Evolution of FU Orionis outbursts in protostellar disks

Applying a fully two-dimensional axisymmetric radiation hydrodynamical sche me, 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 r ate through the disk is 10(-5) M. yr(-1). This mass transfer rate is approp riate 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 accre tion rate the disk is thermally unstable. We examine the evolution of the d isk as it evolves from a quiescent, geometrically thin low state to an outb urst state with a higher mass-inflow rate and a geometrically thick disk. D uring the onset of the instability, a heating front moves slowly outward, s tarting from the inner edge of the disk near the stellar radius; in its wak e, the temperature at the disk midplane undergoes a transition from about 1 0(4) K to a few times 10(5) K. This transition describes the onset of an ou tburst of an FU Ori system. A model light curve and blackbody spectra are c omputed and compared to the outburst of the FU Ori.