Chemical Abundances and Yields from Massive Stars

Stellar rotation produces an internal mixing of the elements due to shear instability and meridional circulation. This leads to observable $N\mathrm{/}C$ enhancements in massive stars above about 7–9 $M_{\odot }$. Rotation also favours mass loss by stellar winds. Mass loss effects dominate for masses above 30 $M_{\odot }$, while mixing dominates below that limit. The effects of mixing are also much larger at lower metallicity $Z$, because the internal $\mathrm{\Omega }$–gradients are steeper. This appears to be in agreement with observations in the SMC. At very low $Z$ and Z=0, mixing between the He–burning core and the H–burning shell leads to the production of primary N in intermediate mass stars. Such enrichments increase the metallicity of the rotating star, also massive Z=0 stars with moderate initial velocities currently reach break–up velocity during a fraction of the MS phase. Both effects favour mass loss in Z=0 stars, which have ejecta with abundance anomalies very similar to those of C–rich very metal poor stars.