29 - Wolf–Rayet populations at high metallicity

Summary

Observed properties of Wolf–Rayet (WR) stars at high metallicity are reviewed. Wolf–Rayet stars are more common at higher metallicity, as a result of stronger mass-loss during earlier evolutionary phases with late-WC-subtypes signatures of Solar metallicity or higher. Similar numbers of early (WC4–7) and late (WC8–9) stars are observed in the Solar neighbourhood, whilst late subtypes dominate at higher metallicities, such as Westerlund 1 in the inner Milky Way and in M83. The observed trend to later WC subtype within metal-rich environments is intimately linked to a metallicity dependence of WR stars, in the sense that strong winds preferentially favour late subtypes. This has relevance to (a) the upper mass limit in metal-rich galaxies such as NGC 3049, due to softer ionizing fluxes from WR stars at high metallicity; and (b) the fact that evolutionary models including a WR metallicity dependence provide a better match to the observed N(WC)/N(WN) ratio. The latter conclusion partially rests upon the assumption of constant line luminosities for WR stars, yet observations and theoretical atmospheric models reveal higher line fluxes at high metallicity.