暗晕次结构的演化

在主流的冷暗物质(cold dark matter,CDM)模型中,暗物质由于引力不稳定性塌缩成位力化的结构,形成所谓的暗物质晕(dark matter halo),然后重子气体由于辐射消耗能量而落入这些暗晕的中心,最终形成星系.暗晕一般是通过等级成团的方式形成,即小质量暗晕先塌缩形成,进而通过并合形成更大尺度的暗晕.并合后的小质量暗晕不会立即消失,而形成暗晕(主暗晕,host halo)的次结构(dark matter halo substructure或subhalo).在次结构中形成的星系是在主暗晕中形成的中心星系的伴星系(satellite galaxy,亦称卫星星系).伴星系因为质量小、光度低而成为观测中的矮星系.     

暗晕的并合

系统介绍暗晕-暗晕并合在理论方面的形成与发展,并简要介绍了其在数值模拟检验(冷暗物质模型)及拟合方面的进展。暗晕-暗晕并合率问题可与星系-星系并合率联系,正确地定义暗晕-暗晕并合率很重要,影响并合率的因素却很多,从数值模拟所得到的拟合公式往往存在较大差异,这些差异主要来自于定义:对并合的定义(并合树的建立),对暗晕质量的定义以及并合暗晕前身质量比的定义。数值模拟输出的时间步长也会对此有所影响,此外暗晕并合率也与环境强烈相关。对这些因素所导致的差异做了简单比较。此外,对于暗晕并合率的统计可分为对不同红移处的拟合和对z=0时刻对其并合历史的拟合,其中后者对不同质量的暗晕往往能给出一个较统一的拟合公式。     

冷暗物质子结构的数值追踪及其湮灭辐射的观测

<正>当前对宇宙的主要观测都指向物质成分为冷暗物质主导的~CDM宇宙学.在冷暗物质的宇宙中,原初的微小扰动在引力作用下增长为位力化的团块,称为暗物质晕.小质量暗晕最先形成,它们之间的相互并合形成了更大质量的暗晕.并合之后,前身暗晕在当前载晕中会以自束缚的子结构形式存活很长时间,称为子晕.在数值模拟中我们可以对子晕的形成和演化进行详细的研究.这种研究为星系形成演化模型提供了基础,同时也影响了我们观测上探测暗物质的方式.     

类星体光度和暗物质晕的统计关系

活动星系核的能量反馈是星系形成理论模型中的一个重要物理过程,与星系所处的暗物质晕质量、星系中央大质量黑洞吸积率等因素有关。当前的半解析模型预测活动星系核反馈机制主要有两种模式:射电模式和类星体模式,前者主要发生在大质量暗晕中央的大质量星系中,后者主要由较小质量星系并合导致。利用斯隆数字化巡天(SDSS)提供的目前最大的类星体光谱观测样本,结合基于SDSS构建的星系群(团)表,从统计上分析了类星体的热光度和暗晕质量的分布情况。初步分析结果显示,在大质量暗晕中,类星体的热光度和所在暗晕的质量没有相关性,类星体所在暗晕的质量分布很广,进一步证实了高光度的类星体并不存在于大质量的暗晕中。     

对N体数值模拟中暗物质晕的卫星星系空间分布的研究

对N体数值模拟中暗物质晕的卫星星系的空间分布进行三轴椭球拟合,以拟合椭球的轴比来衡量该分布的扁平程度,通过比较不同条件下的轴比分布分析样本数量、样本选取方式以及对样本径向分布的归一化对计算结果的影响,并考察了暗晕所在的大尺度结构的空间方向与拟合椭球的空间取向之间的关系.发现对样本径向分布的归一化对计算结果具有较大影响,同时发现大尺度丝状结构中的暗晕的拟合椭球的短轴更趋于与丝状结构的方向垂直,而大尺度片状结构中的暗晕的拟合椭球的短轴则更趋于与片状结构的法线方向平行.     

不同质量比并合对黑洞自旋演化的影响

研究了在仅有并合时黑洞自旋的演化分布.利用了后牛顿近似处理并合后的角动量问题,采用蒙特卡洛方法模拟了不同条件下的并合影响.结果表明主并合不能使黑洞自旋分布稳定,而小并合可使黑洞自旋平稳地降低.当取并合质量比为幂指数形式时黑洞自旋分布可以稳定在-个小自旋占绝对份额的水平,同时,也讨论了射电强活动星系核和黑洞自旋在不同假设下的关系.最后计算并给出了射电噪度的分布情况.     

用强引力透镜研究暗晕中心的物质分布

对矮星系和晚型LSB星系的最新高分辨率自转曲线进行观测,揭示出由CDM主导的物质密度分布中心具有常数密度核。在ΛCDM宇宙学框架下,纯CDM宇宙学N体模拟给出普适的、带尖点的NFW模型,解决尖点-核问题的最佳方案是考虑暗晕中心重子物理过程对暗晕密度分布的影响。到目前为止,考虑重子物理过程后,不管是解析模型还是数值模拟,给出的暗晕密度分布都能很好地符合自转曲线的观测结果,但是却不能通过强引力透镜观测的检验。最后,对基于这两种观测到底能不能用一个统一的暗晕密度分布模型来描述作了简要的讨论,并认为需要更多的强引力透镜样本和分辨率更高的数值模拟,以解决上述问题。     

盘星系并合的数值模拟

使用一系列的模拟,包括了不同的轨道参数、主并合与小并合,以及不同的恒星盘自旋角动量与轨道角动量的耦合方式,来研究盘星系的并合过程中,顺行交会和逆行交会对并合过程会产生怎样的影响.这些影响包括并合时标、轨道形态、星系的恒星及暗物质成分剥离效率、星系的形态变化,以及形成的潮汐结构等.研究发现,由于在并合过程的前期,逆行并合的恒星剥离效率要明显小于顺行并合,因此会形成小得多的潮汐结构.但与通常认为的相反的是,与顺行并合相比,逆行并合并没有显著改变轨道形态和增大并合时标,对恒星成分的剥离效率也没有显著的影响.与顺行与否相比,潮汐半径在小并合时,卫星星系恒星成分被潮汐剥离的过程中扮演着更重要的角色,潮汐半径与恒星盘标长相等的时刻可以被认为是卫星星系恒星盘瓦解的时刻.     

暗晕子结构的数值搜寻与统计性质

宇宙学数值模拟预言的暗晕及其子结构的统计性质,被广泛应用于星系形成模型、近邻宇宙学、红移巡天、宇宙大尺度结构、引力透镜和暗物质搜寻等研究领域。在精确宇宙学时代,随着天文观测能力的不断增长,人们对模型精度提出了更高的要求,因此,数值模拟及其分析工具将继续起到不可替代的巨大作用。数值模拟中物质结构的性质往往与具体的搜寻算法有关,从最初通过数值模拟发现子晕到现在的20年里,结构搜寻算法有了极大的发展,新算法不断涌现。了解不同数值搜寻算法的实现及其差异,有助于分析模拟结果的系统差和不确定性,增强研究的可重复性。通过总结现代子晕数值搜寻的基本方法,并以几种典型的搜寻算法为例,探讨了它们之间的异同,及其对相关研究的影响;回顾了子晕的基本统计性质,包括质量函数、空间分布和密度轮廓等,以及这些统计结果对搜寻算法的依赖。此外,还讨论了重子物理的影响。     

卫星星系动力学状态对星系属性的依赖

基于Sloan Digital Sky Survey(SDSS)数据,采用van den Bosch等发展的自适应方法从星系样本中挑选出中央星系及其卫星星系,然后研究卫星星系速度弥散与中央星系/卫星星系属性的关系.和之前的研究一致,卫星星系平均速度弥散随着中央星系质量增大而增大,并且红中央星系所在星系群的速度弥散比蓝中央星系要大.发现平均而言,在红中央星系周围,卫星星系的速度弥散和其质量之间没有明显关联.但是在蓝中央星系周围,质量大的卫星星系速度弥散偏大,而红卫星星系的速度弥散要比蓝卫星星系偏大.进一步的研究表明,在相同的暗晕中,卫星星系速度弥散和卫星星系质量并不相关.有意思的是,如果红中央星系周围同时有红和蓝卫星星系,那么红卫星星系的速度要小,对其中的物理机制进行了初步的探讨.此外还发现,若中央星系周围只有红或蓝(质量大或小)的卫星星系时,有红(或质量大)的卫星星系所在的暗晕的速度弥散要大于有蓝(或者质量小)的卫星星系的暗晕,这表明卫星星系属性对于暗晕质量测量是有影响的.     

On the origin of the solar system and the exceptional position of the Sun in the Galaxy

The solar system's position in the Galaxy is an exclusive one, since the Sun is close to the corotation circle, which is the place where the angular velocity of the galactic differential rotation is equal to that of density waves displaying as spiral arms. Each galaxy contains only one corotation circle; therefore, it is an exceptional place. In the Galaxy, the deviation of the Sun from the corotation is very small — it is equal to ΔR/R _⊙≈0.03, where ΔR=R _c ?R _⊙,R _c is the corotation distance from the galactic center andR _⊙ is the Sun's distance from the galactic center. The special conditions of the Sun's position in the Galaxy explain the origin of the fundamental cosmogony timescalesT ₁≈4.6×10⁹ yr,T ₂?10⁸ yr,T ₃?10⁶ yr detected by the radioactive decay of various nuclides. The timescaleT ₁ (the solar system's ‘lifetime’) is the protosolar cloud lifetime in a space between the galactic spiral arms. The timescaleT ₂ is the presolar cloud lifetime in a spiral arm.T ₃ is a timescale of hydrodynamical processes of a cloud-wave interaction. The possibility of the natural explanation of the cosmogony timescales by the unified process (on condition that the Sun is near the state of corotation) can become an argument in favour of the fact that the nearness to the corotation is necessary for the formation of systems similar to the Solar system. If the special position of the Sun is not incidental, then the corotation circles of our Galaxy, as well as those of other galaxies, are just regions where situations similar to ours are likely to be found.     

Perturbations by the oblateness of the Earth and by the planets in the motion of the Moon

Perturbations in the motion of the Moon are computed for the effect by the oblateness of the Earth and for the indirect effect of planets. Based on Delaunay's analytical solution of the main problem, the computations are performed by a method of Fourier series operation. The effect of the oblateness of the Earth is obtained to the second order, partly adopting an analytical evaluation. Both in longitude and latitude are found a few terms whose coefficient differs from the current lunar ephemeris based on Brown's theory by about 0.01. While, concerning the indirect effect of planets, several periodic terms in the current ephemeris seem to have errors reaching 0.05.As for the secular variations of and due to the figure of the Earth and the indirect effect of planets, the newly-computed values agree within 1/cy with Brown's results reduced to the same values of the parameters. Further, the accelerations in the mean longitude, and caused by the secular changes in the eccentricity of the Earth's orbite and in the obliquity of the ecliptic are obtained. The comparison with Brown shows an agreement within 0.3/cy² for the former cause and 0.02/cy² for the latter. An error is found in the argument of the principal term for the perturbations due to the ecliptic motion in the current ephemeris.Proceedings of the Conference on Analytical Methods and Ephemerides: Theory and Observations of the Moon and Planets. Facultés universitaires Notre Dame de la Paix, Namur, Belgium, 28–31 July, 1980.     

On the model of the accumulation of the moon compatible with the data on the composition and the age of lunar rocks

It is suggested that the overall early melting of the lunar surface is not necessary for the explanation of facts and that the structure of highlands is more complicated than a solidified anorthositic ‘plot’. The early heating of the interior of the Moon up to 1000K is really needed for the subsequent thermal history with the maximum melting 3.5 × 10⁹ yr ago, to give the observed ages for mare basalts. This may be considered as an indication that the Moon during the accumulation retained a portion of its gravitational energy converted into heat, which may occur only at rapid processes. A rapid (t < 10³ yr) accretion of the Moon from the circumterrestrial swarm of small particles would give necessary temperature, but it is not compatible with the characteristic time 10⁸ yr of the replenishment of this swarm which is the same as the time-scale of the accumulation of the Earth. It is shown that there were conditions in the circumterrestial swarm for the formation at a first stage of a few large protomoons. Their number and position is evaluated from the simple formal laws of the growth of satellites in the vicinity of a planet. Such ‘systems’ of protomoons are compared with the observed multiple systems, and the conclusion is reached that there could have been not more than 2–3 large protomoons with the Earth. The tidal evolution of protomoon orbits was short not only for the present value of the tidal phase-lag but also for a considerably smaller value. The coalescence of protomoons into a single Moon had to occur before the formation of the observed relief on the Moon. If we accept the age 3.9 × 10⁹ yr for the excavation of the Imbrium basin and ascribe the latter to the impact of an Earth satellite, this collision had to be roughly at 30R, whereR is the radius of the Earth, because the Moon at that time had to be somewhere at this distance. Therefore, the protomoons had to be orbiting inside 20–25R, and their coalescence had to occur more than 4.0x10⁹ yr ago. The energy release at coalescence is equivalent to several hundred degrees and even 1000 K. The process is very rapid (of the order of one hour). Therefore, the model is valid for the initial conditions of the Moon.     

On the theory of the oblateness of the Sun

In this paper we first emphasize why it is important to know the successive zonal harmonics of the Sun's figure with high accuracy: mainly fundamental astrometry, helioseismology, planetary motions and relativistic effects. Then we briefly comment why the Sun appears oblate, going back to primitive definitions in order to underline some discrepancies in theories and to emphasize again the relevant hypotheses. We propose a new theoretical approach entirely based on an expansion in terms of Legendre's functions, including the differential rotation of the Sun at the surface. This permits linking the two first spherical harmonic coefficients (J ₂ and J ₄) with the geometric parameters that can be measured on the Sun (equatorial and polar radii). We emphasize the difficulties in inferring gravitational oblateness from visual measurements of the geometric oblateness, and more generally a dynamical flattening. Results are given for different observed rotational laws. It is shown that the surface oblateness is surely upper bounded by 11 milliarcsecond. As a consequence of the observed surface and sub-surface differential rotation laws, we deduce a measure of the two first gravitational harmonics, the quadrupole and the octopole moment of the Sun: J ₂=−(6.13±2.52)×10⁻⁷ if all observed data are taken into account, and respectively, J ₂=−(6.84±3.75)×10⁻⁷ if only sunspot data are considered, and J ₂=−(3.49±1.86)×10⁻⁷ in the case of helioseismic data alone. The value deduced from all available data for the octopole is: J ₄=(2.8±2.1)×10⁻¹². These values are compared to some others found in the literature. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005238718479     

Accounting for the viscosity of the fluid core in the problem of the physical libration of the moon

A two-component theoretical model of the physical libration of the Moon in longitude is constructed with account taken of the viscosity of the core. In the new version, a hydrodynamic problem of motion of a fluid filling a solid rotating shell is solved. It is found that surfaces of equal angular velocity are spherical, and a velocity field of the fluid core of the Moon is described by elementary functions. A distribution of the internal pressure in the core is found. An angular momentum exchange between the fluid core and solid mantle is described by a third-order differential equation with a right-hand side. The roots of a characteristic equation are studied and the stability of rotation is proved. A libration angle as a function of time is found using the derived solution of the differential equation. Limiting cases of infinitely large and infinitely small viscosity are considered and an effect of lag of a libration phase from a phase of action of an external moment of forces is ascertained. This makes it possible to estimate the viscosity and sizes of the lunar fluid core from data of observations.     

On some problems concerning the calculation of the form of the magnetopause and the electric field on the magnetopause in the framework of the continuum medium model

In order to understand the reason of the existence of the electric field in the magnetosphere, and for the theoretical evaluation of its value, it is necessary to find the solution of the problem of determination of the magnetosphere boundary form in the frameworks of the continuum medium model which takes into account part of the magnetospheric plasma movement in supporting the magnetospheric boundary equilibrium. A number of problems for finding the distribution of the pressure, the density, the magnetic field and the electric field on the particular tangential discontinuity is considered in the case when the form of discontinuity is set (the direct problem) and a number of problems for finding the form of the discontinuity and the distribution of the above-mentioned physical quantities on the discontinuity is considered when the law of the change of the external pressure along the boundary is set (for example, with the help of the approximate Newton equation). The problem which is considered here, which deals with the calculation of the boundary form and with the calculation of the distribution of the corresponding physical quantities on the discontinuity of the 1st kind for the compressible fluid with the magnetic field with field lines which are perpendicular to the plane of the flow in question, concerns the last sort of problems. The comparison of the results of the calculation with the data in the equatorial cross-section of the magnetosphere demonstrates that the calculated form of the boundary, the value of the velocity of the return flow and the value of the electric field on the magnetopause, agree satisfactorily with the observational data.