天然卫星CCD精确定位的最新进展

天然卫星CCD精确定位观测对于宇宙飞船探测外行星、行星物理的研究以及天文参考系的连接等方面都有重要的意义．文章介绍了长焦距望远镜CCD观测天然卫星所遇到的各种困难以及解决问题的方案．重点评介了CCD图像处理和归算方法．最后,介绍了影响天然卫星位置测量精度的各种因素及最新观测结果的精度．     

图像移位堆叠方法用于暗天然卫星的天体测量

暗弱天然卫星与主带小行星相比,具有亮度低、速度变化快的特点.在观测这类天体时,不能简单地延长曝光时间来提高其信噪比.尝试观测多幅短曝光的CCD (chargecoupled device)图像,采用移位堆叠(shift-and-add)方法,希望提高目标成像的信噪比,获得暗弱天然卫星的精确测量结果.使用2018年4月9—12日夜间,中国科学院云南天文台1 m望远镜(1 m望远镜)拍摄的木星5颗暗卫星的229幅CCD图像,实施了移位堆叠试验.为了验证结果的正确性,与相近日期中国科学院云南天文台2.4 m望远镜(2.4 m望远镜)观测的相同木卫图像的测量结果进行了比较和分析.位置归算采用了JPL (Jet Propulsion Laboratory)历表.结果表明,对CCD图像使用移位堆叠方法,通过叠加约10幅曝光时间100 s的图像, 1 m望远镜能观测暗至19等星的不规则天然卫星,而且测量的准确度与2.4 m望远镜的测量结果有良好的一致性.     

天王星卫星位置可视化软件设计

为了满足大行星卫星的高精度CCD位置观测与运动理论研究工作的需要 ,采用天王星 5颗主要卫星摄动理论模型 (Gust86 )作为核心 ,设计了一个天王星视位置可视化软件。该软件具有卫星证认 ,最佳观测时段选取 ,精确模拟卫星视运动和实时引导CCD精密定位观测等功能。     

天王星卫星两种定标方法测定结果的比较

利用新发表的高精度、高密度天体测量星表UCAC2,对天王星的五颗主要卫星的CCD观测图像重新进行量测,采用不同方法作定标归算,并使用两种理论模型(GUST86和GUST06模型)计算卫星的理论位置。对不同方法所得到的卫星位置的O-C结果的分析和比较表明,本文获得的卫星位置精度,除天卫五(Miranda)有显著提高,其他4颗卫星的位置精度基本相同。本文中天卫一和天卫三的结果与"亮卫星定标法"的结果在精度上相当,天卫二的位置精度与其他天王星卫星的位置精度具有较好的一致性,这从另一方面证明了我们的"亮卫星定标法"的可靠性。此外我们还获得了天卫四的位置与精度。     

2.4米望远镜天体测量试验

利用文[1]提出的方法,求解出云南天文台丽江观测站2.4m望远镜CCD图像中的几何扭曲模型.将求解的模型应用到同期观测所得Phoebe(土卫九)的116幅CCD图像表明:扣除视场几何扭曲的影响后,该卫星定位精度明显提高.土卫九天体测量精度在每个方向约为0."02～0."03.     

用M15进行CCD视场的校准

用1996年10月所获得的球状星团M15和大距离双星61Cygni的CCD观测资料,分别求解了上海天文台1.56m天体测量望远镜上安装的CCD探测器视场的位置角改正δp和尺度因子ρ。通过对土星主要卫星观测资料0－C的计算分析表明,经球状星团校准视场后所得结果较双星校准后结果的精度要高。该结论为进一步提高计算大行星卫星的位置测量精度打下良好的基础。     

小视场CCD图像的几何定标

针对长焦距望远镜CCD观测天然卫星的情况,导出了用两颗亮星作为定标时,CCD视场几何参数(比例尺和指向)测量的误差公式,进而设计了多定标星定标时正确的归算方案。新方案不仅对天然卫星的位置测量,也对同类型的小视场CCD位置测量具有指导意义。     

小视场CCD图像的几何定标

针对长焦距望远镜CCD观测天然卫星的情况，导出了用两颗亮星作为定标时，CCD视场几何参数(比例尺和指向)测量的误差公式，进而设计了多定标星定标时正确的归算方案。新方案不仅对天然卫星的位置测量，也对同类型的小视场CCD位置测量具有指导意义。     

土星主要卫星天体测量定标试验

在地面望远镜观测土星主要卫星的精确定位中,目前普遍采用CCD成像技术。为了测量视场中暗卫星的位置,常采用4颗主要卫星(土卫三-土卫六)进行定标。鉴于技术的进步,目前CCD视场正在逐步增加,如何精确定标图像值得研究。利用2010年1月20日在云南天文台1 m望远镜上观测的不同取向的105幅CCD图像进行了试验研究。采用了美国喷气推进实验室土星主要卫星历表和美国海军天文台的UCAC4恒星星表进行计算,相对于不同的参考对象进行定标。结果表明,采用4颗主要卫星进行定标并测量离卫星较近的观测对象时具有较好的测量准确度和内部精度。测量离定标星较远的卫星时,这种定标方法具有较大的不确定性。采用6颗主要卫星(土卫三-土卫八)进行定标,所有卫星的测量结果具有更好的外部和内部符合。即便如此,采用四常数模型和六常数模型定标的结果都不是令人满意,这揭示CCD视场存在明显的扭曲效应。如果采用视场中的恒星定标,卫星的外部符合和内部精度明显变差,这说明UCAC4恒星参考星表也不是理想的定标用参考星表。     

小波方法在CCD图像处理中的应用

针对天王星卫星位置观测图像中存在的一些问题,提出了一种新的图像处理方法,即二维墨西哥帽小波变换方法.图像处理结果表明,这种方法能够有效地去除背景梯度和噪声,将接近主星的卫星从主星的光晕中分离出来.     

A survey of orbits of co-orbitals of Mars

Many asteroids with a semimajor axis close to that of Mars have been discovered in the last several years. Potentially some of these could be in 1:1 resonance with Mars, much as are the classic Trojan asteroids with Jupiter, and its lesser-known horseshoe companions with Earth. In the 1990s, two Trojan companions of Mars, 5261 Eureka and 1998 VF₃₁, were discovered, librating about the L₅ Lagrange point, 60° behind Mars in its orbit. Although several other potential Mars Trojans have been identified, our orbital calculations show only one other known asteroid, 1999 UJ₇, to be a Trojan, associated with the L₄ Lagrange point, 60° ahead of Mars in its orbit. We further find that asteroid 36017 (1999 ND₄₃) is a horseshoe librator, alternating with periods of Trojan motion. This asteroid makes repeated close approaches to Earth and has a chaotic orbit whose behavior can be confidently predicted for less than 3000 years. We identify two objects, 2001 HW₁₅ and 2000 TG₂, within the resonant region capable of undergoing what we designate “circulation transition”, in which objects can pass between circulation outside the orbit of Mars and circulation inside it, or vice versa. The eccentricity of the orbit of Mars appears to play an important role in circulation transition and in horseshoe motion. Based on the orbits and on spectroscopic data, the Trojan asteroids of Mars may be primordial bodies, while some co-orbital bodies may be in a temporary state of motion.     

Equation of state and anisotropy of pressure of magnetars

Magnetars are the neutron stars with the highest magnetic fields up to 10¹⁵–10¹⁶ G. It has been proposed that they are also responsible for a variety of extra-galactic phenomena, ranging from giant flares in nearby galaxies to fast radio bursts. Utilizing a relativistic mean field model and a variable magnetic field configuration, we investigate the effects of strong magnetic fields on the equation of state and anisotropy of pressure of magnetars. It is found that the mass and radius of low-mass magnetars are weakly enhanced under the action of the strong magnetic field, and the anisotropy of pressure can be ignored. Unlike other previous investigations, the magnetic field is unable to violate the mass limit of the neutron stars.     

Estimation of the Time Scale of Merging of the Components of Contact Binaries of the W UMa Type

Time scales of the transition from contact binary systems of the W UMa type to single stars of the blue straggler type are estimated. A model of one-way mass transfer from the primary component to the companion inside a common envelope (Eggen et al.) and a model of a contact binary lying on the zero-age main sequence with a companion that is losing mass (F. van't Veer) are used to calculate these scales. The merging times of components were calculated for 304 contact systems of the W UMa type from the Catalog of Approximate Photometric and Absolute Elements of Eclipsing Variable Stars on a scale of loss of mass and orbital angular momentum due to magnetic braking, under the assumption of synchronization of the axial rotation and orbital revolution. On the basis of the resulting statistics, the characteristic merging scales are 0.4-0.6 and 0.5-0.7 Gyr, respectively, for the above two models. The results of a comparison with the work of other authors are discussed.     

淮河洪水的可公度性分析

分析了上两个世纪发生在淮河的洪水事件的可公度性，根据其可公度值及其黄金分割点指出1991年与2003年淮河洪水的不可避免，最后讨论了可公度性的局限及淮河洪水可公度值的可能机制。     

Flattening of galaxies of different morphological types in subclusters of Coma

This paper deals with the observed variation in the flattening of galaxies with the density of galaxies in the subclusters of Coma surrounding NGC 4889, NGC 4874, and NGC 4839 based on data from the Abastumani Combined Catalog of Galaxies. The mean values of the observed ratios of the diameters of the galaxies, as well as histograms of their distributions, indicate that in the central, dense regions of the subclusters within a volume of 0.5h ₇₅ ⁻¹ Mpc³, E and S0 type galaxies are close to spheroidal. A significant reduction in the mean values of the diameters of the galaxies in the subclusters is noted, regardless of their morphology relative to the galaxies in the halo of the Coma cluster. In the subclusters, spiral galaxies are found with a hydrogen deficit that is more than 5 times the hydrogen deficit in spirals within the halo of the cluster. According to their 3-D coordinates, most of the galaxies with a hydrogen deficit are located closer to the south-east edge of the subcluster surrounding NGC 4874 near an extended gas filament in the x-ray region. This may indicate that the subcluster is moving toward a central condensation of faint galaxies in the Coma cluster and a possible merger with it. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 355–368 (August 2007).     

基于STK的寿命末期GEO卫星轨道演化分析

地球同步轨道上的GEO卫星接近寿命末期时,星上的有效载荷和电子元器件等仍能正常应用,此时如果只做东西方向的位置保持,可以大幅度延长卫星的在轨工作寿命。简要介绍了引起GEO卫星轨道变化的主要摄动力,利用STK软件分析寿命末期GEO卫星的运动规律,并给出这类GEO卫星轨道演化的仿真分析流程,用以指导寿命末期GEO卫星的开发应用工作。     

The discovery of Segue 2: a prototype of the population of satellites of satellites

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of   M _v =−2.5  , a half-light radius of 34 pc and a systemic velocity of  ∼−40 km s⁻¹  . Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km s⁻¹ and the possible stream as  ∼7 km s⁻¹  . This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites – survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts   z > 10  and are good candidates for fossils of the reionization epoch.     

Investigation of Times of Minima of Selected Early-Type Eclipsing Binaries

New precise times of minimum light for several early-type eclipsing binaries were obtained at three observatories. The changes of period of the following measured binaries are discussed: V1182 Aql, LY Aur, SZ Cam, FZ CMa, QZ Car, LZ Cen, V606 Cen, AH Cep and TU~Mus.     

Application of KS-transformation in the problem of investigation of the motion of unusual minor planets and comets

A review is presented of the author's results on application of regularizing and stabilizing KS- transformation in the problem of investigation of the motion of unusual minor planets and comets. Two models of the motion of a minor body are considered, viz. the perturbed two body problem and the perturbed restricted three body problem. The variational equations in KS-variables and transformations for obtaining the matrix of partial derivatives of the instantaneous physical parameters of motion with respect to their initial values are presented. The peculiarities of the implementation of the algorithms developed as programs on a computer are described. The original results of the investigation of the efficiency of the developed algorithms and programs are discussed using as an example the motion of unusual minor planets Icarus and Geographos as well as comets Halley, Honda-Mrkos-Pajdusakova and Gehrels 3.