Image-intensified video results from the 1998 Leonid shower: I. Atmospheric trajectories and physical structure
摘要:
Abstract— Two‐station electro‐optical observations of the 1998 Leonid shower are presented. Precise heights and light curves were obtained for 79 Leonid meteors that ranged in brightness (at maximum luminosity) from +0.3 to +6.1 astronomical magnitude. The mean photometric mass of the data sample was 1.4 × 10−6 kg. The dependence of astronomical magnitude at peak luminosity on photometric mass and zenith angle was consistent with earlier studies of faint sporadic meteors. For example, a Leonid meteoroid with a photometric mass of ∼1.0 × 10‐7 kg corresponds to a peak meteor luminosity of about +4.5 astronomical magnitudes. The mean beginning height of the Leonid meteors in this sample was 112.6 km and the mean ending height was 95.3 km. The highest beginning height observed was 144.3 km. There is relatively little dependence of either the first or last heights on mass, which is indicative of meteoroids that have clustered into constituent grains prior to the onset of intensive grain ablation. The height distribution, combined with numerical modelling of the ablation of the meteoroids, suggests that silicate‐like materials are not the principal component of Leonid meteoroids and hints at the presence of a more volatile component. Light curves of many Leonid meteors were examined for evidence of the physical structure of the associated meteoroids: similar to the 1997 Leonid meteors, the narrow, nearly symmetric curves imply that the meteoroids are not solid objects. The light curves are consistent with a dustball structure.
Acetonyltriphenylphosphonium bromide in organic synthesis: a useful catalyst in the cyclotrimerization of aldehydes
作者:Yung-Son Hon、Chia-Fu Lee
DOI:10.1016/s0040-4020(01)00588-9
日期:2001.7
Acetonyltriphenylphosphonium bromide (ATPB) is a useful catalyst for the cyclotrimerization of the aliphatic aldehydes under solvent-free condition. The aldehydes tethered with a variety of functionality such as olefin, ether, ester, bromide, azide and diester could also be cyclotrimerized under the catalysis of ATPB.
Harries; Alefeld, Chemische Berichte, 1909, vol. 42, p. 165
作者:Harries、Alefeld
DOI:——
日期:——
Carriere, Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, 1912, vol. 154, p. 1174
作者:Carriere
DOI:——
日期:——
Aqueous alteration without a pronounced oxygen-isotopic shift: Implications for the asteroidal processing of chondritic materials
作者:P. A. BLAND、M. R. LEE、A. S. SEXTON、I. A. FRANCHI、A. E. T. FALLICK、M. F. MILLER、J. M. CADOGAN、F. J. BERRY、C. T. PILLINGER
DOI:10.1111/j.1945-5100.2000.tb01523.x
日期:2000.11
Abstract— Primitive meteorites exhibit certain features that are consistent with aqueous and thermal alteration on asteroids, but O‐isotopic analyses show only a modest heavy‐isotope shift, interpreted as indicating modification in the nebula. To understand the isotopic effects of asteroidal alteration, we take the L‐group ordinary chondrites weathered in Antarctica as an analogue. The data show that alteration is a two‐stage process, with an initial phase producing only a negligible isotopic effect. Although surprising, a possible explanation is found when we consider the alteration of terrestrial silicates. Numerous studies report pervasive development of channels a few to a few tens of nanometer wide in the incipient alteration of silicates. We observe a similar texture. Alteration involves a restructuring of clay minerals along these narrow channels, in which access of water is restricted. The clay shows a topotactic relationship to the primary grain, which suggests either epitaxial growth of the clay using the silicate as a substrate or inheritance of the original O structure by the clay. Our data suggests the latter: with extensive inheritance of structural polymers by the weathering product, the bulk O‐isotopic composition is comparatively unaffected. This offers an explanation for the lack of an isotopic effect in the weathering of the L chondrites. If substantial modification of chondritic materials may occur without a pronounced isotopic effect, it also reconciles existing O analyses of CV chondrites with an asteroidal model of aqueous alteration.
Carriere, Annales de Chimie (Cachan, France), 1922, vol. <9> 17, p. 122