Observing the r-Process Signature in the Oldest Stars

TL;DR

This paper reviews how metal-poor stars, especially HE 1523-0901, serve as cosmic laboratories for studying the r-process nucleosynthesis, providing insights into the early universe and stellar ages through radioactive element analysis.

Contribution

It introduces the role of metal-poor stars in understanding neutron-capture processes, exemplified by detailed analysis of HE 1523-0901, and discusses the potential of radioactive chronometers for age determination.

Findings

HE 1523-0901 is approximately 13 billion years old.

Radioactive elements Th and U enable stellar age dating.

Few stars with detected U provide crucial data for r-process studies.

Abstract

The abundance patterns of metal-poor stars provide us a wealth of chemical information about various stages of the chemical evolution of the Galaxy. In particular, these stars allow us to study the formation and evolution of the elements and the involved nucleosynthesis processes. Metal-poor stars are the local equivalent of the high-redshift Universe, and thus offer crucial observational constraints on a variety of issues regarding the early Universe. This review presents an introduction to metal-poor stars and their role as "cosmic lab" for the study of neutron-capture nucleosynthesis processes, particularly that of the r-process. The metal-poor star HE 1523-0901 serves as an example for this group of objects. It displays in its spectrum the strongest overabundance of neutron-capture elements associated with the r-process. Heavy neutron-capture elements such as Eu, Os, and Ir were measured, as well as the radioactive elements Th and U. Abundance of Th and U, in conjunction with those of stable elements make possible nucleo-chronomtery, i.e., the determination of stellar ages. HE 1523-0901 appears to be ~13 Gyr old. Age uncertaintites range from 2-5 Gyr for individual chronometers, and are largly due to theoretical uncertainties in the initial production ratio of the employed chronometers. The decay product of the radioactive elements, lead, can be used to constrain r-process calculations. Only few such stars are currently known with detected U. These objects, however, are crucial for the study of this nucleosynthesis process.