Potential of the VLTI for linking stellar frames to ICRF     

Gérard Daigne  Patrick Charlot  Christine Ducourant  Jean-François Lestrade 《Astrophysics and Space Science》2003,286(1-2):255-260

The question of positioning the optical counterparts of the ICRF quasars is outlined in the perspective of future space astrometry missions, which ultimately will bring a new realization of the ICRS in the optical range. Ground-based interferometry with a dual-field observing mode (PRIMA/VLTI),together with the missions DIVA and FAME, will have a key role in building an extragalactic reference frame in the optical/near-IR range with about the same accuracy as that of the present (VLBI) primary frame. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Chaotic Diffusion And Effective Stability of Jupiter Trojans     

Kleomenis Tsiganis  Harry Varvoglis  Rudolf Dvorak 《Celestial Mechanics and Dynamical Astronomy》2005,92(1-3):71-87

It has recently been shown that Jupiter Trojans may exhibit chaotic behavior, a fact that has put in question their presumed long term stability. Previous numerical results suggest a slow dispersion of the Trojan swarms, but the extent of the ‘effective’ stability region in orbital elements space is still an open problem. In this paper, we tackle this problem by means of extensive numerical integrations. First, a set of 3,200 fictitious objects and 667 numbered Trojans is integrated for 4 Myrs and their Lyapunov time, T_L, is estimated. The ones following chaotic orbits are then integrated for 1 Gyr, or until they escape from the Trojan region. The results of these experiments are presented in the form of maps of T_Land the escape time, T_E, in the space of proper elements. An effective stability region for 1 Gyr is defined on these maps, in which chaotic orbits also exist. The distribution of the numbered Trojans follows closely the T_E=1 Gyr level curve, with 86% of the bodies lying inside and 14% outside the stability region. This result is confirmed by a 4.5 Gyr integration of the 246 chaotic numbered Trojans, which showed that 17% of the numbered Trojans are unstable over the age of the solar system. We show that the size distributions of the stable and unstable populations are nearly identical. Thus, the existence of unstable bodies should not be the result of a size-dependent transport mechanism but, rather, the result of chaotic diffusion. Finally, in the large chaotic region that surrounds the stability zone, a statistical correlation between T_LandT_E is found.  相似文献   

Adaptive Optics: Astronomical Results and Perspectives     

P. Léna 《Experimental Astronomy》1997,7(4):281-284

  相似文献   

Palynological evidence for Lower Ordovician rocks (Tremadoc and Àrenig) in the Northern Greywacke Zone (Eastern Alps)     

Erhard Reitz  Rudolf Höll 《地学学报》1992,4(2):198-203

Until recently, only fossils of Caradoc to Carboniferous age have been reported from the Northern Greywacke Zone/Eastern Alps. New medium-preserved and diversified acritarch assemblages from the Lower Wildschönau Formation indicate a Lower Ordovician (Tremadoc and Arenig) age. These fossil spectra define a well-constrained time interval for a lower section of the Northern Greywacke Zone for the first time.  相似文献   

Hα Photometry of Two Contact Binaries     

Szilárd Csizmadia  Zsolt Kővári  Péter Klagyivik 《Astrophysics and Space Science》2006,304(1-4):355-357

We carried out optical and Hα photometry of two contact binaries (V861 Herculis, EQ Tauri). The light curve modeling revealed stellar spots in both contact systems and strong Hα excess in the position of the observed stellar spots. A correlation was found between the V−R and R−H_α colour indices of V861 Her.  相似文献   

Bi-dimensional observation of waves near the mesopause at auroral latitudes     

J. Clairemidi  M. Hersé  G. Moreels 《Planetary and Space Science》1985,33(9):1013-1022

During a campaign of optical observations at high latitude, a bi-dimensional study of the wave structure of the OH layer has been performed in December 1981 from Sodankyla (Finland). This site is one of the three stations of the EISCAT ionosphere sounding system. It has been found that a wave field covering an area of 1 million km² may extend to latitudes as high as 70°N. The OH wave structure shows many similarities with noctilucent clouds. The fairly large horizontal wavelength, of the order of 40 km cannot easily be explained by a wave motion at an interface. The observed wave structure seems to be a result of the propagation of an internal gravity wave in the 80–100 km region. This wave structure was often recorded during the same time as an active aurora was present. As a result, it appears that the perturbation might be correlated with particle precipitations at auroral latitudes.  相似文献   

Polarization of Evolved Stars Envelopes     

C. Kahane  E. Viard  F. Ménard  P. Bastien  N. Manset 《Astrophysics and Space Science》1997,251(1-2):223-226

  相似文献   

Direct generation of solar and stellar radio bursts by energetic electron maser     

P. Louarn  D. Le Quéau  A. Roux 《Solar physics》1987,111(1):201-206

A fully relativistic electron maser is proposed for the explanation of certain non-thermal solar and stellar radio bursts. This mechanism (maser synchrotron) is based on a gyroresonant interaction between waves and electrons of high energies and uses the free energy contained in an electronic distribution function that peaks for energies around 1 MeV.By a calculation of the growth rates of the three electromagnetic modes, we show that the X-mode prevails for values of _p/ _cup to 2 or 3. This result is very different from the standard cyclotron maser case where such values of _p/ _clead to quench the X-mode growth. Hence, the synchrotron maser instability appears to be a direct and efficient amplification process for considerably larger physical conditions than the cyclotron maser. In addition, the radiation, emitted over the second gyroharmonic, freely propagates without a strong reabsorbtion at the 2 _clayer. All these points can constitute major advantages of this mechanism in an astrophysical context.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.  相似文献   

Secular resonances: New results     

Ch. Froeschlé  H. Scholl 《Celestial Mechanics and Dynamical Astronomy》1987,43(1-4):113-117

We integrated numerically, in the frame of the four body problem Sun-Jupiter-Saturn-asteroid the orbit of the asteroid 1974 MA, an Earth-crosser, which is located in a region where three resonances overlap: the two secular resonances ₅ and ₁₆ and the mean motion resonance 5/1. The numerical integration yields a qualitative orbital evolution of this particular region.  相似文献   

Littoral '92     

H. Flanquart  C. Meur-Férec  Ph. Deboudt  V. Morel  A.-P. Hellequin  et al. 《GeoJournal》1991,24(2):223-223

Reports

Littoral '92  相似文献