The Structure and Evolution of Thin Viscous Disks. I. Non-steady Accretion and Excretion

The long-term evolution of solar nebula is a non-steady process. Using the alpha -viscosity model, we computed the long-term non-steady evolution of circumstellar disks isolated from neighboring stars. The inner boundary layer, where the pressure is important, was taken fully into account, while the temperature distribution of the disk was assumed to be constant in time. Computations were made for a variety of angular velocities of a central star. The results indicate that there are three critical angular velocities of the central star, which distinguish between the inward and outward directions of the flows of mass and angular momentum. We also found that if the angular velocity of the central star is sufficiently high, the disk evolves from accretion to excretion. From our results we can expect that a protostar continues to accrete most of the disk mass and reaches a very rapidly rotating state with a critical spin rate, if no efficient mechanism works to carry away angular momentum from the star. On the other hand, a large mass loss is observed for young stars, such as T Tauri stars, and they are rather slow rotators. We thus examine how stellar winds keep a protostar at a low spin rate.