共查询到20条相似文献，搜索用时 31 毫秒
1.
For both asteroids and meteor streams, and also for comets, resonances play a major role for their orbital evolutions but on different time scales. For asteroids both mean motion resonances and secular resonances not only structure the phase space of regular orbits but are mainly at the origin for the inherent chaos of planet crosser objects.For comets and their chaotic routes temporary trapping into orbital resonances is a well known phenomenon. In addition for slow diffusion through the Kuiper belt resonances are the only candidates for originating a slow chaos.Like for asteroids, resonances with Jupiter play a major role for the orbital evolution of meteor streams. Crossing of separatrix like zones appears to be crucial for the formation of arcs and for the dissolution of streams. In particular the orbital inclination of a meteor stream appears to be a critical parameter for arc formation. Numerical results obtained in an other context show that the competition between the Poynting-Robertson drag and the gravitational interaction of grains near the 2/1 resonance might be very important in the long run for the structure of meteor streams. 相似文献
2.
该文基于标准化自协方差函数的组合算法计算了阿仑方差、哈达玛方差的重叠和非重叠形式以及改进阿仑方差、时间方差的等效自由度,并用经验公式计算了总方法估计的等效自由度,在此基础上对各种方差估计不同噪声情况下的等效自由度进行了比较分析,得出如下结论,总方法估计的等效自由度大于相应非总方法估计的等效自由度,重叠和非重叠阿仑方差估计的等效自由度大于相应哈达玛方差估计的等效自由度;对非重叠估计而言,噪声的频率越低,其等效自由度越大;而相应的重叠估计和总方法估计与之相反,噪声的频率越高,其等效自由度越大,这也说明重叠估计和相应的总方法估计对高频噪声等效自由度的提高优于低频噪声. 相似文献
3.
4.
We present the computation of effective refractive coefficients for inhomogeneous two-component grains with three kinds of inclusions with m incl = 3.0 + 4.0i, 2.0 + 1.0i, 2.5 + 0.0001i and a matrix with mm = 1.33 + 0.01i for 11 volume fractions of inclusions from 0 to 50 per cent and wavelengths λ= 0.5 , 1.0, 2.0 and 5.0 μm. The coefficients of extinction for these grains have been computed using a discrete dipole approximation. Computation of the extinction by the same method for grains composed of a matrix material with randomly embedded inclusions has been carried out for different volume fractions of inclusions. A comparison of extinction coefficients obtained for both models of grain material allows one to choose the best mixing rule for a mixture. In cases of inclusions with m incl = 2.0 + 1.0i and 2.5 + 0.0001i the best fit for the whole wavelength range and volume fractions of inclusions from 0 to 50 per cent has been obtained for the Lichtenecker mixing rule. In the case of m incl = 3.0 + 4.0i the fit for the whole wavelength range and volume fractions of inclusions from 0 to 50 per cent is not very significant but the best fit has been obtained for the Hanai rule. For volume fractions of inclusions from 0 to 15 per cent a very good fit has been obtained for the whole wavelength range for Rayleigh and Maxwell-Garnett mixing rules. 相似文献
5.
A. A. Deshpande 《Journal of Astrophysics and Astronomy》1992,13(2):167-173
We discuss here the design details of an inexpensive programmable Sweeping Local Oscillator System (SLOS) built for use in
a ‘swept frequency dedispersion scheme’ for pulsar observations. A useful extension of the basic Divide-and-Add algorithm
for frequency synthesis is developed for this purpose. An SLOS based on this design has been built and used for high time-resolution
observations of pulsars at low radio-frequencies. 相似文献
6.
M. Y&#;ld&#;z 《Monthly notices of the Royal Astronomical Society》2007,374(4):1264-1270
The α Centauri (α Cen) binary system is a well-known stellar system with very accurate observational constraints on the structure of its component stars. In addition to the classical non-seismic constraints, there are also seismic constraints for the interior models of α Cen A and B. These two types of constraint give very different values for the age of the system. While we obtain 8.9 Gyr for the age of the system from the non-seismic constraints, the seismic constraints imply that the age is about 5.6–5.9 Gyr. There may be observational or theoretical reasons for this discrepancy, which can be found by careful consideration of similar stars. The α Cen binary system, with its solar-type components, is also suitable for testing the stellar mass dependence of the mixing-length parameter for convection derived from the binaries of Hyades. The values of the mixing-length parameter for α Cen A and B are 2.10 and 1.90 for the non-seismic constraints. If we prioritize the seismic constraints, we obtain 1.64 and 1.91 for α Cen A and B, respectively. By taking into account these two contrasting cases for stellar mass dependence of the mixing-length parameter, we derive two expressions for its time dependence, which are also compatible with the mass dependence of the mixing-length parameter derived from the Hyades stars. For assessment, these expressions should be tested in other stellar systems and clusters. 相似文献
7.
The European Space Agency's Hipparcos satellite, launched in August 1989, and operated until August 1993, sent back scientific data for about 36 months. The reduction of the first 18 months already shows that the target accuracy for the five astrometric parameters of each star (0.002 arcsec for the position and annual proper motion components, and for the parallax) and for the photometric data (a few milli-magnitudes) will be achieved or even exceeded for most of the 118 000 pre-selected programme stars. These preliminary results will be presented.The contents of the Hipparcos observing programme with respect to red stars and anticipated improvements in the study of red dwarfs will be underlined. 相似文献
8.
Oscillations of magnetic flux tubes are of great importance as they contain information about the geometry and fine structure
of the flux tubes. Here we derive and analytically solve in terms of Kummer’s functions the linear governing equations of
wave propagation for sausage surface and body modes (m=0) of a magnetically twisted compressible flux tube embedded in a compressible uniformly magnetized plasma environment in cylindrical geometry. A general dispersion relation is obtained for such flux
tubes. Numerical solutions for the phase velocity are obtained for a wide range of wavenumbers and for varying magnetic twist.
The effect of magnetic twist on the period of oscillations of sausage surface modes for different values of the wavenumber
and vertical magnetic field strength is calculated for representative photospheric and coronal conditions. These results generalize
and extend previous studies of MHD waves obtained for incompressible or for compressible but nontwisted flux tubes. It is
found that magnetic twist may change the period of sausage surface waves of the order of a few percent when compared to counterparts
in straight nontwisted flux tubes. This information will be most relevant when high-resolution observations are used for diagnostic
exploration of MHD wave guides in analogy to solar-interior studies by means of global eigenoscillations in helioseismology. 相似文献
9.
Andrew J. Barber Peter A. Thomas H. M. P. Couchman C. J. Fluke 《Monthly notices of the Royal Astronomical Society》2000,319(1):267-288
We present the results of weak gravitational lensing statistics in four different cosmological N -body simulations. The data have been generated using an algorithm for the three-dimensional shear, which makes use of a variable softening facility for the N -body particle masses, and enables a physical interpretation for the large-scale structure to be made. Working in three dimensions also allows the correct use of the appropriate angular diameter distances.
Our results are presented on the basis of the filled-beam approximation in view of the variable particle softening scheme in our algorithm. The importance of the smoothness of matter in the Universe for the weak lensing results is discussed in some detail.
The low-density cosmology with a cosmological constant appears to give the broadest distributions for all the statistics computed for sources at high redshifts. In particular, the range in magnification values for this cosmology has implications for the determination of the cosmological parameters from high-redshift type Ia supernovae. The possibility of determining the density parameter from the non-Gaussianity in the probability distribution for the convergence is discussed. 相似文献
Our results are presented on the basis of the filled-beam approximation in view of the variable particle softening scheme in our algorithm. The importance of the smoothness of matter in the Universe for the weak lensing results is discussed in some detail.
The low-density cosmology with a cosmological constant appears to give the broadest distributions for all the statistics computed for sources at high redshifts. In particular, the range in magnification values for this cosmology has implications for the determination of the cosmological parameters from high-redshift type Ia supernovae. The possibility of determining the density parameter from the non-Gaussianity in the probability distribution for the convergence is discussed. 相似文献
10.
We have computed cross-sections and rate coefficients for rovibrational transitions in HD, induced by collisions with atomic and molecular hydrogen. We employed fully quantum-mechanical methods and the potential of Boothroyd et al. for H–HD, and that of Schwenke for H2 –HD. The rate coefficients for vibrational relaxation v =1→0 of HD are compared with the corresponding values for H2 . The influence of vibrationally excited channels on the rate coefficients for rotational transitions within the v =0 vibrational ground state of HD is shown to be small at T =500 K, where T is the kinetic temperature. The rate coefficients, for 100 T 2000 K, are available from http://ccp7.dur.ac.uk/. 相似文献
11.
Andrej G. Sokolsky Akmal A. Vakhidov Nickolay N. Vasiliev 《Celestial Mechanics and Dynamical Astronomy》1995,63(3-4):357-374
The problem of computation of elliptic Hansen coefficients and their derivatives is considered for constructing a motion theory of an artificial Earth satellite with large eccentricity. An algorithm for analytical and numerical computation of these coefficients and their derivatives is described. The recurrence relations for derivatives of the first and second order and initial values for recurrences are obtained. As an example, numerical values of some elliptic Hansen coefficients are given for the orbit with eccentricityk=0.74. 相似文献
12.
Patricia A. Whitelock Michael W. Feast Floor van Leeuwen 《Monthly notices of the Royal Astronomical Society》2008,386(1):313-323
Published data for large-amplitude asymptotic giant branch variables in the Large Magellanic Cloud (LMC) are re-analysed to establish the constants for an infrared ( K ) period–luminosity relation of the form M K =ρ[log P − 2.38]+δ . A slope of ρ=−3.51 ± 0.20 and a zero-point of δ=−7.15 ± 0.06 are found for oxygen-rich Miras (if a distance modulus of 18.39 ± 0.05 is used for the LMC). Assuming this slope is applicable to Galactic Miras we discuss the zero-point for these stars using the revised Hipparcos parallaxes together with published very long baseline interferometry (VLBI) parallaxes for OH masers and Miras in globular clusters. These result in a mean zero-point of δ=−7.25 ± 0.07 for O-rich Galactic Miras. The zero-point for Miras in the Galactic bulge is not significantly different from this value.
Carbon-rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period–luminosity relation zero-points for systems with different metallicity. 相似文献
Carbon-rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period–luminosity relation zero-points for systems with different metallicity. 相似文献
13.
《Astroparticle Physics》2003,18(6):615-627
The MACRO underground detector at Gran Sasso Laboratory recorded 60 million secondary cosmic ray muons from February 1989 until December 2000. Different techniques were used to analyze this sample in search for density excesses from astrophysical point-like sources. No evidence for DC excesses for any source in an all-sky survey is reported. In addition, searches for muon excess correlated with the known binary periods of Cygnus X-3 and Hercules X-1, and searches for statistically significant bursting episodes from known γ-ray sources are also proved negative. 相似文献
14.
Dave Waltham 《Icarus》2011,215(2):518-521
The Earth may have untypical characteristics which were necessary preconditions for the emergence of life and, ultimately, intelligent observers. This paper presents a rigorous procedure for quantifying such “anthropic selection” effects by comparing Earth’s properties to those of exoplanets. The hypothesis that there is anthropic selection for stellar mass (i.e. planets orbiting stars with masses within a particular range are more favourable for the emergence of observers) is then tested. The results rule out the expected strong selection for low mass stars which would result, all else being equal, if the typical timescale for the emergence of intelligent observers is very long. This indicates that the habitable zone of small stars may be less hospitable for intelligent life than the habitable zone of solar-mass stars. Additional planetary properties can also be analyzed, using the approach introduced here, once relatively complete and unbiased statistics are made available by current and planned exoplanet characterization projects. 相似文献
15.
L. Kahl Kristensen 《Astronomische Nachrichten》2001,322(1):47-56
The distribution of observations giving the smallest errors for specific purposes is found in the general case by a numerical procedure. Examples consider the minimum error ellipse for the recovery of a distant asteroid, a main belt object and the position in the target plane for a close approach to the Earth. The observations should be performed only on a few critical nights and inefficient times avoided. The methods are of interest for the optimum strategies for surveys and the determination of initial orbits. 相似文献
16.
The Heitler–Matthews model for hadronic air showers will be extended to all the generations of electromagnetic subshowers in the hadronic cascade. The analysis is outlined in detail for showers initiated by primary protons. For showers initiated by iron primaries the part of the analysis is given for as far as it differs from the analysis for a primary proton. Predictions for shower sizes and the depth of maximum shower size are compared with results of Monte Carlo simulations. The depth of maximum as it follows from the extrapolation of the Heitler–Matthews model restricted to the first generation of electromagnetic subshowers is too small with respect to Monte Carlo predictions. It is shown that the inclusion of all the generations of electromagnetic subshowers leads to smaller predictions for the depth of maximum and to smaller predictions for the elongation rate. The discrepancy between discrete model predictions and Monte Carlo predictions for the depth of maximum can therefore not be explained from the number of generations that is taken into consideration. An alternative explanation will be proposed. 相似文献
17.
In preparation for the Rosetta mission, the location and widths of gravitational resonances surrounding a regularly shaped and possibly complex rotating body are mapped following the second fundamental model of resonance. It is found that for uniaxial rotation of the central body, the surrounding resonances are widest for prograde orbits. If the figure axis is tilted with respect to the spin axis of the central body, an additional number of wide resonances appear with a preference for prograde and inclined orbits, and the occurrence of initial conditions which lie in the globally connected chaotic web is significantly increased. For larger rotational excitations, it is seen how these new additional resonances overlap internally at low eccentricity for very large semi-major axes. However, with exceptions for some excited short-axis rotational modes of the central body, it is argued that most resonances vanish for retrograde orbits lying in the plane normal to the body spin, and that resonant or non-resonant stability therefore can be expected for a wide range of mean orbit eccentricities. 相似文献
18.
J. E. Howard 《Celestial Mechanics and Dynamical Astronomy》2005,92(1-3):219-241
We reexamine the classical virial theorem for bounded orbits of arbitrary autonomous Hamiltonian systems possessing both regular and chaotic orbits. New and useful forms of the virial theorem are obtained for natural Hamiltonian flows of arbitrary dimension. A discrete virial theorem is derived for invariant circles and periodic orbits of natural symplectic maps. A weak and a strong form of the virial theorem are proven for both flows and maps. While the Birkhoff Ergodic Theorem guarantees the existence of the relevant time averages for both regular and chaotic orbits, the convergence is very rapid for the former and extremely slow for the latter. This circumstance leads to a simple and efficient measure of chaoticity. The results are applied to several problems of current physical interest, including the Hénon–Heiles system, weak chaos in the standard map, and a 4D Froeschlé map. 相似文献
19.
Poincaré maps for Hamiltonian systems with 3 degrees of freedom lead to the study of four dimensional symplectic mappings. As a test for the validity of a synthetic mapping of order 3 using gradient informations, we study the evolution with time of Liapounov Indicators in the case of the four dimensional standard map with chaotic and stable zones. Both Liapounov Indicators show the same behaviour for the real and synthetic mappings. They reveal exploding diffusion phenomena for temporarily confined chaotic orbits. The distribution of the time of explosion fits well with a Poisson law for the real mapping, but not for the synthetic one. However the mean time of explosion is essentially the same in both cases. 相似文献
20.
Relativistic shocks provide an efficient method for high-energy particle acceleration in many astrophysical sources. Multiple
shock systems are even more effective and of importance, for example, in the internal shock model of gamma-ray bursts. We
investigate the reacceleration of pre-existing energetic particles at such relativistic internal shocks by the first order
Fermi process of pitch angle scattering. We use a well established eigenfunction method to calculate the resulting spectra
for infinitely thin shocks. Implications for GRBs and relativistic jets are discussed.
Paul Dempsey would like to thank IRCSET for their financial support. 相似文献