GMCs and Star Formation in the Galaxy: I. Effects of an HII region on a GMC

TL;DR

This paper models how HII regions destroy giant molecular clouds (GMCs) in galaxies, analyzing different expansion behaviors, mass loss, and the impact of various parameters on cloud destruction.

Contribution

It provides a theoretical framework for understanding GMC destruction by HII regions, including analytic fits and mass loss predictions based on cloud and OB association properties.

Findings

Associations with S > S_{com} destroy at least 70% of the cloud

Mass loss scales with ionizing luminosity S as S^p, p~0.45-0.75

A critical cloud mass M_{survive} determines if clouds are fully destroyed or not.

Abstract

The destruction of Giant Molecular Clouds is a key component in galaxy evolution. We theoretically model the destruction of GMCs by HII regions, which evaporate ionized gas and eject neutral gas during their expansion. HII regions follow one of three tracks, depending on the EUV luminosity, $S$, of the ionizing OB association: the expansion can stall inside the cloud; it can break out, forming a blister (champagne) flow; or, for $S>S_{\rm com}$, it can result in the formation of a cometary cloud. We present results for the accumulated mass loss, $M_{\rm loss}(t)$, and the final mass loss, $M_{{\rm loss},f}$, by evaporation and ejection for a range of cloud masses ($10^4<M<10^{7}$ M$_\odot$), cloud surface densities ($50<\Sigma<1000$ M$_\odot$ pc$^{-2}$), OB association luminosities ($10^{44}<S<10^{52}$ s$^{-1}$), and off-center position of the association. We do not consider starbursts; our neglect of radiation pressure restricts our treatment to $S<10^{52} [(M/10^6$ M$_\odot)^{0.3})/(\Sigma /100$ M$_\odot$ pc$^{-2}$)] s$^{-1}$, and our neglect of gravity restricts $(M/10^6$ M$_\odot$)($\Sigma/ 100$ M$_\odot$ pc$^{-2}) < 10$. We find that $M_{{\rm loss},f}$ for the range $0.1 < M_{{\rm loss},f}/M < 0.7$ , is proportional to $S^p$, where $p\sim 0.45-0.75$ depends on $M$, $\Sigma$, and association position. We find analytic fits to $S_{\rm com}$ as a function of $\Sigma$, $M$, and association position. $S> S_{\rm com}$ associations destroy at least 70% of the initial cloud. We find a critical cloud mass $M_{\rm survive}$ above which clouds never become cometary and lose $<$ 70% of their mass via a single association. Low mass clouds mostly lose mass via ejection of neutral gas.