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.     

Influence of Interactions on Populations for Hyperons in Neutron Stars

The numerical results of the populations for the baryon octet in neutron star matter have been presented by solving a set transcendental equations in the framework of the relativistic mean field approximation. The influence of the hyperon interactions on hyperon populations in neutron star matter is discussed. The results manifest that when the ratio of the hyperon-to-nucleon couplings increases, all hyperons appear towards low baryon density direction.     

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.     

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.     

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.     

致密物质性质和适用于超新星及中子星研究的状态方程(英文)

采用相对论平均场方法研究了致密物质的性质, 构造了包括较宽温度、 同位旋不对称度和密度范围的适用于超新星模拟研究的状态方程, 均匀物质由相对论平均场理论描述, 非均匀物质由托马斯 费米近似给出。讨论了包含超子自由度的中子星物质的状态方程。 计算结果表明, 包含超子可以有效地软化高密度区的状态方程, Λ超子的超流态有可能存在于大质量中子星内部。The properties of dense matter are studied within the relativistic mean field theory. The equation of state (EOS) of dense matter are constructed covering a wide range of temperature, proton fraction, and density for the use of supernova simulations. The relativistic mean field theory is employed to describe the uniform matter, while the Thomas Fermi approximation is adopted to describe the non uniform matter. The EOS of neutron star matter is discussed with the inclusion of hyperons. It is found that the EOS at high density can be significantly softened by the inclusion of hyperons. The 1S₀ superfluidity of Λ hyperons may exist in massive neutron stars.     

Phase transition to hyperon matter in neutron stars

Recent progress in the understanding of the high density phase of neutron stars advances the view that a substantial fraction of the matter consists of hyperons. The possible impacts of a highly attractive interaction between hyperons on the properties of compact stars are investigated. We find that a hadronic equation of state with hyperons allows for a first order phase transition to hyperonic matter. The corresponding hyperon stars can have rather small radii of R approximately equal 8 km.     

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.     

Antikaon Condensation and In-medium Kaon and Antikaon Production in Protoneutron Stars

Antikaon condensation and kaon and antikaon production in protoneutron stars are investigated in a chiral hadronic model （also referred to as the FST model in this paper）. The effects of neutrino trapping on protoneutron stars are analyzed systematically. It is shown that neutrino trapping makes the critical density of K^- condensation delay to higher density and fifo condensation not occur. The equation of state （EOS） of （proto）neutron star matter with neutrino trapping is stiffer than that without neutrino trapping. As a result, the maximum masses of （proto）neutron stars with neutrino trapping are larger than those without neutrino trapping. If hyperons are taken into account, antikaon does not form a condensate in （proto）neutron stars. Meanwhile, the corresponding EOS becomes much softer, and the maximum masses of （proto）neutron stars are smaller than those without hyprons. Finally, our results illustrate that the Q values for K^＋ and K^- production in （proto）neutron stars are not sensitive to neutrino trapping and inclusion of hyperons.     

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.     

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.     

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.``     

利用密度相关的相对论平均场理论对核物质与中子星的研究(英文)

研究和详细地比较了RMF理论中不同的有效相互作用强度的密度依赖性, 并且讨论了这种密度依赖性对于核物质和中子星性质的影响. 对于核物质, 不同的参数组给出的对称核物质的饱和点非常接近, 基本都在经验值的范围内. 对于中子星, 考虑超子后不同参数组给出的质量极限的范围为1.52—2.06 M☉, 半径为10.24—11.38 km.The density dependencies of various effective interaction strengths in the relativistic mean field and their influences on the properties of nuclear matter and neutron stars are studied and carefully compared. The differences of saturation properties given by various effective interactions are subtle in symmetric nuclear matter. The Oppenheimer Volkoff mass limits of neutron stars calculated from different equations of state are 1.52—2.06 M☉, and the radii are 10.24—11.38 km with hyperons included.     

Quark deconfinement in neutron star cores

Whether or not the deconfined quark phase exists in neutron star cores is an open question. We use two realistic effective quark models, the three-flavor Nambu-Jona-Lasinio model and the modified quark-meson coupling model, to describe the neutron star matter. We show that the modified quark-meson coupling model, which is fixed by reproducing the saturation properties of nuclear matter, can be consistent with the experimental constraints from nuclear collisions. After constructing possible hybrid equations of state (EOSes) with an unpaired or color superconducting quark phase with the assumption of the sharp hadron-quark phase transition, we discuss the observational constraints from neutron stars on the EOSes. It is found that the neutron star with pure quark matter core is unstable and the hadronic phase with hyperons is denied, while hybrid EOSes with a two-flavor color superconducting phase or unpaired quark matter phase are both allowed by the tight and most reliable constraints from two stars Ter 5 I and EXO 0748-676. And the hybrid EOS with an unpaired quark matter phase is allowed even compared with the tightest constraint from the most massive pulsar star PSR J0751+1807.     

Isovector Scalar Field Effects in Asymmetric Nuclear Matter

Density-dependent parametrization models of the nucleon-meson coupfing constants, including the isovector scalar δ-field, are applied to asymmetric nuclear matter. The nuclear equation of state （EOS） and the neutron star properties are studied in a relativistic Lagrangian density, using the relativistic mean field （RMF） hadron theory. It is known that the δ-field in the constant coupling scheme leads to a larger repulsion in dense neutron-rich matter and to a definite splitting of proton and neutron effective masses, finally influences the stability of the neutron stars. We use density-dependent models of the nucleon-meson couplings to study the properties of neutron star matter and to reexamine the （~-field effects in asymmetric nuclear matter. Our calculation shows that the stability conditions of the neutron star matter can be improved in presence of the δ-meson in the density-dependent models of the coupling constants. The EOS of nuclear matter strongly depends on the density dependence of the interactions.     

K0 Condensation in Hyperonic Neutron Star Matter

In the framework of the relativistic mean field theory, we investigate K^0 condensation along with K^- condensation in neutron star matter including the baryon octet. The results show that both K^0 and K^- condensations can occur well in the core of the maximum mass stars for relatively shallow optical potentials of K^- in the range of-100 MeV～ -160 MeV. With the increasing optical potential of K^-, the critical densities of K^- decrease and the species of baryons appearing in neutron stars become fewer. The main role of K^0 condensation is to make the abundances of particles become identical leading to isospin saturated symmetric matter including antikaons, nucleons and hyperons. K^- condensation is chiefly responsible for the softening of the corresponding equation of state, which leads to a large reduction in the maximum masses of neutron stars. In the core of massive neutron stars, neutron star matter including rich particle species, such as antikaons, nucleons and hyperons, may exist.     

Hyperons in neutron stars

Generalized beta equilibrium involving nucleons, hyperons and isobars is examined for neutron star matter. The hyperons produce a considerable softening of the equation of state. It is shown that the observed masses of neutron stars can be used to settle a recent controversy concerning the nuclear compressibility. Compressibilities less than 200 MeV are incompatible with observed masses.     

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]     

\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.