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581.
582.
Mikhail I. PETAEV Anders MEIBOM Alexander N. KROT John A. WOOD Klaus KEIL 《Meteoritics & planetary science》2001,36(1):93-106
Abstract— Thermodynamic analysis of the compositional profiles across large chemically‐zoned Fe, Ni metal grains in the Bencubbin‐like chondrite Queen Alexandra Range (QUE) 94411 suggests that these grains formed by non‐equilibrium gas‐solid condensation under variable oxidizing conditions, isolation degree, and Cr depletion factors. The oxidizing conditions must have resulted from the complete vaporization of nebular regions with enhanced dust/gas ratios (~ 10–40 × solar). Because the origin of each of the metal grains studied requires different condensation parameters (dust/gas ratio, isolation degree, and Cr depletion factor), a high degree of heterogeneity in the formation region of the Bencubbin‐like chondrite metal is required. To preserve compositional zoning of the metal grains and prevent their melting and sulfidization, the grains must have been removed from the hot condensation region into cold regions where the accretion of the Bencubbin‐like asteroidal body took place. 相似文献
583.
Observations of an energy-dependent asymmetry in footpoint hard X-ray emission by RHESSI for the M4.0 solar flare of 17 March
2002 allows us to probe the dynamics of particle transport with energy and time. The presence of such an asymmetry is most
readily explained by the effects of a converging magnetic field with different rates of convergence at the different footpoints,
as would be expected from realistic surface field distributions. Such a geometry has been discussed in the context of a trap-plus-precipitation
model where the transport of energetic particles in the flare is governed by the precipitation out of the coronal trap via
collisions, wave-particle interactions or some other scattering process, into the high-density chromosphere. Comparison of
RHESSI observations with a trap-plus-precipitation model allows us to use the energy dependence of the asymmetry and the observed
ratio of footpoint to coronal emission at the different energies to assess the role of the trapping in the transport of energetic
electrons and to probe the nature of the particle precipitation process inside the loss cone. 相似文献
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587.
Calcium‐aluminum‐rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites
Jrme Alon Alexander N. Krot Kevin D. McKeegan 《Meteoritics & planetary science》2002,37(12):1729-1755
Abstract— Calcium‐aluminum‐rich refractory inclusions (CAIs) in CR chondrites are rare (<1 vol%), fairly small (<500 μm) and irregularly‐shaped, and most of them are fragmented. Based on the mineralogy and petrography, they can be divided into grossite ± hibonite‐rich, melilite‐rich, and pyroxene‐anorthite‐rich CAIs. Other types of refractory objects include fine‐grained spinel‐melilite‐pyroxene aggregates and amoeboid olivine aggregates (AOAs). Some of the pyroxene‐anorthite‐rich CAIs have igneous textures, and most melilite‐rich CAIs share similarities to both the fluffy and compact type A CAIs found in CV chondrites. One major difference between these CAIs and those in CV, CM, and CO chondrites is that secondary mineral phases are rare. In situ ion microprobe analyses of oxygen‐isotopic compositions of 27 CAIs and AOAs from seven CR chondrites demonstrate that most of the CAIs are 16O‐rich (δ17O of hibonite, melilite, spinel, pyroxene, and anorthite < ?22‰) and isotopically homogeneous within 3–4‰. Likewise, forsterite, spinel, anorthite, and pyroxene in AOAs have nearly identical, 16O‐rich compositions (?24‰ < δ17O < ?20‰). In contrast, objects which show petrographic evidence for extensive melting are not as 16O‐rich (δ17O less than ?18‰). Secondary alteration minerals replacing 16O‐rich melilite in melilite‐rich CAIs plot along the terrestrial fractionation line. Most CR CAIs and AOAs are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes, which is reflected in their relatively homogeneous 16O‐rich compositions. It is likely that these objects (or their precursors) condensed in an 16O‐rich gaseous reservoir in the solar nebula. In contrast, several igneous CAIs are not very enriched in 16O, probably as a result of their having melted in the presence of a relatively 16O‐poor nebular gas. If the precursors of these CAIs were as 16O‐rich as other CR CAIs, this implies either temporal excursions in the isotopic composition of the gas in the CAI‐forming regions and/or radial transport of some CAI precursors into an 16O‐poor gas. The absence of oxygen isotope heterogeneity in the primary minerals of melilite‐rich CAIs containing alteration products suggests that mineralogical alteration in CR chondrites did not affect oxygen‐isotopic compositions of their CAIs. 相似文献
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589.
On the basis of the published times of minima and our own observations, we analysed the period change of the Algol-type eclipsing binary CU Pegasi. Over almost seventy years of observations, the parabolic period change has been clearly seen as dP/dt = 1.38 × 10−6 d/year. The estimated mass transfer in the system is about 1 × 10−7 MM⊙/year. 相似文献
590.
Alexander I. Ershkovich 《Earth, Moon, and Planets》1981,25(4):521-522
The method of evaluation of the cometary magnetic field proposed by Podgornyet al. (1980) is shown not to be self-consistent. An alternative method is discussed. 相似文献