The death of massive stars - II. Observational constraints on the
  progenitors of type Ibc supernovae
  The progenitors of many type II core-collapse supernovae have now been
identified directly on pre-discovery imaging. Here we present an extensive
search for the progenitors of type Ibc supernovae in all available
pre-discovery imaging since 1998. There are 12 type Ibc supernovae with no
detections of progenitors in either deep ground-based or Hubble Space Telescope
archival imaging. The deepest absolute BVR magnitude limits are between -4 and
-5. We compare these limits with the observed Wolf-Rayet population in the
Large Magellanic Cloud and estimate a 16 per cent probability we have failed to
detect such a progenitor by chance. Alternatively the progenitors evolve
significantly before core-collapse or we have underestimated the extinction
towards the progenitors. Reviewing the relative rates and ejecta mass estimates
from lightcurve modelling of Ibc SNe, we find both incompatible with Wolf-Rayet
stars with initial masses >25Msun being the only progenitors. We present binary
evolution models that fit these observational constraints. Stars in binaries
with initial masses <20Msun lose their hydrogen envelopes in binary
interactions to become low mass helium stars. They retain a low mass hydrogen
envelope until approximately 10,000 years before core-collapse; hence it is not
surprising that galactic analogues have been difficult to identify.
