Physics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phr...242..297p&link_type=abstract
Physics Reports, Volume 242, Issue 4-6, p. 297-312.
Physics
16
Scientific paper
The thermal evolution of neutron stars has long been regarded as the source of information about the possible physical states of dense matter. Until recently, the general view was that, if matter consisted of nucleons, cooling would be relatively slow, while if matter were in an exotic state (Bose condensates or quark matter), cooling would be faster - so fast, in fact, that thermal emission from the star's surface would be unobservable. Recently, this view has been called into question following the demonstration that ordinary matter can cool by the so-called direct Urca process even more rapidly than matter in an exotic state. The conditions under which very rapid cooling would occur through energy loss via emission of neutrinos from the interior are as follows.
The direct Urca process can occur in neutron stars if the proton concentration exceeds some critical value in the range 11-15%. The proton concentration, which is determined by the poorly known symmetry energy of matter above nuclear density, exceeds the critical value in many current calculations. If it occurs, the direct Urca process enhances neutrino emission and neutron star cooling rates by a large factor compared to any process considered previously.
Direct Urca processes with hyperons and/or nucleon isobars can also occur in dense matter as long as the concentration of Λ hyperons exceeds a critical value that is less than 3% and is typically about 0.1%. The neutrino luminosities from the hyperon Urca processes are about 5-100 times less than the typical luminosity from the nucleon direct Urca process, but they are larger than those expected from other sources. These new direct Urca processes provide avenues for rapid cooling of neutron stars which invoke neither exotic states nor the large proton fraction required for the nucleon direct Urca process.
It is thus likely that all neutron stars will cool rapidly, whether they contain the so-called exotic matter or not. If so, the ramifications are many and are briefly discussed.
Research supported in part by the US Department of Energy Grant No. DE-FG02-88ER40388.
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