Cooling of Small and Massive Hyperonic Stars

TL;DR

This study performs neutron star cooling simulations using new equations of state that align with nuclear physics constraints and observational data, highlighting the impact of hyperons and nuclear symmetry energy on cooling behavior.

Contribution

It introduces new parametrizations of the FSU2 relativistic mean-field functional that accurately reproduce nuclear matter properties and neutron star observations, including hyperonic effects.

Findings

Models FSU2R and FSU2H agree well with cooling data.

Cooling observations favor equations with soft symmetry energy and small radii.

Large stellar masses (>1.8 M_sun) are needed to explain cold neutron stars.

Abstract

We perform cooling simulations for isolated neutron stars using recently developed equations of state for their core. The equations of state are obtained from new parametrizations of the FSU2 relativistic mean-field functional that reproduce the properties of nuclear matter and finite nuclei, while fulfilling the restrictions on high-density matter deduced from heavy-ion collisions, measurements of massive 2$M_{\odot}$ neutron stars, and neutron star radii below 13 km. We find that two of the models studied, FSU2R (with nucleons) and in particular FSU2H (with nucleons and hyperons), show very good agreement with cooling observations, even without including extensive nucleon pairing. This suggests that the cooling observations are more compatible with an equation of state that produces a soft nuclear symmetry energy and, hence, generates small neutron star radii. However, both models favor large stellar masses, above $1.8 M_{\odot}$, to explain the colder isolated neutron stars that have been observed, even if nucleon pairing is present.