Evolution of FU Orionis Outbursts in Protostellar Disks

Applying a fully two-dimensional axisymmetric radiation hydrodynamical scheme, the accretion of matter from a protostellar disk onto the central star is investigated. We follow the evolution of the disk in the vicinity of the protostar during the early epoch of star formation, when the mass inflow rate through the disk is 10^-5 M_solar yr^-1. This mass transfer rate is appropriate for a class of young stellar objects known as FU Orionis stars. These calculations are carried out for about 5000 Keplerian orbital periods near the stellar surface, which is equivalent to 3000 days. For this mass accretion rate the disk is thermally unstable. We examine the evolution of the disk as it evolves from a quiescent, geometrically thin low state to an outburst state with a higher mass-inflow rate and a geometrically thick disk. During the onset of the instability, a heating front moves slowly outward, starting from the inner edge of the disk near the stellar radius; in its wake, the temperature at the disk midplane undergoes a transition from about 10⁴ K to a few times 10⁵ K. This transition describes the onset of an outburst of an FU Ori system. A model light curve and blackbody spectra are computed and compared to the outburst of the FU Ori.