Paper detail

Effects of anisotropic winds on massive stars evolution

Whenever stars are rotating very fast (Omega/Omega_crit > 0.7, with Omega_crit the Keplerian angular velocity of the star accounting for its deformation) radiative stellar winds are enhanced in polar regions. This theoretical prediction is now confirmed by interferometric observations of fast rotating stars.} Polar winds remove less angular momentum than spherical winds and thus allow the star to keep more angular momentum. We quantitatively assess the importance of this effect. First we use a semi-analytical approach to estimate the variation of the angular momentum loss when the rotation parameter increases. Then we compute complete 9 MSun stellar models at very high angular velocities (starting on the ZAMS with Omega/Omega_crit = 0.8 and reaching the critical velocity during the Main Sequence) with and without radiative wind anisotropies. When wind anisotropies are accounted for, the angular momentum loss rate is reduced by less than 4% for Omega/Omega_crit < 0.9 with respect to the case of spherical winds. The reduction amounts to at most 30% when the star is rotating near the critical velocity. These values result from two counteracting effects: on the one hand polar winds reduce the loss of angular momentum, on the other hand, surface deformations imply that the mass which is lost at high co-latitude is lost at a larger distance from the rotational axis and thus removes more angular momentum. In contrast with previous studies, which neglected surface deformations, we show that the radiative wind anisotropies have a relatively modest effect on the evolution of the angular momentum content of fast rotating stars.