Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...287..803m&link_type=abstract
Astronomy and Astrophysics 287, 803-816 (1994)
Astronomy and Astrophysics
Astrophysics
235
Stars: Wolf-Rayet, Stars: Evolution, Stars: Mass Loss, Hertzsprung-Russell Diagram, Binaries: Close, Galaxies: Stellar Content
Scientific paper
We discuss all the relevant data, masses, luminosities, T_eff_, abundances, lifetimes, number ratios WR/O and WC/WN for Wolf-Rayet (WR) star models computed recently by the Geneva group. The results presented may be used directly as input data for population syntheses. The effects of enhanced mass loss rates during the main sequence and the WNL phases on the main outputs of the stellar models are studied in details. For a given metallicity, we find that models with high mass loss predict lower values for the minimum luminosity of the WR stars, lower C/He and O/He ratios at the entrance in the WC phase, and higher values of the WR/O and WC/WN number ratios when integrated over the mass spectrum. Most interestingly, we find that the ratio WC/WN is not a monotonic function of metallicity; these models predict a decrease of the ratio WC/WN for metallicities Z>=0.02 and enhanced mass loss rates. This is due to a long WNL phase obtained in models with high mass loss rates. The comparison of models and WR observations in the Milky Way shows that the mass loss rates during evolutionary phases before the WR stage are likely stronger than generally thought. We also examine the effects of binary mass transfer on the WR/O number ratios at various Z and on the way the WR are distributed among the different WR-subtypes (WNL, WNE and WC). The observed number ratios WR/O and WC/WR in nearby galaxies are particularly well accounted for. We find that at low Z values, nearly 100% of the existing WR stars may owe their existence to binary mass transfer, while this percentage might be relatively low in inner galactic regions, where stellar winds are the dominant factor for forming WR stars and produce a lot of them. The above results are compatible with the fact that a relatively small fraction (~5%) of O-stars have an evolution heavily influenced by binary mass transfer.
Maeder Andre
Meynet George
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