Résumé
A&A 659, A163 (2022) Stars with masses in excess of 100 Msun are observed in the Local Universe,
but they remain rare objects. Because of the shape of the mass function, they
are expected to be present only in the most massive and youngest clusters. They
may thus be formed in number in highly star-forming galaxies. Very massive
stars (VMSs) experience strong stellar winds that are stronger than those of
their less massive OB-type counterparts. These strong winds therefore need to
be taken into account in evolutionary models and synthetic spectra to properly
predict the appearance of VMS. We present evolutionary models computed with the
code STAREVOL. They include a recent mass-loss recipe that is relevant for
VMSs. We subsequently calculated atmosphere models and synthetic spectra along
the resulting tracks with the code CMFGEN. We studied stars with masses between
150 and 400 Msun and focused on a metallicity Z=1/2.5Zsun. We studied the
impact of our VMS spectra on the spectral energy distribution of young
starbursts. We show that the optical and UV range is dominated by HeII 4686 and
HeII 1640 emission for almost the entire main-sequence evolution of VMSs, in
contrast to less massive stars. In the UV spectral range, carbon, nitrogen, and
iron lines shape the spectra of VMSs, which appear for most of their evolution
as WNh objects. The morphology of the synthetic spectra is similar to that of
VMSs in the Large Magellanic Cloud. We show that stars with masses higher than
100 Msun emit nearly as much light as all other stars in young starbursts. The
integrated UV spectrum of these starbursts is significantly affected by the
presence of VMSs.