The growth of H II regions around massive stars: the role of metallicity and dust

Abstract

Gas metallicity $Z$ and the related dust-to-gas ratio $f_\textrm{d}$ can influence the growth of HII regions via metal line cooling and UV absorption. We model these effects in star-forming regions containing massive stars. We compute stellar feedback from photoionization and radiation pressure (RP) using Monte Carlo radiative transfer coupled with hydrodynamics, including stellar and diffuse radiation fields. We follow a $10^5$ M$_\odot$ turbulent cloud with $Z/$Z$_\odot$ = 2, 1, 0.5, 0.1 and $f_\textrm{d} = 0.01 Z/$Z$_\odot$ with a cluster-sink particle method for star formation. The models evolve for at least 1.5 Myr under feedback. Lower $Z$ results in higher temperatures and therefore larger HII regions. For $Z \ge$Z$_\odot$, radiation pressure $P_\textrm{rad}$ can dominate locally over the gas pressure $P_\textrm{gas}$ in the inner half-parsec around sink particles. Globally, the ratio of $P_\textrm{rad}/P_\textrm{gas}$ is around 1 (2 Z$_\odot$), 0.3 (Z$_\odot$), 0.1 (0.5 Z$_\odot$), and 0.03 (0.1 Z$_\odot$). In the solar model, excluding RP results in an ionized volume several times smaller than the fiducial model with both mechanisms. Excluding RP and UV attenuation by dust results in a larger ionized volume than the fiducial case. That is, UV absorption hinders growth more than RP helps it. The radial expansion velocity of ionized gas reaches $+$15 km/s outwards, while neutral gas has inward velocities for most of the runtime, except for 0.1 Z$_\odot$ which exceeds $+$4 km/s. $Z$ and $f_\textrm{d}$ do not significantly alter the star formation efficiency, rate, or cluster half-mass radius, with the exception of 0.1 Z$_\odot$ due to the earlier expulsion of neutral gas.