Actinide Production in Neutron-Rich Ejecta of a Neutron Star Merger

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 fail to account for the observed over-enhancement 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 and explore if varying levels of neutron richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, beta-decay, and mass model. For most cases, the actinides are over-produced 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 a neutron star merger is a likely production site for the formation of actinides, a significant contribution from another site or sites (e.g., the neutron star merger accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.

15 pages, 19 figures

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Cited by in corpus (46)

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• Kilonovae
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• First Multimessenger Observations of a Neutron Star Merger
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• Modeling Kilonova Light Curves: Dependence on Nuclear Inputs
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• Primary fission fragment mass yields across the chart of nuclides
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• Actinide-rich and Actinide-poor $r$-Process Enhanced Metal-Poor Stars do not Require Separate $r$-Process Progenitors
• Fission fragment distributions and their impact on the r-process nucleosynthesis in neutron star mergers
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• Propagation of Statistical Uncertainties of Skyrme Mass Models to Simulations of $r$-Process Nucleosynthesis
• Fission and the r-process nucleosynthesis of translead nuclei in neutron star mergers
• Probing the production of actinides under different r-process conditions
• Actinide opacities for modeling the spectra and light curves of kilonovae
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• $\beta$-Decay Half-Lives of 55 Neutron-Rich Isotopes beyond the N=82 Shell Gap
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• Finding the remnants of the Milky Way's last neutron star mergers
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• First Exploration of Neutron Shell Structure Below Lead and Beyond $\mathbf{N}=126$
• Igniting weak interactions in neutron star postmerger accretion disks
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• The Influence of Beta Decay Rates on r-Process Observables
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• Actinide-boosting r Process in Black Hole-Neutron Star Merger Ejecta
• Sandblasting the $\text{r}$-Process: Spallation of Ejecta from Neutron Star Mergers
• Proposed Lunar Measurements of $r$-Process Radioisotopes to Distinguish Origin of Deep-sea 244Pu
• Gamma-rays from kilonovae and the cosmic gamma-ray background
• KilonovAE: Exploring Kilonova Spectral Features with Autoencoders
• Abundance Analysis of New $r$-Process-Enhanced Stars from the HESP-GOMPA Survey
• A Nuclear Equation of State Inferred from Stellar r-process Abundances
• Characterizing $r$-Process Sites through Actinide Production
• A chemodynamical analysis of bright metal-poor stars from the HESP-GOMPA survey -- Indications of a non-prevailing site for light r-process elements
• Magnetic Field Strength Effects on Nucleosynthesis from Neutron Star Merger Outflows