The Structure of the Boundary Layer in Protostellar Disks

Circumstellar disks are commonly associated with pre-main-sequence stars. Under the action of viscous stress, angular momentum is transferred outward and matter diffuses inward. With the aid of a fully two-dimensional, axisymmetric hydrodynamical scheme we investigate the accretion of matter from protostellar disks onto their central stars. In order to obtain an asymptotic steady state solution we follow the evolution of the flow for over l03 dynamical timescales. For typical ranges of accretion rates we confirm the existence of two quasi-equilibrium states of the disk, separated by thermal instability. For relatively low accretion rates (<10-6 Msun yr-1), typical for classical T Tauri stars, the disk is geometrically thin and fully convective. In the region adjacent to the stellar surface, i.e., in the boundary layer, the disk is optically thin. For relatively high accretion rates (more than a few 10-5 Msun yr 1), typical for FU Orionis systems, the disk is geometrically thick and radiative. The incoming matter spreads rapidly around the protostellar envelope to form a dense, hot shell. Within the stellar envelope, and in the disk for a low mass-inflow rate, we were able to resolve directly several convection cells, which are driven by the ionization of hydrogen. These eddies provide an effective viscous stress such that the accretion of rapidly rotating disk material generally induces an increase in the spin of the outer convective envelope of the star.