TIME EVOLUTION OF VISCOUS CIRCUMSTELLAR DISKS DUE TO PHOTOEVAPORATION BY FAR-ULTRAVIOLET, EXTREME-ULTRAVIOLET, AND X-RAY RADIATION FROM THE CENTRAL STAR

Abstract

We present the time evolution of viscously accreting circumstellar disks as\nthey are irradiated by ultraviolet and X-ray photons from a low-mass central\nstar. Our model is a hybrid of a 1D time-dependent viscous disk model coupled\nto a 1+1D disk vertical structure model used for calculating the disk structure\nand photoevaporation rates. We find that disks of initial mass 0.1M_o around\n1M_o stars survive for 4x10^6 years, assuming a viscosity parameter\n$\\alpha=0.01$, a time-dependent FUV luminosity $L_{FUV}~10^{-2}-10^{-3}$ L_o\nand with X-ray and EUV luminosities $L_X \\sim L_{EUV} ~ 10^{-3}$L_o. We find\nthat FUV/X-ray-induced photoevaporation and viscous accretion are both\nimportant in depleting disk mass. Photoevaporation rates are most significant\nat ~ 1-10 AU and at >~ 30 AU. Viscosity spreads the disk which causes mass loss\nby accretion onto the central star and feeds mass loss by photoevaporation in\nthe outer disk. We find that FUV photons can create gaps in the inner,\nplanet-forming regions of the disk (~ 1-10 AU) at relatively early epochs in\ndisk evolution while disk masses are still substantial. EUV and X-ray photons\nare also capable of driving gaps, but EUV can only do so at late, low\naccretion-rate epochs after the disk mass has already declined substantially.\nDisks around stars with predominantly soft X-ray fields experience enhanced\nphotoevaporative mass loss. We follow disk evolution around stars of different\nmasses, and find that disk survival time is relatively independent of mass for\nstars with M <~ 3M_o; for M >~ 3M_o the disks are short-lived(~10^5 years).\n