Photoevaporation of Molecular Gas Clumps Illuminated by External Massive Stars: Clump Lifetimes and Metallicity Dependence

Abstract

We perform a suite of 3D radiation hydrodynamics simulations of photoevaporation of molecular gas clumps illuminated by external massive stars. We study the fate of solar-mass clumps and derive their lifetimes with varying the gas metallicity over a range of $10^{-3} \,Z_\odot \leq Z \leq Z_\odot $. Our simulations incorporate radiation transfer of far ultraviolet (FUV) and extreme ultraviolet (EUV) photons, and follow atomic/molecular line cooling and dust-gas collisional cooling. Nonequilibrium chemistry is coupled with the radiative transfer and hydrodynamics in a self-consistent manner. We show that radiation-driven shocks compress gas clumps to have a volume that is set by the pressure-equilibrium with the hot ambient gas. Radiative cooling enables metal-rich clumps to condense and to have small surface areas, where photoevaporative flows are launched. For our fiducial set-up with an O-type star at a distance of 0.1 parsec, the resulting photoevaporation rate is as small as $\sim 10^{-5} M_{\odot}/{\rm yr}$ for metal-rich clumps, but is larger for metal-poor clumps that have larger surface areas. The clumps are continuously accelerated away from the radiation source by the so-called rocket effect, and can travel over $\sim$1 parsec within the lifetime. We also study photoevaporation of clumps in a photo-dissociation region. Photoelectric heating is inefficient for metal-poor clumps that contain a smaller amount of grains, and thus they survive for over $10^5$ years. We conclude that the gas metallicity strongly affects the clump lifetime and thus determines the strength of feedback from massive stars in star-forming regions.