We are currently carrying out a spectropolarimetry survey of
Galactic Wolf-Rayet stars in an effort to study the progenitors
of gamma-ray bursts (GRBs). This survey utilizes Gary Schmidt's
SPOL instrument on the Steward Observatory 90-inch and MMT telescopes.
The most widely accepted model of gamma-ray bursts is the
collapsar model in which a massive star undergoes core collapse
resulting in a black hole and an accretion disk. In this model
there is a preferred axis which is generally attributed to rapid
rotation of the progenitor but could also be the result of magnetic
fields or tidal effects of a companion. The gamma-ray burst
is the result of a relativistic jet which propagates along the
preferred axis through the still collapsing star. The collapsar
model requires that progenitor has already shed its hydrogen
envelope, otherwise the jet would lose its energy and no burst
would occur. Wolf-Rayet stars are massive stars which have shed
their hydrogen envelopes and therefore are an excellent candidate
for the progenitors of GRBs.
We are using spectropolarimetry to probe the geometric structure of the
mass loss envelope of Wolf-Rayet stars. Polarization will result from
asymmetries in this structure which can be attributed to the
same physics which produce the preferred axis of a gamma-ray burst.
To date we have observed xx Galactic Wolf-Rayet stars using
spectropolarimetry. The results will be combined with the existing
measurements of just 29 stars to correlate asymmetries with stellar
parameters and environment to identify the likely progenitors of
gamma-ray bursts. Upon completion we will have increased the
measurement of spectropolarimetry of Wolf-Rayet stars by more than 60%.