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