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The favoured progenitors of long-duration gamma-ray bursts (GRBs) are rapidlyrotating Wolf-Rayet (WR) stars. However, most Galactic WR stars are slowrotators, as stellar winds are thought to remove angular momentum. This poses achallenge to the collapsar model. Recent observations indicate that GRBs occurpredominately in low metallicity (Z) environments, which may resolve theproblem: lower Z leads to less mass loss, which may inhibit angular momentumremoval, allowing WR stars to remain rotating rapidly until collapse. We wishto determine whether low Z WR stars rotate on average more rapidly thanGalactic WR stars, and perform a Very Large Telescope (VLT) linearspectropolarimetry survey of WR stars in the low Z environment of the LargeMagellanic Cloud (LMC) and compare our results with the Galactic sample. Wefind that only 2 out of 13 (i.e. 15%) of LMC WR stars show line polarizationeffects, compared to a similarly low fraction of ~15-20% for Galactic WR stars.The low incidence of line polarization effects in LMC WR stars suggests thatthe threshold metallicity where significant differences in WR rotationalproperties occur is below that of the LMC (Z ~ 0.5 Zsun), possibly constrainingGRB progenitor channels to this upper metallicity.

Constraining GRB progenitor models by probing Wolf-Rayet wind geometries in the Large Magellanic Cloud