Description
Neutron stars are born in the violent supernova explosions, when the temperatures reach tens of MeV, replicating in some aspects the environment of the very early Universe. We investigate the faith of MeV dark matter particles
in the freeze-in regime that get produced and gravitationally trapped in the newly born neutron stars. We show that the late time annihilation of dark matter back to the Standard Model states will lead to an additional source of heat generation that may be incompatible with observation of stars' surface temperatures. This sets uniquely nontrivial constraints on the space of dark matter models. We also consider constraints on heavy dark matter capture that turn neutron stars into black holes, and show that the previously derived constraints apply to a much larger class of dark matter models.