Astron. Astrophys. 322, 598-614 (1997)

Combined stellar structure and atmosphere models for massive stars

III. Spectral evolution and revised ionizing fluxes of O3-B0 stars

Daniel Schaerer ¹ and Alex de Koter <sup>2

1</sup> Geneva Observatory, CH-1290 Sauverny, Switzerland
² NASA/GSFC, Advanced Computer Concepts, Code 681, Greenbelt, MD 20771, USA

Received 17 May 1996 / Accepted 25 October 1996

Abstract

We provide an extensive set of theoretical spectral energy distributions of massive stars derived from our "combined stellar structure and atmosphere models". The calculations cover the entire main sequence evolution for initial masses 20 - 120 , corresponding to O3-B0 stars of all luminosity classes.

We predict detailed line blanketed UV spectra along the main sequence evolution. The major result is a systematic study of ionizing fluxes covering the entire parameter space of O and early B stars. We demonstrate the importance of accounting simultaneously for non-LTE effects, line blanketing and stellar winds to obtain an accurate description of the spectra of these stars shortward of the Lyman limit. The main results from our spectra are the following:

• The flux in the He II continuum is increased by 2 to 3 (3 to 6) orders of magnitudes compared to predictions from plane parallel non-LTE (LTE) model atmospheres. This reconfirms the work of Gabler et al.  (1989).
• The flux in the He I continuum is known to be increased due non-LTE effects. However, we find that it is also influenced by wind effects as was previously found by Najarro et al.  (1996) and Schaerer et al.  (1996b). The combined effect of a mass outflow and line blanketing leads to a flatter energy distribution in the He I continuum, which confirms the results of Sellmaier et al.  (1996) for a wider range of stellar parameters.
• The flux in the Lyman continuum is also modified due to line blanketing and the presence of a stellar winds, although to a lesser degree than the spectrum at higher energies.

We derive revised ionizing fluxes for O3 to B0 stars based on the recent temperature and gravity calibrations of Vacca et al.  (1996). The total number of Lyman continuum photons is found to be slightly lower than previous derivations. For most cases the differences are less than 20 %. Due to the increased flux in the He I continuum the hardness ratio of the He I to H continuum is increased by 1.6 to 2.5 depending on spectral type and luminosity class.

In the view of recent EUV and X-ray observations, a critical discussion of current model assumptions (including our own) shows that for stars of spectral types later than approximately B0, which have relatively weak stellar winds, reliable predictions of ionizing fluxes are not yet possible. We identify the most likely physical reasons for this finding.

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

Present address: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA (schaerer@stsci.edu)

Send offprint requests to: D. Schaerer

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Contents

• 1. Introduction
• 2. Model calculations
  • 2.1. Input physics and method
  • 2.2. Selected models
    • 2.2.1. Solar metallicity: Z=0.020
    • 2.2.2. Low metallicity calculations: Z=0.004
• 3. Evolution of the UV line blanketed spectrum at Z=0.020
• 4. Ionizing spectrum
  • 4.1. Comparisons of CoStar and plane parallel models
    • 120 track
    • 60 track
    • 20 track
• 5. Revised ionizing fluxes of O and early B stars
  • 5.1. Integrated photon fluxes
  • 5.2. Metallicity effects
  • 5.3. A new ionizing flux calibration for O3-B0 stars
  • 5.4. Comparison with previous calibrations
• 6. Discussion
  • 6.1. Comparisons with EUVE observations
    • 6.1.1. CMa
    • 6.1.2. CMa
  • 6.2. Current approximations and future improvements
• 7. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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