Neutron Star Core-Crust Transition and Crustal Moment of Inertia: Systematic Implications of Higher-Order Symmetry Energy in the Nuclear Equation of State

TL;DR

This study examines how higher-order symmetry-energy coefficients in the nuclear equation of state influence key neutron star properties, revealing systematic effects on core-crust transition, moment of inertia, and related observables across various EOS stiffness regimes.

Contribution

It provides a comprehensive analysis of the impact of higher-order symmetry-energy coefficients on neutron star structure using a wide range of EOS models, highlighting systematic trends.

Findings

Stiffer EOS increases core-crust transition pressure and neutron star moment of inertia.

Higher-order symmetry-energy coefficients significantly affect neutron star crustal properties.

Opposite effects observed with enhanced higher-order symmetry coefficients, reversing previous trends.

Abstract

We investigate how higher-order symmetry-energy coefficients of the equation of state (EOS) describing $npe\mu$ core matter impact key neutron star (NS) properties at its crust inner edge, its moment of inertia and corresponding crustal fraction, threshold conditions for direct Urca process, adiabatic index, and related structure observables. Our analysis employs a comprehensive set of CDM3Y-IVF1 equations of state, derived from the M3Y-Paris nucleon-nucleon interaction within a non-relativistic Hartree-Fock framework, covering a wide range of nuclear matter stiffness, from soft (${K}_{0}=150$ MeV) to extremely stiff (330 MeV) regimes. Our analysis reveals that the stiffer EOS characterized by higher isoscalar incompressibility (${K}_{0}$) and fourth-order symmetry-energy skewness (${Q}_{4}$) coefficients, coupled with diminished negative values of isoscalar ${Q}_{0}$ and ${H}_{0}$, and isovector ${K}_{4}$, ${K}_{\tau4}$, ${I}_{2,4}$, and ${G}_{2}$ coefficients, systematically increase the estimated core-crust transition pressure, density, and charged-particle ($p,e,\mu$) fractions, and their threshold values for direct Urca process, as well as the total NS moment of inertia and its crustal fraction, and the central adiabatic index. Simultaneously, they yield less central fraction of $p$, $e$, and $\mu$, and reduced fractional crust thickness and moment of inertia for the maximum NS mass supported by a certain EOS. Conversely, enhanced values of the isoscalar kurtosis ${I}_{0}$ and ${G}_{0}$ symmetry coefficients, and of the higher-order ${Q}_{2}$, ${H}_{2}$, ${K}_{6}$ and ${K}_{\tau6}$ coefficients, combined with diminished negative contributions from ${K}_{2}$ and ${K}_{\tau2}$, produce systematically opposite effects on the mentioned NS quantities, reversing their trends with ${K}_{0}$.