Hadronic Phases in High-Density Nuclear Matter and Neutron Stars

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Possible existence of kaon condensation in hyperonic matter, where hyperons are mixed in the ground state of neutron-star matter, has been investigated. P-wave kaon-baryon interactions are included as well as s-wave ones by the use of the SU(3) xSU(3) effective chiral Lagrangian. Baryonic potentials in high-density matter are also taken into account through the nonrelativistic effective baryon-baryon interactions which are based on the recent information from hypernuclear experiments. (1) The dispersion relations of kaons have been obtained. It has been shown that there appears an instability with respect to a pair creation of collective particle-hole modes with K^+ and K^-quantum numbers, stemming from the p-wave kaon-baryon interaction. The onset density of this instability is lower than that of the s-wave K^-condensation. (2) The equation of state (EOS) of the kaon-condensed phase has been studied, and the effects of the EOS on the static properties of neutron stars have been discussed. It has been shown that in the fully-developed phase, the effects of p-wave interactions are small, and that kaon condensation becomes of the s-wave type at higher density. The EOS becomes very soft owing to the s-wave kaon-baryon interactions. In addition to the kaon-condensed branch with a first-order phase transition after Maxwell's construction, there is a kaon-condensed 'self-bound' branch just like a self-bound state for a strange star. The whole star is composed of kaon condensates with high-density. The self-bound star contains a metastable region, so that it eventually decays into a stable star obtained with Maxwell's construction with the total baryon number fixed. The lifetime of the kaon-condensed self-bound star has been estimated in a semi-classical approximation. It is possible that the lifetime is longer than the age of the universe, so that the self-bound star is substantially stable.