1996A&A...314..917L

We have taken λ18cm OH absorption and emission profiles at the VLA, 43m, and Nancay telescopes, toward and around a group of compact mm-wave continuum sources. The interstellar medium is well-characterized along these lines of sight by many recent measurements of H I and various molecules in absorption and emission. VLA OH absorption measurements at 1665 and 1667 MHz show no significant departures from LTE such as have been reported in the past toward molecular gas of moderate density. In comparing the OH emission and absorption lines, we confirm the anomalously low OH excitation temperatures for that limited portion of the gas which is seen both in absorption and emission. Surprisingly, we have discovered that there is a component of λ18cm OH emission which is absent in CO emission, and in OH, HCO⁺, and CO absorption; this OH does however coincide with the deepest λ21cm H I absorption features. We attribute this emission to chemical processes in the denser portions of the interstellar atomic gas. OH therefore exhibits a disparity between its absorption and emission spectra which is similar to H I. For OH, the non-absorbing OH could in principle be segregated in clumps of molecular high column density and low surface covering factor. Alternatively, as for H I, the disparity could arise from blending of components with widely-varying excitation temperature. There is a complex relationship between OH and CO seen in emission and between these and H I absorption. It is commonly the case that stronger ¹²CO emission features are selectively weaker in H I absorption and missing or not outstanding in OH emission. We attribute the relative weakness of the OH emission to low excitation rather than a declining abundance. The relationship to H I absorption can be explained by the presence of a partially molecular envelope around dark material, as observed in OH emission by Wannier et al. (1993ApJ...407..163W).