Cooling of neutron stars with quark core

Abstract

Ordinary neutron stars can undergo two possible scenarios of cooling: with conventional 'standard' neutrino emission processes or with faster 'non-standard' processes. For both of these scenarios various mechanisms have been proposed. As possible nonstandard options, previous detailed studies already explored direct URCA processes involving hyperon-mixed matter and pion condensates. In the current research we explore another possible non-standard scenario - quark cooling where a hybrid star with a quark core undergoes direct URCA cooling. We used the exact evolutionary code originally constructed by Nomoto and Tsuruta (1987) which was modified for quark cooling. We chose a model with a medium equation of state TNI 6, where transition from neutron to quark matter takes place at a critical density of four times the nuclear density. Our results show that low mass stars undergo standard cooling while heavier stars, with mass larger than about 1.45 mass compared to the sun, possess a central core where nonstandard accelerating quark cooling is in operation but it can be suppressed significantly due to density-dependent superfluid property. We showed that our quark cooling scenario can be consistent with the observational data on neutron star temperatures. An important result is that we obtained more realistic cooling behavior than obtained earlier, by adopting a density-dependent superfluid energy gap model, instead of constant gaps employed earlier.