SUPERSONIC INFALL AND CAUSALITY IN ACCRETION DISK BOUNDARY-LAYERS

Previous studies of accretion disk boundary layers have indicated the presence of supersonic radial inflows. Such flows are troubling since they would presumably break causal contact between the accreting central star and the disk, thus calling into question the physical self-consistency of the solutions. We identify certain nonphysical aspects of the standard alpha-viscosity prescription as the cause of this paradoxical behavior, and we develop a more physically realistic model of viscosity. We first modify the viscosity coefficient to account for the reduced radial pressure scale height in the boundary layer. This reduces the radial velocities, but, as noted by previous workers, does not eliminate supersonic infall for large values of alpha. We then include a second factor to allow for the fact that the viscosity coefficient must vanish when the steady-state radial velocity of the flow reaches the maximum speed of the diffusive particles (in this case, turbulent fluid blobs) that produce the viscosity. When this modification is used, we find causally connected, physically self-consistent solutions for all choices of parameters. We thus conclude that information flow between the star and the disk can be maintained in all steady-state accreting systems.