共查询到20条相似文献，搜索用时 593 毫秒
1.
E. Paunzen 《Astrophysics and Space Science》1999,266(3):379-387
In this article the current status of the knowledge forthe group of λ Bootis stars is reviewed. These metal poor objectsare
quite outstanding compared to other chemically peculiar stars of theupper main sequence. Up to now no conclusive theory has
been developedwhich is able to explain the majority of observational results. There is,however, an increasing interest in
observational as well as theoreticalresults in order to develop and test astrophysical models such asaccretion of circumstellar
matter and diffusion processes in stellaratmospheres.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
We compare various objects as the possible parent comet of the1998 June Boötid, by using the Tisserand invariant and the D-discriminant. Furthermore,in order to investigate the behaviour of the meteoric stream orbit, wesimulate the orbital evolution of test particles that are released from7P/Pons–Winnecke. We show firstly that the parent comet of the 1998 June Boötids, is 7P/Pons–Winnecke, and secondly that the meteoroids which constitute 1998 June Boötids were released in 1819 and 1869 from the parent comet. In themid-1900s the meteoroids started to transfer to Earth-colliding orbitsby Jovian perturbations. 相似文献
3.
We report the observation of an outburst of comet Hale–Bopp (C/1995 O1) happened on September 10–11, 1996, carried by the
1.56 m telescope of Shanghai Astronomical Observatory. Two ejecta were found in CCD images during the outburst. According
to the positions of ejecta, we discuss the motion of the ejecta considering dust particles are subjected to the gravity and
the Solar radiation pressure, and conclude that the mean radii of dust grains in the ejecta were about submicron-sized. So
the observed X-ray emission are most likely produced by small size particles scattering the Solar X-ray.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
In this paper, using the energy definition in MØller’s tetrad theory of gravity we calculate the total energy of the universe in Bianchi-type I cosmological models which includes both the matter and gravitational fields. The total energy is found to be zero and this result agrees with a previous works of Banerjee and Sen who investigated this problem using the general relativity version of the Einstein energy-momentum complex and Xulu who investigated same problem using the general relativity versions of the Landau and lifshitz, Papapetrou and Weinberg’s energy-momentum complexes. The result that total energy of the universe in Bianchi-type I universes is zero supports the viewpoint of Tryon. 相似文献
5.
The analysis of application of two dynamical models (``Earth–Moon' and``barycentre' model) in the motion of Near Earth Asteroids was performed. Mainaim was the quantitative estimation of the influence of lunar perturbations on the motionof NEA. Additionally, basic tests of application of numerical methods weremade (RMVS3 and B–S methods). The orbits of 1083 Apollo–Aten–Amor and 7selected AAA objects were adopted as test particles in numerical integrationof the motion. The comparison between results obtained by both dynamicalmodels is discussed in detail. In specific cases, the application of the``Earth–Moon' dynamical model is very important and cannot be neglected incomputations of orbits. 相似文献
6.
The formation and evolution of a large amplitude MHD perturbation propagating perpendicular to the magnetic field in a perfectly conducting low plasma is studied. The perturbation is generated by an abrupt expansion of the source region. Explicit expressions for the time and the distance needed for the transformation of the perturbation's leading edge into a shock wave are derived. The results are applied to coronal conditions and the dynamic spectra of the radio emission excited by the shock are synthesized, reproducing metric and kilometric type II bursts. The features corresponding to the metric type II burst precursor and the moving type IV burst in the case of kilometric type II bursts are identified. A specific radio signature that is sometimes observed at the onset of a metric type II burst is found to appear immediately before the shock wave formation due to the associated growth of the magnetic field gradient. Time delays and starting frequencies of bursts' onsets are calculated and presented as a function of the impulsiveness of the source-region expansion, using different values of the ambient Alfvén velocity and various time profiles of the expansion velocity. The results are confronted with the observations of metric and kilometric type II solar radio bursts. 相似文献
7.
We interpret the historical activity of comet 55P/Tempel–Tuttle in terms of the observed characteristics of present-day short
period comets. In this respect, it is now realized that such comets are liable to undergo significant outburst and mantle
loss events at intervals separated by of order a few hundred years. On this basis one might well expect comet 55P/Tempel–Tuttle
to have undergone several outbursts since its earliest sighing in 1366. The limited absolute magnitude data available for
55P/Tempel–Tuttle is not inconsistent with the suggestion that the comet underwent outbursts during its 1699 and 1865 perihelion
returns. If the outbursts of comet 55P/Tempel–Tuttle are interpreted in terms of mantle loss events then the bright, electrophonic
sound producing fireballs reported during the great Leonid meteor storm of 1833 may have been due to the Earth sampling mantle
material ejected during the outburst of 1699.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
It is shown that an analog of Birkhoff’s theorem of general relativity exists in the Einstein–Cartan–Kalb–Ramond (ECKR) theory
of gravity when Kalb–Ramond (KR) field strength, which occurs in the theory is independent of time. 相似文献
9.
Woodward Charles E. Gehrz R. D. Mason C. G. Jones T. J. Williams D. M. 《Earth, Moon, and Planets》1998,81(3):217-222
Comets, such as C/1995 O1 (Hale-Bopp), are important to studies of the origins of the solar system because they are believed
to be frozen reservoirs of the most primitive pre-solar dust grains and ices. Here, we report 1.2–18.5 μm infrared (IR) spectrophotometric
and polarimetric observations of comet Hale-Bopp. Our measurements of the spectral energy distribution (SED) and IR polarization
near perhelion passage suggest that emission from the coma was dominated by scattering and thermal emission from sub-micron
sized dust grains. Hale-Bopp's surprising brightness may have been largely a result of the properties of its coma grains rather
than the size of its nucleus. The thermal emission continuum from the grains had a superheat of S = Tcolor/TBB ≥ 1.7, the peak of the 10 μm silicate emission feature was 1.7 mags above the carbon grain continuum, and the albedo (reflectivity)
of the grains was ≥ 0.4 at a scattering angles, θ ≥ 135°
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
The gravitational field of a global monopole in the context of Einstein–Cartan theory is investigated. A comparison is made
with the corresponding results predicted by general relativity.
PACS Nos: 98.80cq; 04.20 jb; 04.50 相似文献
11.
Burša Milan Kenyon Steve Kouba Jan Müller Achim Raděj Karel Vatrt Viliam Vojtíšková Marie Vítek Vojtěch 《Earth, Moon, and Planets》1999,84(3):163-176
The TOPEX/POSEIDON (T/P) satellite altimeter data from January 1, 1993to October 24, 1999 (cycles 11–261) was used for investigating
thelong-term variations in the geoidal geopotential W0 and/orin the geopotential scale factor R0 = GM/W0 (GM is theadopted geocentric gravitational constant). The mean valuesdetermined for the whole period covered are: W0 =(62 636 856.161 ± 0.002) m2 s-2, R0 =(6 363 672.5448 ± 0.0002) m. The actual accuracy is limited bythe altimeter calibration error (2–3 cm) and it isestimated
to be about ± 0.5 m2 s-2 (± 5 cm).The yearly variations of the above mean values are at the formalerror level. No long-term trend in W0, representing the oceanvolume change, was found for the seven years period 1993–9 on thebasis of T/P altimeter (AVISO) data.
No sea surface topography modelwas used in the solution.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
H.U.Keller 《紫金山天文台台刊》1996,(3)
The European Space Agency (ESA) launched its science programme Horizon 2000 in themid eighties.this programme encompasses four big missions, the cornerstones,in addition tosmaller missions. The four cornerstones are:SOHO/Cluster a solary observatory and in situmeasurements of the Earth magnetosphere,XMM and X-ray observatory,Rosetta a comet ren-dezvous mission,and FIRST a far infrared observatory.SOHO was successfully launched in 相似文献
13.
A calculation of the equilibrium charge acquired by interstellar grains is given, which takes account of polarization charges that are induced in a grain by incident ions and electrons.Both metal and dielectric grains are considered and photoionization of the latter grains by UV radiation is taken into account where necessary. It is found that the inclusion of the polarization charges in the calculation is only important in gas clouds where the mean charge on a grain is low (<1e); that is, for HI regions and dense molecular clouds. In such clouds, the effect of the polarization charges is to increase the amount of negative charge acquired by a grain. A discussion is given concerning the validity of the classical electrostatic theory employed in the paper for small grains of radius 10–6 cm, and some astrophysical consequences of the modification of the grain charge by polarization effects are considered. 相似文献
14.
《紫金山天文台台刊》1996,(2)
In this paper,we present the main optical observation results of the collisions of comet Shoemaker-Levy 9 and Jupiter at Shanghai Observatory of China. 相似文献
15.
Using polarimetric spectra obtained with the SOFIN spectrograph installed at the Nordic Optical Telescope, we detect a longitudinal magnetic field 〈Bz〉 = –168±35 G in the Of?p star HD 108. This result is in agreement with the longitudinal magnetic field measurement of the order of –150 G recently reported by the MiMeS team. The measurement of the longitudinal magnetic field in the Of?p star HD 191612 results in 〈Bz〉 = +450±153 G. The only previously published magnetic field measurement for this star showed a negative longitudinal magnetic field 〈Bz〉 = –220±38 G, indicating a change of polarity over ∼100 days. Further, we report the detection of distinct Zeeman features in the narrow Ca II and Na I doublet lines for both Of?p stars, hinting at the possible presence of material around these stars. The origin of these features is not yet clear and more work is needed to investigate how magnetic fields interact with stellar wind dynamics (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
16.
17.
Anirudh Pradhan G. S. Khadekar Deepak Srivastava 《Astrophysics and Space Science》2006,305(4):415-421
Implications of cosmological model with a cosmological term of the form , where β is a constant, are analyzed in multidimensional space time. The proper distance, the luminosity distance-redshift, the angular diameter distance-redshift, and look back time-redshift for the model are presented. It has been shown that models are found to be compatible with the present observations. This work, has thus generalized to higher dimensions the well-known result in four dimensional space time. It is found that the difference is significant at least in the principal to the anologous situation in four dimensional space time. 相似文献
18.
G. Contopoulos 《Celestial Mechanics and Dynamical Astronomy》1981,24(4):355-366
We study the resonance 12 = 41 and some near-resonance cases. The main peculiarity of this resonance is that for 12 < 4 the characteristic of the central periodic orbits is broken into two and each part is joined with a resonant characteristic. This behaviour is described theoretically by means of the third integral. It seems that there are infinite families of simple periodic orbits near the escape region. Finally, a comparison is made with the cases near the 12 = 21 resonance. 相似文献
19.
Arbab I.Arbab 《中国天文和天体物理学报》2003,3(2):113-118
Exact solutions for a model with variable G,A and bulk viscosity are obtained,Inflationary solutions with constant(de Sitter-type )and variable energy density are found.An expanding anisotropic universe is found to isotropize during its expansion but a static universe cannot isotropize.The gravitational constant is found to increase with time and the cosmological constant decreases with time as A∝t^-2。 相似文献
20.
In combination with the authors' previous observation about the splitting of Comet Halley in March 1986,the events involving the sharp,straight feature in the antisolar direction observed in the head of Cornet halley in 1910 (such as those occurring on May 14,25 and 31,and June 2) are redis-cussed.The analysis leads to the following scenario;When comet Halley explodes and splits,a fragment jettisoned or thrown off from the nucleus will,after moving in the direction of its tail,develop into a mini-comet.Although not weel developed or permanent,it has its own plasma tail and,sometimes,a dust tail.If Bobrovnikoff's definition of a secondary nucleus is assumed,then the fragment should be considered as a real secondary nucleus.It seems that the current idea of a tailward jet suggested by Sekanina and Larson is a wrong explanation for the plasma tail of a mini-comet and hence the rotation period of 52-53h for Comet Halley is doubtful. 相似文献