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.``     

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.     

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.     

超子耦合常数对混合星性质的影响

用相对论平均场理论描述强子物质, 用NJL模型描述夸克物质研究了超子耦合常数对混合星性质的影响. 结果表明, 随着超子耦合常数的增大, 强子-夸克相变密度变小, 混合相区域的状态方程变硬, 中子星的最大质量及对应的半径变大. 超子耦合常数由0.73增到1.0时, 混合星最大质量由1.68M_⊙增至1.84M_⊙, 相应的半径由11.4km变到12.5km. 该夸克模型下不同强子参数组对应的混合星性质也有较明显的差异. 可知, 其适合于描述大质量而小夸克核心的混合星.     

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.     

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.     

An equation of state for nuclear and higher-density matter based on relativistic mean-field theory

A model stress tensor for high-density matter based on a linearized relativistic quantum field theory is examined. The two coupling constants are fit to nuclear matter. Other properties of nuclear and neutron matter and neutron stars are then implied.     

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.     

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.     

The hyperon coupling constants and the surface gravitational redshift of massive neutron stars

With relativistic mean field theory we examine effect of hyperon coupling constants of hyperon Ξ on the surface gravitational redshift of the massive neutron star (NS) PSR J1614-2230 and NS PSR J0348+0432 as the potential well depth of hyperon Ξ is fixed. We find that the mass and radius of a NS increase with the increase of the coupling constant of hyperon Ξ. With the increase of the coupling constant of the hyperon Ξ, the surface gravitational redshift will decrease for a same NS mass but will increase for a same NS radius. The surface gravitational redshift of the more massive NS PSR J0348+0432 decreases by more than that of the less massive NS PSR J1614-2230. We also find that the value range of the surface gravitational redshift of NS will become narrower with the increase of the coupling constant of hyperon Ξ. The greater the NS mass, the greater the influence of the coupling constant of hyperon Ξ on the value range of the surface gravitational redshift of the NS.     

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.     

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.     

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.     

Two New Hyperon Coupling Models in the Light of the Massive Neutron Star PSR J0348+0432*

In the context of the relativistic mean field theory, we propose two new hyperon coupling models, namely the limitation model and the potential well depth model, in the light of the observed data for the massive neutron PSR J0348+0432. The radius of PSR J0348+0432 given by the limitation model is found to be $12.52 \text{ km}\sim12.97\text{ km}$, while the radius given by the potential well depth model is found to be $12.19\text{ km}\sim12.89 \text{ km}$. We also calculate the gravitational redshift of PSR J0348+0432 within these two models, for which the limitation model gives $0.346\sim0.391$ and the potential well depth model gives $0.350\sim0.409$. Further exploration of these two models shows that, these two models are almost degenerate for neutron stars lighter than $1.85 M_{\odot}$, and start to give different results for massive neutron stars heavier than $1.85 M_{\odot}$. Therefore, the studies of massive neutron stars could be crucial for discriminating these two models and help deepen our understanding of hyper-nuclear interactions.     

Influences of the bag constant on properties of hybrid stars

Influences of the bag constant on the properties of hybrid stars are investigated by using relativistic mean field theory and the MIT bag model to describe the hadron phase and quark phase in the interior of neutron stars, respectively. Our results indicate that the onset of hadron-quark phase transition is put off and the appearance of hyperon species is increased with the increase in bag constant. As a result, the hybrid star equation of state for a mixed phase range stiffens whereas that of the quark phase range softens, and the gravitational mass as well as the corresponding radius of hybrid stars are increased obviously. The gravitational mass of a hybrid star is increased from 1.42 M_⊙ (M_⊙ is solar mass) to 1.63 M_⊙ and the corresponding radius is changed from 9.1 km to 12.2 km when the bag constant (B^1/4) is increased from 170 MeV to 200 MeV. It is interesting to find that hybrid star equations of state become non-smooth when the TM2 parameter sets in the framework of relativistic mean field theory used to describe the hadronic matter, and consequently, the third family of compact stars appear in the mass-radius relations of hybrid stars in the narrow scope of the bag constant from 175 MeV to 180 MeV. These show that the choice of the bag constant in the MIT bag model has significant influence on the properties of hybrid stars.     

Neutron stars with isovector scalar correlations

Neutron stars with the isovector scalar δ-field are studied in the framework of the relativistic mean-field (RMF) approach in a pure-nucleon-plus-lepton scheme. The δ-field leads to a larger repulsion in dense neutron-rich matter and to a definite splitting of proton and neutron effective masses. Both features are influencing the stability conditions of the neutron stars. Two parametrizations for the effective nonlinear Lagrangian density are used to calculate the nuclear equation of state (EOS) and the neutron star properties, and compared to correlated Dirac-Brueckner results. We conclude that in order to reproduce reasonable nuclear structure and neutron star properties within a RMF approach, a density dependence of the coupling constants is required.     

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.     

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.     

A New Determination of the Lambda-Nucleon Coupling Constants in Relativistic Mean Field Theory

A new determination of the Lambda-nucleon coupling constants in relativistic mean field theory is presented by optimizing both hyperon binding energy and spin-orbit splitting.Hypernuclear single particle spectra with the new coupling constants suggest the good agreement between the calculation and available data.The spin-orbit splitting of hyperon in medium mass hypernuclei is systematically larger than that in light-or heavy-mass hypernuclei.The sensitivity of the Lambda spin-orbit splitting to the omega-Lambda-Lambda tensor coupling term is also explored.