Neutron star cooling with $Δ$ dUrca processes

Abstract

The cooling of neutron stars (NSs) provides insight into their composition and equation of state. However, a complete understanding of this phenomenon is still an open question. The early stage of NS cooling is dominated by neutrino emission from the core, which rapidly cools the star. Urca processes are one such mechanism where neutrinos escape from the core, carrying away most of the heat. Nucleonic and hyperonic direct Urca processes are known rapid cooling mechanisms. The possible appearance of $Δ$ baryons in dense matter introduces additional weak-interaction channels, including $Δ$ direct Urca processes. Although such processes have been discussed in the context of $Δ$-admixed NS matter, their neutrino emissivities and impact on NS cooling have not been systematically investigated. In this work, for the first time we incorporate the neutrino emissivities of the relevant $Δ$ direct Urca processes into NS cooling simulations. We investigate the thermal evolution of $Δ$-admixed NSs and assess the impact of these additional neutrino emission channels on their cooling behavior. We find that $Δ$ direct Urca processes can provide an efficient neutrino-cooling mechanism and significantly accelerate the thermal evolution of NSs. Our results demonstrate that $Δ$ baryons and their associated weak-interaction processes can play an important role in explaining NSs with unusually low surface temperatures and provide a new connection between the composition of dense matter and NS cooling observations.