Cooling of young stars growing by disk accretion

Abstract

In the initial formation stages young stars must acquire a significant fraction of their mass by accretion from a circumstellar disk that forms in the center of a collapsing protostellar cloud. Throughout this period mass accretion rates through the disk can reach 10^{-6}-10^{-5} M_Sun/yr leading to substantial energy release in the vicinity of stellar surface. We study the impact of irradiation of the stellar surface produced by the hot inner disk on properties of accreting fully convective low-mass stars, and also look at objects such as young brown dwarfs and giant planets. At high accretion rates irradiation raises the surface temperature of the equatorial region above the photospheric temperature T_0 that a star would have in the absence of accretion. The high-latitude (polar) parts of the stellar surface, where disk irradiation is weak, preserve their temperature at the level of T_0. In strongly irradiated regions an almost isothermal outer radiative zone forms on top of the fully convective interior, leading to the suppression of the local internal cooling flux derived from stellar contraction (similar suppression occurs in irradiated ``hot Jupiters''). Properties of this radiative zone likely determine the amount of thermal energy that gets advected into the convective interior of the star. Total intrinsic luminosity integrated over the whole stellar surface is reduced compared to the non-accreting case, by up to a factor of several in some systems (young brown dwarfs, stars in quasar disks, forming giants planets), potentially leading to the retardation of stellar contraction. Stars and brown dwarfs irradiated by their disks tend to lose energy predominantly through their cool polar regions while young giant planets accreting through the disk cool through their whole surface.