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

Erika M. Holmbeck1,2, Trevor M. Sprouse1, Matthew R. Mumpower2,3, Nicole Vassh1, Rebecca Surman1,2, Timothy C. Beers1,2, and Toshihiko Kawano3
Astrophysical Journal 870, 23 Link to Article [DOI: 10.3847/1538-4357/aaefef ]
1Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
2JINA Center for the Evolution of the Elements, USA
3Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

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.

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