Binary population synthesis models for core-collapse gamma-ray burst
  progenitors
  Long-duration gamma-ray bursts (GRBs) are understood to be the final fate for
a subset of massive, stripped envelope, rapidly rotating stars. Beyond this,
our knowledge of the progenitor systems is limited. Using the BPASS (Binary
Population and Spectral Synthesis) stellar evolution models, we investigate the
possibility that some massive stars in binaries can maintain the angular
momentum required for jet production, while still loosing their outer envelope
through winds or binary interactions. We find that a total hydrogen mass of
less than 0.0005 Msun and a helium ejecta mass fraction of less than 0.20
provide the best thresholds for the supernova type II/Ibc and Ib/Ic divisions
respectively. Tidal interactions in binaries are accounted for by applying a
tidal algorithm to post-process the stellar evolution models output by BPASS.
We show that the observed volumetric gamma-ray burst rate evolution can be
recreated using two distinct pathways and plausible distributions for burst
parameters. In the first pathway, stars are spun up by mass accretion into a
quasi-homogeneous state. In the second, tides maintain rotation where otherwise
the star would spin down. Both lead to type Ic supernova progenitors, and a
metallicity distribution consistent with the GRB host galaxy population. The
inferred core angular momentum threshold for jet production is consistent with
theoretical requirements for collapsars, given the assumptions made in our
model. We can therefore reproduce several aspects of core collapse
supernova/GRB observation and theory simultaneously. We discuss the predicted
observable properties of GRB progenitors and their surviving companions.
