Counterrotating galaxies and accretion disks.

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) tightly-wrapped spiral waves between co and counterrotating stellar components and/or between a corotating stellar component and a counterrotating gaseous component. The instability of counterrotating stellar components has been clearly seen in computer simulations. The unstable two-stream spiral waves can provide an effective viscosity for the gas causing its rapid accretion. Accretion disks consisting of counterrotating gaseous components may exist with an intervening shear layer. Configurations of this type can arise from the accretion of newly supplied counterrotating gas onto an existing corotating gas disk. For example, the gas above the disk midplane can rotate with angular rate +omega(r) while that well below has the same properties but rotates with rate -omega(r). Using the Shakura-Sunyaev alpha turbulence model, we find self-similar solutions where a thin (relative to the full disk thickness) equatorial layer accretes very rapidly, essentially at free-fall speed. This type of accretion flow has now been observed in hydrodynamic simulations.