Hydrodynamic simulations of counterrotating accretion disks

Time-dependent, axisymmetric hydrodynamic simulations have been used to stu dy accretion disks consisting of counterrotating components with an interve ning shear layer(s). Configurations of this type can arise from the accreti on of newly supplied counterrotating matter onto an existing corotating dis k. The grid-dependent numerical viscosity of our hydrocode is used to simul ate the influence of a turbulent viscosity of the disk. First, we consider the case where the gas well above the disk midplane (z > 0) rotates with an gular rate + Omega(r) and that well below (z < 0) has the same properties b ut rotates with rate - Omega(r). We find that there is angular momentum ann ihilation in a narrow equatorial boundary layer in which matter accretes su personically with a velocity that approaches the free-fall velocity. This i s in accord with the recent analytic model of Lovelace & Chou. The average accretion speed of the disk can be enormously larger than that for a conven tional alpha-disk rotating in one direction. Under some conditions the inte rface between the corotating and counterrotating components shows significa nt warping. Second, we consider the case of a corotating accretion disk for r < r(t) and a counterrotating disk for r > r(t). In this case we observed , that matter from the annihilation layer lost its stability and propagated inward, pushing matter of inner regions of the disk to accrete. Third, we investigated the case where counterrotating matter inflowing from large rad ial distances encounters an existing corotating disk. Friction between the inflowing matter and the existing disk is found to lead to fast boundary la yer accretion along the disk surfaces and to enhanced accretion in the main disk. These models are pertinent to the formation of counterrotating disks in gal axies and possibly 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.