2018ApJ...869...77C

We examine new and pre-existing wide-field, continuum-corrected, narrowband images in H₂ 1-0 S(1) and Brγ 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 H₂ and Brγ along interfaces between H II regions and PDRs. Brγ 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, H₂ emission peaks deeper into the cloud relative to Brγ, 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 of the PDRs have spatial offsets between Brγ and H₂ on the order of 10¹⁷cm. 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 Brγ and H₂ 1-0 S(1) will undoubtedly be targeted in future James Webb Space Telescope observations of PDRs, so this work can serve as a guide to interpreting these images.