Jorick S. Vink, G. Graefener, T. J. Harries
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.
View original:
http://arxiv.org/abs/1111.5806
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