Enhanced Neutrino Cooling from Parity-Doubled Nucleons in Neutron Star Cooling Simulations

Abstract

Although restoration of chiral symmetry is predicted by quantum chromodynamics to take place at high baryon density, most modeling of neutron star interiors disregards a chiral phase transition. We model neutron star cores with a parity doublet model, which allows for dynamical chiral symmetry restoration and predicts the appearance of the parity partners of nucleons and hyperons at large densities, as well as deconfined quark matter. We study the thermal evolution of neutron stars, focusing for the first time on the impact of Urca processes involving the parity partners in neutron star cooling simulations. We find that Urca processes for the parity partners of the nucleons significantly affect the thermal evolution of massive stars and allow for improved agreement with observed surface temperature and ages.

Figures (5)

• Figure 1: Properties of the parity doublet chiral mean field (PD-CMF) and parity singlet chiral mean field (PS-CMF) models we consider. (a) Top left: The speed of sound squared as a function of baryon density for neutron star matter described by the PD-CMF and PS-CMF models, showing the onset density of various particles. (b) Top right: The mass--radius predictions for the PD-CMF and PS-CMF models. The shaded regions are astrophysical constraints at the $68\,\%$ confidence level. (c) Bottom left: The particle masses as a function of baryon density in the PD-CMF model. (d) Bottom right: A radial profile of the most massive star predicted by the PD-CMF model showing where the parity partners appear. The dashed curve shows the radius of a $1.4$ M$_\odot$ star and the solid black line the radius of the maximum-mass star.
• Figure 2: Critical temperature as a function of Fermi momentum for the different pairing patterns assumed to take place in the neutron star.
• Figure 3: Neutron star thermal evolution for stars of different masses from the model with the nucleon parity partners (PD-CMF, dashed lines) and without (PS-CMF, solid lines). We show the redshifted surface temperature measured by an observer at infinity as a function of time after formation.
• Figure 4: Cross sections of neutron stars indicating the regions where the direct Urca processes involving the nucleon parity partners (\ref{['eq: n^*d']} and \ref{['eq: n^*d(p^*)']}) are kinematically allowed inside stars of different masses as described by the PD-CMF model. For comparison, the location in the stars where quark matter is present is shown as well.
• Figure 5: The thermal evolution of neutron stars under various assumptions compared with astrophysical observations of isolated, middle-aged neutron stars. (a) Left: Cooling bands from the simulation representing the thermal evolution for stars with masses between $1.4\,\rm{M}_\odot-2.1\,\rm{M}_\odot$. The blue (orange) shaded region is from the model with (without) the nucleon parity partners. The nucleons are the only paired particle species. Observed data is from Ref. Beznogov:2014yia, also see \ref{['table']}. (b) Middle: Same as the left panel but quarks as well as nucleons are paired. (c) Right: Same as the left panel except for stars with an envelope of light, instead of heavy, elements with an envelope mass $\Delta M/M = 10^{-8}$.