Molecular envelope around the HII region RCW 120

Abstract

The H II region RCW 120 is a well-known object, which is often considered as a target to verify theoretical models of gas and dust dynamics in the interstellar medium. However, the exact geometry of RCW 120 is still a matter of debate. In this work, we analyse observational data on molecular emission in RCW 120 and show that 13CO(2-1) and C18O(2-1) lines are fitted by a 2D model representing a ring-like face-on structure. The changing of the C18O(3-2) line profile from double-peaked to single-peaked from the dense molecular Condensation 1 might be a signature of stalled expansion in this direction. In order to explain a self-absorption dip of the 13CO(2-1) and 13CO(3-2) lines, we suggest that RCW 120 is surrounded by a diffuse molecular cloud, and find confirmation of this cloud on a map of interstellar extinction. Optically thick 13CO(2-1) emission and the infrared 8 μm PAH band form a neutral envelope of the H II region resembling a ring, while the envelope breaks into separate clumps on images made with optically thin C18O(2-1) line and far-infrared dust emission.