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 M_i = 20 - 120 M_sun, corresponding to O3-B0 stars of all
We predict detailed line blanketed UV spectra along the main sequence
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
The main results from our spectra are the following:
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 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.
- 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. This reconfirms
the work of Gabler et al. (1989).
- The flux in the HeI 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
HeI 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
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.