Gravitational waves from the axial perturbations of hyperon stars

The eigen-frequencies of the axial w-mode oscillations of hyperon stars are examined.It is shown that as the appearance of hyperons softens the equation of state of the super-density matter,the frequency of gravitational waves from the axial w-mode of hyperon star becomes smaller than that of a traditional neutron star at the same stellar mass.Moreover,the eigenfrequencies of hyperon stars also have scaling universality.It is shown that the EURO thirdgeneration gravitational-wave detector has the potential to detect the gravitational-wave signal emitted from the axial w-mode oscillations of a hyperon star.     

超子耦合参数对热前中子星物质的影响

在相对论平均场理论框架内,利用Λ超子的结合能和中子星质量的观测数据得到超子标量介子耦合参数χσ的范围是0.33—0.77。在这个范围内, 研究了χσ取不同值时, 包含核子, Λ和Ξ超子的热前中子星（固定单个重子熵s=1）的性质。结果表明, 如果超子耦合参数变大, 前中子星核心温度变高, 中微子丰度变低, 前中子星的亚稳态质量范围变小。如果χσ超过了0.75, 前中子星不可能演变成黑洞。联系SN1987A讨论了这一结果的意义。In the framework of the relativistic mean field theory(RMFT), protoneutron stars with hyperons are studied. To be compatible with neutron star masses, the hyperon scalar coupling χσ should lie in the range of 0.33—0.77. As the hyperon scalar coupling increases, in protoneutron star matter, the core temperature increases whereas the abundance of neutrinos decreases. The metastable mass range of protoneutron stars narrows as the temperature increases. It is found that a protoneutron star cannot subside into a low mass black hole when χσ>0.75. Furthermore, the case of SN1987A is discussed.     

Hyperon coupling dependence of hadron matter properties in relativistic mean field model

The properties of hadronic matter at β equilibrium in a wide range of densities are described by appropriate equations of state in the framework of the relativistic mean field model. Strange meson fields, namely the scalar meson field σ＊（975） and the vector meson field φ（1020）, are included in the present work. We discuss and compare the results of the equation of state, nucleon effective mass, and strangeness fraction obtained by adopting the TM1, TMA, and GL parameter sets for nuclear sector and three different choices for the hypcron couplings. We find that the parameter set TM1 favours the onset of hyperons most, while at high densities the GL parameter set leads to the most hyperon-rich matter. For a certain parameter set （e.g. TM1）, the most hyperon-rich matter is obtained for the hyperon potential model. The influence of the hyperon couplings on the effective mass of nucleon, is much weaker than that on the nucleon parameter set. The nonstrange mesons dominate essentially the global properties of dense hyperon matter. The hyperon potential model predicts the lowest value of the neutron star maximum mass of about 1.45 Msun to be 0.4--0.5 Msun lower than the prediction by using the other choices for hyperon couplings.[第一段]     

Hypernuclear physics for neutron stars

The role of hypernuclear physics for the physics of neutron stars is delineated. Hypernuclear potentials in dense matter control the hyperon composition of dense neutron star matter. The three-body interactions of nucleons and hyperons determine the stiffness of the neutron star equation of state and thereby the maximum neutron star mass. Two-body hyperon–nucleon and hyperon–hyperon interactions give rise to hyperon pairing which exponentially suppresses cooling of neutron stars via the direct hyperon URCA processes. Nonmesonic weak reactions with hyperons in dense neutron star matter govern the gravitational wave emissions due to the r-mode instability of rotating neutron stars.     

Effect of f0(975) and φ(1020) Mesons on Neutron Star Mater

In the RMF approach, considering the contributions of the σ^* and φ mesons and the hyperons in the baryon octet {N,P,Λ,∑^－,∑⁰,∑⁺,Ξ^－,Ξ⁰}, the properties of neutron star matter have been investigated. It is found that with the contributions of the σ^* and φ mesons, the critical baryon density of hyperon appearance decreases, the number of hyperons increases, the transition density ρ_0H of hyperon stars decreases, the equation of state turns soft, the maximum mass of neutron star decreases and the corresponding radius increases, the central density, the central energy density and the central pressure are all reduced.     

南开大学奇异性核物理的理论研究

简要介绍了南开大学核物理组在奇异性核物理方面的理论研究工作. 已经完成的工作有核介质内超子平均自由程的理论计算, 奇异性核物理方面已完成的其他4个理论研究课题是, 不同重子杂质对原子核的影响、重味重子超核、核物质内的eta-介子以及K介子原子核的性质.     

Kaon properties in (proto-)neutron star matter

The modification of kaon and antikaon properties in the interior of (proto-)neutron stars is investigated using a chiral SU(3) model. The parameters of the model are fitted to nuclear-matter saturation properties, baryon octet vacuum masses, hyperon optical potentials and low-energy kaon-nucleon scattering lengths. We study the kaon/antikaon medium modification and explore the possibility of antikaon condensation in (proto-)neutron star matter at zero as well as finite temperature/entropy and neutrino content. The effect of hyperons on kaon and antikaon optical potentials is also investigated at different stages of the neutron star evolution.     

Properties of hyperon stars rotating at Keplerian frequency

The structure and properties of a Keplerian rotating hyperon star with an equation of state (EOS) investigated using the relativistic σ--ω--ρ model are examined by employing an accurate numerical scheme. It is shown that there is a clear rotating effect on the structure and properties, and that hyperon star matter cannot support a star with a mass larger than 1.9 M_Θ, even a star rotating at the fastest allowed frequency. The constraints of the two known fastest rotating frequencies (716 Hz and 1122 Hz) on the mass and radius of a hyperon star are also explored. Furthermore, our results indicate that the imprint of the rapid rotation of a hyperon star on the moment of inertia is clear; the backward equatorial redshift, the forward equatorial redshift and the polar redshift can be distinguished clearly, the forward equatorial redshift is always negative; and its figuration is far from a spherical symmetric shape.     

The Effect of Hyperon-Hyperon Interaction on Neutron Stars

The equations ofstate of the neutron star matter are calculated in the relativistic mean-field approximation witl different hyperon coupling constants. The properties of neutron stars are studied by solving the OppenheimerVolkoff equation. It manifests the properties of neutron stars - change explicitly as different hyperon coupling constants are concerned.``     

The Effect of Hyperon-Hyperon Interaction on Neutron Stars

The equations of state of the neutron star matter are calculated in the relativistic mean-field approximation with different hyperon coupling constants. The properties of neutron stars are studied by solving the Oppenheimer-Volkoff equation. It manifests the properties of neutron stars — change explicitly as different hyperon coupling constants are concerned.     

Effect of the mesons σ* and Φ and the variety of U(N)Σ on the transition density of hyperon stars

The effect of the mesons σ* and φ and the variety of U(N)Σ on the transition density of hyperon stars is examined within the framework of relativistic mean field theory for the baryon octet {n,p,∧,∑-,∑0,∑+,Ξ- and Ξ0} system.It is found that,compared with that without considering the mesons δ* and φ,the transition density of hyperon stars decreases,the critical baryon density that hyperons Σ-,Σ0,Ξ+,Ξ- and Ξ0 appears to decrease too,but for A the effect is not obvious.As U(N)Σ goes up,the critical baryon density of Σ+,Σ0 and Σ- increases,that of Ξ0 decreases and that of A and Ξ- is fixed.In addition,it is found that the variety of U(N)Σ almost does not influence the transition density.     

Nuclear Matter in a Chiral SU(3) Quark Mean-Field Model

A quark meson coupling model based on SU(3)_L×SU(3)_R symmetry and scale invariance is proposed. The quarks and mesons get masses through symmetry broken. We apply this SU(3) chiral constituent quark model to investigating the nuclear matter at finite temperature and density. The effective baryon masses, compression modulus and hyperon potentials are all reasonable. The critical temperature of liquid-gas phase transition is also calculated in this model.     

Massive neutron stars and their implications

Recent observations of high mass pulsar PSRJ1614-2230 has raised serious debate over the possible role of exotics in the dense core of neutron stars. The precise measurement of mass of the pulsar may play a very important role in limiting equation of state (EoS) of dense matter and its composition. Indirectly, it may also shape our understanding of the nucleon–hyperon or hyperon–hyperon interactions which is not well known. Within the framework of an effective chiral model, we compute models of neutron stars and analyse the hyperon composition in them. Further related implications are also discussed.     

Neutron Star Structure in a Nonlinear Realization of Chiral SU(3) Spontaneous Breaking Model

We investigate neutron star properties by constructing a chiral SU(3) spontaneous breaking Lagrangian and using relativistic mean-field approximation. The results show that π- condensate appears at some baryon densities,and hyperons ∑- and A exist in neutron star matter at high density. In this model, neutron star's maximum mass is1.12Ms with corresponding radius about 8 km.     

高能强子化过程的自旋转移和轻子诱发反应中超子的极化

指出在轻子诱发反应中测量产生超子的极化是研究高能强子化过程自旋转移的理想途径.在总结这些反应产生的初始夸克或反夸克极化的基础上,给出了超子极化的计算方法.作为应用举例,给出了e⁺e^－湮没过程中LEP能量下各种超子的极化及与已有实验的比较.这些结果表明,测量各类超子的极化可作为检验不同的重子自旋结构图像,何者适用于高能碎裂过程的重要手段.     

Hadron-quark phase transitions in hyperon stars

The Gibbs and Maxwell constructions for the hadron-quark phase transition in neutron and protoneutron stars, including interacting hyperons in the confined phase, are compared. We find that the hyperon populations are suppressed and that neutrino trapping shifts the onset of the phase transition. The effects on the (proto)neutron star maximum mass are explored.     

\bar{K} Condensation in Neutron Star Matter with Δ Quartet

In the framework of relativistic mean field theory, the condensations of K^- and \bar{K}⁰ in neutron star matter including baryon octet and Δ quartet are studied. We find that in this case K^- and \bar{K}⁰ condensations canoccur at relative shallow optical potential depth of \bar{K} from -80 MeV to -160 MeV. Both K^- and \bar{K}⁰ condensations favor the appearances of Δ resonances. With \bar{K} condensations all the Δ quartet can appear well inside the maximum mass stars. The appearances of Δ resonances change the composition and distribution of particles at high densities. The populations ofΔ resonances can enhance K^- condensation. It is found that in the core of massive neutron stars, neutron star matter includes rich particle species, such as antikaons, baryon octet, and Δ quartet. In the presence of Δ resonances and \bar{K} condensation, the EOS becomes softer and results in smaller maximum mass stars. Furthermore the impact of antikaon condensations,hyperons, and Δ resonances on direct Urca process with nucleons is also discussed briefly.     

能通过大质量中子星转动惯量来限定超子参数吗?

在相对论平均场理论（RMFT）框架内,使用GL91参数组结合超核数据和大质量中子星的观测数据限定超子标量耦合参数X_σ=0.59~1.0,用慢转近似计算了大质量中子星和前中子星的转动惯量。当X_σ从0.59增加到1.0,中子星（前中子星）的最大转动惯量增幅达89%（60%）。在同样的变化范围内,用Crab的观测数据,计算得到中子星（前中子星）的最大能量损失（dE/dt）的增幅为44%（25%）,最大磁场增幅为48%（38%）。相比于前中子星,中子星的性质对超子参数更为敏感。当X_σ从0.59增加到1.0,PSR J0348+0432的转动惯量和dE/dt的增幅均为14%,而磁场减幅为10%。如果天文观测能够给出中子星转动惯量的上限,或者同时精确测量中子星的质量和转动惯量,能帮助人们进一步限定超子参数。In the framework of the relativistic mean field theory(RMFT) with GL91 cets, the momentum of inertia (I) of slowly rotating neutron stars is studied by perturbative approach. The scalar hyperon coupling should lie in the range of X_σ=0.59~1.0 to be compatible with massive neutron stars. As X_σ increases from 0.59 to 1.0, the maximum momentum of inertia(I_max) of neutron (protoneutron) stars increases by 89% (60%). According to the data of Crab, the maximum energy loss(dE/dt) of neutron (protoneutron) stars will increase by 44%(25%)and the maximum magnetic field (B) will increase by 48%(38%). I and dE/dt of PSR J0348+0432 both increase by 14%, while B decreases by 10% as X_σ increases from 0.59 to 1.0. So if the upper bound of I, or the accurate values of both the mass and I of neutron stars could be provided by the astronomical observations, the hyperon couplings should be further constrained in the future.     

Effect of the mesons σ~＊ and Φ and the variety of U_Σ~（N） on the transition density of hyperon stars

The effect of the mesons σ＊ and Φ and the variety of UΣ（N ）on the transition density of hyperon stars is examined within the framework of relativistic mean field theory for the baryon octet {n,p,Λ,Σ-,Σ0,Σ＋,Ξand Ξ0} system.It is found that,compared with that without considering the mesons σ＊ and Φ,the transition density of hyperon stars decreases,the critical baryon density that hyperons Σ-,Σ0,Σ＋,Ξand Ξ0 appears to decrease too,but for Λ the effect is not obvious.As U Σ（N ）goes up,the critical baryon density of Σ＋,Σ0 and Σincreases,that of Ξ0 decreases and that of Λ and Ξis fixed.In addition,it is found that the variety of UΣ（N ）almost does not influence the transition density.     

Bose-Einstein condensation of anti-kaons and neutron star twins

We investigate the role of Bose-Einstein condensation (BEC) of anti-kaons on the equation of state (EoS) and other properties of compact stars. In the framework of relativistic mean field model we determine the EoS for β-stable hyperon matter and compare it to the situation when anti-kaons condense in the system. We observe that anti-kaon condensates soften the EoS, thereby lowering the maximum mass of the stars. We also demonstrate that the presence of antikaon condensates in the high density core of compact stars may lead to a new mass sequence beyond white dwarf and neutron stars. The limiting mass of the new sequence stars is nearly equal to that of neutron star branch though they have distinctly different radii and compositions. They are called neutron star twins.