Self-absorption in CII, (12) CO, and HI in RCW 120:

It is a long-lasting question to which extent expanding HII regions that appear circular in IR images are indeed 3D bubbles. We investigated the 3D structure of the archetypical HII region RCW 120, using CII $158\,{\rm \mu m}$ observations from the SOFIA FEEDBACK legacy project, $^{12}$CO and $^{13}$CO (3$\to$2) lines from APEX, the HI 21$\,$cm line and continuum data from the CGPS. Two radiative transfer models were used on the set of observed lines. A line profile analysis with the 1D non-LTE radiative transfer code SimLine shows that the CO emission cannot stem from a spherically symmetric molecular cloud configuration. With the multicomponent two-layer model, we quantified the amount of material located in the warm emitting background layer and the cold absorbing foreground. Hereby we obtained that a substantial amount of the C$^+$ column is hidden in a cold absorbing foreground layer. From HI self-absorption studies (HISA), we find that this surrounding layer is atomic with $T \sim 15-30\,{\rm K}$, has a low density $\sim 100-500\,{\rm cm^{-3}}$ and an extend of about $\sim 5-20\,{\rm pc}$. To determine the spatial distribution of the warm emitting and cold absorbing layer, we apply a Gaussian mixture model to cluster the data cube by spectra with similar spectral shapes. This allows us to initialize the two-layer model for an entire spectral cube with a few initial parameters.

The model results suggest that RCW 120 developed out of a flat molecular cloud, enveloped by HI, and most of the CO is located inside the torus compressed by the stellar wind from the central star.