Wolf–Rayet stars as gamma-ray burst progenitors

The collapsar scenario for long gamma-ray bursts requires rapidly rotating Wolf–Rayet stars as progenitor stars. We highlight two possible ways out of the dilemma that the strong winds of Wolf–Rayet lead to a fast spin-down. One way is to restrict the duration of the Wolf–Rayet phase to a short time span at the end of the star’s evolution. We show that this appears to apply to GRB 021004. The other way is to choose a sub-solar metallicity, thus limiting the Wolf–Rayet wind efficiency. We discuss corresponding stellar evolution models and show that a metallicity limit of about 1/10th solar, as it is required by the models, may be consistent with the empirical gamma-ray burst rate.