A strong case for fast stellar rotation at very low metallicities

We investigate the effect of new stellar models taking rotation into account and computed for a metallicity Z = 10 on the chemical evolution of the earliest phases of the Milky Way. These models were computed under the assumption that the ratio of the initial rotation velocity to the critical velocity of stars is roughly constant with metallicity. This naturally leads to faster rotation at lower metallicity, as metal-poor stars are more compact than metal-rich ones. We find that the new Z = 10 stellar yields have a tremendous impact on the nitrogen enrichment of the interstellar medium for log(O/H)+12 < 7 (or [Fe/H]< −3). We show that upon the inclusion of the Z = 10 stellar yields in chemical evolution models, both high N/O and C/O ratios are obtained in the very-metal poor metallicity range, in agreement with observations. Our results give further support to the idea that stars at very low metallicities could have rotational velocities of the order of 600-800 kms.