Kuiper带小天体动力演化

Kuiper带是指太阳系内位于离太阳 30～ 50AU一个区域。 1 992年该区域陆续发现了一群半径在几十到几百公里的小天体。这些小天体在Kuiper带的分布是极其不均匀的。Kuiper带小天体的发现对人们认识太阳系的形成与演化有重要的意义。本文回顾了近年来国际上在Kuiper带小天体动力演化方面的研究 ,着重分析了目前国际上几种用以解释其非均匀分布的动力学机制 ,并提出目前该领域的一些尚未解决的问题。     

Population of the Scattered Kuiper Belt

We study how the internal structure of dark halos is affected if cold dark matter particles are assumed to have a large cross section for elastic collisions. We identify a cluster halo in a large cosmological N-body simulation and resimulate its formation with progressively increasing resolution. We compare the structure found in the two cases in which dark matter is treated as collisionless or as a fluid. For the collisionless case, the overall ellipticity of the cluster, the central density cusp, and the amount of surviving substructure are all similar to those found in earlier high-resolution simulations. Collisional dark matter results in a cluster that is more nearly spherical at all radii, has a steeper central density cusp, and has less-but still substantial-surviving substructure. As in the collisionless case, these results for a "fluid" cluster halo are expected to carry over approximately to smaller mass systems. The observed rotation curves of dwarf galaxies then argue that self-interacting dark matter can only be viable if intermediate cross sections produce structure that does not lie between the extremes we have simulated.     

Extrasolar Analogues to the Kuiper Belt

Debris disks are found around some 15% of main sequencestars and their dust is thought to be continuallyreplenished in collisions between planetesimals inextrasolar Kuiper belts.While they were discovered in 1984 by IRAS, it is onlywith more recent imaging that their true nature has beenrevealed. This paper discusses recent debris disk images andtheir impact on our understanding of extrasolar systems.Importantly these images confirm the extrasolar Kuiper belthypothesis for most (but not all) debris diskcandidates and show that the planetesimals within thesedisks must have grown to at least a few km.Asymmetries in imaged disk structures also provide informationabout the planetary systems orbiting inside these planetesimalbelts. The impact of debris disk studies on our understandingof the evolution of our own Kuiper belt, as well as theirpotential to solve puzzles such as the origin of the missingmass and the outer edge of the Kuiper belt, is alsodiscussed.     

Opposition Effect of Kuiper Belt Objects: preliminary estimations

Information on the surface structure of the Kuiper Belt objectscan be obtained from studies of their opposition brightening.Although KBOs are observed at a very limited phase angle rangethey represent a unique opportunity to study the backscatteringphenomenon down almost to zero phase angle. Preliminaryestimations of the opposition effect amplitude and width based oncomposite phase curves of four KBOs and two Centaurs showed theexistence of a very narrow opposition surge of about 0.1–0.2 mag at phase angles less than 0.1–0.2 deg. It may indicate a highporosity of the KBOs regoliths. Further observations are needed toconfirm this phenomenon.     

The Kuiper Belt and its Primordial Sculpting

We discuss the structure of the Kuiper belt as it can be inferred from the first decade of observations. In particular, we focus on its most intriguing properties – the mass deficit, inclination distribution, the apparent existence of an outer edge and of a correlation among inclinations, colours and sizes – which clearly show that the belt has lost its pristine structureof a dynamically cold proto-planetary disk. Understanding how the Kuiperbelt acquired its present structure will provide insight into the formationof the outer planetary system and on its early evolution. We outline ascenario of primordial sculpting – issued from a combination of mechanismsproposed by various authors – that seems to explain most of the observedproperties of the Kuiper belt. Several aspects are not yet totallyclear. But, for the first time, we have a view – if not of the detailedsculpture – at least of its rough cast.     

The Shape Distribution Of Kuiper Belt Objects

If we assume that the periodic brightness variations ina Kuiper Belt lightcurve are determined only by theiraspherical shapes and the observing geomtry, the fractionof detectable Kuiper Belt lightcurves and the lightcurveamplitude distribution can be used to constrain the shapesof Kuiper Belt objects. The results indicate that most KuiperBelt objects (～85%) have shapes that are close to spherical(a/b ≤ 1.5$), but there is a small but significant fraction(～12%) possessing highly aspherical shapes (a/b ≥ 1.7).The distribution cannot be well fitted by a gaussian and is betterapproximated by a power law.     

A stability study of Kuiper Belt binaries

The dynamical structure of the orbital element space of seven Kuiper Belt binary systems is studied by numerical methods in the model of the spatial elliptic restricted three‐body problem. It is shown that three systems have an extended region of stability where additional satellites could exist. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kuiper带天体的轨道分布特性

1992年9月,夏威夷大学的D.Jewitt和加利福尼亚大学的J.Lun发现了海王星外绕太阳运行的第一个小天体1992QB1[1],开创了人类对于海王星外天体的实际观测的研究.近10年的接连不断发现,已经证实了海王星轨道外面存在着一个由大量的环绕太阳运动的小天体组成的环带[2].由于G.P.Kuiper曾在1951年的文章中提出过在冥王星的外边可能存在小天体的问题,因此人们一般把这个环带称为Kuiper带,你这些天体为“KuiperBelt　Objects”(KBOs),或从逻辑上称它们为“Trans-NeptunianObjects”(TNOs)[3]     

Kuiper带天体的轨道动力学

主要评述太阳系动力学研究的一个新方向——Kuiper带的轨道动力学。早期的研究是为了探讨短周期彗星的起源。在发现第一颗Kuiper带小天体之后，人们开始将注意力转到Kuiper带共振区的相空间结构上，Morbidelli和Malhotra分别采用不同的模型研究了这些共振区的大小。其中主要研究对象是3:2共振区。冥王星也处在这一共振区中。从冥王星的轨道特性来看，冥王星应是一颗较大的Kuiper带天体，它还拥有另外两种共振——Kozai共振和1：1超级共振。正是由于这些共振的存在，冥王星的运动才得以长期保持稳定。观测表明许多Kuiper带天体也处的海王星的平运动共振中。早期的理论认为这些平运动共振起源于灾难性事件，如碰撞。然而这都是一些小概率事件，无法对共振的形成进行合理的解释。Malhotra通过行星迁移成功地解释了冥王星被共振俘获的机制。这一机制的概率非常大，同样可以用来解释Kuiper带天体共振的形成。     

The origin of the Kuiper Belt high-inclination population

I simulate the orbital evolution of the four major planets and a massive primordial planetesimal disk composed of 10⁴ objects, which perturb the planets but not themselves. As Neptune migrates by energy and angular momentum exchange with the planetesimals, a large number of primordial Neptune-scattered objects are formed. These objects may experience secular, Kozai, and mean motion resonances that induce temporary decrease of their eccentricities. Because planets are migrating, some planetesimals can escape those resonances while in a low-eccentricity incursion, thus avoiding the return path to Neptune close encounter dynamics. In the end, this mechanism produces stable orbits with high inclination and moderate eccentricities. The population so formed together with the objects coming from the classical resonance sweeping process, originates a bimodal distribution for the Kuiper Belt orbits. The inclinations obtained by the simulations can attain values above 30° and their distribution resembles a debiased distribution for the high-inclination population coming from the real classical Kuiper Belt.     

The CFEPS Kuiper Belt Survey: Strategy and presurvey results

We present the data acquisition strategy and characterization procedures for the Canada-France Ecliptic Plane Survey (CFEPS), a sub-component of the Canada-France-Hawaii Telescope Legacy Survey. The survey began in early 2003 and as of summer 2005 has covered 430 square degrees of sky within a few degrees of the ecliptic. Moving objects beyond the orbit of Uranus are detected to a magnitude limit of mR=23-24 (depending on the image quality). To track as large a sample as possible and avoid introducing followup bias, we have developed a multi-epoch observing strategy that is spread over several years. We present the evolution of the uncertainties in ephemeris position and orbital elements of a small 10-object sample of objects tracked through these epochs as part of a preliminary presurvey starting a year before the main CFEPS project. We describe the CFEPS survey simulator, to be released in 2006, which allows theoretical models of the Kuiper belt to be compared with the survey discoveries. The simulator utilizes the well-documented pointing history of CFEPS, with characterized detection efficiencies as a function of magnitude and rate of motion on the sky. Using the presurvey objects we illustrate the usage of the simulator in modeling the classical Kuiper belt. The primary purpose of this paper is to allow a user to immediately exploit the CFEPS data set and releases as they become available in the coming months.     

Planetary Embryos Never Formed in the Kuiper Belt

We study the orbital evolutions of various systems of planetary embryos in the transneptunian region, undergoing mutual scattering and perturbations from the giant planets. We show that about 15-20% of the original embryos should survive in the transneptunian region at the current epoch. The orbital dispersion of the surviving embryos depends on their individual mass, so that only lunar mass embryos could survive with semimajor axis smaller than 50 AU. In all cases, we show by a Monte Carlo model that at least one of the surviving embryos should have already been discovered by one of the most effective Kuiper-belt surveys. This implies that planetary embryos did not form in the transneptunian region (or have been removed by some external and unknown mechanism). Therefore, we conclude that the Kuiper belt was not excited by resident planetary embryos, unlike the asteroid belt. We also compute with the Monte Carlo model that a significant number (order 10) of Pluto-size bodies could exist only on very eccentric and long-periodic orbits, typical of the scattered disk, while the existence of about 30 bodies brighter than absolute magnitude 4 in the classical belt is compatible with the discovery of Varuna by the Spacewatch survey.     

Resonant and Secular Families of the Kuiper Belt

We review ongoing efforts to identify occupants of mean-motion resonances(MMRs) and collisional families in the Edgeworth–Kuiper belt. Directintegrations of trajectories of Kuiper belt objects (KBOs) reveal the 1:1(Trojan), 5:4, 4:3, 3:2 (Plutino), 5:3, 7:4, 9:5, 2:1 (Twotino), and 5:2 MMRsto be inhabited. Apart from the Trojan, resonant KBOs typically have largeorbital eccentricities and inclinations. The observed pattern of resonanceoccupation is consistent with resonant capture and adiabatic excitation bya migratory Neptune; however, the dynamically cold initial conditions priorto resonance sweeping that are typically assumed by migration simulationsare probably inadequate. Given the dynamically hot residents of the 5:2 MMRand the substantial inclinations observed in all exterior MMRs, a fraction ofthe primordial belt was likely dynamically pre-heated prior to resonancesweeping. A pre-heated population may have arisen as Neptune gravitationallyscattered objects into trans-Neptunian space. The spatial distribution of Twotinosoffers a unique diagnostic of Neptune's migration history. The Neptunian Trojanpopulation may rival the Jovian Trojan population, and the former's existence isargued to rule out violent orbital histories for Neptune. Finally, lowest-order seculartheory is applied to several hundred non-resonant KBOs with well-measured orbitsto update proposals of collisional families. No convincing family is detected.     

An Overview on the Kuiper Belt and on the Origin of Jupiter-family Comets

The present paper reviews our current understanding of the dynamical structure of the Kuiper belt and of the origin of Jupiter-family comets. It also discusses the evolutionary scenarios that have been proposed so far to explain the observed structure of the Kuiper belt population. This revised version was published online in July 2006 with corrections to the Cover Date.     

柯伊伯带结构形成动力学

柯伊伯带是指位于海王星轨道外的小天体构成的盘状区域.一般认为柯伊伯带小天体是早期太阳系物质凝聚成各大行星后的残留物,因此这些小天体能够为研究外太阳系的形成与演化提供很多重要的线索.该文首先介绍了柯伊伯带的发现历史及它的主要观测特征,然后回顾了近年来提出的形成这些特征的机制,最后讨论了柯伊伯带中有待解释的主要问题.     

Census of Kuiper Comets-the TAOS Project

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Spectral Models of Kuiper Belt Objects and Centaurs

We present models of the spectral reflectances of groups of outer Solar System objects defined primarily by their colors in the spectral region 0.4–1.2 mu;m, and which have geometric albedo ～0.04 at wavelength 0.55 μm. Our models of the groups with the strongest reflectance gradients (reddest colors) use combinations of organic tholins. We test the hypothesis that metal-reddened igneous rock-forming minerals contribute to the red colors of Centaurs and KBOs by using the space-weathered lunar soil as one of the components of our models. We find that our models can admit the presence of moderate amounts of space-weathered (metal-reddened) minerals, but that they do not require this material to achieve the red colors of the reddest outer Solar System bodies. Our models with organic tholins are consistent with the results of other investigators.     

Research of Small Kuiper Belt Objects by Stellar Occultations

Fast photometric observations of target stars in the ecliptic are a powerful tool to detect small objects in the Kuiper Belt. The various parameters involved in such observations are described. Meter-sized telescopes are able to detect sub-kilometer KBO (Kuiper Belt Objects). A campaign of research of KBO by stellar occultations, organized at the Pic du Midi Observatory is presented. These observations bring the first constraint on the small end of the size distribution of the KBOs.     

Towards initial mass functions for asteroids and Kuiper Belt Objects

Our goal is to understand primary accretion of the first planetesimals. Some examples are seen today in the asteroid belt, providing the parent bodies for the primitive meteorites. The primitive meteorite record suggests that sizeable planetesimals formed over a period longer than a million years, each of which being composed entirely of an unusual, but homogeneous, mixture of millimeter-size particles. We sketch a scenario that might help explain how this occurred, in which primary accretion of 10-100 km size planetesimals proceeds directly, if sporadically, from aerodynamically-sorted millimeter-size particles (generically “chondrules”). These planetesimal sizes are in general agreement with the currently observed asteroid mass peak near 100 km diameter, which has been identified as a “fossil” property of the pre-erosion, pre-depletion population. We extend our primary accretion theory to make predictions for outer Solar System planetesimals, which may also have a preferred size in the 100 km diameter range. We estimate formation rates of planetesimals and explore parameter space to assess the conditions needed to match estimates of both asteroid and Kuiper Belt Object (KBO) formation rates. For parameters that satisfy observed mass accretion rates of Myr-old protoplanetary nebulae, the scenario is roughly consistent with not only the “fossil” sizes of the asteroids, and their estimated production rates, but also with the observed spread in formation ages of chondrules in a given chondrite, and with a tolerably small radial diffusive mixing during this time between formation and accretion. As previously noted, the model naturally helps explain the peculiar size distribution of chondrules within such objects. The optimum range of parameters, however, represents a higher gas density and fractional abundance of solids, and a smaller difference between Keplerian and pressure-supported orbital velocities, than “canonical” models of the solar nebula. We discuss several potential explanations for these differences. The scenario also produces 10-100 km diameter primary KBOs, and also requires an enhanced abundance of solids to match the mass production rate estimates for KBOs (and presumably the planetesimal precursors of the ice giants themselves). We discuss the advantages and plausibility of the scenario, outstanding issues, and future directions of research.