Influence of luminosity bursts on properties of protostellar disks

A (2+ 1)-dimensional numerical model for the formation and evolution of young stellar objects with sub-solar masses is presented. The numerical hydrodynamicall code describing the formation and evolution of a polar disk in a two-dimensional approximation is supplemented by one-dimensional code for the evolution of the star and an algorithm for establishing the vertical structure of the disk. This code is used to investigate the influence of luminosity bursts with intensities similar to those observed in FU Orionis objects (FUors) on the properties and thermal balance of polar disks. A model with gravitational instability and fragmentation of the disk, with subsequent migration of the fragments onto the protostar, is used as a basic model for FUors. Typical FUor bursts (L similar to 100 L-circle dot) can appreciably influence the thermal balance of their disks and parent envelopes, leading to an increase in the disk temperature by more than a factor of two. On the other hand, massive fragments in the disk are only weakly perturbed by such bursts, partially due to screening by the disk and partially due to their high temperature brought about by adiabatic heating. Apart from massive fragments, the characteristic thermal time scales are appreciably shorter than the dynamical time scales throughout the radial extent of the disk and envelope; this enables the use of a stationary radiative-transfer equation when determining the vertical structure of the disk.