Shigeru Nishizaki

Hyperon-Mixed Neutron Star Matter and Neutron Stars ∗)

Effective Σ − n and Σ − Σ − interactions are derived fromthe G-matrix calculations for {n+Σ − } matter and employed in the investigation of hyperon mixing in neutron star matter. The threshold densities ρt(Y ) at which hyperons start to appear are between 2ρ0 and 3ρ0 (where ρ0 is the normal nuclear density) for both Λ and Σ − , and their fractions increase rapidly with baryon density, reaching 10% already for ρ � ρt+ρ0. The mechanism of hyperon mixing and single-particle properties, such as the effective mass and the potential depth, are analyzed taking into account the roles of YN and NN interactions. The resulting equation of state is found to be too soft to sustain the observed neutron star mass Mobs =1 .44M� .W e discuss the reason for this and stress the necessity of the “extra repulsion” for YN and YY interactions to resolve this crucial problem. It is remarked that ρt(Y ) would be as large as 4ρ0 for neutron stars compatible with Mobs. A comment is given regarding the effects on the Y -mixing problem from a less attractive ΛΛ interaction, newly suggested by the NAGARA event.

Effective YN and YY Interactions and Hyperon-Mixing in Neutron Star Matter Y ≡ Λ Case

Effective ΛN and ΛΛ interactions in dense hyperonic nuclear matter are constructed on the basis of the G-matrix calculation with Nijmegen hard-core potentials. With these effective interactions, the mixing of Λ in neutron star matter and the equation of state are analyzed. The Λ-mixed phase is shown to appear in neutron star cores with a baryon number density ρ > ρt(Λ) � (3 − 5)ρ0, where ρt(Λ) is the threshold density for the Λmixing and ρ0 is the normal nuclear matter density. The density ρt(Λ) depends not only on the ΛN but also on the NN interactions. The three-body force introduced in the NN interaction to reproduce the proper nuclear saturation properties enhances the Λ-mixing and drastically softens the equation of state. The resulting equation of state is not consistent with the observed neutron star mass Mobs =1 .44M� . It is found that this crucial problem can be resolved by the introduction of a three-body repulsion also for the ΛN and ΛΛ interactions. The finite-temperature effect on the Λ-mixing is found to be large, especially at lower densities and is expected to affect the properties of neutron stars at birth.

Occurrence of Hyperon Superfluidity in Neutron Star Cores

proach. Numerical results for the equation of state (EOS) with the mixing ratios of the respective components and the hyperon energy gaps including the temperature dependence are presented. These are meant to serve as physical inputs for Y -cooling calculations of NSs. By paying attention to the uncertainties of the EOS and the YY interactions, it is shown that both Λ and Σ − are superfluid as soon as they appear although the magnitude of the critical temperature and the density region where superfluidity exists depend considerably on the YY pairing potential. Considering momentum triangle condition and the occurrence of superfluidity, it is found that a so-called “hyperon cooling”(neutrino-emission from direct Urca process including Y ) combined with Y -superfluidity may be able to account for observations of the colder class of NSs. It is remarked that Λ-hyperons play a decisive role in the hyperon cooling scenario. Some comments are given regarding the consequences of the less attractive ΛΛ interaction recently suggested by the “NAGARA event” 6He.

Double-giant-dipole resonance in ^ Pb

Double-dipole excitations in ^{208}Pb are analyzed within a microscopic model explicitly treating 2p2h-excitations. Collective states built from such 2p2h-excitations are shown to appear at about twice the energy of the isovector giant dipole resonance, in agreement with the experimental findings. The calculated cross section for Coulomb excitation at relativistic energies cannot explain simultaneously the measured single-dipole and double-dipole cross sections, however.

Three-Body Force as an “Extra Repulsion” Suggested from Hyperon-Mixed Neutron Stars

A serious inconsistency between theory and observation for the mass of hyperon-mixed neutron stars strongly suggests the missing of some “extra repulsion” in hypernuclear systems. The mechanism is remarked, and as such microscopic origins of repulsion, the 3-body forces from an extended 2π exchange via isobar Δ excitation type (2πΔ) and from a viewpoint of latent effects for two-baryon overlapping in a string-junction quark model (SJM) are tested. It is remarked that the combined effects from these two processes (2πΔ+SJM) is a promising candidate to solve the confronting problem, keeping the consistency with the saturation property of nuclear matter.

The Possibility of Hyperon Superfluids in Neutron Star Cores

The possible existence of hyperon superfluidity in neutron star cores is studied by using several 1 S0 YY interactions from the OBE baryon-baryon potentials. It is found that not only Λ but also Σ − and Ξ − hyperons could very likely be superfluid. With the increase of the total baryon density ρ toward the center of neutron stars, hyperons (Y )begin to appear as new constituents. The subject of hyperonmixing has gathered much attention from an early stage of theoretical works on neutron stars. 2) - 11) The hyperon fraction yY increases with ρ and in the core region they are important components comparable to nucleons, interestingly affecting the properties of neutron stars. Here we address the question of whether Λ, Σ − and Ξ − admixed could be superfluid. The occurrence of hyperon superfluidity plays a key role in the rapid cooling scenario of neutron stars, i.e., the so-called “hyperon cooling”, 12) - 15) as one of the non-standard cooling scenarios to explain the unusually low surface temperatures observed for some neutron stars. In a preceding work, 16) we concentrated our attention on the case of the Λ superfluid, and we showed that Λ superfluidity exists, though in a limited density region, with the critical temperature Tc ∼ 10 8 −10 9 K. In the present work we discuss the cases of Σ − - and Ξ − -superfluids in reference to the Λ case. Unfortunately, our present knowledge of YY (and also YN )interactions is very limited as compared to the NN interaction. For this reason, we calculate the pairing energy gap by choosing several baryon-baryon (BB) potentials based on the hypothesis of SU(3)invariance and see what can be said regarding the existence or nonexistence of Y superfluids. In this choice, we pay particular attention to the compatibility with hypernuclear data. Although hyperons participate in the high-density region ( ρ> ∼ 2ρ0; where ρ0 = 0.17 nucleons /fm 3 is the nuclear density), the fractional density ρY (≡ yY ρ)is relatively small because yY is not large (10% − 30% at most), and thus the Fermi energy � FY (= ¯ 2 (3π 2 ρY ) 2/3 /2MY , with MY the hyperon mass)is rather low. Therefore, the pairing interaction responsible for Y superfluidity should be that in the 1 S0 pair state. Because there are different Fermi surfaces for every hyperon species, the pairing correlation can be restricted to that of the same Y species. Then the energy gap equation to be solved here has a well-known form of the 1 S0-type. 16)

Superfluidity of Hyperon-Mixed Neutron Stars

Superfluidity of hyperons (Y) admixed in neutron star cores is investigated by a realistic approach. It is found that hyperons such as A and Σ - are likely to be superfluid due mainly to their large effective masses in the medium, in addition to their 1 S 0 -pairing attraction not so different from that of nucleons. Also the existence of nucleon superfluidity at high-density is investigated under a developed Y-contamination. It is found that the density change of nucleon components due to the Y-mixing does not work for the realization of n-superfluid and makes the existence of p-superfluid more unlikely, as compared to the normal case without the Y-mixing.

Universal three‐body repulsion suggested by neutron stars

Because of a serious inconsistency between theory and observation for the mass of hyperon‐mixed neutron stars, it is suggested that some “extra repulsion” is needed in hypernuclear systems. A 3‐body force repulsion is tested for two cases, a 2π‐exchange via Δ‐excitation type (2πΔ) and a string‐junction model (SJM) for the quark structure of baryons. It is found that the 2πΔ model generates an increasing repulsion with increasing density but cannot produce the “extra repulsion” in hyperon‐mixed neutron stars because it does not work on a Λ due to the lack of ΛΛπ direct coupling. It is pointed out that the “extra repulsion” should act universally, i.e., independent of baryon species. The SJM 3‐body repulsion can satisfy the condition of universality because of the flavor‐independence and the {2πΔ+SJM} scheme is shown to be a promising candidate for the “extra repulsion”, maintaining consistency with the empirical saturation property of nuclear matter.

Fluctuation properties of strength functions associated with giant resonances

We performed fluctuation analysis by means of the local scaling dimension for the strength function of the isoscalar (IS) and the isovector (IV) giant quadrupole resonances in

Fluctuation properties of the strength function associated with the giant quadrupole resonance in ^{208}Pb

^{40}\mathrm{Ca}