Unified QMF equation of state for neutron star matter: Static and dynamic properties

TL;DR

This paper develops a unified quark mean field equation of state for neutron star matter, analyzing static and dynamic properties, and compares it with other models to understand neutron star structure, cooling, and instabilities.

Contribution

It introduces a set of unified QMF models calibrated to different symmetry energy slopes, providing insights into neutron star properties and behaviors not previously explored with this approach.

Findings

QMF predicts heavier nuclear clusters and larger crust Wigner-Seitz cells.

Cooling evolution is insensitive to the model and symmetry energy slope without direct Urca process.

Larger symmetry energy slope extends the neutron star instability window.

Abstract

We construct a set of unified equations of state based on the quark mean field (QMF) model, calibrated to different values of nuclear symmetry energy slope at the saturation density ($L_0$), with the aim of exploring both the static properties and dynamical behavior of neutron stars (NSs), and building a coherent picture of their internal structure. We assess the performance of these QMF models in describing the mass-radius relation, the cooling evolution of isolated NSs and X-ray transients, and the instabilities (e.g., the r-mode). In comparison to relativistic mean field (RMF) models formulated at the hadronic level, the QMF model predicts heavier nuclear clusters and larger Wigner-Seitz cell sizes in the NS crust, while the density of the free neutron gas remains largely similar between the two approaches. For the cooling of isolated NSs, the thermal evolution is found to be insensitive to both the many-body model and the symmetry energy slope in the absence of the direct Urca (dUrca) process. However, when rapid cooling via the dUrca process is allowed, in the case of large $L_0$ values (e.g., $L_0 \gtrsim 80$ MeV) in our study, the QMF model predicts a longer thermal relaxation time. Both the QMF and RMF models can reproduce cooling curves consistent with observations of X-ray transients (e.g., KS 1731--260) during their crustal cooling phase, although stellar parameters show slight variations depending on the model and symmetry energy slope. Within our unified framework, a larger $L_0$ value generally results in a wider instability window, while increasing the stellar mass tends to suppress the instability window. We also provide simple power-law parameterizations that quantify the dependence of bulk and shear viscosities on the symmetry energy slope for nuclear matter at saturation density.