排序方式： 共有107条查询结果，搜索用时 343 毫秒
21.
Josep M. TRIGO‐RODRÍGUEZ Domingo Aníbal GARCÍA‐HERNÁNDEZ Maria LUGARO Amanda I. KARAKAS M. van RAAI Pedro García LARIO Arturo MANCHADO 《Meteoritics & planetary science》2009,44(5):627-639
Abstract— We demonstrate that a massive asymptotic giant branch (AGB) star is a good candidate as the main source of short‐lived radionuclides in the early solar system. Recent identification of massive (4–8 M⊙) AGB stars in the galaxy, which are both lithium‐ and rubidium‐rich, demonstrates that these stars experience proton captures at the base of the convective envelope (hot bottom burning), together with high‐neutron density nucleosynthesis with 22Ne as a neutron source in the He shell and efficient dredge‐up of the processed material. A model of a 6.5 M⊙ star of solar metallicity can simultaneously match the abundances of 26Al, 41Ca, 60Fe, and 107Pd inferred to have been present in the solar nebula by using a dilution factor of 1 part of AGB material per 300 parts of original solar nebula material, and taking into account a time interval between injection of the short‐lived nuclides and consolidation of the first meteorites equal to 0.53 Myr. Such a polluting source does not overproduce 53Mn, as supernova models do, and only marginally affects isotopic ratios of stable elements. It is usually argued that it is unlikely that the short‐lived radionuclides in the early solar system came from an AGB star because these stars are rarely found in star forming regions, however, we think that further interdisciplinary studies are needed to address the fundamental problem of the birth of our solar system. 相似文献
22.
E. SKRZYPEK K. SCHULMANN P. ŠTÍPSKÁ F. CHOPIN J. LEHMANN O. LEXA J. HALODA 《Journal of Metamorphic Geology》2011,29(4):473-496
In a Barrovian metamorphic sequence, garnetiferous mica schists document a heterogeneously developed superposition of sub‐orthogonal fabrics and multiple garnet growth episodes. In the variably deformed domains, four types of garnet porphyroblasts have been defined based on inclusion trail patterns. Modelled garnet zoning in the MnNCKFMASHTO system indicates a prograde evolution from 4–4.5 kbar and 490–510 °C to 5–6 kbar and 520–550 °C in the earliest subhorizontal fabric progressing towards 6.5–7.5 kbar and 560–590 °C in the subsequent subvertical foliation. This fabric is heterogeneously deformed into a shallow‐dipping retrograde foliation associated with garnet resorption. In situ electron backscatter diffraction measurements of ilmenite inclusions in individual garnet grains yield precise data on included planar and linear elements. Consistent orientations of internal foliations, lineations and foliation intersection axis sets indicate a superposition of three sub‐orthogonal foliation systems. Weak variations of internal records with increasing intensity of deformation suggest that a moderate buckling stage occurred, but apparent lack of porphyroblast rotation is interpreted as a result of dominant passive flow. Coupling the orientation of internal fabric sets with P–T estimates is used to complement the tectono‐metamorphic evolution of the thickened crust. We demonstrate that garnet porphyroblasts preserve features which reflect large‐scale tectonic processes in orogens. 相似文献
23.
Sergei Yu. Shugarov Vitalij P. Goranskij Nataly A. Katysheva Anatolij V. Kusakin Nataly V. Metlova Igor M. Volkov DrahomÍr Chochol Theodor Pribulla Eugenia A. Karitskaja Alon Retter Ohad Shemmer Yiftah Lipkin 《Astrophysics and Space Science》2005,296(1-4):431-434
UBVRI photoelectric and CCD photometry of the slow nova V723 Cas obtained in the years 1995–2003 is presented. The evolution
of light curves in 1-year intervals, folded with the orbital period 0.69326 days, shows an increase of the amplitude of the
wave-like variations from 0.07 to 1.3 mag during the years 1997–2003. The fact that the shape and amplitude of the orbital
light curves does not depend on wavelength is most probably related to the geometry of eclipses combined with the distribution
of circumstellar matter in the system. 相似文献
24.
We study the evolution of spherically symmetric radiating fluid distributions using the effective variables method, implemented
ab initio in Schwarzschild coordinates. To illustrate the procedure and to establish some comparison with the original method, we integrate
numerically the set of equations at the surface for two different models. The first model is derived from the Schwarzschild
interior solution. The second model is inspired in the Tolman VI solution. 相似文献
25.
GarcÍa R.A. Régulo C. Turck-Chièze S. Bertello L. Kosovichev A.G. Brun A.S. Couvidat S. Henney C.J. Lazrek M. Ulrich R.K. Varadi F. 《Solar physics》2001,200(1-2):361-379
Data recovered from the GOLF experiment on board the ESA/NASA SOHO spacecraft have been used to analyze the low-order low-degree solar velocity acoustic-mode spectrum below =1.5 mHz (i.e., 1n9,l2). Various techniques (periodogram, RLAvCS, homomorphic-deconvolution and RLSCSA) have been used and compared to avoid possible biases due to a given analysis method. In this work, the acoustic resonance modes sensitive to the solar central region are studied. Comparing results from the different analysis techniques, 10 modes below 1.5 mHz have been identified. 相似文献
26.
JOÃO TRABUCHO ALEXANDRE JUAN PEDRO RODRÍGUEZ‐LÓPEZ POPPE L. DE BOER 《Sedimentology》2011,58(5):1217-1246
The Aptian/Albian oceanic anoxic event 1b contains the record of several perturbations in the global carbon cycle and multiple black shale levels, particularly in the Western Tethys. The local lithological expression of an oceanic anoxic event depends on palaeogeographical and depositional setting as well as on regional palaeoclimate. Marine sediments at a particular location may therefore be more or less organic‐rich (or not at all) and they may consist of different lithologies. In most studies, however, much of the lithological variability associated with oceanic anoxic events is left unaccounted for and, thus, the exact processes leading to the enrichment of organic matter in these marine sediments and their subsequent preservation in the geological record are unknown. This study focuses on the local sedimentary processes behind the deposition of organic‐rich sediments at Deep Sea Drilling Project Site 545 and Ocean Drilling Program Sites 1049 and 1276 in the North Atlantic during oceanic anoxic event 1b. Although specifically dealing with the sediments deposited during this particular event at these localities, it is expected that the same processes were responsible for determining the exact sedimentary products at localities in similar settings, as well as during other similar events in the Mesozoic. Here, it is shown that the deposition of organic‐rich sediments during oceanic anoxic event 1b was a consequence of the enhanced productivity favoured by upwelling and by riverine nutrient input, or aeolian fertilization of the euphotic zone depending on geographical location. Slope instability processes resulted in the transfer of part of these organic‐rich sediments from the shelf to deep sea depocentres as mud‐laden organic‐rich turbidites, especially in the northern North Atlantic. The so‐called ‘black shales’ are much more varied than their name implies. The end product of sedimentation during an oceanic anoxic event at a particular location is commonly the result of several equifinal processes acting on a local scale rather than the direct result of basinal or even global mechanisms. 相似文献
27.
Kazuto SAIKI Didier LAPORTE Daniel VIELZEUF Satoru NAKASHIMA Pierre BOIVIN 《Meteoritics & planetary science》2003,38(3):427-444
Abstract— Two types of pallasites can be distinguished on the basis of the grain shape of olivine (rounded or angular). It has been suggested that these two types of textures resulted from different degrees of annealing at high temperature in the parent body. In order to characterize the kinetics of rounding of olivine grains in an Fe‐Ni matrix, we carried out a series of annealing experiments using a mixture of olivine and Fe‐Ni powder. We were able to reproduce, at a miniature scale, the range of textures in pallasites. The rate of rounding was rapid enough to be observed and measured at the scale of a few micrometers to 20 μm, even though the experiments were performed below the solidus of the Fe‐Ni metal. For instance, grains ?14 mm in diameter became nearly spherical within 7 days at 1400°C. For the morphological analysis of olivine grains, we used two independent techniques: the “critical diameter method” and the “Gaussian diffusion‐resample method,” a new technique specially developed for our study. Both techniques indicate that the rounding time scale is proportional to the cube of the grain size and that morphological adjustments in our experiments occurred by volume diffusion in the olivine lattice, not by surface diffusion along the olivine‐metal boundaries. We used our experimental data to estimate the time scales required for the development of olivine‐metal textures in natural pallasites. We determined that small scale rounding of olivine grains in a solid metal matrix can be produced within relatively short time intervals: ?100 years to produce rounded olivine grains 0.1 mm in radius at 1300–1400°C. 相似文献
28.
We have started a survey of galaxies at intermediate redshifts using the HST-STIS parallel fields. Our main goal is to analyse
the morphology of faint galaxies in order to estimate the epoch of formation of the Hubble classification sequence. The high
resolution of STIS images (0.05″) is ideal for this work and enables us to perform a morphological classification and to analyse
the internal structures of galaxies. We find that 40% of the 290 galaxies are early types and that there are more irregulars
and ellipticals at the fainter magnitudes.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
29.
R.A. GarcÍa T. Corbard W.J. Chaplin S. Couvidat A. Eff-Darwich S.J. Jiménez-Reyes S.G. Korzennik J. Ballot P. Boumier E. Fossat C.J. Henney R. Howe M. Lazrek J. Lochard P.L. Pallé S. Turck-Chièze 《Solar physics》2004,220(2):269-285
In the modern era of helioseismology we have a wealth of high-quality data available, e.g., more than 6 years of data collected
by the various instruments on board the SOHO mission, and an even more extensive ground-based set of observations covering
a full solar cycle. Thanks to this effort a detailed picture of the internal rotation of the Sun has been constructed. In
this paper we present some of the actions that should be done to improve our knowledge of the inner rotation profile discussed
during the workshop organized at Saclay on June 2003 on this topic. In particular we will concentrate on the extraction of
the rotational frequency splittings of low- and medium-degree modes and their influence on the rotation of deeper layers.
Furthermore, for the first time a full set of individual |m|-component rotational splittings is computed for modes ℓ≤4 and 1<ν<2 mHz, opening new studies on the latitudinal dependence
of the rotation rate in the radiative interior. It will also be shown that these splittings have the footprints of the differential
rotation of the convective zone which can be extremely useful to study the differential rotation of other stars where only
these low-degree modes will be available. 相似文献
30.
The key outcomes of the 2006 Nairobi Conference on Climate Change are described, with a particular emphasis on adaptation, the Clean Development Mechanism (CDM) and the future of the United Nations climate change regime beyond 2012. Based on an analysis of the key issues being negotiated, the Nairobi Conference can be understood as an important step in the larger process towards a future climate change regime. Its significance is in establishing the confidence and trust between the key players that will be necessary for the successful completion of the current phase of negotiations and agreement on the post-2012 climate change regime. 相似文献