Counterrotating Accretion Disks

We consider accretion disks consisting of counter-rotating gaseous componentswith an intervening shear layer. Configurations of this type may arise from theaccretion of newly supplied counter-rotating gas onto an existing co-rotatinggas disk. For simplicity we consider the case where the gas well above the diskmidplane is rotating with angular rate $+\Omega$ and that well below has thesame properties but is rotating with rate $-\Omega$. Using the Shakura-Sunyaevalpha turbulence model, we find self-similar solutions where a thin (relativeto the full disk thickness) equatorial layer accretes very rapidly, essentiallyat free-fall speed. As a result the accretion speed is much larger than itwould be for an alpha disk rotating in one direction. Counter-rotatingaccretion disks may be a transient stage in the formation of counter-rotatinggalaxies and in the accretion of matter onto compact objects.Comment: 7 pages, 3 figures, aas2pp4.sty, submitted to Ap