2020ApJ...904..136E

We present observations of the four hyperfine structure components of the OH 18 cm transition (1612, 1665, 1667, and 1720 MHz) toward a filamentary dark cloud, the Pipe nebula, with the Green Bank Telescope. A statistical equilibrium analysis is applied to the spectra, and the kinetic temperature of a diffuse molecular gas surrounding dense cores is determined accurately; the derived temperature ranges from 40 to 75 K. From this result, we assess the heating effect on the filamentary structure of the nebula's "stem" region due to UV photons from a nearby star θ-Ophiuchi and a possible filament-filament collision in the interface of the "stem" and "bowl" regions. In the stem region, the gas kinetic temperature is found to be almost independent of the apparent distance from θ-Ophiuchi: the UV-heating effect by the star is not visible. On the other hand, the gas kinetic temperature is raised, as high as ∼75 K, at the interface of the two filamentary structures. This result provides us with additional support for the filament-filament collision scenario in the Pipe nebula.