全文获取类型
收费全文 | 7266篇 |
免费 | 1135篇 |
国内免费 | 1620篇 |
专业分类
测绘学 | 93篇 |
大气科学 | 204篇 |
地球物理 | 798篇 |
地质学 | 4609篇 |
海洋学 | 618篇 |
天文学 | 2133篇 |
综合类 | 329篇 |
自然地理 | 1237篇 |
出版年
2024年 | 32篇 |
2023年 | 109篇 |
2022年 | 318篇 |
2021年 | 326篇 |
2020年 | 299篇 |
2019年 | 300篇 |
2018年 | 283篇 |
2017年 | 276篇 |
2016年 | 261篇 |
2015年 | 315篇 |
2014年 | 317篇 |
2013年 | 373篇 |
2012年 | 339篇 |
2011年 | 351篇 |
2010年 | 305篇 |
2009年 | 556篇 |
2008年 | 445篇 |
2007年 | 498篇 |
2006年 | 552篇 |
2005年 | 419篇 |
2004年 | 417篇 |
2003年 | 458篇 |
2002年 | 368篇 |
2001年 | 309篇 |
2000年 | 309篇 |
1999年 | 258篇 |
1998年 | 281篇 |
1997年 | 189篇 |
1996年 | 142篇 |
1995年 | 127篇 |
1994年 | 132篇 |
1993年 | 84篇 |
1992年 | 84篇 |
1991年 | 44篇 |
1990年 | 47篇 |
1989年 | 25篇 |
1988年 | 18篇 |
1987年 | 22篇 |
1986年 | 10篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
991.
992.
Michael B. Pracy Warrick J. Couch Chris Blake Kenji Bekki Craig Harrison Matthew Colless Harald Kuntschner Roberto de Propris 《Monthly notices of the Royal Astronomical Society》2005,359(4):1421-1432
We present spatially resolved intermediate-resolution spectroscopy of a sample of 12 E+A galaxies in the z = 0.32 rich galaxy cluster AC 114, obtained with the FLAMES multi-integral field unit system on the Very Large Telescope (VLT) of the European Southern Observatory. Previous integrated spectroscopy of all these galaxies by Couch & Sharples had shown them to have strong Balmer line absorption and an absence of [O ii ]λ3727 emission – the defining characteristics of the 'E+A' spectral signature, indicative of an abrupt halt to a recent episode of quite vigorous star formation. We have used our spectral data to determine the radial variation in the strength of Hδ absorption in these galaxies and hence map out the distribution of this recently formed stellar population. Such information provides important clues as to what physical event might have been responsible for this quite dramatic change in star formation activity in the recent past of these galaxies. We find a diversity of behaviour amongst these galaxies in terms of the radial variation in Hδ absorption: four galaxies show little Hδ absorption across their entire extent; it would appear they were misidentified as E+A galaxies in the earlier integrated spectroscopic studies. The remainder show strong Hδ absorption, with a gradient that is either negative (Hδ equivalent width decreasing with radius), flat or positive . By comparison with numerical simulations we suggest that the first of these different types of radial behaviour provides evidence for a merger/interaction origin, whereas the latter two types of behaviour are more consistent with the truncation of star formation in normal disc galaxies with the Hδ gradient becoming increasingly positive with time after truncation. It would seem therefore that more than one physical mechanism is responsible for E+A formation in the same environment. 相似文献
993.
994.
Elizabeth R. Stanway Richard G. McMahon Andrew J. Bunker 《Monthly notices of the Royal Astronomical Society》2005,359(3):1184-1192
We analyse near-infrared Hubble Space Telescope ( HST )/Near-Infrared Camera and Multi-Object Spectrometer F 110 W ( J ) and F 160 W ( H ) band photometry of a sample of 27 i '-drop candidate z ≃ 6 galaxies in the central region of the HST /Advanced Camera for Surveys Ultra Deep Field . The infrared colours of the 20 objects not affected by near neighbours are consistent with a high-redshift interpretation. This suggests that the low-redshift contamination of this i '-drop sample is smaller than that observed at brighter magnitudes, where values of 10–40 per cent have been reported. The J – H colours are consistent with a slope flat in fν ( fλ ∝λ−2 ) , as would be expected for an unreddened starburst. However, there is evidence for a marginally bluer spectral slope ( fλ ∝λ−2.2 ) , which is perhaps indicative of an extremely young starburst (∼10 Myr old) or a top heavy initial mass function and little dust. The low levels of contamination, median photometric redshift of z ∼ 6.0 and blue spectral slope, inferred using the near-infrared data, support the validity of the assumptions in our earlier work in estimating the star formation rates, and that the majority of the i -drop candidates galaxies lie at z ∼ 6 . 相似文献
995.
996.
997.
E. Athanassoula 《Celestial Mechanics and Dynamical Astronomy》2005,91(1-2):9-31
Angular momentum redistribution within barred galaxies drives their dynamical evolution. Angular momentum is emitted mainly
by near-resonant material in the bar region and absorbed by resonant material mainly in the outer disc and in the halo. This
exchange determines the strength of the bar, the decrease of its pattern speed, as well as its morphology. If the galaxy has
also a gaseous component and/or a companion or satellite, then these also take part in the angular momentum exchange. During
the evolution a bar structure forms in the inner parts of the halo as well. This bar is shorter and fatter than the disc bar
and stays so all through the simulation, although its length grows considerably with time. Viewed edge-on, the bar in the
disc component acquires a boxy or peanut shape. I describe the families of periodic orbits that explain such structures and
review the observations showing that boxy/peanut ‘bulges’ are in fact just bars seen edge-on. 相似文献
998.
999.
V. V. Orlov 《Astronomy Letters》2005,31(7):494-499
We consider the evolution of the structure of the velocity field formed by the loops of the trajectory in a generalized Hénon-Heiles model potential. Box-shaped orbits alternating with periodic trajectories are shown to dominate at low values of the energy integral. Signatures of shell-and tube-shaped structures appear as the energy increases. Tube-shaped orbits are associated with stable periodic trajectories with small resonance ratios. Zones of stochastic orbits appear at values of the energy integral close to its critical value, which corresponds to the opening of the zero-velocity contour. 相似文献
1000.
Hilda asteroids and comets are similar from the compositional point of view. The D-taxonomic class prevailing among Hildas has all the characteristics found in cometary spectra. Jupiter Family Comets (JFCs) coming from the trans-neptunian region are under the gravitational control of Jupiter, making them a dynamically unstable population with a mean dynamical lifetime of 104 to 105 years. In contrast, Hilda asteroids residing in the 3:2 mean motion resonance with Jupiter are a very stable population. But once they escape from the resonance, they are dynamically controlled by Jupiter, and in this sense their behavior resembles that of JFC. We performed a numerical simulation to analyze the dynamical evolution that Hildas follow after escaping from the resonance, and their contribution to the JFC population. We found that 8% of the particles leaving the resonance end up impacting Jupiter. 98.7% of the escaped Hildas live at least 1000 years as a JFC, with a mean lifetime of 1.4×106 years. In particular, escaped Hildas stay mainly in the region of perihelion distances greater than 2.5 AU. On the other hand, the number of escaped Hildas reaching the inner Solar System (q<2.5 AU) is negligible. So, there are almost no Hilda asteroids among the NEO population. We also analyzed the possibility that the Shoemaker-Levy 9 were an escaped Hilda asteroid. In this case, it would be possible to give stronger constraints to its pre-capture orbital elements. 相似文献