We study the thermal evolution of isolated neutron stars in scalar-tensor theories for the first time. Whether rapid cooling due to the direct Urca process occurs or not is an interesting question from the viewpoint of the temperature observation of isolated neutron stars. Moreover, investigation of the cooling effect of nucleon superfluidity also has 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 a modification of gravity, the relation between the central density and mass of neutron stars differs from that in general relativity, and the cooling curve is also naively expected to vary. We find that an unscreened fifth force near the surface of neutron stars changes the 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.
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