Can Stellar Mixing Explain the Lack of Type Ib Supernovae in Long-Duration GRBs?

The discovery of supernovae associated with long-duration gamma ray burst observations is primary evidence that the progenitors of these outbursts are massive stars. One of the principle mysteries in understanding these progenitors has been the fact that all of these gamma-ray burst associated supernovae are type Ic supernovae, with no evidence of helium in the stellar atmosphere. Many studies have focused on whether or not this helium is simply hidden from spectral analyses. In this paper, we show results from recent stellar models using new convection algorithms based on our current understanding of stellar mixing. We demonstrate that enhanced convection may lead to severe depletion of stellar helium layers, suggesting that the helium is not observed simply because it is not in the star. We also present light-curves and spectra of these compact helium-depleted stars, compared to models with more conventional helium layers.