共查询到20条相似文献，搜索用时 31 毫秒
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F. Combes 《Astrophysics and Space Science》1999,265(1-4):417-424
Mass accretion is the key factor for evolution of galaxies. It can occur through secular evolution, when gas in the outer
parts is driven inwards by dynamical instabilities, such as spirals or bars. This secular evolution proceeds very slowly when
spontaneous, and can be accelerated when triggered by companions. Accretion can also occur directly through merging of small
companions, or more violent interaction and coalescence. We discuss the relative importance of both processes, their time-scale
and frequency along a Hubble time. Signatures of both processes can be found in the Milky Way. It is however likely that our
Galaxy had already gathered the bulk of its mass about 8–10 Gyr ago, as is expected in hierarchical galaxy formation scenarios.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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在参考文献（２,３）的基础上,提出了含有大山区的复杂陆地地波路径的分段和确定各段路径等效电导率σｃ的方法,给出如何利用接收点场强实测值和已有的理论色散修正曲线（δｔ－ｄ）＾（６）近似计算色散修正值的方法,并对４条含大山区的复杂路径信号传播时延实测值ｔｇ＾－（收）与预测值ｔｇ进行比较,结果表明：路径分为３段等效时的预测准确度要比整段等效高得多。 相似文献
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M.M. Katsova 《Astrophysics and Space Science》1997,252(1-2):427-438
Results of observations of Capella in the infrared He I 10830 line obtained with the Shajn 2.6 m telescope of the Crimean Astrophysical Observatory over 10 years are analyzed. The He I equivalent width varies with the 104-day orbital period, and the amplitude of these variations in September 1993-March 1994 was significantly lower than during our observations in 1985-1988. A comparison of these observations with those obtained by us since 1985 suggests the presence of an activity cycle with a period of at least 5 years. The main He I absorption forms in the chromosphere of the quiescent G6 giant, but near the maximum of the activity cycle, helium absorption forming on the more active, hotter F9 giant becomes observable. These helium data, together with the EUV results for the high-temperature Fe XX-Fe XXIII lines can be explained using a model in which an enhanced magnetized stellar wind originating from the hot corona on the active F9 giant reaches the Lagrangian point and then forms a shock wave in the corona of the quiescent G6 giant. 相似文献
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Edward W. Ng 《Celestial Mechanics and Dynamical Astronomy》1979,20(3):243-249
An efficient algorithm is presented for the solution of Kepler's equationf(E)=E–M–e sinE=0, wheree is the eccentricity,M the mean anomaly andE the eccentric anomaly. This algorithm is based on simple initial approximations that are cubics inM, and an iterative scheme that is a slight generalization of the Newton-Raphson method. Extensive testing of this algorithm has been performed on the UNIVAC 1108 computer. Solutions for 20 000 pairs of values ofe andM show that for single precision (10–8) 42.0% of the cases require one iteration, 57.8% two and 0.2% three. For double precision (10–18) one additional iteration is required. Single- and double-precision FORTRAN subroutines are available from the author. 相似文献
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Literal characterizations are developed for the eigenvalues and eigenvectors of a system of linear time-invariant equations which describes the attitude motion of flexible spacecraft in terms of hybrid coordinates. The eigenproblem is shown to reduce to that of a symmetric and positive definite matrix of lower dimension. For the zero damping case, both analytical and minimax characterization methods prove to be useful in localizing the eigenvalues, and eigenvectors for systems of large dimension are obtained explicitly in terms of a 3×1 matrix whose elements are available from a system of three algebraic equations provided. 相似文献
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A. I. Nazarenko 《Solar System Research》2013,47(7):564-568
The article covers the outcome of the method for optimal filtering of measurements developed by the author and aimed at the determination of the time and place of reentry of the Phobos-Grunt spacecraft. So-called two-line elements (TLE) of the orbit of the American Space Surveillance System are used as measurements. 相似文献
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A. A. Akopian 《Astrophysics》1996,39(4):331-333
A method is suggested for the determination of the distribution function for the mean frequency of supernova explosions, which is similar to the Ambartsumian method for the determination of the distribution function for the mean flare frequency of flare stars. This method has not been applied because of the lack of the required data.Translated fromAstrofizika, Vol. 39, No. 4, pp. 561–566, November, 1996. 相似文献
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In this paper we present Runge-Kutta-Nyström (RKN) pairs of orders 4(3) and 6(4). We choose a test orbit from the Kepler problem to integrate for a specific tolerance. Then we train the free parameters of the above RKN4(3) and RKN6(4) families to perform optimally. For that we form a neural network approach and minimize its objective function using a differential evolution optimization technique. Finally we observe that the produced pairs outperform standard pairs from the literature for Pleiades orbits and Kepler problem over a wide range of eccentricities and tolerances. 相似文献
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harold C. Urey 《Astrophysics and Space Science》1972,16(2):311-323
The problem of the accumulation of the Moon is discussed on the assumption that the Moon is a captured object. If it is such, it is highly improbable that it is the only object of this kind present in the early solar system. Evidence indicating that other massive objects were present at that time is presented. Also, it is pointed out that the interior of the Moon must contain normal solar proportions of the elements of intermediate volatility in the lunar interior, if the Moon accumulated in a gas sphere. 相似文献
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Giles A. Graham Nick E. Teslich Anton T. Kearsley Frank J. Stadermann Rhonda M. Stroud Zurong Dai Hope A. Ishii Ian D. Hutcheon Saša Bajt Christopher J. Snead Peter K. Weber John P. Bradley 《Meteoritics & planetary science》2008,43(3):561-569
Abstract— Sample preparation is always a critical step in the study of micrometer‐sized astromaterials available for study in the laboratory, whether their subsequent analysis is by electron microscopy or secondary ion mass spectrometry. A focused beam of gallium ions has been used to prepare electron transparent sections from an interplanetary dust particle (IDP), as part of an integrated analysis protocol to maximize the mineralogical, elemental, isotopic, and spectroscopic information extracted from one individual particle. In addition, focused ion beam (FIB) techniques have been employed to extract cometary residue preserved on the rims and walls of microcraters in 1100 series aluminum foils that were wrapped around the sample tray assembly on the Stardust cometary sample collector. Non‐ideal surface geometries and inconveniently located regions of interest required creative solutions. These include support pillar construction and relocation of a significant portion of sample to access a region of interest. Serial sectioning, in a manner similar to ultramicrotomy, is a significant development and further demonstrates the unique capabilities of focused ion beam microscopy for sample preparation of astromaterials. 相似文献
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In the algebraA of functions periodic in the mean anomaly we relate the problem of integrating over the mean anomaly with that of decomposing an element ofA as the direct sum of two functions, one in the kernel of the Lie derivative in the Keplerian flow and one in the image of this Lie derivative. We propose recursive rules amenable to general purpose symbolic processors for accomplishing such decomposition in a wide subclass ofA. We introduce the dilogarithmic function to express in exact terms quadratures involving the equation of the center. 相似文献
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N. R. Ikhsanov 《Astronomy Letters》2005,31(9):586-590
The state of a subsonic propeller in the evolutionary tracks of magnetized compact stars is intermediate between the states of a supersonic propeller and an accretor. We show that neutron stars in this state would manifest themselves as accretion-powered pulsars of low (or moderate) luminosity. The criteria that allow subsonic propellers to be distinguished from accretors include a soft X-ray spectrum, a limited range of admissible spin periods, and a rapid spindown. 相似文献
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Vicent J. Martínez María-Jesús Pons-Bordería Rana A. Moyeed & Matthew J. Graham 《Monthly notices of the Royal Astronomical Society》1998,298(4):1212-1222
We introduce a statistical quantity, known as the K function, related to the integral of the two-point correlation function. It gives us straightforward information about the scale where clustering dominates and the scale at which homogeneity is reached. We evaluate the correlation dimension, D 2 , as the local slope of the log–log plot of the K function. We apply this statistic to several stochastic point fields, to three numerical simulations describing the distribution of clusters and finally to real galaxy redshift surveys. Four different galaxy catalogues have been analysed using this technique: the Center for Astrophysics I, the Perseus–Pisces redshift surveys (these two lying in our local neighbourhood), the Stromlo–APM and the 1.2-Jy IRAS redshift surveys (these two encompassing a larger volume). In all cases, this cumulant quantity shows the fingerprint of the transition to homogeneity. The reliability of the estimates is clearly demonstrated by the results from controllable point sets, such as the segment Cox processes. In the cluster distribution models, as well as in the real galaxy catalogues, we never see long plateaus when plotting D 2 as a function of the scale, leaving no hope for unbounded fractal distributions. 相似文献