Actinide-rich and Actinide-poor r-process-enhanced Metal-poor Stars Do Not Require Separate r-process Progenitors

Abstract

The astrophysical production site of the heaviest elements in the universe remains a mystery. Incorporating heavy element signatures of metal-poor, r -process enhanced stars into theoretical studies of r -process production can offer crucial constraints on the origin of heavy elements. In this study, we introduce and apply the "Actinide-Dilution with Matching" model to a variety of stellar groups ranging from actinide-deficient to actinide-enhanced to empirically characterize r -process ejecta mass as a function of electron fraction. We find that actinide-boost stars do not indicate the need for a unique and separate r -process progenitor. Rather, small variations of neutron richness within the same type of r -process event can account for all observed levels of actinide enhancements. The very low-Y e , fission-cycling ejecta of an r -process event need only constitute 10-30% of the total ejecta mass to accommodate most actinide abundances of metal-poor stars. We find that our empirical Y e distributions of ejecta are similar to those inferred from studies of GW170817 mass ejecta ratios, which is consistent with neutron-star mergers being a source of the heavy elements in metal-poor, r -process enhanced stars.

TheR-Process Alliance: Fourth Data Release from the Search forR-process-enhanced Stars in the Galactic Halo

“…Future data releases by the RPA will continue to increase the number of stars with identified r-process signatures and perhaps reveal new ones for investigations of the various proposed nucleosynthetic sites. Fresh investigations of actinide production, for example, are being used to distinguish between specific r-process sites and the conditions that produce these heavy elements (Eichler et al 2019;Holmbeck et al 2019a). Furthermore, the identification of dynamical groups that include r-process-enhanced stars are useful to constrain theoretical models of r-process production; see, e.g., Holmbeck et al (2019b) Opticon; Strasbourg Observatory; and the Universities of Groningen, Heidelberg and Sydney.…”

129 I and 247 Cm in meteorites constrain the last astrophysical source of solar r-process elements

“…To do so, we employed an iterative random sampling procedure (see also Ref. [30]). In each iteration, we randomly selected ten trajectories, summed the abundances of 129 I and 247 Cm, and calculated the integrated 129 I / 247 Cm abundance ratio.…”

“…Similarly, Ref. [30] showed that dynamical ejecta overproduce the Th/Eu elemental ratio derived from the surface of actinide-boost stars, which suggests that very neutron-rich conditions cannot entirely be responsible for the enrichment of these stars. Although current merger simulations tend to predict the presence of very neutron-rich material, these simulations still an active area of research, such that the dominance of very neutron-rich conditions in dynamical ejecta remains debatable.…”

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“…We note, however, that such ratios cannot be reliably used for actinide-boost r-process stars (Schatz et al 2002), which are known to exhibit unusually high Th and U abundances as compared to stable elements, such as Eu. Interestingly, the actinide element ratio, Th/U (if measured simultaneously in a star), seems to remain a robust tool to determine the ages of actinide-boost r-process stars (Holmbeck et al 2019).…”