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Astron. Astrophys. 322, 615-623 (1997)

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Combined stellar structure and atmosphere models for massive stars

IV. The impact on the ionization structure of single star H II regions

Grayna Stasiska 1 and Daniel Schaerer 2

1 DAEC, Observatoire de Meudon, F-92195 Meudon Cedex, France (grazyna@obspm.fr)
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA (schaerer@stsci.edu)

Received 6 August 1996 / Accepted 25 October 1996


We study the impact of modern stellar atmospheres that take into account the effects of stellar winds, departures from LTE and line blanketing ("CoStar " models) on the ionization structure of H II regions. Results from a large grid of photoionization models are presented. Due to 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 [FORMULA] /O [FORMULA] and Ne [FORMULA] /O [FORMULA] can be safely used as direct indicators of N/O and Ne/O abundance ratios in H II regions, over a wide range of astrophysical situations.

The roughly constant observed value of Ne [FORMULA] /O [FORMULA] ionic ratios in Galactic H II regions is naturally reproduced by photoionization models using CoStar fluxes, while Kurucz models at solar metallicity fail to reproduce this behaviour. This gives support 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.

Key words: H ii regions – ISM: abundances – stars: atmospheres – stars: mass – loss – stars: early-type – ultraviolet: stars

Send offprint requests to: G. Stasiska


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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998