Ionising feedback from an O star formed in a shock-compressed layer

We develop a simple analytic 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 HII Region (HIIR). The model is characterised by three parameters: the half-thickness of the undisturbed layer, Zlay, the mean number-density of hydrogen molecules in the undisturbed layer, nlay, and the (collective) ionising output of the star(s), NdotLyC. The radius of the circular hole is given by WIF ~ 3.8 pc [Zlay/0.1pc]^{-1/6} [nlay/10^4cm^{-3}]^{-1/3} [NdotLyC/10^{49} s^{-1}]^{1/6} [t/Myr]^{2/3}. Similar power-law expressions are obtained for the rate at which ionised gas is fed into the bipolar lobes; the rate at which molecular gas is swept up into a dense ring by the shock front (SF) that precedes the ionisation front (IF); and the density in this dense ring. We suggest that our model might be a useful zeroth-order representation of many observed HIIRs. From viewing directions close to the midplane of the layer, the HIIR 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.