Structure of a merged H II region/PDR model, for H II region electron density ﹩n_{e}=10﹩ cm⁻³ and number of H‐ionizing photons ﹩\Phi _{i}=10^{49}﹩ s⁻¹. The star cluster is to the left, and the H II region extends to ~﹩3.3\times 10^{19}﹩ cm. The transition region (see § 3.1) is indicated by the dotted lines. Top: Temperature and density. Middle: Fraction of H nuclei that are ionized (H II), neutral atomic (H I), or molecular (H₂). Bottom: Fractional abundances of C⁺, Si⁺, and Fe⁺.