Cooling of neutron stars with quark-hadron continuity

Abstract

Neutron stars are high-density objects formed by the gravitational collapse of massive stars, and the whole star can be likened to a giant nucleus. The interior of a neutron star is considered to contain exotic particles and states which do not appear in a normal nucleus. The internal states are constrained by observations of masses and radii via the equation of state of highly dense nuclear matter. Within these constraints, a variety of exotic states have been discussed. The internal state of neutron stars is closely related to its neutrino emission process, which cools the star from the inside. This effect can be compared with observations of the surface temperature of neutron stars. However, despite the wide range of observations of neutron stars, the nature of the neutron star matter remains uncertain. We consider quark matter as an exotic state and perform cooling calculations for neutron stars, incorporating the effects of nucleon superfluidity and quark colour superconductivity.We take into account the “quark-hadron continuity”, in which the neutron superfluidity is succeeded by thedquark pairing. Furthermore, we obtained the range of the neutron star cooling curve, taking into account the difference in surface temperature due to the composition of the surface layer. We found that the existence of quark matter causes strong neutrino emission from quarks, which is moderately suppressedbysuperfluidity and superconductivity, and canexplain the cold surface temperature of neutron stars.