全文获取类型
收费全文 | 37016篇 |
免费 | 7148篇 |
国内免费 | 10291篇 |
专业分类
测绘学 | 1988篇 |
大气科学 | 12176篇 |
地球物理 | 7796篇 |
地质学 | 14942篇 |
海洋学 | 4444篇 |
天文学 | 5480篇 |
综合类 | 1955篇 |
自然地理 | 5674篇 |
出版年
2024年 | 192篇 |
2023年 | 558篇 |
2022年 | 1272篇 |
2021年 | 1559篇 |
2020年 | 1597篇 |
2019年 | 1794篇 |
2018年 | 1526篇 |
2017年 | 1666篇 |
2016年 | 1695篇 |
2015年 | 1856篇 |
2014年 | 2423篇 |
2013年 | 3018篇 |
2012年 | 2434篇 |
2011年 | 2469篇 |
2010年 | 2300篇 |
2009年 | 2918篇 |
2008年 | 2816篇 |
2007年 | 2877篇 |
2006年 | 2766篇 |
2005年 | 2375篇 |
2004年 | 2009篇 |
2003年 | 1831篇 |
2002年 | 1507篇 |
2001年 | 1394篇 |
2000年 | 1294篇 |
1999年 | 1079篇 |
1998年 | 963篇 |
1997年 | 737篇 |
1996年 | 671篇 |
1995年 | 620篇 |
1994年 | 520篇 |
1993年 | 378篇 |
1992年 | 324篇 |
1991年 | 223篇 |
1990年 | 180篇 |
1989年 | 130篇 |
1988年 | 146篇 |
1987年 | 41篇 |
1986年 | 58篇 |
1985年 | 44篇 |
1984年 | 52篇 |
1983年 | 24篇 |
1982年 | 18篇 |
1981年 | 12篇 |
1980年 | 19篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 23篇 |
1976年 | 2篇 |
1954年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
82.
The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b) = (0.7°,−0.07°), with a radius of 5 pc and a total mass of 1.0 × 105 to 1.6 × 106 M. The expansion velocity of the Ring is estimated to be 30–40 km s−1, which gives an expansion time scale of 1.5 × 105 year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN− from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring. 相似文献
83.
84.
85.
86.
A. S. Oliveira J. E. Steiner D. Cieslinski 《Monthly notices of the Royal Astronomical Society》2003,346(3):963-967
The star WR 7a, also known as SPH 2, has a spectrum that resembles that of V Sagittae stars although no O vi emission has been reported. The Temporal Variance Spectrum – TVS – analysis of our data shows weak but strongly variable emission of O vi lines which is below the noise level in the intensity spectrum.
Contrary to what is seen in V Sagittae stars, optical photometric monitoring shows very little, if any, flickering. We found evidence of periodic variability. The most likely photometric period is Pphot = 0.227(±14) d , while radial velocities suggest a period of P spec = 0.204(±13) d . One-day aliases of these periods can not be ruled out. We call attention to similarities with HD 45166 and DI Cru (= WR 46), where multiple periods are present. They may be associated to the binary motion or to non-radial oscillations.
In contrast to a previous conclusion by Pereira et al., we show that WR 7a contains hydrogen. The spectrum of the primary star seems to be detectable as the N v 4604 Å absorption line is visible. If so, it means that the wind is optically thin in the continuum and that it is likely to be a helium main sequence star.
Given the similarity to HD 45166, we suggests that WR 7a may be a qWR – quasi Wolf–Rayet – star. Its classification is WN4h/CE in the Smith, Shara & Moffat three-dimensional classification system. 相似文献
Contrary to what is seen in V Sagittae stars, optical photometric monitoring shows very little, if any, flickering. We found evidence of periodic variability. The most likely photometric period is P
In contrast to a previous conclusion by Pereira et al., we show that WR 7a contains hydrogen. The spectrum of the primary star seems to be detectable as the N v 4604 Å absorption line is visible. If so, it means that the wind is optically thin in the continuum and that it is likely to be a helium main sequence star.
Given the similarity to HD 45166, we suggests that WR 7a may be a qWR – quasi Wolf–Rayet – star. Its classification is WN4h/CE in the Smith, Shara & Moffat three-dimensional classification system. 相似文献
87.
88.
昆仑山口大地震与地形变异常的讨论 总被引:7,自引:5,他引:2
针对昆仑山口大地震,总结了多种地形变(大地测量)手段所显示的异常变化及其时空分布,结果显示:8.1级大震前存在空间尺度大,时间尺度的地形变前兆异常,简要介绍了相关的异常图像,给出了初步解释,并对未来震情的发展进行了探讨,认为近期内强震活动向华北迁移的可能性不大。 相似文献
89.
We present the results of a systematic investigation of spectral evolution in the Z source GX 349+2, using data obtained during 1998 with the Proportional Counter Array (PCA) on-board the RXTE satellite. The source traced a extended normal branch (NB) and flaring branch (FB) in the colour–colour diagram (CD) and the hardness-intensity diagram (HID) during these observations. The spectra at different positions of the Z-track were best fitted by a model consisting of a disc blackbody and a Comptonized spectrum. A broad (Gaussian) iron line at ∼6.7 keV is also required to improve the fit. The spectral parameters showed a systematic and significant variation with the position along the Z-track. The evolution in spectral parameters is discussed in view of the increasing mass accretion rate scenario, proposed to explain the motion of Z sources in the CD and the HID. 相似文献
90.
We investigate the role of seasonal variations of Titan’s stratospheric composition on the temperature. We use a general circulation model coupled with idealized chemical tracers that reproduce variations of ethane (C2H6), acetylene (C2H2), and hydrogen cyanide (HCN). Enhancement of the mole fractions of these compounds, at high latitudes in the winter hemisphere relative to their equatorial values, induces a relative decrease in temperature above approximately 0.2 mbar, with a peak amplitude around −20 K, and a relative increase in temperature below, around 1 mbar, with a peak amplitude around +7 K. These thermal effects are mainly due to the variations of the cooling to space induced by the varying distributions. The ethane, acetylene, and hydrogen cyanide variations affect the cooling rates in a similar way, with the dominant effect being due to ethane, though its latitudinal variations are small. 相似文献