Spin-down of massive rotating stars

Models of rapidly rotating massive stars at low metallicities show significantly different evolution and higher metal yields compared to non-rotating stars. We estimate the spin-down time-scale of rapidly rotating non-convective stars supporting an I?-I? dynamo. The magnetic dynamo gives rise to mass-loss in a magnetically controlled stellar wind and hence stellar spin-down owing to loss of angular momentum. The dynamo is maintained by strong horizontal rotation-driven turbulence which dominates over the Parker instability. We calculate the spin-down time-scale and find that it could be relatively short, a small fraction of the main-sequence lifetime. The spin-down time-scale decreases dramatically for higher surface rotations suggesting that rapid rotators may only exhibit such high surface velocities for a short time, only a small fraction of their main-sequence lifetime