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991.
A.V. Myasnikov N.A. Belov B. Gustafsson K. Eriksson 《Astrophysics and Space Science》2000,274(1-2):231-241
Gasdynamic features of detached shells around carbon stars with variable mass loss rate are investigated in detail numerically.
It is shown that a shell is unstable and also, 2D perturbations are less developed that 3D ones. The structure of perturbed
flows corresponding to different evolution scenarios is compared. The results obtained seem to be promising for interpretation
of observations, in particular, the recently obtained detailed data of TT Cyg.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
992.
Non-existence of higher dimensional anisotropic cosmological model in bimetric theory of gravitation
A Five dimensional spherically symmetric space-time model in Rosen's (1973) bimetric theory of gravitation is considered.
It is shown that, in this theory, the geometry of the five dimensional space-time does not admit perfect fluid source. Hence
a vacuum model is presented and studied. It is observed that in the case of stiff fluid we again get the vacuum model.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
993.
N. A. Andreeva Yu. L. Zyskin O. R. Kalekin Yu. I. Neshpor A. A. Stepanyan V. P. Fomin N. N. Chalenko V. G. Shitov 《Astronomy Letters》2000,26(4):199-203
We present two-year-long observations of the flux of very-high-energy (~1012 eV) gamma rays from the active galactic nucleus Mk 501 performed with a Cherenkov detector at the Crimean Astrophysical Observatory. A gamma-ray flux from the object was shown to exist at confidence levels of 11 and 7 standard deviations for 1997 and 1998, respectively. The flux varied over a wide range. The mean flux at energies >1012 eV, as inferred from the 1997 and 1998 data, is (5.0±0.6)×10?11 and (3.7±0.6)×10?11 cm?2 s?1, respectively. The errors are the sum of statistical observational and modeling errors. The mean power released in the form of gamma rays is ~2×1043 erg s?1 sr?1. 相似文献
994.
Observations of the X-ray pulsar Vela X-1 with the ART-P telescope onboard the Granat Observatory are presented. Variability on a time scale of several thousand seconds was detected; intensity variations are shown to be accompanied by changes in the source’s spectrum. The hardness was also found to be highly variable on a scale of one pulsation period. The source’s spectrum exhibits an absorption feature at energy ~7 keV, which is apparently attributable to cyclotron scattering/absorption in the neutron-star magnetic field. Weak persistent emission was detected during an X-ray eclipse, which probably resulted from the scattering of pulsar emission in the stellar wind from an optical star. 相似文献
995.
The Franck-Condon factors andr-centroids, which are very closely related to vibrational transition probabilities, have been evaluated by the more reliable
numerical integration procedure for the bands of A
2 Δ - X2 πr system of astrophysical molecule GeH, using a suitable potential. The dissociation energy for the electronic ground state
of astrophysical molecule GeH has been estimated precisely as D
0
0 = 2.69 ± 0.05 eV by fitting the empirical potential function to the experimental potential energy curve using correlation
coefficient.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
996.
Boice D. C. Soderblom L. A. Britt D. T. Brown R. H. Sandel B. R. Yelle R. V. Buratti B. J. Hicks Nelson Rayman Oberst J. Thomas N. 《Earth, Moon, and Planets》2000,89(1-4):301-324
NASA's Deep Space 1 (DS1) spacecraft successfully encountered comet 19P/Borrelly near perihelion and the Miniature Integrated Camera and Spectrometer (MICAS) imaging system onboard DS1 returned the first high-resolution images of a Jupiter-family comet nucleus and surrounding environment. The images span solar phase angles from 88° to 52°, providing stereoscopic coverage of the dust coma and nucleus. Numerous surface features are revealed on the 8-km long nucleus in the highest resolution images(47–58 m pixel). A smooth, broad basin containing brighter regions and mesa-likestructures is present in the central part of the nucleus that seems to be the source ofjet-like dust features seen in the coma. High ridges seen along the jagged terminator lead to rugged terrain on both ends of the nucleus containing dark patches and smaller series of parallel grooves. No evidence of impact craters with diameters larger thanabout 200-m are present, indicating a young and active surface. The nucleus is very dark with albedo variations from 0.007 to 0.035. Short-wavelength, infrared spectra from 1.3 to 2.6 μm revealed a hot, dry surface consistent with less than about10% actively sublimating. Two types of dust features are seen: broad fans and highlycollimated “jets” in the sunward hemisphere that can be traced to the surface. The source region of the main jet feature, which resolved into at least three smaller “jets” near the surface, is consistent with an area around the rotation pole that is constantly illuminated by the sun during the encounter. Within a few nuclear radii, entrained dustis rapidly accelerated and fragmented and geometrical effects caused from extended source regions are present, as evidenced in radial intensity profiles centered on the jet features that show an increase in source strength with increasing cometocentric distance. Asymmetries in the dust from dayside to nightside are pronounced and may show evidence of lateral flow transporting dust to structures observed in the nightside coma. A summary of the initial results of the Deep Space 1 Mission is provided, highlighting the new knowledge that has been gained thus far. 相似文献
997.
A. N. KROT A. MEIBOM M. I. PETAEV K. KEIL M. E. ZOLENSKY A. SAITO M. MUKAI K. OHSUMI 《Meteoritics & planetary science》2000,35(6):1249-1258
Abstract— The CH carbonaceous chondrites contain a population of ferrous (Fe/(Fe + Mg) ? 0.1‐0.4) silicate spherules (chondrules), about 15–30 μm in apparent diameter, composed of cryptocrystalline olivinepyroxene normative material, ±SiO2‐rich glass, and rounded‐to‐euhedral Fe, Ni metal grains. The silicate portions of the spherules are highly depleted in refractory lithophile elements (CaO, Al2O3, and TiO2 <0.04 wt%) and enriched in FeO, MnO, Cr2O3, and Na2O relative to the dominant, volatile‐poor, magnesian chondrules from CH chondrites. The Fe/(Fe + Mg) ratio in the silicate portions of the spherules is positively correlated with Fe concentration in metal grains, which suggests that this correlation is not due to oxidation, reduction, or both of iron (FeOsil ? Femet) during melting of metal‐silicate solid precursors. Rather, we suggest that this is a condensation signature of the precursors formed under oxidizing conditions. Each metal grain is compositionally uniform, but there are significant intergrain compositional variations: about 8–18 wt% Ni, <0.09 wt% Cr, and a sub‐solar Co/Ni ratio. The precursor materials of these spherules were thus characterized by extreme elemental fractionations, which have not been observed in chondritic materials before. Particularly striking is the fractionation of Ni and Co in the rounded‐to‐euhedral metal grains, which has resulted in a Co/Ni ratio significantly below solar. The liquidus temperatures of the euhedral Fe, Ni metal grains are lower than those of the coexisting ferrous silicates, and we infer that the former crystallized in supercooled silicate melts. The metal grains are compositionally metastable; they are not decomposed into taenite and kamacite, which suggests fast postcrystallization cooling at temperatures below 970 K and lack of subsequent prolonged thermal metamorphism at temperatures above 400–500 K. 相似文献
998.
TH. STELZNER K. HEIDE A. BISCHOFF D. WEBER P. SCHERER L. SCHULTZ M. HAPPEL W. SCHR
N U. NEUPERT R. MICHEL R. N. CLAYTON T. K. MAYEDA G. BONANI I. HAIDAS S. IVY-OCHS M. SUTER 《Meteoritics & planetary science》1999,34(5):787-794
Abstract— Weathering effects on meteorite finds from the Acfer region were studied by various analytical techniques and in dependence on the depth of sampling. In thin sections of weathered meteorites, weathering effects usually decrease from the outside to the interior of the meteorite. The results of evolved gas analysis indicate that variation in weathering between surface and core is not significant in respect to the formation of Fe-oxyhydroxides. The secondary alteration effects in the noble gases are distributed unevenly throughout the specimens, as seen in the nonsystematic differences observed for the heavy noble gases. Chemical analyses show significant enrichment of Ba and Sr in the outer parts of the weathered samples due to element contamination through aqueous solution. Iron, Ni, and Co are partly flushed from the system as the metal oxidation proceeds. Oxygen isotopes show increases in δ18O and δ17O with increasing terrestrial age. For a set of H3 chondrites, the degree of weathering determined from the water content was correlated with terrestrial ages and is discussed with respect to possible weathering mechanisms. 相似文献
999.
1000.