Cooling of Isolated Neutron Stars with Hyperon-mixed Kaon-Condensation Matter

We investigate the thermal evolution of isolated neutron stars containing hyperon--mixed kaon--condensed matter, focusing on the role of proton superconductivity. The equation of state utilized for cooling calculation is based upon the minimal relativistic mean--field framework supplemented by chiral SU(3) dynamics for kaon condensation with an additional component on the three-baryon force, which ensures stiffness at high densities enough to meet astrophysical constraints on neutron-star masses and radii. We show that the nucleonic direct Urca processes operate at relatively low stellar masses ($M \gtrsim 1.3\,M_\odot$), erasing any observable signature of strangeness in the absence of superfluidity. However, if the proton $^1{\rm S}_0$ superconductivity works, because of suppression of fast neutrino cooling processes, the cooling scenario could become relevant with the strangeness, depending on the density regions of the pairing gap. In particular, if the proton superconductivity is so strong in high-density regions ($T_{c,p}\sim10^{10}~{\rm K}$), the nucleon and hyperon direct Urca processes shut down, which makes the kaon-induced Urca processes dominant in massive neutron stars. This scenario is in good agreement with several cold isolated neutron stars identified recently. Hence, we suggest that strong proton superconductivity can render kaon condensation observationally visible through cold neutron-star observations, providing a potential signature of strangeness in dense matter.