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 most frequent r-process in the galaxy. In this article, we explore the sensitivity of actinide production in r-process calculations on the hydrodynamical conditions as well as on the nuclear physics. We find that the initial electron fraction Ye 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 be distinguished from the "regular" composition also in other ways, most notably in the second r-process peak abundances.