RCW 120: A possible case of hit and run, elucidated by multi temperature dust mapping

Abstract: 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.