COMBINED STELLAR STRUCTURE AND ATMOSPHERE MODELS FOR MASSIVE STARS .4. THE IMPACT ON THE IONIZATION STRUCTURE OF SINGLE STAR H-II REGIONS

We study the impact of modern stellar atmospheres that take into accou nt the effects of stellar winds, departures from LTE and line blanketi ng (''CoStar'' models) on the ionization structure of H II regions. Re sults from a large grid of photoionization models are presented. Due t o a flatter energy distribution in the He I continuum, compared to the widely used Kurucz models, generally higher ionic ratios are obtained . We find that N+/O+ and Ne++/O++ can be safely used as direct indicat ors of N/O and Ne/O abundance ratios in H II regions, over a wide rang e of astrophysical situations. The roughly constant observed value of Ne++/O++ ionic ratios in Galactic H rt regions is naturally reproduced by photoionization models using CoStar fluxes, while Kurucz models at solar metallicity fail to reproduce this behaviour. This gives suppor t to ionizing fluxes from non-LTE atmospheres including stellar winds and line blanketing. However, we also point out that tests of stellar atmosphere models from observations of H II regions are hampered by a lack of strong constraints on the ionization parameter.