Contributed Talk - Splinter ISM

Wednesday, 23 September 2020, 14:22   (virtual room F)

SOFIA FEEDBACK [CII] observations of RCW 120: a new paradigm of HII region bubble formation

S. Kabanovic, N. Schneider, C. Guevara, R. Simon, M. Luisi, L. Anderson, J. Stutzki and A. Tielens
1. Physikalisches Institut Uni Köln, Department of Physics and Astronomy West Virginia University

Stellar feedback of young massive stars by strong irradiation and winds impact their natal molecular cloud by forming HII regions and possibly triggering star formation. Observationally, the energetics and dynamics are traced by the main cooling line of gas: the [CII] 158 and [OI] 63 micron fine structure lines and the mid- to high-J CO lines. The SOFIA Legacy Program FEEDBACK (PIs N. Schneider and A. Tielens) focusses on studying stellar feedback effects by mapping 11 galactic high-mass star-formation regions in the [CII] and [OI] lines. We here present first results of the prototype bubble region RCW120, complemented by observations of 12CO (3-2) and 13CO (3-2) using APEX. The velocity resolved [CII] spectra have the unique nature to be a direct tracer of the expanding bubble velocity (Pabst et al., 2019, Nature, 565, 618). However, the [CII] fine structure line can be strongly affected by optical depth effects which can lead to misinterpretations of the observed kinematics. An analysis of the hyperfine transition line of the ionized carbon isotope [13CII] indeed revealed high optical depths along the bubble edge, where the expanding bubble merges with the surrounding molecular cloud. We model the optically thick [CII] emission by solving the radiative transfer equation for multiple components distributed between two layers. This analysis reveals a cold [CII] layer which would be undetected otherwise. We observe that the CO emission originates from photo-dissociated surfaces of clumps at the bubble edges, but the bubble interior is devoid of CO emission. This finding can be explained if the HII region starts to break out of a dense filament, similar to simulations presented in Haid et al. (2019, MNRAS, 482, 406). We propose that this is a common mechanisms observed for many HII region bubbles whose CII and CO emission could not be explained consistently before.