Exotic cooling on neutron stars with different surface compositions

The thermal evolution of isolated neutron stars depends on neutrino emission process, equation of state (EOS) of nuclear matter, and surface composition. Focusing on the neutrino emission process, we can classify the cooling models in two groups. First one is the "standard cooling model", which includes modified URCA process and bremsstrahlung process; these processes are believed to operate inside neutron stars. Others include some exotic processes at high density or high temperature, such as pion condensation or quark beta decay, which is named as "exotic cooling model". Since exotic models result in high neutrino emission, exotic neutron stars cool much faster than standard ones. We investigate the thermal evolution of isolated neutron stars, using both the standard and the exotic cooling processes, and two kinds of surface composition, He and Fe. Furthermorewe employ nucleon superfluiditymodels, and adopt a critical temperature as a density-independent parameter. We find that there exist parameter regions which can be consistent with the observational results.