The s Process and Beyond

TL;DR

This paper reviews the s process of neutron capture in stars, explores processes beyond it, and discusses observational and nuclear physics insights into heavy element formation in the universe.

Contribution

It provides a comprehensive overview of the s process, its astrophysical sites, uncertainties, and evidence for processes beyond the classical s and r processes.

Findings

Identification of neutron-capture processes operating at intermediate neutron densities.

Discussion of physical uncertainties in s-process modeling.

Compilation of observational evidence from stars and meteorites.

Abstract

Neutron captures produce the vast majority of abundances of elements heavier than iron in the Universe. Beyond the classical slow (s) and rapid (r) processes, there is observational evidence for neutron-capture processes that operate at neutron densities in between, at different distances from the valley of $\beta$ stability. Here, we review the main properties of the s process within the general context of neutron-capture processes and the nuclear physics input required to investigate it. We describe massive stars and asymptotic giant branch stars as the s-process astrophysical sites and discuss the related physical uncertainties. We also present current observational evidence for the s process and beyond, which ranges from stellar spectroscopic observations to laboratory analysis of meteorites.