环双星系统中行星-行星之间的散射

流体模拟的研究表明,如果多颗行星形成在环绕双星的气体盘中,行星之间的会聚迁移会导致行星之间的散射.本文系统研究了环双星的行星系统中两颗等质量行星(P型行星)之间的散射,目的是探讨散射对P型行星系统构型的影响.数值模拟的研究表明散射后只剩一颗行星的几率最大,一般80%.从某种意义上说,伴星的存在有利于行星的存活,尤其是在靠近双星的地方.我们发现散射会导致行星的向外迁移,这和单恒星系统中的散射现象相反,此现象可以用来解释最近通过成像方法发现的远距离P型行星.即便对于等质量的行星,散射位置的不同会造成偏心率分布的多样性.在靠近双星的位置,幸存行星的偏心率较小;在远离双星的位置,剩余行星的偏心率较大.此外,P型行星之间的散射可使P型行星转变为围绕一颗主星运行的S型行星.     

引入塑料行星轮对行星齿轮传动动态特性的影响

为了深入了解钢质行星齿轮传动系统引入塑料行星轮后的动态特性,建立了钢/塑齿轮组合行星传动的动力学分析模型和实验模型,对含塑料行星轮行星齿轮传动系统的动态特性进行了理论分析与实验研究,分析了塑料行星轮的引入对行星齿轮传动动态特性的影响。数值仿真与实验研究结果表明:塑料行星轮的引入对轮齿动态特性影响很大,显著地减小了太阳轮—行星轮和内齿圈—行星轮的啮合动载荷;显著地降低了行星齿轮传动系统的转子不平衡工频及其谐波振动、齿轮啮合振动及其谐波振动和高频带振动;在很大程度上降低了行星齿轮传动系统的振动强度。     

行星曲线及应用

本文讨论了行星机构中行星轮运动轨迹的矩阵方程的建立方法,研究并提出了行星曲线封闭的条件.同时也提出了行星曲线的计算机模拟方法及数学模型,分析并讨论了行星曲线的特征及变化规律.最后,简单介绍了行星曲线在工程中的应用.     

行星齿轮变速器的档位分析

本文通过对单个行星排进行分析,阐明了行星齿轮机构的传动原理,并提供了一种行星排传动规律简单易记的方法;另外,根据行星排传动规律的特点,对行星齿轮变速器进行了挡位分析,从而使行星齿轮变速器的传力路线变得客易理解.     

行星齿轮参数有限元分析与优化

文章针对车辆选择性输出双离合自动变速器,为了验证行星齿轮系行星齿轮的具体结构参数及承载能力,利用通用有限元分析软件Ansys对所设计的行星齿轮进行静态有限元分析,发现行星齿轮在传动中齿面接触应力及齿根弯曲应力超过了材料的许用应力范围;为达到符合行星齿轮材料及工作要求,对行星齿轮进行齿面修形,修形后的行星齿轮经分析符合材料要求。     

钢/塑齿轮组合行星传动的振动特性研究

为了深入了解钢质行星齿轮传动系统引入塑料齿轮后的振动特性,文中建立了钢/塑齿轮组合行星传动的动力学分析模型和实验模型,对4种钢/塑齿轮组合行星传动的振动特性进行了理论分析与实验研究,分析了组合方式对行星传动振动特性的影响。数值仿真与实验研究结果表明：塑料齿轮的引入对行星齿轮传动的振动特性影响很大,显著地减小了太阳轮-行星轮和内齿圈-行星轮的啮合动载荷;有效地抑制了行星齿轮传动的齿轮啮合频带振动和高频带振动;组合方式对行星齿轮传动的振动特性影响显著,合理地采用钢/塑齿轮组合行星传动结构可以极大地降低啮合动载荷,从而显著地降低传动系统的振动和噪声。     

钢／塑齿轮组合行星传动系统的振动特性

为了解钢质行星齿轮传动系统引入塑料齿轮后的振动特性,建立了钢/塑齿轮组合行星传动的动力学分析模型和实验模型,对4种钢/塑齿轮组合行星传动系统的振动特性进行了理论分析与实验研究,分析了组合方式对行星传动振动特性的影响.数值仿真与实验研究结果表明:塑料齿轮的引入对行星齿轮传动的振动特性影响很大,显著减小了太阳轮-行星轮和内齿圈-行星轮的啮合动载荷,有效抑制了行星齿轮传动的齿轮啮合频带振动和高频带振动;组合方式对行星齿轮传动的振动特性影响显著,合理地采用钢/塑齿轮组合行星传动结构可以极大地降低啮合动载荷,从而显著地降低传动系统的振动和噪声.     

风电机组行星架设计及动态特性分析

以某兆瓦级风电机组主齿轮箱为研究对象,设计了齿轮箱的行星传动机构。通过三维建模软件Pro-E建立行星架的计算模型,利用ANSYS有限元软件对行星架进行模态分析,选择Block Lanczos提取方法,提取前6阶模态;分析各阶模态的固有频率和振型;在此基础上,对行星架进行谐响应分析,有效地判断出行星架的振动特性。该分析结果为行星架的动力学研究提供了基础,也为风电机组行星架及齿轮传动的设计提供了理论依据。     

钢/塑齿轮组合行星传动系统的振动与噪声特性(英文)

为了深入了解钢质行星齿轮传动系统引入塑料齿轮后的振动和噪声特性,建立了钢/塑齿轮组合行星传动的动力学分析模型和实验模型,对4钢/塑齿轮组合行星传动系统的振动与噪声特性进行了理论分析与试验研究,分析了组合方式对行星传动振动特性的影响.数值仿真与实验研究结果表明:塑料齿轮的引入对行星齿轮传动的振动特性影响很大,显著地减小了太阳轮与行星轮和内齿圈与行星轮的啮合动载荷;有效地抑制了行星齿轮传动的齿轮啮合频带振动和高频带振动;组合方式对行星齿轮传动的振动特性影响显著,合理地采用钢/塑齿轮组合行星传动结构可以极大地降低啮合动载荷,从而有效地抑制了传动系统的振动和噪声.研究成果对降低工程机械中传动系统的振动和噪声具有一定的应用价值.     

超环面行星蜗杆传动系统自由振动特性分析

分析了超环面行星蜗杆传动系统的自由振动特性,通过用三种振动模式来简化超环面行星蜗杆传动系统的动力学模型：行星蜗轮振动模式;滚动体振动模式;轴向振动模式。得出三种不同的振动模式下超环面行星蜗杆传动系统的特征值以及各行星蜗轮振型比例关系。     

An abundance of small exoplanets around stars with a wide range of metallicities

The abundance of heavy elements (metallicity) in the photospheres of stars similar to the Sun provides a 'fossil' record of the chemical composition of the initial protoplanetary disk. Metal-rich stars are much more likely to harbour gas giant planets, supporting the model that planets form by accumulation of dust and ice particles. Recent ground-based surveys suggest that this correlation is weakened for Neptunian-sized planets. However, how the relationship between size and metallicity extends into the regime of terrestrial-sized exoplanets is unknown. Here we report spectroscopic metallicities of the host stars of 226 small exoplanet candidates discovered by NASA's Kepler mission, including objects that are comparable in size to the terrestrial planets in the Solar System. We find that planets with radii less than four Earth radii form around host stars with a wide range of metallicities (but on average a metallicity close to that of the Sun), whereas large planets preferentially form around stars with higher metallicities. This observation suggests that terrestrial planets may be widespread in the disk of the Galaxy, with no special requirement of enhanced metallicity for their formation.     

55Cancri系中类海王星行星的轨道稳定性

55 Cancri系是已发现的太阳系外众多行星系中的一个,其中心天体类似太阳,周围有4颗巨行星.最内部的行星e是2004年新发现的,质量与海王星类似,但到中心天体的距离却仅为水星到太阳的1/10.这类行星在外太阳系具有典型性.为了探索该类行星的轨道稳定性问题,本文对行星e进行了106年的轨道演化数值模拟.结果表明:除了与相邻行星的2:11共振点外,行星e半长径在小于0.0388AU范围内的轨道都是稳定的.该结果意味着,在类太阳恒星的周围,类海王星质量的行星可以稳定在近距离的轨道上.     

One or more bound planets per Milky Way star from microlensing observations

Most known extrasolar planets (exoplanets) have been discovered using the radial velocity or transit methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17-30% (refs 4, 5) of solar-like stars host a planet. Gravitational microlensing, on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing. These planets are at least as numerous as the stars in the Milky Way. Here we report a statistical analysis of microlensing data (gathered in 2002-07) that reveals the fraction of bound planets 0.5-10?AU (Sun-Earth distance) from their stars. We find that 17(+6)(-9)% of stars host Jupiter-mass planets (0.3-10?M(J), where M(J) = 318?M(⊕) and M(⊕) is Earth's mass). Cool Neptunes (10-30?M(⊕)) and super-Earths (5-10?M(⊕)) are even more common: their respective abundances per star are 52(+22)(-29)% and 62(+35)(-37)%. We conclude that stars are orbited by planets as a rule, rather than the exception.     

Images of a fourth planet orbiting HR 8799

High-contrast near-infrared imaging of the nearby star HR 8799 has shown three giant planets. Such images were possible because of the wide orbits (>25?astronomical units, where 1?au is the Earth-Sun distance) and youth (<100?Myr) of the imaged planets, which are still hot and bright as they radiate away gravitational energy acquired during their formation. An important area of contention in the exoplanet community is whether outer planets (>10?au) more massive than Jupiter form by way of one-step gravitational instabilities or, rather, through a two-step process involving accretion of a core followed by accumulation of a massive outer envelope composed primarily of hydrogen and helium. Here we report the presence of a fourth planet, interior to and of about the same mass as the other three. The system, with this additional planet, represents a challenge for current planet formation models as none of them can explain the in situ formation of all four planets. With its four young giant planets and known cold/warm debris belts, the HR 8799 planetary system is a unique laboratory in which to study the formation and evolution of giant planets at wide (>10?au) separations.     

The role of chaotic resonances in the Solar System

Our understanding of the Solar System has been revolutionized over the past decade by the finding that the orbits of the planets are inherently chaotic. In extreme cases, chaotic motions can change the relative positions of the planets around stars, and even eject a planet from a system. Moreover, the spin axis of a planet-Earth's spin axis regulates our seasons-may evolve chaotically, with adverse effects on the climates of otherwise biologically interesting planets. Some of the recently discovered extrasolar planetary systems contain multiple planets, and it is likely that some of these are chaotic as well.     

A giant planet orbiting the 'extreme horizontal branch' star V 391 Pegasi

After the initial discoveries fifteen years ago, over 200 extrasolar planets have now been detected. Most of them orbit main-sequence stars similar to our Sun, although a few planets orbiting red giant stars have been recently found. When the hydrogen in their cores runs out, main-sequence stars undergo an expansion into red-giant stars. This expansion can modify the orbits of planets and can easily reach and engulf the inner planets. The same will happen to the planets of our Solar System in about five billion years and the fate of the Earth is matter of debate. Here we report the discovery of a planetary-mass body (Msini = 3.2M(Jupiter)) orbiting the star V 391 Pegasi at a distance of about 1.7 astronomical units (au), with a period of 3.2 years. This star is on the extreme horizontal branch of the Hertzsprung-Russell diagram, burning helium in its core and pulsating. The maximum radius of the red-giant precursor of V 391 Pegasi may have reached 0.7 au, while the orbital distance of the planet during the stellar main-sequence phase is estimated to be about 1 au. This detection of a planet orbiting a post-red-giant star demonstrates that planets with orbital distances of less than 2 au can survive the red-giant expansion of their parent stars.     

A low mass for Mars from Jupiter's early gas-driven migration

Jupiter and Saturn formed in a few million years (ref. 1) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ～100,000 years (ref. 2). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later, and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 au is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 au, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 au; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.     

Origin of the obliquities of the giant planets in mutual interactions in the early Solar System

The origin of the spin-axis orientations (obliquities) of the giant planets is a fundamental issue because if the obliquities resulted from tangential collisions with primordial Earth-sized protoplanets, then they are related to the masses of the largest planetesimals out of which the planets form. A problem with this mechanism, however, is that the orbital planes of regular satellites would probably be uncorrelated with the obliquities, contrary to observations. Alternatively, they could have come from an external twist that affected the orientation of the Solar System plane; but in this model, the outer planets must have formed too rapidly, before the event that produced the twist. Moreover, the model cannot be quantitatively tested. Here I show that the present obliquities of the giant planets were probably achieved when Jupiter and Saturn crossed the 1:2 orbital resonance during a specific migration process: different migration scenarios cannot account for the large observed obliquities. The existence of the regular satellites of the giant planets does not represent a problem in this model because, although they formed soon after the planetary formation, they can follow the slow evolution of the equatorial plane it produces.     

用密度波理论描述旋涡星系的演化方向和太阳系的八大行星模式

基于密度波理论的非线性方程,用定性分析理论可导出旋涡星系的演化方向和由观测可得到的演化条件,根据最新的观测结果,作者定量论证了太阳系的八大行星模式：它是对称的四个类地行星和小行星带;四个类木行星和柯伊伯带(包括冥王星)。