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
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 /O and Ne /O 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 /O 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
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998