 Ionization-bounded nebula

An ionization-bounded nebula contains enough absorbing material to trap the relevant ionizing photons, unlike a density-bounded region that ends before its ionization front. In dust-free hydrogen at steady photoionization equilibrium, the absorbed stellar photon rate equals the integrated Case B recombination rate. Escape, absorption by dust, and competition from other species must be accounted for before identifying the absorbed hydrogen rate with the entire stellar ionizing luminosity.