Evolution of gamma-ray burst progenitors at low metallicity

Abstract

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 remain 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 = Zsolar/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.