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

Journal Article · · The Astrophysical Journal (Online)

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. Here, 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.

Research Organization:
Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
Sponsoring Organization:
USDOE Office of Science (SC). Nuclear Physics (NP); USDOE National Nuclear Security Administration (NNSA); National Science Foundation (NSF)
Grant/Contract Number:
89233218CNA000001; AC52-07NA27344; SC0013039; SC0018232; AC52-06NA25396; PHY 14-30152
OSTI ID:
1768516
Report Number(s):
LA-UR-18-29300; TRN: US2206607
Journal Information:
The Astrophysical Journal (Online), Vol. 870, Issue 1; ISSN 1538-4357
Publisher:
Institute of Physics (IOP)Copyright Statement
Country of Publication:
United States
Language:
English

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Cited By (13)

GW170817 --the first observed neutron star merger and its kilonova: Implications for the astrophysical site of the r-process journal November 2019
The Common Envelope Jet Supernova (CEJSN) r -process Scenario journal June 2019
r -process nucleosynthesis: connecting rare-isotope beam facilities with the cosmos journal July 2019
GW170817$-$the first observed neutron star merger and its kilonova: implications for the astrophysical site of the r-process text January 2019
Probing the Production of Actinides under Different r-process Conditions journal July 2019
The lanthanide fraction distribution in metal-poor stars: a test of neutron star mergers as the dominant r-process site text January 2019
Gamma-rays from kilonova: a potential probe of r-process nucleosynthesis text January 2019
Gamma Rays from Kilonova: A Potential Probe of r -process Nucleosynthesis journal February 2020
Finding the Remnants of the Milky Way's Last Neutron Star Mergers journal July 2019
Kilonovae journal December 2019
Actinide-rich and Actinide-poor r -process-enhanced Metal-poor Stars Do Not Require Separate r -process Progenitors journal August 2019
Actinide-rich and Actinide-poor $r$-Process Enhanced Metal-Poor Stars do not Require Separate $r$-Process Progenitors text January 2019
The Lanthanide Fraction Distribution in Metal-poor Stars: A Test of Neutron Star Mergers as the Dominant r -process Site journal August 2019

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