Strangeness in Neutron Star Cooling

We study the thermal evolution of neutron stars in the presence of hyperons or kaons in the core. Our results indicate that the nucleon and the hyperon direct Urca processes play crucial roles for macroscopic cooling behavior of neutron stars. The presence of hyperons drives fast cooling mechanisms in two ways: 1) it allows the hyperon direct Urca prior to the nucleon direct Urca, and 2) it makes the nucleon direct Urca more feasible by reducing the neutron Fermi momentum. All of the equations of state (EOSs) used in this work satisfy the maximum mass constraints and the allowed areas of mass and radii of neutron stars. We found that the neutron star EOS with hyperons can still be consistent with both mass and temperature observations if the original neutron star’s EOS without hyperons is stiff enough. On the other hand, we found that the neutron star’s EOS with kaons can be consistent only with mass observations, but can hardly explain the cooling behavior if we take into account the statistical distribution of observed neutron star masses.