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S. Kabanovic, N. Schneider, V. Ossenkopf-Okada, F. Falasca, R. Simon, M. Luisi, L. Anderson, J. Stutzki, R. Güsten, C. Guevara and A.G.G.M. Tielens
1. Physikalisches Institut, Universität zu Köln

SOFIA FEEDBACK observations reveal the distribution and physical conditions of the cold material in front of RCW 120

Stellar feedback from young massive stars in the form of radiation and wind impact their environment, clearing out the natal molecular cloud and creating HII regions in bubble-like structures. Feedback can lead to a compressed molecular shell that fragments to dense clumps that collapse to form stars. The dynamics and energetics of UV-illuminated gas is best traced by the [CII] 158 and [OI] 63 micron fine structure lines and the SOFIA Legacy program FEEDBACK (PIs N. Schneider and A. Tielens) focusses on mapping 11 galactic high-mass star-formation regions in these tracers. We here present a study of the [CII] line emission of the prototypical bubble RCW 120, complemented by 12CO (3-2) and 13CO (3-2) data from APEX. The [CII] spectra reveal an expanding bubble (Luisi et al. 2021), but a mostly static molecular gas distribution in CO, arranged in a ring-like structure in the plane of the sky, and a deficit of CO emission along the line-of-sight to the bubble center. The [CII] line is strongly self-absorbed at velocities of the bulk emission, and corresponds to self-absorption features in atomic hydrogen (HISA). To disentangle the geometry of background and absorbing foreground, we solve the radiative transfer equations for a two-layer multicomponent model. For CO, we employ the gaussian mixture model, which is an unsupervised machine learning approach to cluster spectra. The derived column densities of the cold absorbing layer can not explain the observed molecular deficit towards the center of the ring, suggesting that RCW 120 emerged from a sheet like geometry and that the molecular cloud has a flat geometry. The large column densities of cold [CII] can be explained by an extended (~5 pc) atomic cloud in which carbon is ionized by cosmic rays and X- rays. We propose that these HI envelopes around molecular clouds are the gas component responsible for the observed large amounts of cold [CII].