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S. E. Woosley, A. Heger (Astronomy and Astrophysics Department, UCSC)
We investigate possible progenitor stars of Type I collapsars that are assumed to be responsible for the long class of GRBs. Our basic assumption is that in close binary systems a bare massive helium core is formed. This may occur either: (1) by losing the envelope by Roche-Lobe overflow to the companion, or (2) by a merger. For the first case, we will assume 10% of Keplerian rotation of the helium core, as predicted by simulations of rotating massive single stars. In the second case, even faster rotation may occur and we adopt 30%. We then follow the evolution of these cores, initially 15 solar masses of 10% solar metal content, from onset of central helium burning until onset of core collapse. In order to obtain the angular momentum distribution in the pre-collapsed stellar models we investigate different physical models for angular momentum transport and compare the results with what would be required to from a centrifugally supported disk around a central compact object. We also study angular momentum reduction by mass loss. We find that, if only rotationally induced instabilities (Heger et al. 2000) are considered, or magnetic fields (Spruit 2001) and no mass loss, sufficient angular momentum may remain in the core to power a collapsar. However, when this description of magnetic torques is combined with mass loss, the resulting angular momentum is too low. Magnetic field torques as described by Spruit & Phinney (1998) result in too low spin rates in any case studied. This work has been supported by the NSF (AST-9731569) and the DOE (B347885).