MOPRA CO OBSERVATIONS OF THE BUBBLE H ii REGION RCW 120

We use the Mopra radio telescope to test for expansion of the molecular gas associated with the bubble H ii region RCW 120. A ring, or bubble, morphology is common for Galactic H ii regions, but the three-dimensional geometry of such objects is still unclear. Detected near- and far-side expansion of the associated molecular material would be consistent with a three-dimensional spherical object. We map the J = 1 → 0 transitions of 12CO, 13CO, C18O, and C17O, and detect emission from all isotopologues. We do not detect the 00 → 1−1E masing lines of CH3OH at 108.8939 GHz. The strongest CO emission is from the photo-dissociation region (PDR), and there is a deficit of emission toward the bubble interior. We find no evidence for expansion of the molecular material associated with RCW 120 and therefore can make no claims about its geometry. The lack of detected expansion is roughly in agreement with models for the time-evolution of an H ii region like RCW 120, and is consistent with an expansion speed of ≲ 1.5 km s−1. Single-position CO spectra show signatures of expansion, which underscores the importance of mapped spectra for such work. Dust temperature enhancements outside the PDR of RCW 120 coincide with a deficit of emission in CO, confirming that these temperature enhancements are due to holes in the RCW 120 PDR. H-alpha emission shows that RCW 120 is leaking ∼5% of the ionizing photons into the interstellar medium (ISM) through PDR holes at the locations of the temperature enhancements. Hα emission also shows a diffuse “halo” from leaked photons not associated with discrete holes in the PDR. Overall ∼25% ± 10% of all ionizing photons are leaking into the nearby ISM.