Evolution of a Viscous Protoplanetary Disk with Convectively Unstable Regions

Abstract

The role of convection in the gas-dust accretion disk around a young star is studied. The evolution of a Keplerian disk is modeled using the Pringle equation, which describes the time variations of the surface density under the action of turbulent viscosity. The distributions of the density and temperature in the polar directions are computed simultaneously in the approximation that the disk is hydrostatically stable. The computations of the vertical structure of the disk take into account heating by stellar radiation, interstellar radiation, and viscous heating. The main factor governing evolution of the disk in this model is the dependence of the viscosity coefficient on the radius of the disk. The computations of this coefficient take into account the background viscosity providing the continuous accretion of the gas and the convective viscosity, which depends on the parameters of the convection at a given radius. The results of computations of the global evolution and morphology of the disk obtained in this approach are presented. It is shown that, in the adopted model, the accretion has burst-like character: after the inner part of the disk ( R < 3 AU) is filled with matter, this material is relatively fast discharged onto the star, after which the process is repeated. Our results may be useful for explaining the activity of young FU Ori and EX Lup objects. It is concluded that convection may be one of the mechanisms responsible for the non-steady pattern of accretion in protostellar disks.