The effect of ultra-violet photon pumping of H2 in dust-deficient protoplanetary disks
Abstract
We perform radiation hydrodynamics simulations to study the structure and evolution of a photoevaporating protoplanetary disk. Ultraviolet and X-ray radiation from the host star heats the disk surface, where H2 pumping also operates efficiently. We run a set of simulations with varying the amount of dust grains, or the dust-to-gas mass ratio, which determines the relative importance between photoelectric heating and H2 pumping. We show that H2 pumping and X-ray heating contribute stronger to the mass-loss of the disk if the dust-to-gas mass ratio is D≤10-3.The disk mass loss rate decreases with a lower dust amount, but remains around 10-10-11 M yr-1. In such dust-deficient disks, H2 pumping enhances photoevaporation from the inner disk region and shapes the disk mass-loss profile. We thus argue that the late-stage disk evolution is affected by the ultra-violet H2 pumping effect. The mass-loss rates derived from our simulations can be used in the study of long-term disk evolution.