Two-dimensional Hydrodynamic Simulations of Convection in Radiation-dominated Accretion Disks

The standard equilibrium for radiation-dominated accretion disks has long been known to be viscously, thermally, and convectively unstable, but the nonlinear development of these instabilities--and hence the actual state of such disks--has not yet been identified. By performing local two-dimensional hydrodynamic simulations of disks, we demonstrate that convective motions can release heat sufficiently rapidly as to substantially alter the vertical structure of the disks. If the dissipation rate within a vertical column is proportional to its mass, the disk settles into a new configuration that is thinner than the standard radiation-supported equilibrium by a factor of 2. If, on the other hand, the vertically integrated dissipation rate is proportional to the vertically integrated total pressure, the disk is subject to the well-known thermal instability. Convection, however, biases the development of this instability toward collapse. The end result of such a collapse is a gas-pressure-dominated equilibrium at the original column density.