In pursuit of gamma-ray burst progenitors: the identification of a sub-population of rotating Wolf-Rayet stars

TL;DR

This study identifies a sub-population of young, rotating Wolf-Rayet stars with ejecta nebulae as the most promising progenitors for long gamma-ray bursts, using spectropolarimetry to detect stellar rotation.

Contribution

The paper presents the first significant correlation between spectropolarimetric signatures of rotation and ejecta nebulae in Wolf-Rayet stars, pinpointing potential GRB progenitors.

Findings

80% of WR stars have spherically symmetric winds and rotate slowly.

20% of WR stars show signs of rapid rotation.

Most rotating WR stars are surrounded by ejecta nebulae, indicating youth and potential as GRB progenitors.

Abstract

Long gamma-ray bursts involve the most powerful cosmic explosions since the Big Bang. Whilst it has been established that GRBs are related to the death throes of massive stars, the identification of their progenitors has proved challenging. Theory suggests that rotating Wolf-Rayet stars are the best candidates, but their strong stellar winds shroud their surfaces, preventing a direct measurement of their rotation. Fortunately, linear spectropolarimetry may be used to probe the flattening of their winds due to stellar spin. Spectropolarimetry surveys show that an 80% majority of WR stars have spherically symmetric winds and are thus rotating slowly, yet a small 20% minority display a spectropolarimetric signature indicative of rotation. Here we find a highly significant correlation between WR objects that carry the signature of stellar rotation and the subset of WR stars with ejecta nebulae that have only recently transitioned from a red sugergiant or luminous blue variable phase. As these youthful WR stars have yet to spin-down due to mass loss, they are the best candidate GRB progenitors identified to date. When we take recently published WR ejecta nebula numbers we find that five out of the six line-effect WR stars are surrounded by ejecta nebulae. The statistics imply that the null hypothesis of no correlation between line-effect WR stars and ejecta nebulae can be rejected at the 0.0004% level. Given that four line-effect and WR ejecta nebula have spectroscopically been confirmed to contain nucleosynthetic products, we argue that the correlation is both statistically significant and physically convincing. The implication is that we have identified a WR sub-population that fulfills the necessary criteria for making GRBs. Finally, we discuss the potential of identifying GRB progenitors via spectropolarimetry with extremely large telescopes.