Minimal Cooling of Neutron Stars: A New Paradigm

TLDR

The authors introduce a new classification of neutron star cooling into minimal and enhanced scenarios and compare model predictions with observed thermal emissions to test for the presence of enhanced cooling. Minimal cooling replaces the standard scenario by incorporating Cooper pair breaking and formation neutrino emission, excludes direct Urca processes, and is modeled as a four‑parameter family (equation of state, superfluid properties, envelope composition, and mass) explored through extensive parameter variations. The study finds that Cooper pair emission dominates over modified Urca near the superfluid critical temperature, all detected neutron stars are broadly consistent with minimal cooling, but PSR 0833‑45, PSR 1706‑44, and the upper limits for PSR J0205+6449 and RX J0007.0+7302 suggest possible enhanced neutrino emission, while PSR 0656+14, PSR 1055‑52, Geminga, and RX J0720.4‑3125 may require additional internal heating.

Abstract

A new classification of neutron star cooling scenarios, involving either ``minimal'' cooling or ``enhanced'' cooling is proposed. The minimal cooling scenario replaces and extends the so-called standard cooling scenario to include neutrino emission from the Cooper pair breaking and formation process. This emission dominates that due to the modified Urca process for temperatures close to the critical temperature for superfluid pairing. Minimal cooling is distinguished from enhanced cooling by the absence of neutrino emission from any direct Urca process, due either to nucleons or to exotica. Within the minimal cooling scenario, theoretical cooling models can be considered to be a four parameter family involving the equation of state of dense matter, superfluid properties of dense matter, the composition of the neutron star envelope, and the mass of the neutron star. Consequences of minimal cooling are explored through extensive variations of these parameters. Results are compared with the inferred properties of thermally-emitting neutron stars in order to ascertain if enhanced cooling occurs in any of them. All stars for which thermal emissions have been clearly detected are at least marginally consistent with the lack of enhanced cooling. The two pulsars PSR 0833-45 (Vela) and PSR 1706-44 would require enhanced cooling in case their ages and/or temperatures are on the lower side of their estimated values whereas the four stars PSR 0656+14, PSR 1055-52, Geminga, and RX J0720.4-3125 may require some source of internal heating in case their age and/or luminosity are on the upper side of their estimated values. The new upper limits on the thermal luminosity of PSR J0205+6449 and RX J0007.0+7302 are indicative of the occurrence of some enhanced neutrino emission beyond the minimal scenario.