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

TL;DR

This study models how metallicity and dust influence the growth of HII regions around massive stars, revealing that lower metallicity leads to larger ionized regions and that radiation pressure and dust absorption have complex roles.

Contribution

It introduces a comprehensive simulation of star-forming regions incorporating metallicity-dependent cooling, dust effects, and feedback mechanisms, advancing understanding of HII region evolution.

Findings

Lower metallicity results in higher temperatures and larger HII regions.

Radiation pressure can dominate locally in high-metallicity environments.

Dust UV absorption can hinder HII region growth more than radiation pressure promotes it.

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.