  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.

Astrophysics