RCW 120: A possible case of hit and run, elucidated by multi temperature
  dust mapping
  We present resolution-enhanced images of warm dust at multiple temperatures
and opacity index values in the star-forming bubble/HII region, RCW 120. The
image set, representing a 4D hypercube of differential column density, was
obtained using our Bayesian procedure, ppmap. The cool peripheral material
($\sim16$-22 K) exhibits ragged clumpy structure as noted previously by others.
However, at higher temperatures ($\stackrel{>}{_\sim}26$ K) the geometry
changes dramatically, showing a bubble boundary which is accurately circular in
projection, except for the previously-reported opening in the north. Comparison
with Spitzer 8 $\mu$m data suggests that the $\sim26$-30 K dust seen by
Herschel resides in the photodissociation region (PDR) surrounding the HII
region. Its projected radial profile is consistent with that of a spherical
shell, thus arguing against previous suggestions of cylindrical or planar
geometry. The inferred geometry is, in fact, consistent with previous
interpretations of the HII region as a classical Str\"omgren sphere, except for
the fact that the ionising star (CD -38.11636; O8V) is displaced by more than
half a radius from its geometric centre. None of the previously published
models has satisfactorily accounted for that displacement. It could, however,
be explained by proper motion of the O star at $\sim2$-4 km s$^{-1}$ since its
formation, possibly due to a cloud-cloud collision. We suggest that the current
spherical bubble constitutes the fossilised remnant of the initial expansion of
the HII region following the formation of the star, which now continues to flee
its formation site.
