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A Nuclear Equation of State Inferred from Stellar r-Process Abundances
By Erika M. Holmbeck, Richard O'Shaughnessy, Vera Delfavero, Krzysztof Belczynski
Published in The Astrophysical Journal 926, 196 (Thursday, February 24, 2022)


Binary neutron star mergers (NSMs) have been confirmed as one source of the heaviest observable elements made by the rapid neutron-capture (r-) process. However, modeling NSM outflows -- from the total ejecta masses to their elemental yields -- depends on the unknown nuclear equation of state (EOS) that governs neutron-star structure. In this work, we derive a phenomenological EOS by assuming that NSMs are the dominant sources of the heavy-element material in metal-poor stars with r-process abundance patterns. We start with a population synthesis model to obtain a population of merging neutron star binaries and calculate their EOS-dependent elemental yields. Under the assumption that these mergers were responsible for the majority of r-process elements in the metal-poor stars, we find parameters representing the EOS for which the theoretical NSM yields reproduce the derived abundances from observations of metal-poor stars. For our proof-of-concept assumptions, we find an EOS that is slightly softer than, but still in agreement with, current constraints, e.g., by the Neutron Star Interior Composition Explorer (NICER), with R1.4=12.25±0.03~km and MTOV of 2.17±0.03~M⊙(statistical uncertainties, neglecting modeling systematics).