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

Abstract

Context. The favoured progenitors of long-duration gamma-ray bursts (GRBs) are rapidly rotating Wolf-Rayet (WR) stars. However, most Galactic WR stars are slow rotators, as stellar winds are thought to remove angular momentum. This poses a serious challenge to the collapsar model. Recent observations indicate that GRBs occur predominately in low metallicity (Z) environments, which may resolve the problem: lower Z leads to less mass loss, which may inhibit angular momentum removal, allowing WR stars to remain rotating rapidly until collapse. Aims. We wish to determine whether low Z WR stars rotate on average more rapidly than Galactic WR stars. Methods. We perform a Very Large Telescope (VLT) linear spectropolarimetry survey of WR stars in the low Z environment of the Large Magellanic Cloud (LMC) and compare our results with the Galactic sample of Harries et al. (1998, MNRAS, 296, 1072). Results. We find that only 2 out of 13 (i.e. 15%) of LMC WR stars show line polarization effects, compared to a similarly low fraction of ∼15-20% for Galactic WR stars. Conclusions. The low incidence of line polarization effects in LMC WR stars suggests that the threshold metallicity where significant differences in WR rotational properties occur is below that of the LMC (Z ∼ 0.5 Z ⊙ ), possibly constraining GRB progenitor channels to this upper metallicity.