THE BURST MODE OF ACCRETION AND DISK FRAGMENTATION IN THE EARLY EMBEDDED STAGES OF STAR FORMATION

Abstract

We revisit our original papers on the burst mode of accretion by\nincorporating a detailed energy balance equation into a thin-disk model for the\nformation and evolution of circumstellar disks around low-mass protostars.Our\nmodel includes the effect of radiative cooling, viscous and shock heating, and\nheating due to stellar and background irradiation. Following the collapse from\nthe prestellar phase allows us to model the early embedded phase of disk\nformation and evolution. During this time, the disk is susceptible to\nfragmentation, depending upon the properties of the initial prestellar core.\nGlobally, we find that higher initial core angular momentum and mass content\nfavors more fragmentation, but higher levels of background radiation can\nmoderate the tendency to fragment. A higher rate of mass infall onto the disk\nthan that onto the star is a necessary but not sufficient condition for disk\nfragmentation. More locally, both the Toomre Q-parameter needs to be below a\ncritical value _and_ the local cooling time needs to be shorter than a few\ntimes the local dynamical time. Fragments that form during the early embedded\nphase tend to be driven into the inner disk regions, and likely trigger mass\naccretion and luminosity bursts that are similar in magnitude to\nFU-Orionis-type or EX-Lupi-like events. Disk accretion is shown to be an\nintrinsically variable process, thanks to disk fragmentation, nonaxisymmetric\nstructure, and the effect of gravitational torques. The additional effect of a\ngeneric \\alpha-type viscosity acts to reduce burst frequency and accretion\nvariability, and is likely to not be viable for values of \\alpha significantly\ngreater than 0.01.\n