Binary Models for the Solar Neighborhood and Magellanic Cloud W-R Populations

Details Binary Models for the Solar Neighborhood and Magellanic Cloud W-R Populations Binary Models for the Solar Neighborhood and Magellanic Cloud W-R Populations

Jul 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...448..369d&link_type=abstract

Astrophysical Journal v.448, p.369

Statistics

20

Galaxies: Magellanic Clouds, Stars: Abundances, Stars: Binaries: Close, Stars: Early-Type, Stars: Evolution, Stars: Statistics, Stars: Wolf-Rayet

Scientific paper

New population synthesis models of binary Wolf-Rayet (W-R) and 0 stars are constructed for a variety of metallicities (Z = 0.001, 0.008, and 0.020). We consider the formation of W-R stars both by stellar wind mass loss and mass loss within binary star systems. We use recent stellar models for the single stars, and an extensive library of 5544 binary evolutionary tracks for each metallicity. We show that the binary star formation channel is an important, and perhaps even dominant, mode for W-R formation. We show that binary mergers probably do not contribute significantly to the W-R formation rate. We find that the predicted formation rate of single W-R stars decreases rapidly with decreasing metallicity, while the formation rate of 0 stars increases somewhat. The formation rate of binary W-R stars is nearly independent of abundances. As a result of these trends, the predicted ratio W-R/O of Wolf-Rayet stars to 0 stars decreases dramatically as the metallicity decreases. At the same time, the fraction of W-R stars which are binaries is predicted to increase very rapidly as the abundance decreases. We model the W-R and 0 star populations in the solar neighborhood, and in the Large and Small Magellanic Clouds. Our models agree very well with the number ratio of W-R to 0 stars (W-R/O) in the solar neighborhood, and the W-R/O ratio and the absolute numbers of W-R stars in the Magellanic Clouds. The predicted number of single W-R stars is much too low to explain the observed total numbers of W-R stars in all three of these environments. Our prediction for the binary frequency among the solar neighborhood W-R stars (˜70%) is considerably higher than the observed value. Our prediction agrees with the observed binary fraction among bright W-R stars (bright in apparent magnitude), but very few faint W-R stars have been identified as binaries. Using a variety of statistical methods, we argue that the lack of faint binary W-R stars is probably an observational bias, and we predict that better spectral observations will show that the majority of these are also binaries. Because of the increasing dominance of the binary formation channel as the metallicity is decreased, we calculate that the W-R binary frequency in the low Z Magellanic Clouds is even higher than in the solar neighborhood. Using a standard set of star formation parameters, we predict binary frequencies of roughly 74%, 80%, and 92% in the solar neighborhood (Z = 0.020), the Large Magellanic Cloud (Z = 0.0057), and the Small Magellanic Cloud (Z = 0.002), respectively.

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