Optically Thick [Oi] and [Cii] Emission toward NGC 6334A

This work focuses on [O I] and [C II] emission towards NGC 6334 A, anembedded H+ region/PDR only observable at infrared or longer wavelengths. Ageometry where nearly all the emission escapes out the side of the cloud facingthe stars, such as Orion, is not applicable to this region. Instead, we findthe geometry to be one where the H+ region and associated PDR is embedded inthe molecular cloud. Constant density PDR calculations are presented whichpredict line intensities as a function of AV (or N(H)), hydrogen density (nH),and incident UV radiation field (G0). We find that a single component modelwith AV ~650 mag, nH = 5x10^5 cm-3, and G0 = 7x10^4 reproduces the observed [OI] and [C II] intensities, and that the low [O I] 63 to 146 micron ratio is dueto line optical depth effects in the [O I] lines, produced by a large columndensity of atomic/molecular gas. We find that the effects of a density-lawwould increase our derived AV, while the effects of an asymmetric geometrywould decrease AV, with the two effects largely canceling. We conclude thatoptically selected H+ regions adjacent to PDRs, such as Orion, likely have adifferent viewing angle or geometry than similar regions detected through IRobservations. Overall, the theoretical calculations presented in this work haveutility for any PDR embedded in a molecular cloud.Comment: 27 pages, 8 figures, accepted for publication in the Astrophysical Journa