Mopra CO Observations of the Bubble HII Region RCW120
  We use the Mopra radio telescope to test for expansion of the molecular gas
associated with the bubble HII region RCW120. A ring, or bubble, morphology is
common for Galactic HII 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
photodissociation 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 HII region like RCW120, and is consistent with an
expansion speed of $< 1.5\, {\rm km\, s^{-1}}$. 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 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 $\sim5\%$ of the ionizing photons into the interstellar
medium (ISM) through PDR holes at the locations of the temperature
enhancements. H-alpha emission also shows a diffuse "halo" from leaked photons
not associated with discrete holes in the PDR. Overall $25\pm10\%$ of all
ionizing photons are leaking into the nearby ISM.
