Neutron star cooling in modified gravity theories
  We study thermal evolution of isolated neutron stars in scalar-tensor
theories for the first time. Whether the rapid cooling due to the direct Urca
process occurs or not is an interesting question in the viewpoint of the
temperature observation of isolated neutron stars. Moreover, investigation of
the cooling effect of nucleon superfluidity also has the large uncertainties
though it is important in modern cooling theory. The cooling effect is
typically influenced by the proton fraction and the central density. If a fifth
force is mediated due to modification of gravity, the relation between the
central density and mass of neutron stars differs from one in general
relativity, and the cooling curve is also naively expected to be varied. We
find that an unscreened fifth force near the surface of neutron stars changes
mass-central density relation, and the direct Urca process can be triggered
even for neutron stars with smaller mass. We also present cooling curves
including nucleon superfluidity under the scalar-tensor theory. These results
show that it might be useful to test gravitational theories with cooling
observations of neutron stars.
