Minimal Cooling of Neutron Stars: A New Paradigm

A new classification of neutron star cooling scenarios, involving either``minimal'' cooling or ``enhanced'' cooling is proposed. The minimal coolingscenario replaces and extends the so-called standard cooling scenario toinclude neutrino emission from the Cooper pair breaking and formation process.This emission dominates that due to the modified Urca process for temperaturesclose to the critical temperature for superfluid pairing. Minimal cooling isdistinguished from enhanced cooling by the absence of neutrino emission fromany direct Urca process, due either to nucleons or to exotica. Within theminimal cooling scenario, theoretical cooling models can be considered to be afour parameter family involving the equation of state of dense matter,superfluid properties of dense matter, the composition of the neutron starenvelope, and the mass of the neutron star. Consequences of minimal cooling areexplored through extensive variations of these parameters. Results are comparedwith the inferred properties of thermally-emitting neutron stars in order toascertain if enhanced cooling occurs in any of them. All stars for whichthermal emissions have been clearly detected are at least marginally consistentwith the lack of enhanced cooling. The two pulsars PSR 0833-45 (Vela) and PSR1706-44 would require enhanced cooling in case their ages and/or temperaturesare on the lower side of their estimated values whereas the four stars PSR0656+14, PSR 1055-52, Geminga, and RX J0720.4-3125 may require some source ofinternal heating in case their age and/or luminosity are on the upper side oftheir estimated values. The new upper limits on the thermal luminosity of PSRJ0205+6449 and RX J0007.0+7302 are indicative of the occurrence of someenhanced neutrino emission beyond the minimal scenario.Comment: Version to appear in ApJ Supplements. Minor modifications in text and discussion of updated data with new figure