1996A&A...305L..37S

By means of realistic model atmospheres for O-type stars, we investigate the influence of EUV radiation on the ionization structure of HII regions. Our model calculations are based on a detailed multilevel NLTE treatment including radiation-driven wind theory and a consistent calculation of NLTE line blocking opacities for all contributing ionization stages. This is especially important since the spectral distribution of our ionizing fluxes is mainly influenced by the two partly compensating processes of (i) metal line blocking and (ii) the effect of a stellar wind. Even very low mass-loss rates affect the subsonic density structure and, consequently, the emergent flux. By applying our emergent fluxes to a standard nebular code we find, compared to other investigations, an improved match between the observed and calculated ionization structure. In particular, the relatively high ionization fraction of Ne⁺⁺ is well reproduced by our calculations. Thus, our emergent ionizing fluxes provide a solution to the persistent [NeIII] problem in H II regions. In addition, the fractions of O⁺⁺ and S⁺⁺ are changed with the consequence that the relation between the ratio O⁺⁺/S⁺⁺ and the temperature of the ionizing source is modified.