Probing the Production of Actinides under Different r-process Conditions

Abstract

Several extremely metal-poor stars are known to have an enhanced thorium abundance. These actinide-boost stars have likely inherited material from an r-process that operated under different conditions than the r-process that is reflected in most other metal-poor stars with no actinide enhancement. In this article, we explore the sensitivity of actinide production in r-process calculations to the hydrodynamical conditions as well as the nuclear physics. We find that the initial electron fraction Y e is the most important factor determining the actinide yields and that the abundance ratios between long-lived actinides and lanthanides like europium can vary for different conditions in our calculations. In our setup, conditions with high entropies systematically lead to lower actinide abundances relative to other r-process elements. Furthermore, actinide-enhanced ejecta can also be distinguished from the "regular" composition in other ways, most notably in the second r-process peak abundances.