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Combined stellar structure and atmosphere models for massive stars. III. Spectral evolution and revised ionising fluxes of O3-B0 stars

We provide an extensive set of theoretical spectral energy distributions of massive stars derived from our "combined stellar structure and atmosphere models". The calculations covering the entire main sequence evolution for initial masses M_i=20 - 120 Msun (O3-B0 stars of all luminosity classes) are used for a systematic study of the ionizing fluxes of O and early B stars. We demonstrate the importance of accounting simultaneously for non-LTE effects, line blanketing and stellar winds. The main results from our spectra are the following: (1) The flux in the HeII continuum is increased by 2 to 3 (3 to 6) orders of magnitudes compared to predictions from plane parallel non-LTE (LTE) model atmospheres. (2) The flux in the HeI continuum is increased due to non-LTE effects. However, we find that it is also influenced by wind effects. The combined effect of a mass outflow and line blanketing leads to a flatter energy distribution in the HeI continuum, which confirms the results of Sellmaier et al. (1996) for a wider range of stellar parameters. (3) 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 Teff and gravity calibrations of Vacca et al. (1996). The total number of Lyman continuum photons is found to be slightly lower than previous derivations. Due to the increased flux in the HeI continuum the hardness ratio of the HeI to H continuum is increased by 1.6 to 2.5 depending on spectral type and luminosity class. A critical discussion of current model assumptions shows that for stars of types later than B0, having weak stellar winds, reliable predictions of ionizing fluxes are not yet possible.