Contributed Talk - Splinter Stars

Wednesday, 15 September 2021, 10:00   (virtual Stars)

The importance of self-absorption in [CII], HI, and CO lines

Slawa Kabanovic and the FEEDBACK consortium
1. Physikalisches Institut Uni Köln

Large-scale mapping of the [CII] 158μm line in the RCW120 HII region, obtained with the SOFIA legacy program FEEDBACK, revealed an expanding bubble, alongside strong self-absorption effects in the [CII] line. The self-absorbtion is observed at velocities of the bulk emission, and corresponds to self-absorbtion features in atomic hydrogen (HISA) and 12CO (3-2). We also observe a deficit of CO emission along the line-of-sight towards the bubble center. To disentangle the self-absorbtion effects from the geometry of the region, we solve the radiative transfer equations for a two-layer multicomponent model. For the CO spectral cube, 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 can be the gas component responsible for the so far unexplained but observed large amounts of cold [CII].