Temperature Structure of the Pipe Nebula Studied by the Intensity Anomaly of the OH 18 cm

TL;DR

This study uses OH 18 cm line observations to determine the temperature structure of the Pipe nebula, revealing localized heating likely due to filament interactions rather than UV radiation from nearby stars.

Contribution

It provides the first detailed temperature mapping of the Pipe nebula using OH hyperfine lines, supporting filament-filament collision as a heating mechanism.

Findings

Diffuse gas temperature ranges from 40 K to 75 K.

UV heating from $ heta$-Ophiuchi is not significant in the stem.

Elevated temperatures at filament interfaces support collision scenario.

Abstract

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 K to 75 K. From this result, we assess the heating effect on the filamentarystructure of the nebula's "stem" region due to UV photons from a nearby star $\theta$-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 $\theta$-Ophiuchi: the UV-heating effect by the star is not visible. On the other hand, the gas kinetic temperature is raised, as high as $\sim$75 K, at the interface of the two filamentary structures. This result provides us with an additional support to the filament-filament collision scenario in the Pipe nebula.