29 - Stellar abundances: the r-process and supernovae

Summary

Abstract

Stellar abundance observations are providing important clues about the relationship between supernovae (SNe) and the rapid neutron capture process (i.e., the r-process). Although the site for the r-process is still not identified, events in and around SNe have long been suspected. Abundances of heavy neutron-capture elements in a number of stars suggest a robust r-process operating over billions of years, constraining astrophysical and nuclear conditions in supernova models. Variations in lighter n-capture element abundances — observed only very recently in any stars — could be explained as a signature of certain supernova models, or might require multiple r-process sites with different mass ranges or frequencies of SNe. Recent observations of elemental abundance scatter in the early Galaxy are consistent with earlier suggestions of a restricted range of SNe responsible for the r-process.

Introduction

The elements heavier than iron are synthesized in neutron processes, either in the (s)-low or (r)-apid process. In the s-process the timescale for neutron capture (τ_n) is much longer than the electron (beta)-decay (τ_β) timescale. For the r-process, however, τ_n << τ_β with many neutrons captured in a very short time period. As a result, neutron captures proceed into very neutron-rich regions far from the stable nuclei, where very little experimental nuclear data is available. This element synthesis is intimately connected to the late stages of stellar evolution, with the s-process occurring in the thermally pulsing helium shells of asymptotic giant branch (AGB) stars of low- and intermediate-mass (M ∼ 0.8–8 M_⊙) (see, e.g., the review by Busso, Gallino, & Wasserburg 1999).