Photoevaporation of Molecular Clouds in Regions of Massive Star Formation as Revealed Through H$_2$ and Br$\gamma$ Emission

TL;DR

This study analyzes H$_2$ and Br$\gamma$ emission in regions of massive star formation to understand photoevaporation processes in photodissociation regions, revealing consistent spatial offsets and their dependence on radiation and geometry.

Contribution

It provides the first detailed measurements of spatial emission profiles in diverse PDRs and models the offsets using simple photoevaporative flow theories.

Findings

Br$\gamma$ emission declines rapidly with curvature radius.

H$_2$ peaks deeper into molecular clouds than Br$\gamma$.

Offsets between emissions are consistently around 10$^{17}$cm across regions.

Abstract

We examine new and pre-existing wide-field, continuum-corrected, narrowband images in H$_2$ 1-0 S(1) and Br$\gamma$ of three regions of massive star formation: IC 1396, Cygnus OB2, and Carina. These regions contain a variety of globules, pillars, and sheets, so we can quantify how the spatial profiles of emission lines behave in photodissociation regions (PDRs) that differ in their radiation fields and geometries. We have measured 450 spatial profiles of H$_2$ and Br$\gamma$ along interfaces between HII regions and PDRs. Br$\gamma$ traces photoevaporative flows from the PDRs, and this emission declines more rapidly with distance as the radius of curvature of the interface decreases, in agreement with models. As noted previously, H$_2$ emission peaks deeper into the cloud relative to Br$\gamma$, where the molecular gas absorbs far-UV radiation from nearby O-stars. Although PDRs in IC 1396, Cygnus OB2, and Carina experience orders of magnitude different levels of ionizing flux and have markedly differing geometries, all the PDRs have spatial offsets between Br$\gamma$ and H$_2$ on the order of $10^{17}$cm. There is a weak negative correlation between the offset size and the intensity of ionizing radiation and a positive correlation with the radius of curvature of the cloud. We can reproduce both the size of the offsets and the dependencies of the offsets on these other variables with simple photoevaporative flow models. Both Br$\gamma$ and H$_2$ 1-0 S(1) will undoubtedly be targeted in future JWST observations of PDRs, so this work can serve as a guide to interpreting these images.