简并度对引力不稳定性的影响

本讨论了无碰撞粒子系统中简并度对引力不稳定性的影响,计算了塌缩时间的Ｌａｕｄａｕ阻尼时间。     

8M_■恒星演化过程中湍流应力作用效果的分析

以恒星结构与演化理论中常用的混合程理论为基础,将湍流作用表现出来的宏 观应力引入恒星结构与演化模型中的流体静力学平衡方程．通过计算8M(?)恒星从主序星 到早期AGB星演化过程中湍流应力梯度与引力的比值来研究湍流作用对恒星演化与结构 的影响．结果发现:在核燃烧阶段其比值很小,湍流作用几乎可以忽略;但在RGB和早 期AGB演化阶段,发现在恒星外部存在一个湍流应力梯度为引力的几倍到几十倍的很小 区域,而该小区域以外的对流区内湍流作用力能达到引力的65％,这些对AGB星的中心 温度变化与热脉动发生的时间等恒星结构与演化规律有不同程度的影响．     

非均匀椭球的引力坍缩

本文讨论了密度不均匀性对椭球引力坍缩的影响。如果椭球初始存在一个从中心到边缘的微小密度梯度,就会产生一个在坍缩后期不可忽视的压力。这个压力起抗衡引力坍缩的作用,作用的大小受热力学过程影响。满足多方过程pocp~v(v～5/3)的非均匀椭球会出现振荡。但对于初始偏心率很大的长椭球,反而会加强Lin,Mestal & Shu于1965年提出的不稳定性。     

8M⊙恒星演化过程中湍流应力作用效果的分析

以恒星结构与演化理论中常用的混合程理论为基础，将湍流作用表现出来的宏观应力引入恒星结构与演化模型中的流体静力学平衡方程．通过计算8M⊙恒星从主序星到早期AGB星演化过程中湍流应力梯度与引力的比值来研究湍流作用对恒星演化与结构的影响．结果发现：在核燃烧阶段其比值很小，湍流作用几乎可以忽略；但在RGB和早期AGB演化阶段，发现在恒星外部存在一个湍流应力梯度为引力的几倍到几十倍的很小区域，而该小区域以外的对流区内湍流作用力能达到引力的65％，这些对AGB星的中心温度变化与热脉动发生的时间等恒星结构与演化规律有不同程度的影响．     

关于太阳系起源过程中尘层的厚度和演化

本文讨论了在尘层沉降后期的一些动力学过程。假定在沉降后期由于湍流的阻碍而出现缓慢沉降(准平衡),求出了开始产生径向引力不稳定性的临界厚度,并给出三种尺度之间的确定的数值关系 <|Z|>_c:h_*:λ_r=0.02107:0.1592:1,其中<|Z|>_c为开始产生径向引力不稳定时尘粒的平均高度,h_*的定义是:当半厚度等于h_*时,在Z=h_*处尘层自引力恰等于太阳引力的Z分量,λ_r则为尘层发生环形不稳定扰动的最大波长。本文并计算了尘层成环所需的时间尺度,结果表明,成环时间远远短于沉降时间。     

速度分布和银盘厚度效应对正常脉冲星Z向分布的影响

基于彭秋和等1978年得到的无扰动有限厚盘引力势,根据两个引力势参数(h_(z1)= 0．001kpc,h_(z2)=0．325 kpc)和两个速度分布参数(σ_(v1)=115km/s,σ_(v2)=200 km/s)组合的4种情况,对4×10~5颗脉冲星做了三维蒙特卡洛模拟．4×10~5颗脉冲星按着10~8年间隔被分成20组,计算了每组“逃逸”脉冲星所占比例．对每组中“未逃逸”脉冲星,分别在0＜r＜25kpc和5＜r＜11kpc两个区域中对|z|分布做了统计,研究了“未逃逸”脉冲星在这两个区域中的分段标高和累积标高的演化特征．“未逃逸”脉冲星的分段标高在一定空间范围内的变化不规则,但累积标高的变化却比较平滑．在相对长的时间里(＞10~8年),银盘厚度对脉冲星累积标高演化有显著的复杂影响；在相对短的时间中(＜2×10~7年),这种效应很小,而且累积标高对时间的依赖关系是线性的．在长时间里,初始速度分布对累积标高也有显著影响．在引力势h_(z2)=0．325 kpc的条件下,在径向范围5＜r＜11kpc和时间t=9．22×10~6年以及σ_(v2)=200km/s的情况下,模拟的累积标高是0．596±0．005 kpc．     

喷流吸积与小行星的形成

前文论述了我们对小行星起源的基本观点,即小行星区的初始物理-化学条件导致该区星子生长慢、可吸积物质被转移到木星区,因而星子生长停顿于“半成品”状态,不能形成大行星,而只能形成小行星.本文利用前文得出的星云盘条件和引力不稳定性公式,把由引力不稳定性所形成的环体作为“喷流”,用喷流吸积理论计算星子的生长,讨论可吸积物质转移问题,给我们的基本观点作了进一步的论证.     

MOND理论和暗物质模型的检验

从理论上探讨在地球引力系统内，修正动力学的引力理论以及暗物质模型的预言．着重研究修正动力学(MOND)的引力理论中一些常用的模型，对其中一个最简单模型，给出了球对称情况下引力势的一般表达式，计算了地球引力场中这些模型预言的卫星角速度，发现不同模型给出的角速度是不相同的，并且将这些值分别与牛顿理论的角速度值相比较．虽然这些模型的角速度与牛顿理论角速度的差异都很小，但简单模型的差异更大一些．对于月球作为卫星的情况，目前的技术有可能对这个角速度差进行实际观测．最后估算暗物质模型中月球绕地球运动角速度所受到的影响，证明它远远小于MOND理论的效应．由此对这个角速度差的观测，就构成检验MOND理论与暗物质模型的一个判据．     

自引力磁均分和磁守恒吸积盘模型的稳定性分析

对自引力磁均分和磁守恒两种模型的径向振荡稳定性的研究表明：在同时考虑自引力和磁场作用的情况下,吸积盘在磁均分和磁守恒模型中均存在着三种振荡模式,其中粘滞模式总是稳定的,磁声模式是不稳定的,中性模式在图中较长波段范围趋于稳定,在较短波段范围是不稳定的．这些结果有利于解释活动天体的Mark421和Pks2155—304的长周期光变现象．同时阐明了自引力在两种模型中对三种模式的影响有相同的趋势,而对磁场则分别起着相反的作用．这个结论表明磁均分吸积盘模型在解释光变现象时更为有利．     

金牛T星在慢引力收缩阶段抛射物质和引力收缩作用下质量和半径的演化时标及其对自转角速度变化的影响

研究了带有引力能源和抛射物质的引力收缩星如金牛T星处于慢引力收缩阶段后,在抛射物质和引力收缩联合作用下对质量和半径的改变及其对自转角速度变化产生的影响。给出了质量和半径随时间演变的联立微分方程组及其解。利用解计算了金牛T星的质量和半径的演化时标以及对自转角速度变化产生的影响。给出数值结果,并讨论了理论和数值结果。     

On the influence of degeneracy on gravitational instability

The influence of degeneracy on gravitational instability in a collisionless particle system with a semi-degenerate distribution function is discussed. The collapse time and the Landau damping time are calculated. The influence of degeneracy on Jeans wave number and on stability have important implications in the theory of galaxy formation in a universe dominated by fermionic dark matter.     

Neutrino degeneracy and the primordial abundance of helium and deuterium

Recent observations indicate that the primordial abundance of⁴He could be smaller than 0.24. It may then be necessary to invoke neutrino degeneracy in the early universe to explain the primordial abundances of helium and deuterium. It is shown here that the necessary degeneracy, though small, gives rise to a large asymmetry between the present number densities of neutrinos and antineutrinos. The effect of degeneracy on the upper limit to the neutrino masses is also considered.     

Modulational instability of electron-acoustic waves in relativistically degenerate quantum plasma

Using the Quantum hydrodynamic (QHD) model the modulational instability of electron-acoustic waves (EAWs) has been examined theoretically by deriving a nonlinear Schrodinger equation in a two-electron-populated relativistically degenerate super dense plasma. Through numerical analysis it is shown that the relativistic degeneracy parameter significantly influence the stability conditions and the formation and properties of the envelop solitons.     

Gravitational collapse of a rotating iron stellar core: A model with Fermi-Dirac neutron degeneracy

The effect of Fermi-Dirac neutron degeneracy on the collapse process is studied. Two computations of a spherically symmetric quasi-one-dimensional model are considered: the case with a full allowance for neutrino deposition and the case with total transparency of the star to neutrino emission. The hindering character of neutron degeneracy with respect to collapse has been revealed quantitatively. A weak sensitivity of both thermodynamics and neutrino characteristics of a rotating collapsar to this effect has been established. However, the hydrostatically equilibrium neutron star obtained is composed of a strongly degenerate neutron gas by more than half. This undoubtedly confirms that the performed computations of a quasi-one-dimensional model with neutron degeneracy are topical.     

Polarization of AGN in UV spectral range

We employ the Sagdeev pseudo-potential method to investigate the propagation of nonlinear ion waves in a relativistically degenerate electron-ion plasmas. The matching criteria for existence of such nonlinear excitations are numerically investigated in terms of the relativity measure (relativistic degeneracy parameter) of electrons and the allowed Mach-number range for propagation of such waves is evaluated. It is shown that the electron relativistic degeneracy parameter has significant effects on nonlinear wave dynamics in superdense degenerate plasmas such as that encountered in white dwarfs and the cores of massive planets.     

Partially degenerate nonrelativistic isothermal gas spheres in the presence of a magnetic field

The effect of a weak poloidal magnetic field on the equilibrium structure of a partially degenerate nonrelativistic isothermal gas sphere has been studied. Models have been computed for varying degrees of central degeneracy and the effect of the magnetic field on the various parameters, e.g., mass, central condensation, moment of inertia, and oblateness has been looked into. It is found that as the central degeneracy parameter is increased, the central condensation in general decreases and, hence, the eccentricity of the configuration decreases.     

Criculation in rotating degenerate objects

It is shown that due to degeneracy rather high Eddington-Vogt circulation velocities occur in cooling white dwarfs. The conclusion is that in fast rotating white dwarfs angular momentum is redistributed within a time scale short compared to the cooling time of the star. This might be important for the theory of very oblate rotating white dwarfs.It is shown that similar mechanisms work in the interior of degenerate, rotating cores of evolved stars, where circulations are driven by contraction energy and by neutrino losses. The circulations redistribute the core's matter within time scales shorter than or comparable with the evolution time scale.     

Non-parametric strong lens inversion of Cl 0024+1654: illustrating the monopole degeneracy

The cluster lens Cl 0024+1654 is undoubtedly one of the most beautiful examples of strong gravitational lensing, providing five large images of a single source with well-resolved substructure. Using the information contained in the positions and the shapes of the images, combined with the null space information, a non-parametric technique is used to infer the strong lensing mass map of the central region of this cluster. This yields a strong lensing mass of  1.60 × 10¹⁴ M_⊙  within a 0.5  arcmin radius around the cluster centre. This mass distribution is then used as a case study of the monopole degeneracy, which may be one of the most important degeneracies in gravitational lensing studies and which is extremely hard to break. We illustrate the monopole degeneracy by adding circularly symmetric density distributions with zero total mass to the original mass map of Cl 0024+1654. These redistribute mass in certain areas of the mass map without affecting the observed images in any way. We show that the monopole degeneracy and the mass-sheet degeneracy together lie at the heart of the discrepancies between different gravitational lens reconstructions that can be found in the literature for a given object, and that many images/sources, with an overall high image density in the lens plane, are required to construct an accurate, high-resolution mass map based on strong lensing data.     

Resolving the Degeneracy: Experimental Tests of the New Self Creation Cosmology and a Heterodox Prediction for Gravity Probe B

The new theory of Self Creation Cosmology has been shown to yield a concordant cosmological solution that does not require inflation, exotic non-baryonic Dark matter or unknown Dark Energy to fit observational constraints. In vacuo there is a conformal equivalence between this theory and canonical General Relativity and as a consequence an experimental degeneracy exists as the two theories predict identical results in the standard tests. However, there are three definitive experiments that are able to resolve this degeneracy and distinguish between the two theories. Here these standard tests and definitive experiments are described. One of the definitive predictions, that of the geodetic precession of a gyroscope, has just been measured on the Gravity Probe B satellite, which is at the present time of writing in the data processing stage. This is the first opportunity to falsify Self Creation Cosmology. The theory predicts a ‘frame-dragging’ result equal to GR but a geodetic precession of only 2/3 the GR value. When applied to the Gravity Probe B satellite, Self Creation Cosmology predicts an E–W gravitomagnetic/frame-dragging precession, equal to that of GR, of 40.9 milliarcsec/yr but a N–S gyroscope (geodetic + Thomas) precession of just 4.4096 arcsec/yr.     

Asteroseismology and the degeneracy of model solutions for subgiant stars

Stellar evolutionary models are essential for the determination of stellar masses and ages. Several parameters can be used to characterize the structure of subgiant stars. Amongst others we can find: the helium and metal abundances, mixing length and core overshooting. In many cases, these parameters are scaled taking into account the solar values. Yet, the universality of such scalings has been put to question. As shown here, by allowing some freedom to these parameters we end up with a degeneracy of model solutions. This puts serious limitations on the determination of mass and age. Asteroseismology may provide a valuable help to break such a degeneracy. This is illustrated with an application to the subgiant star μ Her.