Actinide Production in the Neutron-rich Ejecta of a Neutron Star Merger

Abstract

Abstract The rapid neutron-capture (“ r -”) process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r -process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far they fail to account for the observed overenhancement of actinides, present in about 30% of r -process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger (NSM) and explore whether varying levels of neutron-richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, β -decay, and mass model. For most cases, the actinides are overproduced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of an NSM are likely production sites for the formation of actinides, a significant contribution from another site or sites (e.g., the NSM accretion disk wind) is required to explain abundances of r -process-enhanced, metal-poor stars.