Synthetic spectra of massive stars as tool for the spectral analysis of stars and stellar clusters

Using an improved model code, EUV spectral energy distributions
(SEDs) have been computed for a large grid of stellar models
spanning the parameter range observed for O and early B stars.
These SEDs have been incorporated into an evolutionary population
synthesis code to investigate the time-dependence of the integrated
SEDs from evolving clusters of massive stars. Purpose of these
calculations is to provide a crucial ingredient for the simulations
of the photoionized gas in star-forming regions, which then yield
information about the star-formation history of observed clusters.
The new method used for computing the SEDs renders the influence of
spectral lines on the EUV radiation field in identical quality as
the high-resolution synthetic spectra used for comparison with
observed UV spectra. By means of exemplary UV analyses of
individual O stars it has been shown that the models reproduce most
features of the observed UV spectra. As the appearance of the
observable UV spectrum depends strongly on the spectral shape of
the EUV radiation field, this result represents strong evidence
that the computed SEDs are on a realistic level, an essential
requirement for their application in photoionization calculations.
Some minor discrepancies still remain, however, to be resolved in
future work. The mass loss rates and terminal velocities from
models with consistently calculated hydrodynamics have been shown
to reproduce the theoretically predicted wind-momentum--luminosity
relation, as well as the predicted metallicity dependence thereof,
showing no distinct differences for dwarfs and supergiants.
Comparison of the observed UV spectra of a sample of galactic O
stars with the synthetic spectra of two sets of models, one based
on selfconsistent hydrodynamics, the other on wind parameters
derived from an analysis of optical lines, shows discrepancies that
are consistent with the scenario of a fragmented stellar wind,
although an in-depth investigation of other possible explanations,
such as non-solar abundance patterns, remains to be performed.
(This will require a detailed spectral analysis and comparative
study of a sample of Galactic, LMC, and SMC stars.) The different
relations previously obtained for dwarfs and supergiants from
analyses of the H-alpha line might therefore be the result of
inadequate assumptions made in modelling the optical lines.