Disruption of Molecular Clouds by Expansion of Dusty H II Regions

TL;DR

This study models how expanding dusty H II regions disrupt molecular clouds, revealing that radiation pressure and star formation efficiency are key factors in cloud dispersal, with disruption occurring rapidly after star cluster formation.

Contribution

It introduces a semi-analytic and numerical approach to analyze dusty H II region expansion and cloud disruption, highlighting the roles of radiation pressure and star formation efficiency.

Findings

Typical giant molecular clouds require less than 10% star formation efficiency for disruption.

Massive cluster-forming clumps need over 50% efficiency for cloud dispersal.

Disruption occurs on a timescale comparable to the free-fall time.

Abstract

Dynamical expansion of H II regions around star clusters plays a key role in dispersing the surrounding dense gas and therefore in limiting the efficiency of star formation in molecular clouds. We use a semi-analytic method and numerical simulations to explore expansion of spherical dusty H II regions and surrounding neutral shells and the resulting cloud disruption. Our model for shell expansion adopts the static solutions of Draine (2011) for dusty H II regions and considers the contact outward forces on the shell due to radiation and thermal pressures as well as the inward gravity from the central star and the shell itself. We show that the internal structure we adopt and the shell evolution from the semi-analytic approach are in good agreement with the results of numerical simulations. Strong radiation pressure in the interior controls the shell expansion indirectly by enhancing the density and pressure at the ionization front. We calculate the minimum star formation efficiency $\epsilon_{min}$ required for cloud disruption as a function of the cloud's total mass and mean surface density. Within the adopted spherical geometry, we find that typical giant molecular clouds in normal disk galaxies have $\epsilon_{min} \lesssim 10$%, with comparable gas and radiation pressure effects on shell expansion. Massive cluster-forming clumps require a significantly higher efficiency of $\epsilon_{min} \gtrsim 50$% for disruption, produced mainly by radiation-driven expansion. The disruption time is typically of the order of a free-fall timescale, suggesting that the cloud disruption occurs rapidly once a sufficiently luminous H II region is formed. We also discuss limitations of the spherical idealization.