Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991a%26a...241...77s&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 241, no. 1, Jan. 1991, p. 77-86.
Astronomy and Astrophysics
Astrophysics
109
Metallicity, Milky Way Galaxy, Stellar Evolution, Stellar Models, Wolf-Rayet Stars, Abundance, Emission Spectra, Magellanic Clouds, Spectral Line Width, Star Distribution, Stellar Mass, Stars: Evolution, Wolf-Rayet, Mass Loss, Galaxies: Evolution, Stars: Abundances
Scientific paper
Models of Wolf-Rayet stars are identified with WC subtypes via the surface composition of (C + 0)/He, as observed by Smith and Hummer (1988), which increases to early subtypes. New models by Maeder (1990) predict that the lower the initial metallicity Z, the higher the (0+ C)/He ratio when the star first becomes a WC star (hence the earlier the WC subtype). This is due to the fact that at low Z, the mass loss rates are lower and the newly synthesized C and 0 are revealed later in the evolution. Thus, for any given value of metallicity Z, only WC stars earlier than some corresponding limiting subtype (i.e. with (C + 0)/He ratios larger than some limit) can be found. This explains the outer limit of galactic radius to which each WC subtype is observed. The models also give a general agreement with the inner radius limit for the subtypes.
Having arrived at the WC phase, a star evolves by continued mass loss to lower mass and luminosity, and higher (0+ C)/He, i.e. to earlier subtype. To first approximation, stars (at a given Z) of any initial mass follow the same mass-abundance (subtype) relation. Thus, at a given Z, stars with higher (C + 0)/He (earlier subtype) will be fainter. However, the relationship is Z-dependent; at lower Z, stars of a given surface abundance (subtype) will be more massive and more luminous. This explains the general trend of MV with subtype in the Galaxy and the observation that LMC WC 4 stars are brighter than Galactic WC 5-6 stars.
WO stars, the extension of the WC sequence to high abundance and excitation, fit logically into the sequence, dominating the WC population at low Z.
Maeder Andre
Smith Lisa F.
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