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Accretion Disks THEORY

Hydrodynamic Simulations of Counterrotating Accretion Disks

[abstract]  [full text] [plots from the paper]

Time-dependent, axisymmetric hydrodynamic simulations have been used to study Keplerian accretion disks consisting of counterrotating components. 

The simulation code used for the study has a numerical viscosity that is calibrated by the study of the accretion of a disk rotating in one direction. Different grid resolutions were used to study the influence of the numerical viscosity. Our simulation model does not include radiative cooling. Therefore, different values of the specific heat ratio g from 1.01 to 5/3 were used in order to assess the influence of heating due to the numerical viscosity. A small value of g-1 corresponds to almost isothermal conditions.

Different cases were considered. In model I, the gas well above the disk midplane rotates in one direction and that well below has the same properties but rotates in the opposite direction. In this case, there is angular momentum annihilation in a narrow equatorial boundary layer in which matter accretes supersonically with a velocity that approaches the free-fall velocity. The average accretion speed of the disk can be enormously larger than that for a conventional a-viscosity disk rotating in one direction. For a much lower viscosity (when we took a small part of the region and calculated it with a grid resolution of 200 x 200), the interface between the corotating and counterrotating components shows signifcant warping, which is probably a type of Kelvin-Helmholtz instability. We observed that a large viscosity suppresses this instability. 

In model II, we considered the case where the inner part of the disk corotates while the outer part counterrotates. In this case a new equilibrium inner disk forms with a low- density gap between inner and outer disks. In model III we investigated the case where low-density counterrotating
matter inflowing from large radial distances encounters an existing corotating disk. Friction between the inflowing matter and the existing disk is found to lead to fast boundary layer accretion along the disk surfaces, whereas interaction of the disk with counterrotating matter at large radii
leads to enhanced accretion in the main body of the disk. We observed that the boundary layer accretion is a temporary phenomenon, because the interaction of the dense disk and low-density counterrotating gas leads to heating of this gas. However, the interaction at large radii is more steady and leads to continuous enhanced accretion in the main disk.

These models are pertinent to the formation of counter-rotating disks in galaxies, in active galactic nuclei, and in X-ray pulsars in binary systems. For galaxies, the high accretion speed allows counterrotating gas to be transported into the central regions of a galaxy in a time much less than
the Hubble time. 

created by O. Toropina, 2000-2004 Your comments are welcome
2000-2011, last updated on 19.03.11