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

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 about 0.6 dex at the lowest metallicities to about 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] about0.0 across-3.0 [Fe/H] -1.5, as well as across stars with 0.0 [Eu/Fe] 2.5. Even so, the absolute range of ε (Th/Eu) is 1.02 dex, with an observed standard deviation of plus or minus0.20 dex and an intrinsic standard deviation of plus or minus0.11 dex at the lowest metallicities. We infer that 68% of r-process events have ε (Th/Eu) yields that only vary within a factor of plus or minus1.3 or plus or minus30%, while 5% of r-process events have ε (Th/Eu) yields that vary by factors >3.3 approaching about 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 ε (Th/Eu) ratio is a challenge for current models.