional CIP -MOCCT simulations of Magneto Rotational Instability (MRI) in accretion disks with nonzero ohmic resistivity. An accretion disk is formed as the gas around a protostar accretes to the central star. In the disk the ...

Accretion disks are invoked to explain a host of astrophysical phenomena, from protostellar objects to AGN. And yet the mechanism allowing accretion disks to operate are completely unknown. This proposal seeks to observe the ``smoking gun'' signature of magnetically-driven viscosity in ...

We investigate the global nonaxisymmetric Rossby vortex instability (RVI) in a differentially rotating, compressible magnetized accretion disk with radial density structures. Equilibrium magnetic fields are assumed to have only the toroidal component. Using linear theory analysis, we show that the density structure can be unstable to ...

We investigate the global nonaxisymmetric Rossby vortex instability (RVI) in a differentially rotating, compressible magnetized accretion disk with radial density structures. Equilibrium magnetic fields are assumed to have only the toroidal component. Using linear theory analysis, we show that the density structure can be unstable to ...

We assess the suitability of various numerical MHD algorithms for astrophysical accretion disk simulations with the PLUTO code. The well-studied linear growth of the magneto-rotational instability is used as the benchmark test for a comparison between the implementations within PLUTO and against the ZeusMP code. The results demonstrate ...

Forming giant planets by disk instability requires a gaseous disk that is massive enough to become gravitationally unstable and able to cool fast enough for self-gravitating clumps to form and survive. Models with simplified disk cooling have shown the critical importance of the ratio of the cooling to the orbital ...

The disk instability model for dwarf nova eruptions is investigated by computing the spectral development of the accretion disk through a complete limit cycle. Observed stellar spectra are used to model the radiation emitted by optically thick annuli within the disc. The general findings agree with those of Smak ...

Non-ideal MHD effects play an important role in the gas dynamics in protoplanetary disks (PPDs). This paper addresses the influence of non-ideal MHD effects on the magnetorotational instability (MRI) and angular momentum transport in PPDs using the most up-to-date results from numerical simulations. We perform chemistry calculations using a complex ...

We will observe soft X-ray transients (SXTs) as targets of opportunity using the FOS at UV wavelengths. This is part of an extensive multiwavelength campaign to monitor newly discovered SXTs in outburst and throughout the subsequent decline. Recent work establishes that a high proportion of SXTs harbor black holes, so these observations provide an unrivalled opportunity to obtain high quality data ...

Accretion disks are three-dimensional, turbulent, often self-gravitating, magnetohydrodynamic (MHD) flows, which can be modeled with numerical simulations. In this paper, we present a new algorithm that is based on a spectral decomposition method to simulate such flows. Because of the high order of the method, we can solve the induction equation in terms ...

The local thermal stability of accretion disks around supermassive black holes in active galactic nuclei is examined. Such disks are unstable at radii where the surface temperature is several thousand degrees. Supermassive disks therefore should undergo limit-cycle outbursts similar to those believed to occur in ...

For a non-magnetized white dwarf the accretion disk extends all the way to the surface, where matter impacts and spreads as new matter continuously piles up behind it. Using the RIEMANN code we have simulated the boundary layer on an axisymmetric grid to see how its structure depends on magneto-rotational instabilities (MRI) and ...

A broad class of astronomical accretion disks is presently shown to be dynamically unstable to axisymmetric disturbances in the presence of a weak magnetic field, an insight with consequently broad applicability to gaseous, differentially-rotating systems. In the first part of this work, a linear analysis is presented of the ...

We revisit the flip-flop instability of two-dimensional planar accretion using high-fidelity numerical simulations. By starting from an initially steady-state axisymmetric solution, we are able to follow the growth of this overstability from small amplitudes. In the small-amplitude limit, before any transient accretion ...

Symbiotic stars appear almost certainly to be interacting binaries with a cool giant losing mass from its wind or by Roche lobe overflow to a much more compact companion. The latter is thought to be usually a white dwarf, but may sometimes be a main sequence star and perhaps even a neutron star. Symbiotic stars go through activity phases, which are thought to be sometimes due to ...

During the T Tauri phase, there is a differentially rotating region attached to the top of the stellar convective layer connecting the star with the accretion disk that rotates significantly faster than the stellar surface; rotation periods during the TT phase are about 7-8 days (?_* = 0.8-0.9 day^{-1}) while the Keplerian frequency is: ?_k = 11.1 day^{-1} ...

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 ...

Electromagnetic streaming instabilities of multicomponent collisional magnetized accretion disks are studied. Sufficiently ionized regions of the disk are explored where there is strong collisional coupling of neutral atoms with both ions and dust grains simultaneously. The steady state is investigated in detail ...

The accretion of hot slowly rotating gas onto a supermassive black hole is considered. Rotation velocities at the Bondi radius r_B are small in comparison with speed of sound c_s. The centrifugal barrier at a depth r_c = l^2/G M_BH << r_B hinders supersonic accretion. We take into account saturated electron thermal conductivity and Bremsstrahlung ...

The dynamics of flat, rapidly and nonuniformly rotating self-gravitating systems has now been studied quite thoroughly. This research has aimed above all to explain the origin of various observed structures: spiral formation in galaxies, the rapid planetary formation, the fine-scale ~ 100 m structure of Saturn's main rings. It has been shown analytically that the evolution of these systems are ...

Stars form by the flow of matter through an accretion disk. An efficient mechanism of angular momentum transport is required to drive this flow. The magneto rotational instability (MRI) is the leading candidate (along with self-gravity in the early stages of growth) to drive turbulent momentum transport in disks. I ...

Axisymmetric MRI in viscous accretion disks is investigated. The linear growth of the viscous MRI is characterized by the Reynolds number RMRI ? v2A/??, where vA is the Alfv�n velocity, ? is the kinematic viscosity, and ? is the angular velocity of the disk. Although the linear growth of the MRI is suppressed as the Reynolds number ...

The accretion disk thermal instability (limit cycle) mechanism for dwarf novalike outbursts is applied to the old nova GK Per in order to find values of the input parameters which reproduce the observed outbursts and to constrain the physics associated with the disk and the mass transfer process. It is found that ...

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 most efficient energy sources known in the Universe are accretion disks. Those around black holes convert 5-40 per cent of rest-mass energy to radiation. Like water circling a drain, inflowing mass must lose angular momentum, presumably by vigorous turbulence in disks, which are essentially inviscid. The origin of the turbulence is ...

Doppler surveys have shown that more massive stars have significantly higher frequencies of giant planets inside ~3 AU than lower mass stars, consistent with giant planet formation by core accretion. Direct imaging searches have begun to discover significant numbers of giant planet candidates around stars with masses of ~1 M sun to ~2 M sun at orbital distances of ~20 AU to ...

In an earlier work we identified a global, nonaxisymmetric instability associated with the presence of an extreme in the radial profile of the key function L(r){identical_to}({sigma}{omega}/{kappa}{sup 2})S{sup 2/{gamma}} in a thin, inviscid, nonmagnetized accretion disk. Here {sigma}(r) is the surface mass density of the ...

We examine the small-scale dynamics of black hole accretion disks in which radiation pressure exceeds gas pressure. Local patches of disk are modeled by numerically integrating the equations of radiation MHD in the flux-limited diffusion approximation. The shearing-box approximation is used, and the vertical component of gravity is ...

The final stage of terrestrial planet formation is known as the giant impact stage where protoplanets collide with one another to form planets. So far this stage has been mainly investigated by N-body simulations with an assumption of perfect accretion in which all collisions lead to accretion. However, this assumption breaks for collisions with high ...

A limit cycle driven by thermal instabilities in the accretion disk may account for the periodic outbursts of cataclysmis variables and certain x-ray transients. The outbursts will depend on specific properties of the vertical accreation disk structure such as convection, partial ionization opacity, and viscosity. ...

This thesis describes in detail a two-dimensional, axisymmetric computer code for calculating fully relativistic ideal gas hydrodynamics around a Kerr black hole. The aim is to study fully dynamic inviscid fluid accretion onto black holes, as well as to study the evolution and development of nonlinear instabilities in pressure supported ...

Jul 20, 2009... protoplanetary disks, radiative transfer, T-TAURI DISKS, ACCRETION DISKS, YOUNG OBJECTS, CIRCUMSTELLAR DISKS, STARS, GROWTH, EARTH, DUST ...

... studies to, eg, measure inner disk truncation radii, probe the nature of the disk accretion process, and chart the evolution in the gas content of disks. ...

We study the nonlinear evolution of the Rossby wave instability in thin disks using global two-dimensional hydrodynamic simulations. The detailed linear theory of this nonaxisymmetric instability was developed earlier by Lovelace et al. and Li et al., who found that the instability can be excited when there is an ...

The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster pres...

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...

Geometrically thin, optically thick, turbulent accretion disks are believed to surround many stars. Some of them are the compact components of close binaries, while the others are throught to be T Tauri stars. These accretion disks must be magnetized obje...

The astrophysical importance of the new class of theoretical models for accretion disks concentrating mainly on the applications to active galactic nuclei are discussed. The slim accretion disks model describes several important astrophysical processes wh...

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...

Mar 12, 2005 ... Accretion Disk Simulation Credit: Michael Owen, John Blondin ... Such accretion disks power bright x-ray sources within our own galaxy. ...

Accretion, the process by which matter collects into a central object, is ubiquitous and often dynamically important for astrophysical objects on the scale of compact object disks (� 1010 cm) up to that of galactic clusters (� 1024 cm). In order for matter to accrete, it must lose angular momentum. The central issue in ...

We propose a Galactic model for the isotropic component of gamma-my bursts (GB`s) based upon high-velocity neutron stars (NS`s) that have accretion disks. The fast NS`s are formed in tidally locked binaries, leading to a unique fast NS population. The tidal locking occurs due to the meridional circulation caused by the conservation of angular momentum of ...

The recent discoveries of massive planets on ultra-wide orbits of HR 8799 and Fomalhaut present a new challenge for planet formation theorists. Our goal is to figure out which of three giant planet formation mechanisms-core accretion (with or without migration), scattering from the inner disk, or gravitational instability-could be ...

Nov 9, 2003 ... Consequently accretion disks have fundamental roles in the formation of planets, stars and compact objects, and are the engines driving the ...

... Understanding of accretion disks, accretion flows, X-ray induced winds, compact obect evolution are all in a primitive state. ...

Material falling (accreting) onto a black hole in a binary system, or at the center of a galaxy, forms an accretion disk about the event horizon, with the inner ...

... unique opportunities to study the details of the accretion flow in the cases of normal white dwarfs with accretion disks and magnetic white dwarfs. ...

Radial oscillations resulting from axisymmetric perturbations in viscous accretion disks surrounding

The accretion-induced collapse (AIC) of a white dwarf (WD) may lead to the formation of a protoneutron star and a collapse-driven supernova explosion. This process represents a path alternative to thermonuclear disruption of accreting white dwarfs in Type Ia supernovae. Motivated by the need for systematic predictions of the GW signature of AIC, we present ...

We invesgitage the interaction between the stellar magnetic field and the accreting plasma at the magnetospheric boundary of a neutron star accreting matter from a disk. The stellar magnetic field penetrates the inner part of the disk via the Kelvin-Helmholtz instability, turbulent diffusion, ...

"Transitional" T Tauri disks have optically thin holes with radii gsim10 AU, yet accrete up to the median T Tauri rate. Multiple planets inside the hole can torque the gas to high radial speeds over large distances, reducing the local surface density while maintaining accretion. Thus multi-planet systems, together with reductions in ...

Whether protoplanetary disks accrete at observationally significant rates by the magnetorotational instability (MRI) depends on how well ionized they are. Disk surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates, ranging from ~0.01 ?m sized grains to angstrom-sized ...

Young stellar objects radiate large numbers of keV-photons that are able to penetrate significant columns of gas and thus ionize and heat circumstellar material, including the accretion disks and outflows of very young stars. The unshielded ionization rates are high. At a distance of 1 AU from a young stellar object with a moderately strong X-ray source, ...

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 ...

Radial transport of poloidal and toroidal angular momentum is rapid during a global reconnection event in the MST reversed field pinch experiment. Theoretical explanation has evolved for transport from Maxwell and Reynolds stresses from multiple nonlinearly coupled tearing modes. Comparing stresses from multimode computation with those for a single mode shows that nonlinear coupling (multiple ...

How are accretion disks in Seyfert Galaxies oriented relative to the disk of the spiral galaxy which hosts them? This angle, ?, serves as a link between the innermost workings of the black hole plus fueling accretion disk and the larger galaxy, either as the memory of the activity-provoking ...

The spectral evolution of accretion disks in X-ray binaries containing black holes is studied, based

The standard model of an accretion disk is considered. The temperature in the inner region is computed. It is assumed that the radiated power derives from comptonized photons, produced in a homogeneous single temperature plasma, supported by radiation pre...

Jan 22, 2011 ... Title: The Production of Jets From Magnetized Accretion Disks: Simulation of the Blandford-Payne Mechanism. Authors: Meier, D. ...

Jan 22, 2011 ... Title: The Production of Jets From Magnetic Accretion Disks: Simulation of the Blandford-Payne Mechanism. Authors: Meier, David L. ...

of a collimated jet ejected from the center of the accretion disk. The jet velocity is a strong function of the strength of the initial magnetic field: ...

Feb 23, 2004 ... "Accretion disks are known to flow around many objects in the universe ... Under normal conditions, accretion disks appear far too minute to ...

Jan 31, 2005 ... KERRBB is a multi-temperature blackbody model for a thin, steady state, general relativistic accretion disk around a Kerr black hole. ...

The thesis consists of an introduction and summary, and five research papers. The introduction and summary provides the background in accretion disk physics and magnetohydrodynamics. The research papers describe numerical studies of magnetohydrodynamical ...

First � Previous � Next � Last � Index � Home � Text. Long-term Periodicities and Accretion Disks. Slide 4 of 22.

Jan 22, 2010 ... Many astrophysical objects, including neutron stars and accretion disks, are powered by a combination of very strong magnetic fields and ...

Jun 11, 2010 ... Relativistic Illumination Law from Accretion Disks ... Relativistic Iron Fluorescent Line from Accretion Disks ...

Quasar Jets and Accretion Disks. Questions: " Does the most compact optical emission from an AGN come from an accretion disk or from a relativistic jet? ...

Dec 19, 1999 ... Here gas from a blue giant star is shown being stripped away into an accretion disk around its compact binary companion. ...

Sep 4, 1998 ... In the case of a HMXB with a large ( $\\mathrel{\\hbox{\\rlap{\\hbox{\\lower4pt\\ ) magnetic field the accretion disk cannot extend all the way down to ...