Constraining the high-density behavior of nuclear symmetry energy with direct Urca processes

TL;DR

This paper investigates how the density dependence of nuclear symmetry energy affects the onset of direct Urca processes in neutron stars, using relativistic models and Bayesian analysis to constrain the conditions for rapid star cooling.

Contribution

It introduces modified relativistic models that keep isoscalar properties fixed while exploring symmetry energy effects, and uses Bayesian inference to constrain Urca process thresholds in neutron stars.

Findings

Direct Urca processes likely occur only in stars above ~1.8 solar masses.

Symmetry energy slope above 60 MeV favors Urca process onset.

The proposed parametrization breaks the previously observed correlation between symmetry energy slope and incompressibility.

Abstract

The density dependence of the symmetry energy in relativistic mean-field models with density dependent couplings is discussed in terms of the possible opening of nucleonic direct Urca processes inside neutron stars, which induce a very rapid cooling of the star. The modification of the parametrization of the isospin channel of two models, DD2 and DDMEX, keeping the same isoscalar properties is considered and the implications are discussed. Within the models discussed it is not possible the onset of nucleonic direct Urca processes in stars with a mass below $\sim1.6\,M_\odot$ if chiral effective field theory constraints for neutron matter are imposed. A Bayesian inference calculation confirms the low probability that nucleonic direct Urca processes occur inside stars with masses below 1.8$M_\odot$, considering the isoscalar channel of the equation of state described by DD2 or DDMEX and the same symmetry energy at saturation. The lowest masses allowing direct Urca processes are associated with a slope of the symmetry energy above $60$ MeV and most likely a positive symmetry energy incompressibility. It is shown that the parametrization of the isospin channel proposed destroys the correlation between symmetry energy slope and incompressibility previously identified in several works.