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.