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

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