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