Introduction

Wolf-Rayet (WR) stars are the final phase in the evolution of very massive stars prior to core-collapse, in which the H-rich envelope has been stripped away via either stellar winds or close binary evolution, revealing products of H-burning (WN sequence) or He-burning (WC sequence) at their surfaces, i.e. He, N or C, O (Crowther 2007).

Stellar winds of WR stars are significantly denser than those of O stars, as illustrated in Figure 29.1, so their visual spectra are dominated by broad emission lines,

The Metal-rich Universe, eds. G. Israelian and G. Meynet. Published by Cambridge University Press. © Cambridge University Press 2008.

HD 96548 (WN8)

Figure 29.1. Comparisons between stellar radii at Rosseland optical depths of 20 (R*, black) and 2/3 (R2/3, grey) for HD 66811 (O4 If), HD 96548 (WN8) and HD 164270 (WC9), shown to scale, together with the wind region corresponding to the primary optical wind line-forming region, 1011 cm-3 < ne < 1012cm-3 (hatched) in each case, illustrating the highly extended winds of WR stars with respect to O stars (Crowther 2007).

notably He 1114686 (WN stars) and C11114647-51, C11115696, and C iv 15801-12 (WC stars). The spectroscopic signature of WR stars may be seen individually in Local Group galaxies (e.g. Massey & Johnson 1998), within knots in local star-forming galaxies (e.g. Hadfield & Crowther 2006) and in the average rest-frame UV spectrum of Lyman-break galaxies (Shapley et al. 2003).

In the case of a single massive star, the strength of stellar winds during the main-sequence and blue supergiant phases scales with the metallicity (Vink et al. 2001). Consequently, one expects a higher threshold for the formation of WR stars at lower metallicity, and indeed the SMC has a smaller ratio of WR to O stars than is found in the Solar neighbourhood. Alternatively, the H-rich envelope may be removed during the Roche-lobe overflow phase of close binary evolution, a process that is not expected to depend upon metallicity.

Wolf-Rayet stars are the prime candidates for Type-Ib/c core-collapse supernovae and long, soft gamma-ray bursts (GRBs). This is due to their immediate

Milky Way (<3 kpc)

Figure 29.2. Subtype distribution of Milky Way (<3 kpc), LMC and SMC WR stars, in which known binaries are shaded (Crowther 2007).

Figure 29.2. Subtype distribution of Milky Way (<3 kpc), LMC and SMC WR stars, in which known binaries are shaded (Crowther 2007).

progenitors being associated with young massive-stellar populations, compact in nature and deficient either in hydrogen (Type Ib) or in both hydrogen and helium (Type Ic). For the case of GRBs, some of which have been associated with Type-Ic hypernovae (Galama et al. 1998; Hjorth et al. 2003), a rapidly rotating core is a requirement for the collapsar scenario in which the newly formed black hole accretes via an accretion disc (MacFadyen & Woosley 1999). Indeed, WR populations have been observed within local GRB host galaxies (Hammer et al. 2006).

In this review article, the observed properties of WR stars at high metallicity are presented and discussed.

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