The<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>s</mml:mi></mml:math>process: Nuclear physics, stellar models, and observations

TLDR

The $s$ process occurs in the He‑burning layers of low‑mass AGB stars and in the He‑ and C‑burning phases of massive stars, accounting for roughly half of the solar Cu–Bi abundances, with abundance patterns that vary with stellar mass and metallicity and are constrained by growing detailed observations. Using updated nuclear physics data, the authors aim to solve ab initio the stellar convection and mixing processes that govern the $s$‑process. They review progress in observations, nuclear and atomic physics, and stellar modeling, illustrating the interplay through general abundance patterns beyond iron and the effects of branching points along the $s$‑process path. The resulting strong metallicity dependence of $s$‑process efficiency has significant implications for galactic chemical evolution.

Abstract

Nucleosynthesis in the $s$ process takes place in the He-burning layers of low-mass asymptotic giant branch (AGB) stars and during the He- and C-burning phases of massive stars. The $s$ process contributes about half of the element abundances between Cu and Bi in solar system material. Depending on stellar mass and metallicity the resulting $s$-abundance patterns exhibit characteristic features, which provide comprehensive information for our understanding of the stellar life cycle and for the chemical evolution of galaxies. The rapidly growing body of detailed abundance observations, in particular, for AGB and post-AGB stars, for objects in binary systems, and for the very faint metal-poor population represents exciting challenges and constraints for stellar model calculations. Based on updated and improved nuclear physics data for the $s$-process reaction network, current models are aiming at an ab initio solution for the stellar physics related to convection and mixing processes. Progress in the intimately related areas of observations, nuclear and atomic physics, and stellar modeling is reviewed and the corresponding interplay is illustrated by the general abundance patterns of the elements beyond iron and by the effect of sensitive branching points along the $s$-process path. The strong variations of the $s$-process efficiency with metallicity bear also interesting consequences for galactic chemical evolution.