2022MNRAS.517.4940W

We develop a simple analytical model for what happens when an O star (or compact cluster of OB stars) forms in a shock-compressed layer and carves out an approximately circular hole in the layer, at the waist of a bipolar H II region (H IIR). The model is characterized by three parameters: the half-thickness of the undisturbed layer, $Z_{_{\rm LAYER}}$, the mean number density of hydrogen molecules in the undisturbed layer, $n_{_{\rm LAYER}}$, and the (collective) ionizing output of the star(s), $\dot{\cal N}_{_{\rm LyC}}$. The radius of the circular hole is given by $W_{_{\rm IF}}(t)\sim 3.8\, {\rm pc}\, [Z_{_{\rm LAYER}}/0.1\,{\rm pc}]^{-1/6}[n_{_{\rm LAYER}}/10^4\,{\rm cm^{-3}}]^{-1/3}[\dot{\cal N}_{_{\rm LyC}}/10^{49}\,{\rm s^{-1}}]^{1/6}[t/{\rm Myr}]^{2/3}$. Similar power-law expressions are obtained for the rate at which ionized gas is fed into the bipolar lobes, the rate at which molecular gas is swept up into a dense ring by the shock front that precedes the ionization front, and the density in this dense ring. We suggest that our model might be a useful zeroth-order representation of many observed H IIRs. From viewing directions close to the mid-plane of the layer, the H IIR will appear bipolar. From viewing directions approximately normal to the layer, it will appear to be a limb-brightened shell but too faint through the centre to be a spherically symmetric bubble. From intermediate viewing angles, more complicated morphologies can be expected.