Mopra CO observations of the bubble H II region RCW 120
Résumé
We use the Mopra radio telescope to test for expansion of the molecular gas associated with the bubble H II region RCW120. 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 $\rightarrow$ 0 transitions of $^{12}$CO, $^{13}$CO,C$^{18}$O and C$^{17}$O, and detect emission from all isotopologues. We do not detect the 0$_0$ $\rightarrow$ 1$_{-1}E$ masing lines of CH$_3$OH 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 RCW120 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 RCW120, and is consistent with an expansion speed of less than or similar to 1.5km 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 RCW120 coincide with a deficit of emission in CO, confirming that these temperature enhancements are due to holes in the RCW120 PDR. H-alpha emission shows that RCW120 is leaking similar to 5% of the ionizing photons into the interstellar medium (ISM) through PDR holes at the locations of the temperature enhancements. Ha emission also shows a diffuse "halo" from leaked photons not associated with discrete holes in the PDR. Overall similar to 25% +/- 10% of all ionizing photons are leaking into the nearby ISM.
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