  Recent models of rotating massive stars including magnetic fields prove it
difficult for the cores of single stars to retain enough angular momentum to
produce a collapsar and gamma-ray burst. At low metallicity, even very massive
stars may retain a massive hydrogen envelope due to the weakness of the stellar
winds, posing an additional obstacle to the collapsar model. Here, we consider
the evolution of massive, magnetic stars where rapid rotation induces almost
chemically homogeneous evolution. We find that in this case, the requirements
of the collapsar model are rather easily fulfilled if the metallicity
sufficiently small: 1) Rapidly rotating helium stars are formed without the
need to remove the hydrogen envelope, avoiding mass-loss induced spin-down. 2)
Angular momentum transport from the helium core to hydrogen envelope by
magnetic torques is insignificant. We demonstrate this by calculating
evolutionary models of massive stars with various metallicities, and derive an
upper metallicity limit for this scenario based on currently proposed mass loss
rates. Our models also suggest the existence of a lower CO-core mass limit of
about 10 Msun -- which relates to an initial mass of only about 20 Msun within
our scenario -- for GRB production. We argue that the relative importance of
the considered GRB progenitor channel, compared to any channel related to
binary stars, may increase with decreasing metallicity, and that this channel
might be the major path to GRBs from first stars.
