Recent Developments on Kaon Condensation and Its Astrophysical Implications

TL;DR

This paper explores theoretical models of kaon condensation in dense matter, its implications for neutron star collapse into black holes, and the resulting astrophysical phenomena, supported by recent observations and theoretical consistency.

Contribution

It introduces three approaches to kaon condensation using hidden local symmetry and links this to neutron star collapse and black hole formation.

Findings

Kaon condensation occurs at about 3 times nuclear matter density.

Neutron stars above 1.5 solar masses likely collapse into black holes due to kaon condensation.

Binary neutron stars tend to have nearly equal masses, influencing black hole and neutron star populations.

Abstract

We discuss three different ways to arrive at kaon condensation at n_c = 3 n_0 where n_0 is nuclear matter density: (1) Fluctuating around the n=0 vacuum in chiral perturbation theory, (2) fluctuating around n_VM near the chiral restoration density n_chi where the vector manifestation of hidden local symmetry is reached and (3) fluctuating around the Fermi liquid fixed point at n_0. They all share one common theoretical basis, "hidden local symmetry." We argue that when the critical density n_c < n_chi is reached in a neutron star, the electrons turn into K^- mesons, which go into an S-wave Bose condensate. This reduces the pressure substantially and the neutron star goes into a black hole. Next we develop the argument that the collapse of a neutron star into a black hole takes place for a star of M = 1.5 M_sun. This means that Supernova 1987A had a black hole as result. We also show that two neutron stars in a binary have to be within 4% of each other in mass, for neutron stars sufficiently massive that they escape helium shell burning. For those that are so light that they do have helium shell burning, after a small correction for this they must be within 4% of each other in mass. Observations support the proximity in mass inside of a neutron star binary. The result of strangeness condensation is that there are 5 times more low-mass black-hole, neutron-star binaries than double neutron-star binaries although the former are difficult to observe.