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Type Ibc supernovae
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Title: The death of massive stars - II. Observational constraints on the progenitors of type Ibc supernovae
Authors: John J. Eldridge, Morgan Fraser, Stephen J. Smartt, Justyn R. Maund, R. Mark Crockett

We present an extensive search for the progenitors of type Ibc supernovae in all available pre-discovery imaging since 1998. To date there have been 12 type Ibc supernovae with either deep ground-based imaging or Hubble Space Telescope archival imaging of their explosion sites. Our analysis shows there is no detection of any progenitor in these images, with the deepest limits probing absolute magnitudes of between -4 and -5 in broad-band B, V and R. We compare the magnitude limits with the observed Wolf-Rayet population in the Large Magellanic Cloud. There is only a 15 per cent probability that this is actually the progenitor population and that we have failed to detect a Wolf-Rayet progenitor by chance selection. If SN Ibc progenitors are drawn from such populations then either Wolf-Rayet stars evolve significantly in the last stages of their lives, or we have underestimated circumstellar dust and extinction towards the progenitors. We review the relative rates of Ibc SNe, finding agreement with previous work that they occur too frequently to originate only from progenitors with initial masses >25 solar masses. Hence we propose that a Wolf-Rayet origin for the bulk of the normal Ibc population is unlikely. We present results from binary stellar evolution calculations that show that stars less than approximately 20 solar masses, in interacting binaries can fit all of these observational constraints. These stars lose their hydrogen envelopes through either Roche-lobe overflow or common envelope evolution and end their lives as low mass helium stars. We show that such helium stars in model binaries retain a low mass hydrogen envelope until after the end of core helium burning and lose the last of the hydrogen approximately 10^4 years before core-collapse. Hence it is not surprising that they have been difficult to identify, but some viable Ibc progenitor systems exist.

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