Two-Dimensional Accretion Disk Models: Inner Accretion Disks of FU Orionis Objects

Abstract

To examine the structure and dynamics of a geometrically thick accretion disk, we performed two-dimensional axisymmetric calculations by solving a set of fully non-linear hydrodynamic equations coupled with radiation transport. Focusing on the inner accretion disks of FU Orionis objects, we obtained geometrically thick and quasi-steady accretion disks which are surrounded by an optically thin, rarefied and high-temperature atmosphere. For a non-rotating central star, we obtained a rather broad boundary layer between the central star and the accretion disk. After the quasi-steady disk is established within a few hundred Keplerian orbital periods, further continuous accretion forms a static and dense envelope just above the stellar surface. As a result, the luminosity gradually decreases on a long time scale, because the gravitational potential well becomes shallow. On the other hand, for a rapidly rotating star with 0.8 breakup angular velocity, convective flow is formed near to the stellar surface instead of a broad boundary layer and a static envelope. As soon as the increasing convective mass flux attains a critical value, a high-velocity jet is formed in the rarefied polar region and the luminosity increases sharply. The bursting luminosity lasts for a few months and recurs approximately every eight months.