A numerical model of a steady state, thin and isothermal accretion disk is presented. We demonstrate that 'zero torque' inner boundary condition is a reasonable approximation provided that the disk thickness, including the thickness of the torquing magnetic fields, is small. It is likely that this conclusion is correct also for non-isothermal and non-steady disks, as long as the total thickness...

Recently Ghosh (1998) reported a new regime of instability in Keplerian accretion disks which is caused by relativistic effects. This instability appears in the gas pressure dominated region when all relativistic corrections to the disk structure equations are taken into account. We show that he uses the stability criterion in completely wrong way leading to inappropriate conclusions. We perfor...

Our understanding of the structure and properties of accretion disks in quiescent Dwarf Novae and X-ray Transients is very limited. Observations during quiescence challenge some of the standard Disk Instability Model (DIM) predictions. Our ignorance of the nature of viscosity may be the main source of uncertainties in quiescent disk models. A “layered accretion” alternative to the DIM, in which...

The existence of bimodal disks is investigated. Following a simple argument based on energetic considerations we show that stationary, bimodal accretion disk models in which a Shakura–Sunyaev disk (SSD) at large radii matches an advection dominated accretion flow (ADAF) at smaller radii are never possible using the standard slim disk approach, unless some extra energy flux is present. The same ...

We use three dimensional magnetohydrodynamic simulations, in a pseudo-Newtonian potential, to study geometrically thin accretion disc flows crossing rms, the marginally stable circular orbit around black holes. We concentrate on vertically unstratified and isothermal disk models, but also consider a model that includes stratification. In all cases, we find that the sonic point lies just inside ...

Thin, Keplerian accretion disks generically become gravitationally unstable at large radius. I investigate the nonlinear outcome of such instability in cool disks using razorthin, local, numerical models. Cooling, characterized by a constant cooling time τc, drives the instability. I show analytically that, if the disk can reach a steady state in which heating by dissipation of turbulence balan...

We present an exact three-dimensional wave solution to the shearing sheet equations of motion. The existence of this solution argues against transient amplification as a route to turbulence in unmagnetized disks. Moreover, because the solution covers an extensive dynamical range in wavenumber space, it is an excellent test of the dissipative properties of numerical codes. Subject headings: accr...

We consider the effects of eccentricity on the fragmentation of gravitationally unstable accretion disks, using numerical hydrodynamics. We find that eccentricity does not affect the overall stability of the disk against fragmentation, but significantly alters the manner in which such fragments accrete gas. Variable tidal forces around an eccentric orbit slow the accretion process, and suppress...