Evolution of Photoevaporating Protoplanetary Disks

A crucial time-scale issue in planet formation is the timing required for the loss of protoplanetary gas material, which dominates the total disk mass. The outcome for a particular planetary system might be very different if the parent disk is dispersed sooner or later than in our solar system. We model the evolution of protoplanetary disks under the influence of viscous accretion and photoevaporation by the central star. Previous studies are extended by considering the evolution of disks around different types of parent stars in which extrasolar planets have been discovered. We consider stellar masses in the range 1 to 4.7 M\sun, and extreme ultraviolet (EUV) fluxes, φ, in the range 1040 to 1043 photons s-1. The disk evolves on the viscous diffusion time scale at the gravitational radius, the disk location inside which ionized hydrogen is gravitationally bound to the central star. Photoevaporation is initially powered by the diffuse EUV flux arising from recombinations in the gravitationally bound region. Following the analysis of Alexander et al. (2007), we include the direct contribution from the direct stellar EUV flux as the gravitationally bound inner disk is removed. The disk is removed in ~30 Myr for a 4 M\sun central star and φ=1042 s-1 . Increasing the EUV flux results in shorter disk life times, as expected. Reducing the stellar mass results in shorter disk lifetimes for two reasons. First, photoevaporation is more easily accomplished since material is less tightly bound to the central star (the gravitational radius moves inward). Second, the viscous diffusion time scale at the smaller gravitational radius decreases, speeding up the overall disk evolution.