Hyperon-Mixed Neutron Stars

Hyperon mixing in neutron star matter is investigated by the G-matrix-based effective interaction approach under the attention to use the YN and the YY potentials compatible with hypernuclear data and is shown to occur at densities relevant to neutron star cores, together with discussions to clarify the mechanism of hyperon contamination. It is remarked that developed Y-mixed phase causes a dramatic softening of the neutron star equation of state and leads to the serious problem that the resulting maximum mass Mmax for neutron star model contradicts the observed neutron star mass (Mmax≪Mobs = 1.44 M⊙), suggesting the necessity of some “extra repulsion” in hypernuclear system. It is shown that the introduction of three-body repulsion similar to that in nuclear system can resolve the serious situation and under the consistency with observation (Mmax > Mobs) the threshold densities for Λ and Σ- are pushed to higher density side, from ∼2ρ0 to ∼4ρ0 (ρ0 being the nuclear density). On the basis of a realistic Y-mixed neutron star model, occurrence of Y-superfluidity essential for “hyperon cooling” scenario is studied and both of Λ- and Σ--superfluids are shown to be realized with their critical temperatures 108-9 K, meaning that the “hyperon cooling” is a promising candidate for a fast non-standard cooling demanded for some neutron stars with low surface temperature. A comment is given as to the consequence of less attractive ΛΛ interaction suggested by the “NAGARA event” 6ΛΛHe.