Optically thick [O I] and [C II] emission toward NGC 6334A

This work focuses on [O I] and [C II] emission toward NGC 6334A, an embedded H⁺ region/PDR only observable at infrared or longer wavelengths. A geometry in which nearly all the emission escapes out the side of the cloud facing the stars, such as Orion, is not applicable to this region. Instead, we find the geometry to be one in which the H⁺ region and associated PDR is embedded in the molecular cloud. Constant-density PDR calculations are presented which predict line intensities as a function of A_V [or N(H)], hydrogen density (n_H), and incident UV radiation field (G₀). We find that a single-component model with A_V ∼ 650 mag, n_H = 5 × 10⁵ cm ³, and GO = 7 × 10⁴ reproduces the observed [O I] and [C II] intensities, and that the low [O II 63 to 146 μm ratio is due to line optical depth effects in the [O I] lines, produced by a large column density of atomic/molecular gas. We find that the effects of a density law would increase our derived A_V, while the effects of an asymmetric geometry would decrease A_V, with the two effects largely canceling. We conclude that optically selected H⁺ regions adjacent to PDRs, such as Orion, likely have a different viewing angle or geometry than similar regions detected through IR observations. Overall, the theoretical calculations presented in this work have utility for any PDR embedded in a molecular cloud.