The $R$-Process Alliance: Actinide Abundances, Variation, and Evolution in Metal-Poor Stars

TL;DR

This study analyzes thorium abundances in 47 metal-poor stars to understand the variation and evolution of actinides produced by the $r$-process, providing constraints for astrophysical models.

Contribution

It presents the largest homogeneous dataset of Th abundances in metal-poor stars and quantifies the variation in $r$-process yields, informing models of nucleosynthesis.

Findings

Th and lanthanides Eu and Dy are well co-produced across metallicities.

The variation in Th/Eu yields among $r$-process events is within a factor of 1.3 for 68% of cases.

A small fraction of $r$-process events show yield variations greater than a factor of 3.3.

Abstract

The actinides, including thorium (Th), are the heaviest observable elements synthesized in the universe, holding clues to the extremes of the astrophysical and nuclear conditions of $r$-process sites. We present Th abundances based on high-resolution spectroscopy for 47 metal-poor stars, the largest homogeneously analyzed sample to date. The chemical evolution of Th exhibits a decrease in dispersion in [Th/H] and [Th/Fe] from $\sim$0.6 dex at the lowest metallicities to $\sim$0.2 dex at higher metallicities. We also find that Th and the lanthanides Eu and Dy are co-produced remarkably well, with average [Th/Eu]$\sim0.0$ across $-3.0 \lesssim$ [Fe/H] $\lesssim -1.5$, as well as across stars with $0.0\lesssim$ [Eu/Fe] $\lesssim2.5$. Even so, the absolute range of $\log\epsilon$(Th/Eu) is 1.02 dex, with an observed standard deviation of $\pm0.20$ dex and an intrinsic standard deviation of $\pm0.11$ dex at the lowest metallicities. We infer that $68\%$ of $r$-process events have $\log\epsilon$(Th/Eu) yields that only vary within a factor of $\pm1.3$ or $\pm30\%$, while $5\%$ of $r$-process events have $\log\epsilon$(Th/Eu) yields that vary by factors $>3.3$ approaching $\sim$10. This serves as a strong constraint for the nuclear and astrophysical models of $r$-process sites, and suggests that achieving an $r$-process site that is both prompt and produces a robust $\log\epsilon$(Th/Eu) ratio is a challenge for current models.