Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence

Authors:
Nakatani et al

Abstract:

Protoplanetary disks are thought to have lifetimes of 3−6 million years 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 photoevaporation of protoplanetary disks to study the disk structure and its long-term evolution of ∼10000 years, and the metallicity dependence of mass-loss rate. Our simulations follow hydrodynamics, extreme and far ultraviolet radiative transfer, and non-equilibrium 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 gas metallicity over a wide range of 10−4Z⊙≤Z≤10 Z⊙. For our fiducial model with a 0.5 M⊙ central star with solar metallicity, the time-averaged photoevaporation rate is M˙ph=1.38×10−8M⊙yr−1. The photoevaporation rate is lower with higher metallicity in the range of 10−0.5Z⊙≲Z≲10 Z⊙, because dust shielding effectively prevents far-ultra violet (FUV) photons from penetrating into and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in 10−1Z⊙≲Z≲10−0.5Z⊙, because FUV photoelectric heating is not efficient any more to raise the gas temperature and to drive outflows. At 10−4Z⊙≤Z≲10−1Z⊙, HI photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with roughly a 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.