Recent papers on astro-ph
The Origin of Jets
Accretion Disks THEORY
Hydrodynamic Simulations of Counterrotating Accretion Disks
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
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