Asteroid identification over apparitions     

Mikael Granvik  Karri Muinonen 《Icarus》2008,198(1):130-137

We present a new method for the linking of scarce asteroid astrometry over apparitions, and apply it both to simulated and real data to prove its feasibility. Up to date, there has not been a robust method available to search for linkages between the approximately 50,000 provisionally designated sets of asteroid astrometry spanning less than two days. Unless such a scarce set of astrometry is linked to another set of astrometry, the underlying object can be considered lost as the ephemeris uncertainties are substantial. The new method, which can tackle the challenges, is based on Ranging, which is a fully nonlinear, statistical orbital inversion method. Ranging properly treats astrometric uncertainties and propagates the uncertainty to the resulting orbital-element probability density, which is sampled by a set of orbits. The new orbital-element-space multiple-address-comparison (oMAC) method uses dimensionality-reduction techniques and tree structures to efficiently search for overlapping probability densities in the orbital-element phase space. Overlapping probability densities indicate a candidate linkage between astrometric observation sets. To accept a candidate linkage, we have to find a many-body orbital solution which reproduces the observed positions within the observational uncertainties. To find the linking orbit, we use a multi-step approach starting from a Monte-Carlo generation of possible orbits in a reduced volume of the orbital-element phase space and ending with a least-squares orbital solution, which, in addition to the Sun's gravitation, also takes into account the gravitational influence of the relevant planets. The new multiple-address-comparison method has a loglinear computational complexity, that is, it scales as O(nlogn), where n is the number of included observation sets. It has recently also been implemented for the ephemeris-space multiple-address-comparison (eMAC) method, which is optimized for the short-term linking of scarce astrometry.  相似文献   

A new outcome of binary–binary scattering     

Douglas C. Heggie 《Monthly notices of the Royal Astronomical Society》2000,318(4):L61-L63

A recently discovered solution of the three-body problem represents a new possible byproduct of binary–binary scattering. This is demonstrated by an explicit example. A way of detecting the new orbit automatically is discussed, and we give estimates related to the probability of its formation.  相似文献   

Fluctuations in isothermal spheres     

Joseph Katz  Isao Okamoto 《Monthly notices of the Royal Astronomical Society》2000,317(1):163-169

Isolated isothermal spheres of N gravitationally interacting points with equal mass are believed to be stable when density contrasts do not exceed 709. That stability limit does not, however, take into consideration fluctuations of temperature near the onset of instability. These are important when N is finite.
Here we correlate global mean quadratic temperature fluctuations with the onset of instability. We show that such fluctuations trigger instability when the density contrast reaches a value near 709×exp(−3.3 N ^−1/3). These lower values of limiting density contrasts are significantly smaller than 709 when N is not very large, and this suggests (i) that numerical calculations with small N may not reflect correctly the onset of core collapse in clusters with large N , and (ii) that a greater number of globular clusters than is normally believed may already be in an advanced stage of core collapse, because most of the observed globular clusters with parameters that fit quasi-isothermal configurations are close to marginal stability.  相似文献   

Analytical solutions for the Newtonian gravitational field induced by matter within axisymmetric boundaries     

John T. Conway 《Monthly notices of the Royal Astronomical Society》2000,316(3):540-554

An analytical method originally applied to the problem of the actuator disc in fluid mechanics has been applied to the closely analogous problem of constructing the classical Newtonian potential and attractions. The method can treat axisymmetric problems and also non-axisymmetric cases where matter is confined within axisymmetric boundaries. The potential and attractions for the generalized thin finite disc can be given in closed form in terms of elliptic integrals and elementary functions. For the general case of matter within an axisymmetric boundary, the potentials and attractions can be evaluated as one-dimensional integrals of albeit complex analytical expressions. These expressions represent the fields induced by matter in an extended region as a distribution of gravitating discs. For certain special cases, such as matter bounded by a circular cylinder and also for matter distributed in a spherical region, closed-form solutions can be given that appear to be new. Some non-axisymmetric results are also given for the thin disc of infinite radial extent.  相似文献   

Studies of multiple stellar systems – III. Modulation of orbital elements in the triple-lined system HD 109648     

Saurabh Jha  Guillermo Torres  Robert P. Stefanik  David W. Latham  Tsevi Mazeh 《Monthly notices of the Royal Astronomical Society》2000,317(2):375-384

The triple-lined spectroscopic triple system HD 109648 has one of the shortest periods known for the outer orbit in a late-type triple, 120.5 d, and the ratio between the periods of the outer and the inner orbits is small, 22:1. With such extreme values, this system should show orbital element variations over a time-scale of about a decade. We have monitored the radial velocities of HD 109648 with the CfA Digital Speedometers for 8 yr, and have found evidence for modulation of some orbital elements. While we see no definite evidence for modulation of the inner binary eccentricity, we clearly observe variations in the inner and the outer longitudes of periastron, and in the radial velocity amplitudes of the three components. The observational results, combined with numerical simulations, allow us to put constraints on the orientation of the orbits.  相似文献   

The time-scale of escape from star clusters     

T. Fukushige  D. C. Heggie 《Monthly notices of the Royal Astronomical Society》2000,318(3):753-761

In this paper a cluster is modelled as a smooth potential (due to the cluster stars) plus the steady tidal field of the Galaxy. In this model there is a minimum energy below which stars cannot escape. Above this energy, however, the time-scale on which a star escapes varies with the orbital parameters of the star (mainly its energy) in a way which we attempt to quantify, with both theoretical arguments and computer simulations. Within the limitations of the model we show that the time-scale is long enough to complicate the interpretation of full N -body simulations of clusters, and that stars above the escape energy may remain bound to the cluster for about a Hubble time.  相似文献   

刚性承压板下深部岩体压缩蠕变参数反演   总被引：4，自引：1，他引：3  

杨文东  张强勇  张建国  曾纪全  贺如平 《岩土力学》2009,30(3):762-768

为获得岩体深部的压缩蠕变力学参数,进行了现场刚性承压板中心孔变形试验。推导出了圆形刚性承压板在均布荷载作用下中心孔深部岩体广义Kelvin模型的瞬时弹性变形。基于黏弹性理论,经过拉普拉斯变换和逆变换,进一步得到了黏弹性变形的计算公式。采用了以瞬时弹性模量、黏弹性模量和黏弹性系数为反演值的迭代优化反演法,并将计算公式程序化。该方法应用于一大型水电站坝区边坡工程,有效获得了坝区软弱岩体深部的压缩蠕变力学参数,克服了以往仅以岩体表面位移进行力学参数反演的缺陷。  相似文献   

滚石冲击碰撞恢复系数研究   总被引：21，自引：3，他引：18  

何思明  吴永  李新坡 《岩土力学》2009,30(3):623-627

在研究滚石的运动规律过程中,滚石的碰撞恢复系数是重要的控制参数,它不仅与滚石的冲击速度、质量有关,还与滚石、坡面覆盖层的物理力学性质密切相关,目前尚无合理的计算理论,这是滚石灾害研究的一个难点问题。根据Hertz接触理论、Cattaneo & Mindlin切向接触理论,在考虑材料弹塑性特性的基础上研究了滚石法向碰撞恢复系数和切向碰撞恢复系数的计算模式,给出了具体的计算公式,并阐明了影响滚石碰撞恢复系数的主要控制因素。通过一算例的计算分析并与已有结论的比较,表明该计算模式的合理性。  相似文献   

孔隙气压对核废物地质处置热-流-固耦合影响的有限元分析     

张玉军 《岩土力学》2009,30(5):1451-1457

为考察孔隙气体压力对高放废物地质处置中的热-流-固耦合过程的影响,借用Leiws等建立的可变形孔隙介质中非等温空气流和水流模型,在其水连续性方程中加入了温度梯度引起的水分扩散项,研制出相应的热-水-气-应力耦合弹塑性二维有限元程序。针对一个假定的高放废物地质处置库模型,在相同的初始温度、孔隙水压力和岩体应力条件下,取3种缓冲层中的初始孔隙气体压力,通过数值模拟考察了处置库近场的主应力、饱和度、气和水的流速、温度和孔隙压力的分布与变化。结果显示,当缓冲层中的初始孔隙气压力较高时,其对围岩中应力影响较大。  相似文献   

基于强度折减原理的地下洞室群整体安全系数计算方法探讨   总被引：18，自引：2，他引：16  

江权  冯夏庭  向天兵 《岩土力学》2009,30(8):2483-2488

为解决大型地下洞室群的整体稳定性评价这一难题,吸收边坡强度折减有限元的优点,提出了基于强度折减技术的地下洞室群整体安全系数求解方法。该方法采用Mohr-Coulomb屈服准则,以洞室之间的等效塑性应变贯通作为洞室群整体失稳的临界判据。洞群整体安全系数计算是通过搜索折减系数F,将岩土材料的力学强度参数（黏聚力c和内摩擦角?）按F值折减代入数值计算中,使计算模型的洞室间正好出现明显的贯通性等效塑性应变来实现。拉西瓦水电站地下发电枢纽洞室群和锦屏二级水电站A、B辅助洞两个典型工程实例的整体安全系数计算表明,该方法可以便捷、客观地获得洞室群的整体安全系数,可为大型地下多洞室的稳定性评价和开挖与支护设计提供定量指标和科学依据。  相似文献