1997年3月4日海尔-波普彗星的小规模分裂

１　引言彗核分裂是彗星的最重要特征之一．至１９８２年止文献记录共观测到２２个彗核分裂,３３个次核［１］．１９８６年哈雷彗星回归时观测到彗核的分裂［２］．１９９６年紫金山天文台２００ｍｍ赤道望远镜和青岛观象台同时观测到百武彗星的分裂［３］．海尔波普彗星从１９９５年８月到１０月曾爆发５次［４］．１９９６年３月至１２月不断有喷流射出,彗核活动日趋剧烈［５］．１９９７年３月４日我们观测到彗核的小规模分裂及喷流特征,３月３日我们观测到海尔波普彗星近核照片上有喷流,这可能是３月４日彗核小块分裂的前奏．３…     

哈雷彗星与彗星研究

彗星是太阳系的一种奇特天体。在太阳辐射和太阳风作用下,彗星发生一系列物理过程和化学过程,表现为外貌形态(彗发、彗尾)、亮度、光谱及运动特性等多种变化。彗星研究是天文学和空间科学的重要课题,对于了解太阳系起源、有机分子仍至生命起源、太阳风及等离子体过程都有重要意义。 著名的哈雷彗星将在1985—86年再次回归到内太阳系。它有多种彗星现象,从地面到近地空间及空间会合的各种观测及样品分析,预料会取得彗星研究的新进展。本文先概述国际哈雷彗星联测(IHW)的计划;然后,评述彗星研究的一些进展,包括:(1)彗星的化学组成和化学过程;(2)尘埃彗尾和颗粒;(3)等离子体彗尾;(4)彗发;(5)彗星的亮度、爆发和分裂;(6)彗核的蒸发、寿命及非引力效应;(7)彗核,其中也包括我国的彗星研究情况。     

彗星的CCD成像观测

近年来，对一系列彗星进行的宽带或带ＣＣＤ测光得到了许多有趣的结果，为深入地理解彗星的物理性质，结构，起源和演化等提供了丰富的信息。文中简要地介绍了利用ＣＣＤ成像观测在测量彗核的自转，大小，开头，质量和研究彗核的活动以及彗发的形成和演化等方面的进展。     

哈雷彗星的彗核分裂

彗核分裂是最重要的彗星性质之一。这是因为,不仅这一性质是支持P.L.Whipple的冰冻团块模型的强有力的证据,而且,对彗核分裂现象的理解亦将为研究彗星本质提供大量的信息。迄今为止,发现有彗核分裂的彗星总数已超过20。至于关于哈雷彗星的彗核分裂,上一次回归(1910年)提供的报告极少。国际天文界对此将信将疑,并无定论。     

1994年4月的SL9彗星观测

１９９４年４月１２日晨，我们对Ｓ９彗星进行了成功的观测．观测使用云南天文台一米望远镜和加了缩焦器的Ⅱ号ＣＣＤ系统．照相机像素为５１２×５１２，视场约７’×７’．在近两小时的不同时刻的四张照片中，观测到了彗星２１颗碎核中的主要部份及其运动．本文介绍了此次观测情况和资料处理，并给出对ＳＬ９各彗核的位置和亮度的测量结果．     

1996年8月5日—1997年5月15日Hale—Bopp彗星的照相观测

在1996年8月5日——1997年5月15日对Hale-Bopp彗星进行了照相观测,多次观测到Hale-Bopp彗星彗核的小规模分裂,喷流,壳层结构及彗发的膨胀现象。     

彗木碰撞时刻及位置的确定

Ｓｈｏｅｍａｋｅｒ－Ｌｅｖｙ０号彗星于１９９４年７月与木星碰撞已成为事实。本文依据众多碰撞时刻的观测报道，并结合碰撞预报，对其进行了系统研究。分析了用不同观测技术和观测手段所给出的碰撞时刻的报道，强调了碰撞物理过程的重要性。最后，按照统一的物理模式初步确定了各个碎核的碰撞时刻。并归算出木星南纬碰撞带区的自转速度约为９ｈ５５ｍｉｎ一周，说明它没有受到彗核连续撞击的影响。本文还将各彗核撞击点在木面上的     

还在发光的海尔-波普彗星

8年前,曾让天文学家眩目的海尔-波普彗星虽然现在距太阳有21天文单位之遥,但依然可以探测到。2005年1月8日,麻省理工学院天文学家 AndrewS.Rivkin 和 Richard P.Binzel 用麦哲伦天文台的6.5米 Clay 望远镜观测了这颗彗星。Rivkin 和 Binzel 观测发现彗发已经没有了,但彗核亮得还可以看到。Rivkin 说,彗核光谱是很难拍     

太阳风的发现之路

人类对太阳风最早的实验观测来自于对彗星彗尾的观测。在1943年,霍夫迈斯特(Hoffmeister)发现离子彗尾的方向与彗核和太阳连线方向(径向)有一个系统的且通常小于5度的偏离,起初天文学家们试图用太阳光的光压来解释这一偏离,但光压太弱,不足以造成这么大的偏差。另一个早期使人们费解的现象是在一些彗星中观测到的所谓“断尾事件”,即在彗星的彗尾中有时会观测到彗尾有很明显的不连续性(见图1)。为了解释霍夫迈斯特的发现,德国天体物理学家比尔曼(Biermann)于1951年提出了     

小行星与彗星—近地小行星,主带小行星和彗星之间相互关系探讨

小行星和彗星都是太阳系中的小天体,而近地小行星又是小行星中特殊的一类。近年来对近地小行星的观测和研究已全面开展,这已成为当今太阳系研究的前沿领域之一。本文初步探讨太阳系中不同小天体之间的关系:1.小行星与短周期彗星之间的关系;2.近地小行星与主带小行星和短周期彗星之间的关系。最近的观测研究表明某些小行星可能是由彗星演变来的,而某些近地小行星可能来源于主带小行星和死亡的彗核。     

Solar rotation measurements at Mount Wilson

We examine the background velocity fields of the Sun as observed at Mount Wilson. The method of velocity reduction of the full-disk Mount Wilson data is outlined. We describe a number of tests that have been carried out in order to find an instrumental origin for short-term rotation variations and a large-scale background line-shift - the ears. No instrumental cause can be found for this ear effect, although such a cause cannot yet be ruled out.Operated jointly by the Carnegie Institution of Washington and the California Institute of Technology.     

Anomalous particle-production thresholds through systematic and non-systematic quantum-gravity effects

A growing number of studies is being devoted to the identification of plausible quantum properties of spacetime which might give rise to observably large effects. The literature on this subject is now relatively large, including studies in string theory, loop quantum gravity and non-commutative geometry. It is useful to divide the various proposals into proposals involving a systematic quantum-gravity effect (an effect that would shift the main/average prediction for a given observable quantity) and proposals involving a non-systematic quantum-gravity effect (an effect that would introduce new fundamental uncertanties in some observable quantity). The case of quantum-gravity-induced particle-production-threshold anomalies, a much studied example of potentially observable quantum-gravity effect, is here used as an example to illustrate the differences to be expected between systematic and non-systematic effects.     

Modelling the circumnuclear coma of comets: objectives,methods and recent results

We investigate whether the modelling of the immediate vicinity of an active nucleus—currently unobservable—can, as the modelling of the outer, observable coma, be based on unrealistic simple assumptions such as those of nucleus and dust grains sphericity. We point out the inconsistency of models based on such assumptions, which, to manage compatibility with the observations, have to introduce additional assumptions that conflict with the previous ones, such as the existence of active areas of the nucleus. We argue that, while the outer coma models being phenomenological in nature, can perhaps tolerate such inconsistencies, the circumnuclear coma models must be predictive, having to obviate the lack of observational data, and therefore must exclude implausible and ad hoc assumptions, and advocate only well-understood physical processes and duly validated modelling methods. We describe the first steps of development of a predictive circumnuclear coma model, and present a set of results obtained with parameters fitted to comet P/Wirtanen, the target of the Rosetta mission, but of a quite general significance. Considering, first an inhomogeneous spherical nucleus with spherical dust grains, and then an aspherical homogeneous nucleus with spherical dust grains, we show that, in both cases (1) the surface temperature and initial gas parameters differ considerably from the Hertz–Knudsen values; (2) the near-surface gas and dust flows are not in general vertical, (3) the gas and dust density do not always monotonically decrease outwards, (4) the gas and dust velocity vary strongly from point to point, (5) shock structures are formed, which result in the formation of pseudo-jets of dust grains originating from various points of the surface. No simple method to distinguish between dust structures created by the surface inhomogeneity and by the surface orography is found. We show, for the first time, the deformation of the near-nucleus dust coma during a full rotation of an homogeneous, aspherical nucleus. We also show that identical active regions located at different points of an inhomogeneous spherical nucleus produce very different dust distributions, suggesting that the dust distribution is also strongly deformed during the rotation of such a nucleus. Finally, we consider, for the first time, a spherical homogeneous nucleus emitting aspherical dust grains. We show that, in such a case, the terminal grain velocity depends upon the shape, initial position, and even possibly upon the initial orientation of the grain at the surface, so that there cannot exist a precise relation between terminal velocity and dust grain mass. We conclude that, far from giving an approximate or average representation of the circumnuclear coma, the classical modelling approach yields in this region predictions that are in total conflict with the real behaviour of the gas and dust. As a most dramatic consequence, the use of this classical approach may have obscured completely the significance of the few direct and of the many indirect informations acquired hitherto on the nucleus activity.     

Dynamics of the Jupiter Trojans with Saturn’s perturbation when the two planets are in migration

In a previous paper (Hou et al. in Celest Mech Dyn Astron 119:119–142, 2014a), the problem of dynamical symmetry between two Jupiter triangular libration points (TLPs) with Saturn’s perturbation in the present configuration of the two planets was studied. A small short-time scale spatial asymmetry exists but gradually disappears with the time going, so the planar stable regions around the two Jupiter TLPs should be dynamically symmetric from a longtime perspective. In this paper, the symmetry problem is studied when the two planets are in migration. Several mechanisms that can cause asymmetries are discussed. Studies show that three important ones are the large short-time scale spatial asymmetry when Jupiter and Saturn are in resonance, the changing orbits of Jupiter and Saturn in the planet migration process, and the chaotic nature of Trojan orbits during the planet migration process. Their joint effects can cause an observable difference to the two Jupiter Trojan swarms. The thermal Yarkovsky effect is also found to be able to cause dynamical differences to the two TLPs, but generally they are too small to be practically observed.     

Small-scale dust structures in Halley's coma: II. Disintegration of large dust bodies

Small-scale dust structures, SDSs, altogether ∼35 events with extent ∼30-220 km, have been recognized owing to electric field records, mostly near the closest approach of Vega-2 to Halley's nucleus. Several (8-9) morphological forms of SDS have been identified, and all they make one family. Among the family members, the key form (with respect to which, all other forms can be regarded as degenerate) is a sequence of 3-5 dust clouds. The morphological forms represent various Vega-2 passes through SDSs at different stages of development. SDSs observable as the key form consisted of several fairly regularly spaced dust subpopulations, whose plane of symmetry was parallel to the comet orbit plane. That regularity together with specific features of morphological forms strongly constrain disintegration scenarios and dynamics of fragments, and allow to draw a number of conclusions, the main of which are: SDS parent bodies were ice-free dust aggregates lifted from the nucleus near the comet perihelion, whose masses were in the range ∼0.1-1 of the biggest emitted mass (mass of a body accelerated to the escape velocity, i.e., ∼300-1500 kg); the disintegration scenario comprised a few steps, and the first-step disintegration consisted mainly in consecutive detachments of biggest first-step fragments (BF-SFs) from the parent body; a SDS observable as the key form included the dust minitail of parent body and a few BF-SF minitails, the former one being longer than the latter ones; SDS parent bodies had a fractal-like internal structure, and the BF-SF mass was a few percent of the parent body mass; the thermal conductivity of SDS parent body was less than ∼0.4 W m⁻¹ K⁻¹ or so, while the latent heat of gluing organics was roughly 80 kJ mol⁻¹; the disintegration mechanism was a combination of sintering and sublimation of organics. The multistep disintegration of SDS parent bodies can be reconciled with the basically one-step disintegration of aggregates responsible for the dust boundary (Oberc, P., Icarus 1996, 124, 195-208). The fractal-like structure and the relation between BF-SF mass and parent body mass are in agreement with predictions from the Weidenschilling model of comet formation. Large ice-free dust bodies, in particular SDS parent bodies, can be identified with refractory boulders postulated by some comet nucleus models.     

Meteor Outburst Profiles and Cometary Ejection Models

The spatial structure of meteor streams, and the activity profiles of their corresponding meteor showers, depend firstly on the distribution of meteoroid orbits soon after ejection from the parent comet nucleus, and secondly on the subsequent dynamical evolution. The latter increases in importance as more time elapses. For younger structures within streams, notably the dust trails that cause sharp meteor outbursts, it is the cometary ejection model (meteoroid production rate as a function of time through the several months of the comet’s perihelion return, and velocity distribution of the meteoroids released) that primarily determines the shape and width of the trail structure. This paper describes how a trail cross section can be calculated once an ejection model has been assumed. Such calculations, if made for a range of ejection model parameters and compared with observed parameters of storms and outbursts, can be used to constrain quantitatively the process of meteoroid ejection from the nucleus, including the mass distribution of ejected meteoroids.     

Planetary dynamos: effects of electrically conducting flows overlying turbulent regions of magnetic field generation

A fully three-dimensional, nonlinear, time-dependent, multi-layered spherical kinematic dynamo model is used to study the effect on the observable external magnetic field of flow in an electrically conducting layer above a spherical turbulent dynamo region in which the α effect generates the magnetic field. It is shown that the amplitude and structure of an observable planetary magnetic field are largely determined by the magnitude and structure of the flow in the overlying layer. It is also shown that a strong-field planetary dynamo can be readily produced by the effect of an electrically conducting flow layer at the top of a convective core. The overlying layer and the underlying convective region constitute a magnetically strongly coupled system. Such overlying layers might exist at the top of the Earth's core due to chemical or thermal causes, in the cores of other terrestrial planets for similar reasons, and in Saturn due to the differentiation of helium from hydrogen. An electrically conducting and differentially rotating layer could exist above the metallic hydrogen region in Jupiter and affect the jovian magnetic field similar to the overlying layers in other planets. Lateral temperature gradients resulting in thermal winds drive the flow in the overlying layers. All planetary magnetic fields could be maintained by similar turbulent convective dynamos in the field-generation regions of planets with the differences among observable magnetic fields due to different circulations in the overlying electrically conducting layers.     

On the observability of radiation forces acting on near-Earth asteroids

We consider the perturbations on near-earth asteroid orbits due to various forces stemming from solar radiation. We find that the existence of precise radar astrometric observations at multiple apparitions, spanning periods on the order of 10 years, allows the detection of such forces on bodies as large as kilometer across. Indeed, the perturbations are so substantial that certain of the forces can be essential to fit an orbit to the observations. In particular, we show that the recoil force of thermal radiation from the asteroid, known as the Yarkovsky effect, is the most important of these unmodeled perturbations. We also show that the effect of reflected light can be important if even moderate albedo variations are present, while moderate changes in oblateness appear to have a far smaller effect. An unexpected result is that the Poynting–Robertson effect, typically only considered for submillimeter dust particles, could be observable on smaller asteroids with high eccentricity, such as 1566 Icarus. Finally, we also study the possibility of improving the orbit uncertainty through well-timed optical observations which might help in better detection of these nongravitational perturbations.     

Lineaments on the Surface of the Consolidated Material of the Comet 67P/Churyumov−Gerasimenko Nucleus

Morphologic analysis of the fragments (500 × 500 pixels) of images of comet 67P/Churyumov? Gerasimenko obtained with the OSIRIS camera for 18 regions, where the consolidated material is exposed on the surface, has been carried out. In terms of resolution, the images form a series from 0.04 to 1.29 m/pixel; consequently, the areas covered by these fragments vary from 400 to ~400000 m². In all of the regions, lineaments resembling tension fractures—several dozens of structures per region—are seen; and their number scarcely changes when passing from high-resolution images to lower-resolution ones. It is clear that relatively small lineaments cease to be reliably distinguished when the image resolution worsens, but the largest and well-defined ones remain observable, while the number of larger lineaments grows proportionally to the survey area increasing. Undoubtedly, this is an observational effect, which was demonstrated by examples with an artificially worsened resolution. However, on the other hand, this means that the tension fractures of the consolidated nucleus material represent a hierarchic population of smaller (meters long) to larger (decameters and longer) features. The lineaments seen in the analyzed fragments of images were counted, their lengths were measured, and the spacings (the mean distances between lineaments) and, from them, the depths of penetration of fractures into the nucleus material were estimated. It has been shown that the mean length of lineaments within each studied region depends on its area (which here correlates with the image resolution) and the depths of fracture penetration into the nucleus body depends on the mean length of lineaments. Both dependences are close to a power law. In the images of four regions covering the areas from 100000 to 400000 m² with a resolution of 0.66?1.29 m/pixel, the structures that look like layering or sheet jointing are seen in addition to fracture lineaments. The tension fractures are apparently formed due to seasonal and diurnal variations of the temperature, while the nature of the formation of the layer-like structures is not yet fully understood.     

Dynamical evolution of the nucleus of comet Hartley 2 and Asteroid Itokawa

Cometary nuclei are being actively studied using spacecrafts. In November 2010, the Deep Impact spacecraft (EPOXI project, NASA) approached the nucleus of comet 103P/Hartley 2 and returned images of this small celestial body having a dumbbell shape with a smooth neck. Since rotation of the nucleus leads to centrifugal forces, it is assumed that the dumbbell neck appeared as the result of their effect and the neck is lengthening slowly but continuously, which should eventually result in fragmentation of the nucleus. This paper considers dynamical evolution of the nucleus of comet Hartley 2. Calculations show that centrifugal forces exceed gravitational forces in the narrow part, and the nucleus can indeed undergo upcoming breakup and fragmentation into two parts. If there are no external perturbations, both parts of the celestial body will drift apart to a distance of less than 1 km from each other. The nucleus of comet Hartley 2 is an observed example of breakup of a celestial body. Asteroid Itokawa is considered, which has a similar feature but does not seem to undergo breakup.