Pre-supernova Evolution and Nucleosynthesis in Massive Stars and Their Stellar Wind Contribution

Abstract

In this chapter, we review the modelling and pre-supernova evolution of massive stars with a particular emphasis on the effects of rotation and mass loss. We then present the stellar wind contribution to nucleosynthesis and the production of weak s-process at various metallicities (Z). We also review the transition between intermediate-mass and massive stars and the major nuclear and stellar uncertainties involved. Rotation and mass loss both have a strong impact on the evolution and nucleosynthesis in massive stars. The effects of rotation on pre-supernova models are most spectacular for stars between 15 and 25 M _⊙. For M > 30M _⊙, mass loss dominates over the effects of rotation. Massive stars near solar metallicity lose more than half their initial mass for stars more massive than 20 M _⊙. The stellar wind contribution to nucleosynthesis consists mostly of hydrogen-burning products and to a smaller extent helium-burning products since mass loss is generally small during the advanced phases.

At low and very low Z, one expects mass loss and the production of secondary elements like ¹⁴N to decrease and gradually become negligible. Rotation changes this picture. For the most massive stars ($M\gtrsim 60\,M_{\odot }$), primary production of CNO elements raises the overall metallicity of the surface drastically, and significant mass loss may occur during the red supergiant stage. The production of primary ¹⁴N and also ²²Ne in rotating massive stars at low Z opens the door to produce s-process elements at low Z. The strong dependence of the production of the barium peak on metallicity and initial rotation rate means that rotating models provide a natural explanation for the observed scatter in the strontium over barium ratio ([Sr/Ba]) at low metallicities.