Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks by Ultraviolet Radiation: Metallicity Dependence

Protoplanetary disks are thought to have lifetimes of several million yr in the solar neighborhood, but recent observations suggest that the disk lifetimes are shorter in a low-metallicity environment. We perform a suite of radiation hydrodynamics simulations of photoevaporating protoplanetary disks to study their long-term evolution of ∼10,000 yr and the metallicity dependence of mass-loss rates. Our simulations follow hydrodynamics, extreme and far-ultraviolet (FUV) radiative transfer, and nonequilibrium chemistry in a self-consistent manner. Dust-grain temperatures are also calculated consistently by solving the radiative transfer of the stellar irradiation and grain (re-)emission. We vary the disk metallicity over a wide range of {10}^-4 {Z}_⊙ ≤slant Z≤slant 10 {Z}_⊙ . The photoevaporation rate is lower with higher metallicity in the range of {10}^-1 {Z}_⊙ ≲ Z≲ 10 {Z}_⊙ , because dust shielding effectively prevents FUV photons from penetrating and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in {10}^-2 {Z}_⊙ ≲ Z≲ {10}^-1 {Z}_⊙ , because FUV photoelectric heating becomes less effective than dust-gas collisional cooling. The temperature in the neutral region decreases, and photoevaporative flows are excited only in an outer region of the disk. At {10}^-4 {Z}_⊙ ≤slant Z≲ {10}^-2 {Z}_⊙ , H I photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with a roughly constant rate. The typical disk lifetime is shorter at Z = 0.3 {Z}_⊙ than at Z={Z}_⊙ , being consistent with recent observations of the extreme outer galaxy.