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901.
902.
F. De Luise D. Perna E. Dotto I.N. Belskaya G.B. Valsecchi A. Rossi P. Paolicchi 《Icarus》2007,191(2):628-635
In this paper we present the observational campaign carried out at ESO NTT and VLT in April and May 2006 to investigate the nature and the structure of the near-Earth object (144898) 2004 VD17. In spite of a great quantity of dynamical information, according to which it will have a close approach with the Earth in the next century, the physical properties of this asteroid are largely unknown. We performed visible and near-infrared photometry and spectroscopy, as well as polarimetric observations. Polarimetric and spectroscopic data allowed us to classify 2004 VD17 as an E-type asteroid. A good agreement was also found with the spectrum of the aubrite meteorite Mayo Belwa. On the basis of the polarimetric albedo (pv=0.45) and of photometric data, we estimated a diameter of about 320 m and a rotational period of about 2 h. The analysis of the results obtained by our complete survey have shown that (144898) 2004 VD17 is a peculiar NEO, since it is close to the breakup limits for fast rotator asteroids, as defined by Pravec and Harris [Pravec, P., Harris, A.W., 2000. Icarus 148, 12-20]. These results suggest that a more robust structure must be expected, as a fractured monolith or a rubble pile in a “strength regime” [Holsapple, K.A., 2002. Speed limits of rubble pile asteroids: Even fast rotators can be rubble piles. In: Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids, Washington, September, 2002]. 相似文献
903.
M. Ćwiok W. Dominik K. Małek L. Mankiewicz J. Mrowca-Ciułacz K. Nawrocki L. W. Piotrowski P. Sitek M. Sokołowski G. Wrochna A. F. Żarnecki 《Astrophysics and Space Science》2007,309(1-4):531-535
Experiment “Pi of the Sky” is designed to search for prompt optical emission from GRB sources. 32 CCD cameras covering 2 steradians will monitor the sky continuously. The data will be analysed on-line in search for optical flashes. The prototype with 2 cameras operated at Las Campanas (Chile) since 2004 has recognised several outbursts of flaring stars and has given limits for a few GRB. 相似文献
904.
A large fraction of the anticipated source detections by the Gamma-ray Large Area Space Telescope (GLAST-LAT) will initially be unidentified. We argue that traditional approaches to identify individuals and/or populations of gamma ray sources will encounter procedural limitations. Those limitations are discussed on the background of source identifications from EGRET observations. Generally, our ability to classify (faint) source populations in the anticipated GLAST dataset with the required degree of statistical confidence will be hampered by sheer source wealth. A new paradigm for achieving the classification of gamma ray source populations is discussed. 相似文献
905.
Zs. Kvri F. Vilardell I. Ribas K. Vida L. van Driel‐Gesztelyi C. Jordi K. Olh 《Astronomische Nachrichten》2007,328(9):904-908
We present B and V light curves of a large stellar flare obtained with the Wide Field Camera at the Isaac Newton 2.5‐m telescope (La Palma). The source object is a faint (mV = 21.38) foreground star in the field of the Andromeda galaxy, with its most probable spectral type being dM4. We provide an estimate of the total flare energy in the optical range and find it to be of the order of 1035 erg. The cooling phase of the large flare shows three additional weak flare‐like events, which we interpret as results of a triggering mechanism also observed on the Sun during large coronal mass ejections. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
906.
The effects of Bohm potential on the head-on collision between two quantum electron-acoustic solitary waves (QEASWs) in two
electron species quantum plasma have been investigated using the extended Poincaré–Lighthill–Kuo (PLK) method. The analytical
phase shifts after the head-on collision of the two QEASWs are derived. Numerically, in two cases (i.e., the dense solid state
plasma and the dense astrophysical environments), the results show that the cold electron-to-hot electron number density ratio,
the quantum corrections of diffraction and Fermi temperature of hot electrons have strong effects on the nature of the phase
shifts and the trajectories of two QEASWs after collision. 相似文献
907.
We investigate analytically and numerically a nonlinear modification of the magnetospheric plasma density under the action of the ponderomotive force induced by ULF traveling waves, using the nonlinear stationary force balance equation. This equation is applied to both the dipole and dayside magnetosphere having one and two minima of the geomagnetic field near the magnetospheric boundary. The separate and joint actions of the ponderomotive, centrifugal, and gravitational forces on the density distribution are shown. 相似文献
908.
F. A. Chekhonadskikh 《Kinematics and Physics of Celestial Bodies》2012,28(3):128-136
Using high-resolution spectra, we study 31 yellow supergiants of the Large and Small Magellanic Clouds by the method of atmosphere models. Abundance values of 20 chemical elements are determined. It is shown that ??-elements are in a slight excess and neutron-capture elements have an excess up to 0.60 dex. Approbation of a new technique for the determination of absolute stellar magnitudes of late-type supergiants is performed. The technique is based on the use of the spectroscopic criterion, namely, depth-line ratios for iron. Absolute stellar magnitudes of nine supergiants of the Large Magellanic Cloud are calculated using this technique. A value of the distance modulus of the Large Magellanic Cloud is estimated: m ? M = 18.4 ± 0.3 m . 相似文献
909.
J. Schlüter L. Schultz F. Thiedig B. O. Al‐Mahdi A. E. Abu Aghreb 《Meteoritics & planetary science》2002,37(8):1079-1093
Abstract— As of July 2001, 1238 Libyan meteorites have been reported. Most were found in two areas called Dar al Gani and Hamadah al Hamra. Dar al Gani is located on a plateau of marine carbonate rocks with marly components. Eight‐hundred and sixty‐nine meteorites between 6 g and 95 kg totalling 687 kg have been found here but the calculated mean recovery density is comparatively low with one meteorite on 6.5 km2. Dar al Gani is a perfect site for the recognition and preservation of meteorites. The existence of meteorites is the result of a combination of specific geological and geomorphological conditions: there is a bright‐colored, old limestone plateau (<2 Ma), under arid weather conditions over long periods of time, with rapid elimination of surface water if present and low erosion rates. The preservation of meteorites is guaranteed through the absence of quartz sand on the plateau, strongly reducing wind erosion and a basic environment emerging from the carbonate ground retards rusting of metallic meteorite components. A supposed soil cover during pluvial times has probably protected older meteorites and led to a concentration of meteorites of different periods. An evaluation of Dar al Gani meteorites suggests the existence of at least 26 strewnfields and 26 meteorite pairs reducing the number of falls to, at most, 534. Shock and weathering grades as a tool for the recognition of pairings turned out to be problematic, as several strewnfields showed paired meteorites which had been classified to different shock and weathering grades. 相似文献
910.
Abstract— From 2001 June 17 to 25, we held the first international workshop in Erice, Italy, dedicated to the determination of geological and geophysical properties of near‐Earth objects (NEOs). The goal was to develop a roadmap for determining the physical and chemical properties of NEOs in the coming decades to meet the scientific requirements for development of Earth collision avoidance technology. We identified many properties that are desired, but four measurements are needed most critically for any potentially hazardous NEO: (1) its mass, (2) its mass distribution, (3) its material strengths, and (4) its internal structure. Global (whole‐body) properties, such as material strengths and internal structure, can be determined best from the analyses of permeating waves: artificially initiated seismology and multifrequency reflection and transmission radio tomography. Seismology provides the best geophysical (material strengths) data of NEOs composed of consolidated materials while radio tomography provides the best geological data (e.g., the state of fracture) of electrically nonconducting media. Thus, the two methods are complementary: seismology is most suitable for stony and metallic asteroids, while radio tomography is most appropriate for comet nuclei and carbonaceous asteroids. The three main conclusions are (1) remote sensing for physical characterization should be increased, (2) several dedicated NEO missions should be prepared for geophysical and geological investigations, and (3) that it is prudent to develop and prove the technology to make geophysical measurements on NEOs now. 相似文献