共查询到20条相似文献，搜索用时 62 毫秒
1.
Stanislav D. Furta 《Celestial Mechanics and Dynamical Astronomy》1992,53(3):255-266
The author considers a problem of Lyapunov's stability of relative equilibria of a flexible nonstretchable thread attached to the satellite moving in a circular Keplerian orbit in the first approximation. When it is in the position of relative equilibrium, the thread is known to be situated either along the radius vector of the orbit (the radial equilibrium) or along the circular orbit (the tangential equilibrium) and in each case the thread can be in a folded state. The author shows that folded radial equilibria of the thread are always unstable while tangential ones are unstable if the thread is sufficiently short in comparison with the radius of the orbit. The generalized Chetaev functional has been constructed to prove the instability. 相似文献
2.
Fred L. Whipple 《Earth, Moon, and Planets》1995,71(3):69-78
This study is based primarily on the calculations of comet orbits over ~ 106 years for 160 short-period comets by Harold F. Levison and Martin J. Duncan from which there are calculated ablation AGES. There are positive statistical correlations (having many deviations) with radial nongravitational forces, comet activity measures, and dust-to-gas ratios in the spectra, in the sense that comets of greater AGES tend to be less active and to show less dust in their spectra than comets of lesser AGES.Harvard-Smithsonian Center for Astrophysics 相似文献
3.
V. L. Oknyanskij 《Astrophysics and Space Science》1994,222(1-2):157-169
We have used long light curves in the X-ray (ARIEL V, EXOSAT, Ginga and others), UV (IUE), and optical (data of intensive ground-based monitoring) ranges of NGC 4151 to investigate connections between variations in these spectral fields. Applying Fourier (CLEAN algorithm) and modern cross-correlation analysis, we revealed the different character of the variability and the cross-correlation between high and low states of the nucleus. In contrast to earlier results, we found that X-ray and optical fluxes correlate in the active state without any apparent delay.In the power spectrum of the optical and X-ray light curves in the high state we found the existence of the same period of about 65 days, which disappeared in the low state.We confirmed previous data that X-ray variations correlate more strongly with UV and optical continuum variability during the low state, but it is possible that the optical variations appear to lag behind those involving X-rays by several tens of days.Strong variations in H line profile and [Fe X]6374 line intensity variations correlate with the observed X-ray covering dip in 1990 (Yaqoobet al., 1993).We discuss briefly the implications of these results. 相似文献
4.
This article reviews observational data on cataclysmic variables (CVs) whose orbital periods fall within the so-called period gap between 2 and 3 hours. The orbital period distribution of cataclysmic variables and the filling of the period gap by various types of CVs are demonstrated. Roughly half the stars in the period gap are magnetic, of which the majority are polars, while the other half (except a few stars) are SU UMa-type stars characterized by superbursts and superhumps. It is also interesting to note that those intermediate polars whose periods fall within the gap are among the stars with constant superhumps. The height above the galactic plane is estimated for the nonmagnetic stars and it is shown that, contrary to predictions, the stars in the period gap do not belong (except for one) to the spherical component of the galaxy. Light curves, some characteristic times, burst amplitudes, etc. are given for a number of Su UMa-type stars observed by the authors. The evolution of superbursts from the stars NY Ser, V725 Aql, and Var73 Dra is described. Observational data are presented on the classical nova V Per, a star whose period lies right in the middle of the period gap but whose status (magnetic or nonmagnetic) has still unclear. 相似文献
5.
Through the use of Jacobi's formulation of the least action principle, differential equations for Szebehely's problem extended to a holonomic system with n degrees of freedom are obtained.
Sunto Si riottengono le equazioni differenziali relative al problema di Szebehely esteso ad un sistema olonomo ad n gradi di libertà utilizzando il principio di minima azione nella formulazione di Jacobi.相似文献
6.
In this paper the star characteristics (luminosities, spectra, masses, absolute and relative radii of the components, mass ratio, major semiaxis, and orbital inclination) of close binary systems (CBS) with detached components are compared element-by-element. Svechnikov and Perevozkina's Catalog of the orbital elements, masses, and luminosities of pre-main sequence-type eclipsing variable stars with known photometric and spectroscopic elements and Svechnikov and Kuznetsova's Catalog of approximate photometric and absolute elements of eclipsing variable stars are used as initial data. The orbital elements contained in the first catalog are determined from solutions of the known photometric light curves and the known radial velocity curves, while the orbital elements for the second catalog were estimated using data from the General Catalog of Variable Stars (GCVS) IV employing approximate statistical relationships such as mass-radius, mass-luminosity, and mass-spectrum, along with the dependence of the orbital inclination on the depth of the principal minimum and others. Possible reasons for discrepancies among the compared quantities are discussed. 相似文献
7.
Jacques M. Beckers 《Experimental Astronomy》1991,2(1):57-71
Maximum sensitivity for optical interferometers is achieved only when the optical pathlengths between the different arms can be equalized without using interference fringes on the research object itself. This is called blind operation of the interferometer. In this paper I examine different options to achieve this, focussing on the application to the Very Large Telescope Interferometer (VLTI). It is proposed that blind operation should be done using a so-called coherence autoguider, working on an unresolved star of magnitude V=11 to 13 within the isoplanatic patch for coherencing, which has a diameter of about 1 degree. Estimates of limiting magnitudes for the VLTI are also derived. 相似文献
8.
M. G. Abrahamyan 《Astrophysics》2002,45(2):205-214
The stability of Riemann S ellipsoids inside an oblate halo with respect to the second form of oscillations is investigated. It is shown that some ellipsoids with reverse internal circulation of matter, which are stable inside a spherical halo or in its absence, become unstable with respect to second odd forms of oscillation inside an oblate halo. Here there is asymmetry between conjugate ellipsoids from the standpoint of their stability. Only those conjugate ellipsoids that correspond to higher frequencies of reverse circulation of matter than their corresponding angular rotation rates are unstable. The domains of instability of light and heavy conjugate embedded ellipsoids are obtained as a function of the oblateness measure and relative density of the halo. 相似文献
9.
In this paper we compute differentially rotating polytropic models distorted by toroidal magnetic field. In particular, we study rotating sequences, which do not terminate with a critical rotation. In the computations we use the so-called complex-plane strategy and multiple partition technique, which are numerical methods developed recently by the first author. 相似文献
10.
We study the so-called inverse planetary problem (i.e., given the distances from the centre, masses, and radii of, say, three planets of a planetary system, find the optimum polytropic index, mass, and radius of their star, and also other quantities of interest, which depend either explicitly or implicitly on the foregoing ones, e.g., central and mean density, central and mean pressure, central and mean temperature, etc.) for the system of satellites of Jupiter. In particular, Jupiter is considered as star and its satellites as planets of a proper planetary system, which is then treated numerically on the basis of the so-called global polytropic model, developed recently by the first author. 相似文献
11.
Data on solar emission variations in the extreme ultraviolet range <1300 (EUV-range) performed on board the Prognoz satellites and the Phobos spacecraft by the thermoluminescent method are presented. Flux variations from the 11-years cycle are factors of 2–2.5, and that by the 27-days cycle do not exceed 30%. 相似文献
12.
The results of the spectroscopic observation of 23 galaxies with a UV excess from Kazarian's lists are presented. The spectra were obtained on the 2.6-m telescope of Byurakan Observatory with a Byu FOSC-2 camera. Two grisms, red and green, were used in the observations. Emission lines are observed in the spectra of 21 of the galaxies. 相似文献
13.
Ioan Todoran 《Astrophysics and Space Science》1993,201(2):281-285
The effects of the radiation pressure in the restricted three-body problem are considered and the existence of the out-of-plane equilibrium points is analyzed. It is found that within the framework of the stellar stability, the five Lagrangian points are the only equilibrium points, at least as far as the force of the radiation pressure is taken into account. 相似文献
14.
W. J. Maciel 《Astrophysics and Space Science》1995,229(2):203-213
Measurements of the equivalent width of the interstellar Lyman line from IUE spectra in the direction of the planetary nebulae NGC 7009 and BD+30°3639 are used to infer the H column density in these directions. Hydrogen 21 cm profiles are also used in connection with the ultraviolet data so that the expected rotation velocities and distances can be determined. The results are compared with recently published distances, in an attempt to distinguish between the short and long PN distance scales as applied to these objects. 相似文献
15.
Joseph L. Hora Lynne K. Deutsch William F. Hoffmann Giovanni G. Fazio 《Astrophysics and Space Science》1995,224(1-2):361-364
We have obtained near diffraction-limited images of three bipolar PPN at UKIRT in October, 1993: AFGL 915 (the Red Rectangle), AFGL 618, and AFGL 2688 (the Egg Nebula). Images were taken at unidentified infrared (UIR) emission feature wavelengths and at several continuum wavelengths in the 10 and 20µm atmospheric windows. In all three PPN the emission is dominated by a central point source with fainter emission extending for several arcsec. In AFGL 2688, the mid-IR emission is extended in the same direction as the main optical lobes. In AFGL 915, the UIR feature emission is spatially separated from the central source. The spikes that have been observed at 2µm and give the nebula its rectangular appearance are also visible at 10µm. 相似文献
16.
As a check on the differentiation processes of planetesimals the experiment Tsarev was carried out. The main problems of the experiment were: investigation of melting and liquation processes in the melt of primitive meteorite substance; analysis of the composition of metallic and silicate phases, including the distribution of rare-earth elements and comparison of the obtained phase with iron and differentiated meteorite composition. The heating of a sample of the L-chondrite Tsarev (volume 15 cm3) was produced by intensive microwave radiation because this method of heating provides the possibility of uncontact entry of energy into a sufficiently large sample. Upon the heating of the meteorite Tsarev (with the maximum temperature 1500±50K) this sample was melted and two phases appeared and were separated in the gravity field: the composition of Fe-Ni-S phase includes 15 rare-earth elements and the silicate phase is composed of the main oxides. There is also clear evidence for the presence of liquation processes of second order, e.g. separation of Fe-Ni from Fe-S component. 相似文献
17.
The application of Fourier analysis to study leaky waves has the advantage of simplicity, but it is not clear why the complex roots of the dispersion relations represent leaky waves, nor how the leakage occurs. We investigate the different kinds of leakage that can occur in a three layer plasma, and to which Fourier pseudomodes they are associated. We find four basic mechanisms, called surface mode ..., single interface ..., trapped wave ... and lost insulation leakage. These mechanisms appear in pure form only near certain limiting cases, in which the parameters of the problem take some special values. As soon as the parameters depart significantly from the limiting values, the behavior of the leaky wave complicates and mixtures of the mechanisms occur, in varying amounts. In consequence different points of the same complex branch of the spectrum may correspond to different mechanisms. All the complex branches of the spectrum correspond to leaky waves, but in general it is not possible to classify them according to type of leakage, except close to a limiting case. Since in a three layer configuration there are many of these, the spectrum of leaky waves is very complicated. 相似文献
18.
A. H. Jupp 《Celestial Mechanics and Dynamical Astronomy》1987,43(1-4):127-138
The critical inclination problem in artificial satellite theory, first discovered 30 years ago, has aroused great interest and provoked much discussion and controversy in the intervening years. It was this problem which essentially provided the seed corn for the development of the theory of the Ideal Resonance Problem (IRP). The latter theory provides good first-approximation solutions to a number of important resonance problems in celestial mechanics. It is not applicable, however, to certain other interesting resonant systems within the solar system. For these resonances a new fundamental mathematical model of resonance, in the spirit of the IRP, has recently been formulated and successfully applied.This paper reviews the history of the critical inclination problem and highlights the controversies it has generated over the years. The Problem's strong connection with the IRP is outlined with both thenormal andabnormal forms featuring. Finally, with reference to the critical inclination problem, the essential properties of the newer fundamental model are described and compared with the IRP. A strong correspondence is established between recent independent investigations of a variety of resonance problems and earlier work of Andoyer. 相似文献
19.
S. V. M. Clube 《Earth, Moon, and Planets》1995,71(3):433-440
The relationship between punctuated equilibrium and impact crises is critically examined in the light of our present knowledge of asteroids and comets. It turns out that the emphasis on relatively narrow epochs associated with occasional NEO impacts is probably misplaced. Rather priority should be given to the wider and more frequent epochs associated with multiple NEO debris impacts which result in so-called punctuational crises afflicting the planets. These comprise the global coolings, super-Tunguska events and generally enhanced fireball flux produced by the larger orbital debris whenever an active, dormant or dead comet fragments and produces a trail. Taken as a whole and in conjunction with the target, the response function is inevitably complex. Nevertheless we broadly expect that the strength of a punctuational crisis will vary as the progenitor comet mass, the inverse dispersion of its debris and the inverse delay since fragmentation. The encounter of P/SL-9 with Jupiter may be taken as representing an extreme punctuational crisis where the dispersion and delay were exceptionally small. The more familiar crises affecting the Earth with less extreme values of dispersion and delay, which have resulted in civilization being disturbed a good many times during recent millennia, are no less important however. Indeed, the next such threat to civilization ostensibly has a roughly 1 in 4 lifetime chance. Any support for the Spaceguard programme which detracts from consideration of these punctuational crises, whatever their strength, would seem now to be peculiarly wide of the mark. 相似文献
20.
Elma S. Parsamian 《Astrophysics》2002,45(1):15-19
It is shown that to derive the distribution function of the average frequency of stellar flares in a case in which the number of flares observed in an aggregatation is small, a chronology of third flares can be used to confirm the chronology of first flares. A concrete solution for faint stars of the Pleiades cluster is given. 相似文献