Actinide Abundances, Variation, and Evolution in Metal-Poor Stars featuring Shivani Shah (North Carolina State University)

Hosted by: Aldana Grichener (University of Arizona and Observatory)

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. I 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 0.6 dex at the lowest metallicities to 0.25 dex at higher metallicities. I will discuss how Th closely tracks lanthanides Eu and Dy across metallicities of -3.0 ≲ [Fe/H] ≲ -1.5, as well as across r-process enrichment levels of 0.0 ≲ [Eu/Fe] ≲ 2.5. While the absolute range of logε(Th/Eu) is 1.02 dex, the average intrinsic scatter of the ratio is only ±0.10 dex. This implies that 68% of r-process events have actinide-to-lanthanide yields that vary within a factor of only ±1.26 or ±26%, while 5% of r-process events have yields that vary by a factor of ≳ 2.5, approaching ~10. I will discuss how this result serves as a strong constraint for nuclear and astrophysical models, as well as implications for r-process sites.

Website: https://www.shivanipshah.com/