The metallicity dependence of photoevaporation out of a protoplanetary disk
Ryohei Nakatani, University of Tokyo) - 12.6.17
Motivated by recent observations suggesting that a disk has shorter lifetime in lower-metallicity environments (e.g., Yasui et al. 2010), we present here the results of our recent radiation-hydrodynamic simulations of the photoevaporation of protoplanetary disks with various metallicities. We solve the time-dependent hydrodynamics with various chemical and thermal processes, also along with the transfer both of the extreme and far ultraviolet (EUV and FUV) irradiation from a central star. The grain temperature is determined by the balance between the irradiation and (re-)emission, which are solved within a hybrid scheme using radial ray-tracing combined with flux-limited diffusion. Our simulations cover a broad range of different metallicities, from 10-4 Zsun to 10 Zsun.
As a result, the photoevaporation rate, or the disk lifetime, largely varies with different metallicities owing to different efficiencies of the FUV photoevaporation. Interestingly, the resultant photoevaporation rate does not show a monotonic trend of the metallicity dependence, but has a peak around the sub-solar metallicity around 0.3 Zsun. For the upper side of Z > 0.3 Zsun, the photoevaporation rate decreases with increasing metallicity because the FUV photons cannot enter the deep interior of the disk due to larger dust extinction. For Z < 0.3 Zsun, on the other hand, the photoevaporation rate sharply declines with decreasing metallicity because the photoelectric heating becomes inefficient with the lower amount of grains. With extreme low metallicities of Z < 10-3 Zsun, the evaporation rate takes an almost constant value that is set by the EUV photoionization only.
Our results are in good agreement with the observations showing the short disk lifetimes in an outer part of the Galaxy where the metallicity is Z ~ 0.2 Zsun. We predict that the disk lifetime should become longer towards even lower metallicities.