Thermal evolution of neo-neutron stars. II. Temperature-dependent crusts

Abstract

The neo-neutron star phase is an intermediate stage in the evolution from a hot lepton-rich compact object, which is formed in the aftermath of a core-collapse supernova or an accretion-induced collapse of a white dwarf, to a cold deleptonized neutron star (NS). Alternatively, this phase can occur after a binary NS merger if the final compact object does not collapse immediately into a black hole. Radial temperature profiles provided by numerical simulations of proto-NS evolution suggest that the composition and mechanical structure of the star's core at the beginning of the neo-NS phase are, as a good approximation, identical to those of a cold NS. In contrast, the composition and mechanical structure of the outer layers (i.e., the crust) are expected to change as the star cools. The thermal evolution of neo-NSs with temperature-dependent (inner) envelopes was considered by Beznogov et al. (2020). Here, we investigated what happens if the equation of state (EoS) of the entire crust, including its composition, depends on temperature. We analyzed the thermal, mechanical, and chemical evolution of neo-NSs by further developing and employing neo-NSCool, our NS thermal evolution code. First, we proved that thermally dripped neutrons slightly slow down the cooling during the early thermal relaxation stage. Then, we showed that the EoSs with exotic light species in the deepest layers of the crust result in significantly slower cooling in the photon cooling era compared with the EoSs that favor massive nuclei. The crust's composition also impacts the crystallization process as well as the way in which the crust contracts while it cools. The EoSs with light nuclei cause the innermost layers to remain liquid for a longer time than the EoSs with heavy nuclei.