Evolution of Gamma-Ray Burst Progenitors at Low Metallicity
  Despite the growing evidence that long Gamma-Ray Bursts (GRBs) are associated with deaths of Wolf-Rayet stars, the evolutionary path of massive stars to GRBs and the exact nature of GRB progenitors remained poorly known. However, recent massive star evolutionary models indicate that -- for sufficiently low metallicity -- initially very rapidly rotating stars can satisfy the conditions for collapsar formation. Even though magnetic torques are included in these models, a strong core spin-down is avoided through quasi-chemically homogeneous evolution induced by rotational mixing. Here, we explore for which initial mass and spin-range single stars of Z=Zsun/20 are expected to produce GRBs. We further find a dichotomy in the chemical structure of GRB progenitors, where lower initial masses end their lives with a massive helium envelope which still contains some amounts of hydrogen, while higher initial masses explode with C/O-dominated hydrogen-free atmospheres. 
Sung-Chul Yoon and Norbert Langer