Metallicity-dependent Wolf-Rayet winds

Summary

Observational and theoretical evidence in support of metallicity-dependent winds for Wolf-Rayet stars is considered. Well-known differences in Wolf-Rayet subtype distributions in the Milky Way, LMC and SMC may be attributed to the sensitivity of subtypes to wind density. Implications for Wolf-Rayet stars at low metallicity include a hardening of ionizing flux distributions, an increased WR population due to reduced optical line fluxes, plus support for the role of single WR stars as gamma-ray burst progenitors.

Introduction

Wolf-Rayet (WR) stars represent 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).

WR stellar winds are significantly denser than O stars, as illustrated in Figure 1, so their visual spectra are dominated by broad emission lines, notably He II λ4686 (WN stars) and C III λ4647–51, CIII λ5696, C IV λ5801–12 (WC stars). The spectro-scopic 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).