Kavli Affiliate: Anna Frebel
| Summary:
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 $0.0lesssim$ [Eu/Fe] $lesssim2.5$. Even so, the absolute range of $logε$(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 $logε$(Th/Eu) yields that only vary within a factor of $pm1.3$ or $pm30%$, while $5%$ of $r$-process events have $logε$(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 $logε$(Th/Eu) ratio is a challenge for current models.
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