The Effect of Ultraviolet Photon Pumping of H2 in Dust-deficient Protoplanetary Disks

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 in which we varied the number 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 more strongly to the mass loss of the disk when the dust-to-gas mass ratio is ${ \mathcal D }\leqslant {10}^{-3}$ . The disk mass-loss rate decreases with a lower dust amount, but remains around 10^‑10‑11 M _⊙yr^‑1. In these 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 ultraviolet H₂ pumping effect. The mass-loss rates derived from our simulations can be used in the study of long-term disk evolution.