The H+ region contribution to [C II] 158-μm emission

The [CII] 158-mu m line is an important emission-line diagnostic in photodissociation regions (PDRs), but this emission line can also emerge from ionized gas. This work calculates the contribution of [C II] emission from ionized gas over a wide range of parameter space by considering the simplified case of an H+ region and PDR in pressure equilibrium. Additionally, these calculations also predict the strong correlation observed between [N II] 205-mu m emission and [C II] discussed by previous authors. Overall, the results of these calculations have wide-ranging applications to the interpretation of [C II] emission in astrophysical environments.