Nitrogen rises to the top: Evidence of enhanced mixing in very massive stars

Recent observations of young galaxies in the high-redshift Universe have revealed signs of early enrichment of nitrogen. Extremely massive stars (M ≳ 10^ 2 -10^3,M_⊙) with strong stellar winds have been proposed as a potential driver of this phenomenon. We show that the observed fraction of nitrogen-rich stars with masses ≳ 100M_⊙ cannot be explained by mass loss alone. Significantly more efficient mixing beyond their convective cores is required than is accounted for in current evolutionary models. We compiled a representative sample of 122 stars in the Tarantula nebula of the Large Magellanic Cloud (LMC) with masses M ≳ 30,M_⊙ . Nearly all stars with masses M ≳ 100,M_⊙ exhibit strong nitrogen enrichment by factors of ≳ 5-10. We demonstrate that this trend cannot be reproduced by varying assumptions on binary fraction, star formation history, or mass-loss rates within the ranges predicted by current empirical and theoretical models. In contrast, enhanced core overshooting of α_ ̊m ov ≳ 1 can account for the observed enrichment, but leads to a quasi-chemically homogeneous evolution that is inconsistent with the observed Hertzsprung–Russell diagram. The origin of this discrepancy remains unclear. Our results, in combination with observational and theoretical constraints on mass-loss rates, however, suggest that efficient early mixing operates during or shortly after the formation of very massive stars. Mixing models like this are currently not included in stellar evolution models. These findings have immediate implications for the formation, radial expansion, evolution, and final fates of stars at the upper mass end, and they provide a potential pathway for explaining the rapid nitrogen enrichment observed in the high-redshift Universe.