Composition, structure and evolution of neutron stars with kaon condensates

Abstract

We investigate the possibility of kaon condensation in the dense interior of neutron stars through the s-wave interaction of kaons with nucleons. We include nucleon-nucleon interactions by using simple parametrizations of realistic forces, and include electrons and muons in β-equilibrium. The equation of state above the condensate threshold is derived in the mean field approximation. The conditions under which kaon condensed cores undergo a transition to quark matter containing strange quarks are also established. The critical density for kaon condensation lies in the range (2.3–5.0) ϱ 0 where ϱ 0 = 0.16 fm −3 is the equilibrium density of nuclear matter. The critical density depends largely on the value of the strangeness content of the proton, the size of which is controversial. For too large a value of the strangeness content, matter with a kaon condensate is not sufficiently stiff to support the lower limit of 1.44 M ⊙ for a neutron star. Kaon condensation dramatically increases the proton abundance of matter and even allows positrons to exist inside the core. We also consider the case when neutrinos are trapped in the matter, a situation that applies to newly-formed neutron star matter that is less than about 10 s old. Neutrino trapping shifts both kaon condensation and the quark matter transition to higher densities than in the case of cold, catalyzed matter. A newly-formed neutron star is expected to have a rather low central density, the density rising only after mass accretion onto the star ends after a few seconds. Thus, it is likely that if kaon condensation and/or the quark-hadron phase transition occur, they do so only during or after the mass accretion and neutrino trapping stages. We suggest that neutrino observations from a galactic supernova may provide direct evidence for or against a condensate and/or a quark-hadron transition.