Nucleosynthesis Clocks and the Age of the Galaxy

TL;DR

This paper discusses how radioactive isotopes like thorium and uranium are used as cosmic clocks to determine the age of the galaxy, highlighting recent stellar dating methods and their uncertainties.

Contribution

It reviews the application of nucleocosmochronology using actinide isotopes to estimate galactic and stellar ages, emphasizing recent stellar dating results and their implications.

Findings

Thorium/europium dating yields ages around 15 Gyr with uncertainties.

Lower age limits for some stars are 10-11 Gyr based on thorium and uranium abundances.

Th/U dating of star CS 31082-00 suggests an age of 12.5 Gyr.

Abstract

Nucleocosmochronology involves the use of the abundances of radioactive nuclear species and their radiogenic decay daughters to establish the finite age of the elements and the time scale for their formation. While there exist radioactive products of several specific nucleosynthesis mechanisms that can reveal the histories of these mechanisms, it is the long lived actinide isotopes {232}Th, {235}U, and {238}U, formed in the r-process, that currently play the major role in setting the time scale of Galactic nucleosynthesis. Age determinations in the context of Galactic chemical evolution studies are constrained by intrinsic model uncertainties. Recent studies have rather taken the alternative approach of dating individual stars. Thorium/europium dating of field halo stars and globular cluster stars yields ages on the order of 15 +/- 4 Gyr. A solid lower limit on stellar ages is available as well, for stars for which one both knows the thorium abundance and has an upper limit on the uranium abundance. For the cases of the two extremely metal deficient field halo stars CS 22892-052 and HD 115444, lower limits on the nuclear age of Galactic matter lie in the range 10-11 Gyr. Th/U dating of the star CS 31082-00 gives an age of 12.5 +/- 3 Gyr. Observations of thorium and uranium abundances in globular cluster stars should make possible nuclear determinations of the ages of clusters that can be compared with ages from conventional stellar evolution considerations.