100-metre-diameter moonlets in Saturn's A ring from observations of 'propeller' structures

Saturn's main rings are composed predominantly of water-ice particles ranging between about 1 centimetre and 10 metres in radius. Above this size range, the number of particles drops sharply, according to the interpretation of spacecraft and stellar occultations. Other than the gap moons Pan and Daphnis (the provisional name of S/2005 S1), which have sizes of several kilometres, no individual bodies in the rings have been directly observed, and the population of ring particles larger than ten metres has been essentially unknown. Here we report the observation of four longitudinal double-streaks in an otherwise bland part of the mid-A ring. We infer that these 'propeller'-shaped perturbations arise from the effects of embedded moonlets approximately 40 to 120 m in diameter. Direct observation of this phenomenon validates models of proto-planetary disks in which similar processes are posited. A population of moonlets, as implied by the size distribution that we find, could help explain gaps in the more tenuous regions of the Cassini division and the C ring. The existence of such large embedded moonlets is most naturally compatible with a ring originating in the break-up of a larger body, but accretion from a circumplanetary disk is also plausible if subsequent growth onto large particles occurs after the primary accretion phase has concluded.     

The determination of the structure of Saturn's F ring by nearby moonlets

Saturn's narrow F ring exhibits several unusual features that vary on timescales of hours to years. These include transient clumps, a central core surrounded by a multistranded structure and a regular series of longitudinal channels associated with Prometheus, one of the ring's two 'shepherding' satellites. Several smaller moonlets and clumps have been detected in the ring's immediate vicinity, and a population of embedded objects has been inferred. Here we report direct evidence of moonlets embedded in the ring's bright core, and show that most of the F ring's morphology results from the continual gravitational and collisional effects of small satellites, often combined with the perturbing effect of Prometheus. The F-ring region is perhaps the only location in the Solar System where large-scale collisional processes are occurring on an almost daily basis.     

A belt of moonlets in Saturn's A ring

The origin and evolution of planetary rings is one of the prominent unsolved problems of planetary sciences, with direct implications for planet-forming processes in pre-planetary disks. The recent detection of four propeller-shaped features in Saturn's A ring proved the presence of large boulder-sized moonlets in the rings. Their existence favours ring creation in a catastrophic disruption of an icy satellite rather than a co-genetic origin with Saturn, because bodies of this size are unlikely to have accreted inside the rings. Here we report the detection of eight new propeller features in an image sequence that covers the complete A ring, indicating embedded moonlets with radii between 30 m and 70 m. We show that the moonlets found are concentrated in a narrow 3,000-km-wide annulus 130,000 km from Saturn. Compared to the main population of ring particles (radius s < 10 m), such embedded moonlets have a short lifetime with respect to meteoroid impacts. Therefore, they are probably the remnants of a shattered ring-moon of Pan size or larger, locally contributing new material to the older ring. This supports the theory of catastrophic ring creation in a collisional cascade.     

An extrasolar giant planet in a close triple-star system

Hot Jupiters are gas-giant planets orbiting with periods of 3-9 days around Sun-like stars. They are believed to form in a disk of gas and condensed matter at or beyond approximately 2.7 astronomical units (au-the Sun-Earth distance) from their parent star. At such distances, there exists a sufficient amount of solid material to produce a core capable of capturing enough gas to form a giant planet. Subsequently, they migrate inward to their present close orbits. Here I report the detection of an unusual hot Jupiter orbiting the primary star of a triple stellar system, HD 188753. The planet has an orbital period of 3.35 days and a minimum mass of 1.14 times that of Jupiter. The primary star's mass is 1.06 times that of the Sun, 1.06 M(\circ). The secondary star, itself a binary stellar system, orbits the primary at an average distance of 12.3 au with an eccentricity of 0.50. The mass of the secondary pair is 1.63 M(\circ). Such a close and massive secondary would have truncated a disk around the primary to a radius of only approximately 1.3 AU (ref. 4) and might have heated it up to temperatures high enough to prohibit giant-planet formation, leaving the origin of this planet unclear.     

Origin of the Moon's orbital inclination from resonant disk interactions

The Moon is generally believed to have formed from the debris disk created by a large body colliding with the early Earth. Recent models of this process predict that the orbit of the newly formed Moon should be in, or very near, the Earth's equatorial plane. This prediction, however, is at odds with the known history of the lunar orbit: the orbit is currently expanding, but can be traced back in time to reveal that, when the Moon formed, its orbital inclination relative to the Earth's equator was I approximately = 10 degrees. The cause of this initial inclination has been a mystery for over 30 years, as most dynamical processes (such as those that act to flatten Saturn's rings) will tend to decrease orbital inclinations. Here we show that the Moon's substantial orbital inclination is probably a natural result of its formation from an impact-generated disk. The mechanism involves a gravitational resonance between the Moon and accretion-disk material, which can increase orbital inclinations up to approximately 15 degrees.     

A low density of 0.8 g cm(-3) for the Trojan binary asteroid 617 Patroclus

The Trojan population consists of two swarms of asteroids following the same orbit as Jupiter and located at the L4 and L5 stable Lagrange points of the Jupiter-Sun system (leading and following Jupiter by 60 degrees ). The asteroid 617 Patroclus is the only known binary Trojan. The orbit of this double system was hitherto unknown. Here we report that the components, separated by 680 km, move around the system's centre of mass, describing a roughly circular orbit. Using this orbital information, combined with thermal measurements to estimate the size of the components, we derive a very low density of 0.8(- 0.1)+0.2 g cm(-3). The components of 617 Patroclus are therefore very porous or composed mostly of water ice, suggesting that they could have been formed in the outer part of the Solar System.     

Planet-planet scattering in the upsilon Andromedae system

Doppler spectroscopy has detected 152 planets around nearby stars. A major puzzle is why many of their orbits are highly eccentric; all planets in our Solar System are on nearly circular orbits, as is expected if they formed by accretion processes in a protostellar disk. Several mechanisms have been proposed to generate large eccentricities after planet formation, but so far there has been little observational evidence to support any particular model. Here we report that the current orbital configuration of the three giant planets around upsilon Andromedae (upsilon And) probably results from a close dynamical interaction with another planet, now lost from the system. The planets started on nearly circular orbits, but chaotic evolution caused the outer planet (upsilon And d) to be perturbed suddenly into a higher-eccentricity orbit. The coupled evolution of the system then causes slow periodic variations in the eccentricity of the middle planet (upsilon And c). Indeed, we show that upsilon And c periodically returns to a very nearly circular state every 6,700 years.     

Origin of the orbital architecture of the giant planets of the Solar System

Planetary formation theories suggest that the giant planets formed on circular and coplanar orbits. The eccentricities of Jupiter, Saturn and Uranus, however, reach values of 6 per cent, 9 per cent and 8 per cent, respectively. In addition, the inclinations of the orbital planes of Saturn, Uranus and Neptune take maximum values of approximately 2 degrees with respect to the mean orbital plane of Jupiter. Existing models for the excitation of the eccentricity of extrasolar giant planets have not been successfully applied to the Solar System. Here we show that a planetary system with initial quasi-circular, coplanar orbits would have evolved to the current orbital configuration, provided that Jupiter and Saturn crossed their 1:2 orbital resonance. We show that this resonance crossing could have occurred as the giant planets migrated owing to their interaction with a disk of planetesimals. Our model reproduces all the important characteristics of the giant planets' orbits, namely their final semimajor axes, eccentricities and mutual inclinations.     

建筑物与设备互动减震体系的动力响应分析

着重讨论了利用建筑物内部的使用荷载或设备作为子结构的主子结构互动减震体系的动力响应。文中将建筑物内部的使用荷载设备作为子结构，将建筑物作为主结构，讨论了由主子结构形成的互动减震体系地震结构，探讨了互动减震体系系统参数对结构响应的影响。     

组态d~N在D_(3h)对称场中稳定化能的推求方法

介绍一种推求组态 d N在 D3h对称场中稳定化能的方法 ,它是根据配体场势的可合性和可分性以及立体构型的可合性和可分性两条原则 ,利用简单的代数运算方法 ,由常见对称场中各 d轨道的相对能量算得 D3h对称性的 ML3和 ML5构型配合物中各 d轨道相对能量 ,尔后求出组态 d N在 D3h对称场中的稳定化能。     

A high-velocity black hole on a Galactic-halo orbit in the solar neighbourhood

Only a few of the dozen or so known stellar-mass black holes have been observed away from the plane of the Galaxy. Those few could have been ejected from the plane as a result of a 'kick' received during a supernova explosion, or they could be remnants of the population of massive stars formed in the early stages of evolution of the Galaxy. Determining their orbital motion should help to distinguish between these options. Here we report the transverse motion (in the plane of the sky) for the black-hole X-ray nova XTE J1118+480 (refs 2, 3, 4, 5), from which we derive a large space velocity. This X-ray binary system has an eccentric orbit around the Galactic Centre, like most objects in the halo of the Galaxy, such as ancient stars and globular clusters. The properties of the system suggest that its age is comparable to or greater than the age of the Galactic disk. Only an extraordinary 'kick' from a supernova could have launched the black hole into an orbit like this from a birthplace in the disk of the Galaxy.     

锶同位素地层学在海相地层定年中的潜在价值

锶同位素地层学在海相地层的对比和定年中具有十分重要的作用。随着锶同位素地层学的深入研究，人们清楚地认识到海水的锶同位素组成是全球变化的灵敏指示剂。显生宙以来全球海水的^87Sr／^86Sr比值是时间的函数，可以把锶同位素组成用于海相地层的定年和进行全球对比，估计沉积间断持续时间、生物带的时间范围和发展阶段，从而为重要地球化学周期的预测提供基础参数。当锶同位素地层学用于定年时，其地质年龄的精确度取决于几个因素，包括样品原始锶同位素组成的保存情况、分析精度、用于建立数据库样品年龄的可信度以及^87Sr／^86Sr比值—年龄曲线的最佳拟合方法等。     

绕射激波和反射激波冲击下的Richtmyer-Meshkov不稳定性

利用高速纹影和平面片光测试技术,实验研究低马赫数入射激波绕圆柱体后冲击N2/SF6平面界面,以及来自固壁的反射激波再冲击过程的R-M不稳定性特征.在竖直激波管采用重气体尾部充入,轻气体上部充入,狭缝流出的方式,在实验段狭缝处生成准静止稳定的N2/SF6平面界面.激波与圆柱作用后的流场是复杂的,包括初始的入射波、弯曲反射波、马赫波、由马赫反射产生的滑移线.研究这些复杂流场对界面的作用,对认识界面扰动的生成具有较大帮助.与平面激波作用不同的是,在柱体绕射后的激波冲击下,界面会生成局部扰动.实验结果显示,入射激波作用下界面宽度增长缓慢,而反射激波再冲击后,局部扰动会产生大的"尖钉"和"气泡"结构;反射激波与边界层相互作用产生壁面涡,会加剧湍流混合区的增长;来自尾部固壁的反射稀疏波会再次加剧湍流混合区的增长.     

角度重迭模型的应用(Ⅰ)

本文依据角度重迭模型的基本原理,首先计算了d过渡系金属离子与单齿配体形成的不同构型配合物的d~*轨道能量:接着,为了使参数单一化,将配体分为三类,计算了各类配合物的d~*轨道能量。得到了一套非常实用的数据。     

论弗吉尼亚·伍尔夫对西尔维亚·普拉斯的影响

弗吉尼亚·伍尔夫(1882-1941)和西尔维亚·普拉斯(1932-1963)是西方现代文学史上两位声名显赫的女作家.事实上,伍尔夫那种力图"杀死房中的天使"以摆脱传统性别观念束缚的思想,对普拉斯有着极其深刻的影响,激励着普拉斯成为一名积极表现女性生活经历和感受的女作家,为促进女性文学的发展贡献力量.但是,这也使她和伍尔夫一样,最终倒在了"杀死房中的天使"的战斗中."房中的天使"代表的是强大男权文化霸权对女作家的迫害,因此伍尔夫"杀死房中的天使"的思想具有极其重要的意义.     

泡沫增强聚合物体系配方研究

在分析了聚合物驱提高采收率技术在实际应用中的局限性基础上,提出了采用就地起泡形成泡沫的泡沫增强聚合物体系以减小聚合物沿大通道窜流的问题。为了有效确定该体系的配方,实验中采用了均匀设计实验方法安排实验,并运用DPS数据处理软件对实验数据进行了二次多项式逐步回归处理,找出了影响该体系性能的主要因素。     

STEINER 3—系的代数结构

本文引入宽线性空间的概念,并借助于此来讨论Steiner 3—系,证明了每个GF(3)上的宽线性空间,确定一个Steiner 3—系.反之,每个Steiner 3—系也确定一个GF(3)上的宽线性空间,作为宽线性空间概念的应用,对一种Steiner 3—系的自同构群的结构作了讨论。     

MEHMO法计算F+H_2共线反应势能面

用改进的EHMO(MEHMO)法计算了F+H_2共线反应的势能面,本方法在H矩阵的对角元中引入了一项描述不同原子间排斥力的经验项,基组选用F原子的2p_(?)轨道和H原子的1s轨道,参数值B为0.64,在得到的势能面上,于R_(UH)~*=2.54 a.u.和P_(HH)~*=2.18a.u.处出现的位垒高度为15.72kJ/mo1.反应产物和作用物的能量差为-144.21kJ/mo1.本文所得结果与实验测定、从头计算法的结果一致。     

一类三重积分的新算法

主要讨论有关某一类的三重积分的计算问题。积分是微积分学与数学分析里的一个非常重要的核心概念,积分的运算方法也因此一直被不断的总结与研究。随着积分变量的增加,重积分就变的更加难以计算。三重积分的计算问题更一直被很多人所关注,一般情况下是把三重积分化为三次定积分来计算。近年来,出现了一些新的方法和手段去解决三重积分的计算问题,这里提出一种结合《概率论与数理统计》的相关内容,根据服从[0-1]区间上均匀分布的顺序统计量的有关知识,利用顺序统计量的数学期望以及条件数学期望,直接把三重积分的计算变为一重积分的计算方法,进而达到简化计算三重积分的目的。     

三重相关匹配滤波技术的研究

分析了实现二重相关和三重相关的方法，讨论了根据三重相关的结果来重建信号的算法，探讨了以二重相关匹配滤波和三重相关为基础的三重相关匹配滤波技术．并以随机脉冲的相关检测为例，进行了计算机模拟．实验结果表明，三重相关匹配滤波在信噪比改善方面优于二重相关和三重相关．