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.

中文翻译：

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修正重力理论中的中子星冷却

我们首次在标量张量理论中研究孤立中子星的热演化。从孤立的中子星温度观测的角度来看，是否发生直接的 Urca 过程导致的快速冷却是一个有趣的问题。此外，核子超流冷却效应的研究也存在很大的不确定性，尽管它在现代冷却理论中具有重要意义。冷却效果通常受质子分数和中心密度的影响。如果第五种力是由于引力的改变而产生的，那么中子星的中心密度和质量之间的关系就不同于广义相对论中的关系，而且冷却曲线也会发生变化。我们发现中子星表面附近未经筛选的第五力改变了质心密度关系，即使对于质量较小的中子星，也可以触发直接的 Urca 过程。我们还提出了标量张量理论下的冷却曲线，包括核子超流体。这些结果表明，通过对中子星的冷却观测来检验引力理论可能是有用的。