Possibility of rapid neutron star cooling with a realistic equation of state

Whether fast cooling processes occur or not is crucial for the thermal evolution of neutron stars. In particular, the threshold of the direct Urca process, which is one of the fast cooling processes, is determined by the interior proton fraction Y_p, or the nuclear symmetry energy. Since recent observations indicate the small radius of neutron stars, a low value is preferred for the symmetry energy. In this study, simulations of neutron star cooling are performed adopting three models for the equation of state (EoS): Togashi, Shen, and LS220 EoSs. The Togashi EoS has been recently constructed with realistic nuclear potentials under finite temperature, and found to account for the small radius of neutron stars. As a result, we find that, since the direct Urca process is forbidden, the neutron star cooling is slow with use of the Togashi EoS. This is because the symmetry energy of Togashi EoS is lower than those of other EoSs. Hence, in order to account for observed age and surface temperature of isolated neutron stars with the use of the Togashi EoS, other fast cooling processes are needed regardless of the surface composition.