Proton gaps and cooling of neutron stars with a stiff hadronic EoS

TL;DR

This paper investigates neutron star cooling using a stiff hadronic equation of state, demonstrating that the nuclear medium cooling scenario can explain observed cooling data without requiring direct Urca processes, by adjusting proton gaps and medium effects.

Contribution

It introduces the use of a very stiff DD2 hadronic EoS to model neutron star cooling, showing that it can account for observed cooling behaviors without direct Urca reactions.

Findings

The DD2 EoS supports neutron star masses over 2 solar masses.

Cooling curves can be explained by medium modified Urca processes and proton gap profiles.

Fast cooling of young neutron stars like Cas A is consistent with the model.

Abstract

The recent measurements of the masses of the pulsar J00737-3039B and of the companion J1756-2251 and pulsars PSR J1614-2230, PSR J0348-0432 demonstrate the existence of compact stars with masses in a broad range from 1.2 to 2 $M_\odot$. To fulfill the constraint $M_{\rm max}>2M_{\odot}$ and to demonstrate the possibility of cooling scenarios for purely hadronic and further for hybrid stars we exploit the stiff DD2 hadronic equation of state producing a maximum neutron star mass $M\simeq 2.43 M_{\odot}$. We show that the "nuclear medium cooling" scenario for neutron stars comfortably explains the whole set of cooling curves just by a variation of the star masses without the necessity for the occurrence of the direct Urca reaction. To describe the cooling data with the very stiff DD2 equation of state we select a proton gap profile from those exploited in the literature and allow for a variation of the effective pion gap controlling the efficiency of the medium modified Urca process. Fast cooling of young neutron stars like it is seen in the data for Cas A is explained with the DD2 equation of state when the following conditions are provided: the presence of an efficient medium modified Urca process, and a large proton gap at densities $n\le 2n_0$ vanishing for $n\ge (2.5 - 3) n_0$, where $n_0$ is the saturation nuclear density.