高密物质的一种状态方程和中子星的结构

我们用改进的平均场方法计算了核密度以上物质的状态方程,并用中子星结构方程计算了中子星结构,得出引力质量M_G=1.7M_⊙,转动惯量I=1.62×10~(45)g-cm~2。结果表明,这些值与观测值符合的较好;高密物质相互作用模型的选取直接影响到中子星结构的特征量,同时可看出,用改进的平均场方法可使非相对论和相对论理论所推算的中子星的质量和转动惯量之间的差距明显地缩小。     

中子星内部中微子辐射的相对论修正

通过相对论平均场理论和相关的弱相互作用冷却理论,在有、无超子两种情况下的中子星物质中研究含相对论效应的中子星冷却性质,并且与非相对论情况进行对比分析.考虑到极微小尺度上的引力会偏离牛顿引力,引入引力修正效应.结果表明中微子辐射的相对论效应降低了中微子发射率、发光度以及星体的冷却速度.在考虑引力修正的无超子的中子星物质中,相对论效应所引起的星体冷却速度降幅最大,对于两倍太阳质量的传统物质中子星可达56%,而超子物质中降幅最小,约为38%.     

中子星的相对论平均场描述

从相对论平均场理沦出发,考虑核子、超子和介子的相互作用,研究了中子星的结构和性质以及超子耦合常数对中子星性质的影响.发现当密度较高时,中子星的核心区主要由超子组成,即中子星转变成以超子为主要成分的奇异中子星,并且这种转变受到超子相互作用的影响.当超子耦合常数与核子耦合常数的比值为0.65时,中子星转变为奇异中子星所对应的密度最小,此时计算的中子星的最大质量为1.4 M⊙,与天文观测结果较好符合.     

大质量中子星PSR J0348+0432原生星的组成与结构

在相对论平均场理论中考虑重子八重态,选取每重子熵S=1或S=2,集中研究讨论了熵效应对大质量中子星PSR J0348+0432原生星性质的影响.利用GL85核子耦合参数组计算了PSR J0348+0432的质量,将该参数组推广来计算每重子熵S=1或S=2时原生中子星的性质.结果发现原生中子星较零温中子星会有更多超子出现,且原生中子星的温度越往内部温度越高,超子的出现会降低内部温度.熵会增大大质量原生中子星的质量,这种增加效应超过超子出现减小大质量中子星质量的效应.熵会增加原生中子星的半径,即原生中子星的冷却是一个星体收缩的过程.     

研究高密物质的平均场方法的改进

本文对以前提出的改进的平均场模型(简称 MMFT)做了进一步的讨论。发现该模型有新的解 ml=2 ; 加入ρ介子,计算了核物质对称能 α_4;用新的参数计算了加ρ与未加ρ的中子物质的态方程,将二者作了比较,并与ml=4.3的结果做了比较;引用二体关联函数对矢量介子平均场的形式做了初步的理论探讨;用中子星结构方程计算了相应于各种态方程的中子星最大质量,结果是: MMFT-2(ml=2): M_(max)=2.22M(不含ρ介子) M_(max)=2.27M(加入ρ介子) MMFT-1(ml=4.3):M_(max)=1.69M(不含ρ介子) M_(max)=1.89M(加入ρ介子)     

慢旋转星体结构的微扰方法

本文用微扰的方法求解在广义相对论框架下，均匀慢旋转的轴对称的稳定星体的结构方程，对旋转的处理精确到角速度的二级近似．并用１０种可能的中子星的物态方程，计算了相应的稳定星体的结构．通过对物态方程Ｇ的详细计算结果，具体讨论了旋转对中子星的结构参量及各种性质的影响．     

强磁场下中子星外壳的组分和状态方程

本文计算和讨论了强磁场下由冷的催化物质组成的中子星外壳的组份和状态方程。文中考虑了晶格能和强磁场下均匀电子气体的交换能的贡献.得出结论:(1)强磁场使低密度区的状态方程变软;(2)强磁场对高密度区的状态方程几乎没有影响;(3)核质量公式对外壳的组份影响较明显.     

用中子星限制暗物质粒子散射截面

中子星可以通过重子物质和暗物质的相互作用吸积暗物质,且在一定条件下, 中子星吸积的暗物质粒子可以引发自引力塌缩形成小型黑洞, 生成的黑洞可能会进一步吞噬中子星.依据文献已有模型, 基于以上物理过程给出了在暗物质粒子不同质量下对暗物质粒子--中子的散射截面的限制.使用弱相互作用大质量粒子(Weakly Interacting Massive Particle, WIMP)模型, 并考虑暗物质粒子是玻色子的情形, 讨论了暗物质粒子有无自相互作用以及有无湮灭等条件下对限制暗物质参数的影响.既考虑了已发现的两个中子星系统来给出对暗物质参数空间的限制,也考虑了两个可能存在的年老中子星来预测未来观测可能对暗物质参数空间的限制.对于考虑玻色--爱因斯坦凝聚(Bose-Einstein Condensate, BEC)的玻色子暗物质, 在无自相互作用或有弱自相互作用, 无湮灭或有很小湮灭截面的条件下,中子星给出的间接观测对暗物质粒子-中子散射截面的限制的强度比XENON1T直接探测实验来得更强.未来, 如果在银心附近能观测到年老中子星, 其观测结果可以提升模型给出的对暗物质粒子--中子散射截面的限制, 从而帮助人们进一步理解暗物质.     

γ爆的磁流管模型

本文讨论了一种基于老年中子星的γ爆的磁流管模型.这个模型认为中子星表面磁场包含有两种成分,一种为弱的偶极背景磁场,另一种为细管状局域强磁场.此模型可以同时解释回旋吸收线和高能尾巴的存在.本文还计算了磁场为1.7×10~(12)G时的Compton散射截面。对于GB880205,根据吸收线的深度定出磁流管内电子数密度与管的直径之积约为10~(21)cm~(-2).     

Be／X射线双星的理论模型

为解释Ｂｅ／Ｘ射线双星波段联合观测结果，已发展了许多理论模型。在本文中简述这些Ｂｅ／Ｘ射线双星理论模型的研究现状，包括枞两个正常的Ｂ型星组成的密近双星演化成为Ｂｅ／Ｘ射线双星的演化模型，描述Ｂｅ星气壳的物理模型，Ｂｅ星和中子星的性质所决定的中子星吸积方式的吸积量及Ｂｅ／Ｘ射线双星Ｘ射线源光变曲线的理论解释。     

A numerical study of the r-mode instability of rapidly rotating nascent neutron stars

The first results of numerical analysis of classical r-modes of rapidly rotating compressible stellar models are reported. The full set of linear perturbation equations of rotating stars in Newtonian gravity is solved numerically without the slow rotation approximation. A critical curve of gravitational wave emission induced instability, which restricts the rotational frequencies of hot young neutron stars, is obtained. Taking the standard cooling mechanisms of neutron stars into account, we also show the 'evolutionary curves' along which neutron stars are supposed to evolve as cooling and spinning down proceed. Rotational frequencies of 1.4-M_⊙ stars suffering from this instability decrease to around 100 Hz when the standard cooling mechanism of neutron stars is employed. This result confirms the results of other authors, who adopted the slow rotation approximation.     

Almost analytic models of ultramagnetized neutron star envelopes

Recent ROSAT measurements show that the X-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields B  ∼ 10¹⁵ − 10¹⁶ G, which are relevant to models that invoke 'magnetars' to account for soft γ-ray emission from some repeating sources.     

Cooling of Accretion-Heated Neutron Stars

We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.     

Neutron stars and the dense matter equation of state

Neutron stars provide a unique laboratory with which to study cold, dense matter. The observational quantities of primary astrophysics interest are the maximum mass and the typical radius of a neutron star. These quantities are related to the relative stiffness of neutron-rich matter at supernuclear densities and the density dependence of the nuclear symmetry energy near the nuclear saturation density. The measurements of these nuclear properties via nuclear systematics and structure, heavy-ion collisions and parity-violating electron scattering from neutron-rich nuclei, are discussed. Several new observations, including mass measurements of binary pulsars and a confirmed distance determination for a nearby cooling neutron star, will be summarized. Additionally addressed will be observations of thermal emissions from cooling neutron stars in globular clusters and thermonuclear explosions from accreting stars. It will be demonstrated how this astrophysical data is shedding light on the pressure-density relation of extremely dense matter.     

Multidimensional thermal structure of magnetized neutron star envelopes

Recently launched X-ray telescopes have discovered several candidate isolated neutron stars. The thermal radiation from these objects may potentially constrain our understanding of nuclear physics in a realm inaccessible to terrestrial experiments. To translate the observed fluxes from neutron stars into constraints, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We describe models of the thermal structure of the envelopes of neutron stars with magnetic fields up to 10¹⁴ G. Unlike earlier work, we infer the properties of envelope models in two dimensions and precisely account for the quantization of the electron phase-space. Both dipole and uniformly magnetized envelopes are considered.     

Dim isolated neutron stars, cooling and energy dissipation

The cooling and reheating histories of dim isolated neutron stars (DINs) are discussed. Energy dissipation due to dipole spindown with ordinary and magnetar fields, and due to torques from a fallback disk are considered as alternative sources of reheating which would set the temperature of the neutron star after the initial cooling era. Cooling or thermal ages are related to the numbers and formation rates of the DINs and therefore to their relations with other isolated neutron star populations. The possibility of energy dissipation at ages greater than about 10⁶ yrs is a potentially important factor in determining the properties of the DIN population. Interaction with a fallback disk, higher multipole fields and activity of the neutron star are briefly discussed.      

Neutron stars in globular clusters: Formation and observational manifestations

Population synthesis is used to model the number of neutron stars in globular clusters that are observed as low-mass X-ray sources and millisecond radio pulsars. The dynamical interactions between binary and single stars in a cluster are assumed to take place only with a continuously replenished “background” of single stars whose properties keep track of the variations in parameters of the cluster as a whole and the evolution of single stars. We use the hypothesis that the neutron stars forming in binary systems from components with initial masses of ～8–12 M _⊙ during the collapse of degenerate O-Ne-Mg cores through electron captures do not acquire a high space velocity. The remaining neutron stars (from single stars with masses >8 M _⊙ or from binary components with masses >12 M _⊙) are assumed to be born with high space velocities. According to this hypothesis, a sizeable fraction of the forming neutron stars remain in globular clusters (about 1000 stars in a cluster with a mass of 5 × 10⁵ M _⊙). The number of millisecond radio pulsars forming in such a cluster in the case of accretion-driven spinup in binary systems is found to be ～10, in agreement with observations. Our modeling also reproduces the observed shape of the X-ray luminosity function for accreting neutron stars in binary systems with normal and degenerate components and the distribution of spin periods for millisecond pulsars.     

Young Close-By Neutron Stars: The Gould Belt Vs. The Galactic Disc

We present new population synthesis calculations of close young neutron stars. In comparison with our previous investigation we use a different neutron star mass spectrum and different initial spatial and velocity distributions. The results confirm that most of ROSAT dim radioquiet isolated neutron stars had their origin in the Gould Belt. We predict that about several tens of young neutron stars can be identified in ROSAT All Sky Survey data at low galactic latitudes. Some of these sources also can have counterparts among EGRET unidentified sources.     

Thermal evolution of rotating hybrid stars

As a neutron star spins down, the nuclear matter is continuously converted into quark matter due to the core density increase, and then latent heat is released. We have investigated the thermal evolution of neutron stars undergoing such deconfinement phase transition. We have taken into account the conversion in the frame of the general theory of relativity. The released energy has been estimated as a function of changed rate of deconfinement baryon number. The numerical solutions to the cooling equation are seen to be very different from those without the heating effect. The results show that neutron stars may be heated to higher temperatures which is well matched with pulsar's data despite the onset of fast cooling in neutron stars with quark matter cores. It is also found that the heating effect has a magnetic field strength dependence. This feature could be particularly interesting for high temperatures of low-field millisecond pulsars at a later stage. The high temperature could fit the observed temperature for PSR J0437−4715.     

Signal of quark deconfinement in thermal evolution neutron stars with deconfinement heating

As neutron stars spin-down and contract, the deconfinement phase transition can continue to occur, resulting in energy release (so-called deconfinement heating) in case of the first-order phase transition. The thermal evolution of neutron stars is investigated to combine phase transition and the related energy release self-consistently. We find that the appearance of deconfinement heating during spin-down result in not only the cooling delay but also the increase of surface temperature of stars. For stars characterized by intermediate and weak magnetic field strength, a period of increasing surface temperature could exist. Especially, a sharp jump in surface temperature can be produced as soon as quark matter appears in the core of stars with a weak magnetic field. We think that this may serve as evidence for the existence of deconfinement quark matter. The results show that deconfinement heating facilitates the emergence of such characteristic signature during the thermal evolution process of neutron stars.