RCW 120: a possible case of hit and run, elucidated by multitemperature dust mapping

We present resolution-enhanced images of warm dust at multiple temperatures and opacity index values in the star-forming bubble/H II region, RCW 120. The image set, representing a four-dimensional hypercube of differential column density, was obtained using our Bayesian procedure, PPMAP. The cool peripheral material (∼16–22 K) exhibits ragged clumpy structure as noted previously by others. However, at higher temperatures (≲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 μμm data suggests that the ∼26–30 K dust seen by Herschel resides in the photodissociation region (PDR) surrounding the H II 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 H II region as a classical Strömgren sphere, except for the fact that the ionizing 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 ∼2–4 km s^(−1) since its formation, possibly due to a cloud–cloud collision. We suggest that the current spherical bubble constitutes the fossilized remnant of the initial expansion of the H II region following the formation of the star, which now continues to flee its formation site.