Recent observations have found shorter lifetimes of protoplanetary disks (PPDs) in low-metallicity environments than in the solar neighborhood (Yasui et al. 2009, 2010). It suggests a more efficient disk dispersal with decreasing metallicity. Prior studies have shown that photoevaporation is one of the essential disk-dispersing mechanisms that can yield sufficient mass-loss rates consistent with observed disk lifetimes. Ercolano & Clarke (2010) have demonstrated that EUV/X-ray photoevaporation potentially explains the shorter disk lifetimes for low-metallicity PPDs.
In our studies, we implement photoelectric heating due to FUV as well as photoionization heating due to EUV/X-ray and examine the effects on thermochemical structures PPDs. We perform a suite of radiation hydrodynamics simulations, varying disk metallicities, to study the effects of metallicity on thermochemical structures and photoevaporation. Our simulations self-consistently solve hydrodynamics, radiative transfer, and nonequilibrium chemistry. We also consistently determine grain temperatures with 2D radiative transfer.
The results show increasing mass-loss rates as metallicity decreases at sub-solar metallicities owing to the reduced opacity of the disk. It is consistent with the observational trend that the lifetimes are shorter in low metallicity environments. At even lower metallicities, dust-gas collisional cooling remains efficient compared to FUV photoelectric heating. The disk temperatures are too low to drive strong photoevaporation regardless of FUV heating. For further lower metallicities, dynamical time is shorter than the heating or cooling timescale, and thus the atmosphere of PPDs becomes effectively adiabatic. Overall, our results show metallicity significantly affects the thermochemical structures and dynamics of the PPD atmosphere.