全文获取类型
收费全文 | 601篇 |
免费 | 39篇 |
国内免费 | 63篇 |
专业分类
测绘学 | 9篇 |
大气科学 | 47篇 |
地球物理 | 36篇 |
地质学 | 187篇 |
海洋学 | 24篇 |
天文学 | 380篇 |
综合类 | 7篇 |
自然地理 | 13篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 6篇 |
2021年 | 7篇 |
2020年 | 16篇 |
2019年 | 11篇 |
2018年 | 6篇 |
2017年 | 9篇 |
2016年 | 13篇 |
2015年 | 11篇 |
2014年 | 20篇 |
2013年 | 21篇 |
2012年 | 17篇 |
2011年 | 11篇 |
2010年 | 10篇 |
2009年 | 51篇 |
2008年 | 43篇 |
2007年 | 41篇 |
2006年 | 54篇 |
2005年 | 53篇 |
2004年 | 40篇 |
2003年 | 45篇 |
2002年 | 30篇 |
2001年 | 30篇 |
2000年 | 25篇 |
1999年 | 28篇 |
1998年 | 38篇 |
1997年 | 11篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 11篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1980年 | 2篇 |
排序方式: 共有703条查询结果,搜索用时 15 毫秒
91.
92.
We present the results based on multiwavelength imaging observations of the prominent dust lane starburst galaxy NGC 1482 aimed to investigate the extinction properties of dust existing in the extreme environment. (B-V) colour-index map derived for the starburst galaxy NGC 1482 confirms two prominent dust lanes running along its optical major axis and are found to extend up to ∼11 kpc. In addition to the main lanes, several filamentary structures of dust originating from the central starburst are also evident. Though, the dust is surrounded by exotic environment, the average extinction curve derived for this target galaxy is compatible with the Galactic curve, with RV = 3.05, and imply that the dust grains responsible for the optical extinction in the target galaxy are not really different than the canonical grains in the Milky Way. Our estimate of total dust content of NGC 1482 assuming screening effect of dust is ∼2.7 × 105 M⊙, and provide lower limit due to the fact that our method is not sensitive to the intermix component of dust. Comparison of the observed dust in the galaxy with that supplied by the SNe to the ISM, imply that this supply is not sufficient to account for the observed dust and hence point towards the origin of dust in this galaxy through a merger like event.Our multiband imaging analysis reveals a qualitative physical correspondence between the morphologies of the dust and Hα emission lines as well as diffuse X-ray emission in this galaxy. Spatially resolved spectral analysis of the hot gas along outflows exhibit a gradient in the temperature. Similar gradient was also noticed in the measured values of metallicity, indicating that the gas in the halo is not yet enriched. High resolution, 2-8 keV Chandra image reveals a pair of point sources in the nuclear region with their luminosities equal to 2.27 × 1039 erg s−1 and 9.34 × 1039 erg s−1, and are in excess of the Eddington-limit of 1.5 M⊙ accreting source. Spectral analysis of these sources exhibit an absorbed-power law with the hydrogen column density higher than that derived from the optical measurements. 相似文献
93.
94.
95.
Brice Ménard Daniel Nestor David Turnshek Anna Quider Gordon Richards Doron Chelouche Sandhya Rao 《Monthly notices of the Royal Astronomical Society》2008,385(2):1053-1066
Using a sample of almost 7000 strong Mg ii absorbers with W 0 > 1 Å and 0.4 < z < 2.2 detected in the SDSS DR4 data set, we investigate the gravitational lensing and dust extinction effects they induce on background quasars. After carefully quantifying several selection biases, we isolate the reddening effects as a function of redshift and absorber rest equivalent width, W 0 . We find the amount of dust to increase with cosmic time as τ( z ) ∝ (1 + z )−1.1±0.4 , following the evolution of cosmic star density or integrated star formation rate. We measure the reddening effects over a factor of 30 in E ( B − V ) and we find that τ∝ ( W 0 )1.9±0.1 , providing us with an important scaling for theoretical modelling of metal absorbers. We also measure the dust-to-metal ratio and find it similar to that of the Milky Way. In contrast to previous studies, we do not detect any gravitational magnification by Mg ii systems. We measure the upper limit μ < 1.10 and discuss the origin of the discrepancy. Finally, we estimate the fraction of absorbers missed due to extinction effects and show that it rises from 1 to 50 per cent in the range 1 < W 0 < 6 Å . We parametrize this effect and provide a correction for recovering the intrinsic ∂ N /∂ W 0 distribution. 相似文献
96.
J. H. Hough D. K. Aitken D. C. B. Whittet A. J. Adamson A. Chrysostomou 《Monthly notices of the Royal Astronomical Society》2008,387(2):797-802
We present spectropolarimetry of the solid CO feature at 4.67 μm along the line of sight to Elias 16, a field star background to the Taurus dark cloud. A clear increase in polarization is observed across the feature with the peak of polarization shifted in wavelength relative to the peak of absorption. This shows that dust grains in dense, cold environments (temperatures ∼20 K or less) can align and produce polarization by dichroic absorption. For a grain model, consisting of a core with a single mantle, the polarization feature is best modelled by a thick CO mantle, possibly including 10 per cent water-ice, with the volume ratio of mantle to bare grain of ∼5. Radiative torques could be responsible for the grain alignment provided the grain radius is at least 0.5 μm. This would require the grain cores to have a radius of at least 0.3 μm, much larger than grain sizes in the diffuse interstellar medium. Sizes of this order seem reasonable on the basis of independent evidence for grain growth by coagulation, as well as mantle formation, inside dense clouds. 相似文献
97.
98.
M. A. Burlak 《Astronomy Letters》2008,34(4):249-255
Using photometric data for Galactic novae, we compared the Sharov (1963) and Schlegel et al. (1998) interstellar extinction maps. We found the distances and extinctions for 64 novae. 相似文献
99.
当代生物多样性剧减与古-中生代之交生物绝灭的对比 总被引:1,自引:0,他引:1
18世纪以来人类活动对环境造成了强烈破坏,给生物多样性带来了巨大损失.为了了解生物多样性的变化趋势,利用"绝灭物种统计法"、"种-区曲线法"和化石记录对400年以来的生物多样性和古-中生代之交的生物绝灭进行了分析.当代生物多样性剧减表现为物种的快速绝灭(从8×104种/Ma增大到1.2×106种/Ma),而属的绝灭几乎为0;古-中生代之交生物绝灭的初始阶段也表现为物种的快速绝灭(从40种/Ma增大到150种/Ma),并且这个阶段的属的绝灭速率远低于绝灭高峰阶段的相应值(即66属/Ma远小于465属/Ma),可见当代生物多样性剧减相当于二叠-三叠系之交生物绝灭的初始阶段. 相似文献
100.
PTB全球最大生物灭绝事件后,早三叠世被认为是微生物发育繁殖的黄金时期,浅水环境是微生物活动及微生物岩形成的最有利环境。借助高分辨率扫描电镜和能谱分析,在川东北地区开江—梁平海槽西侧下三叠统飞仙关组浅水碳酸盐岩台地边缘的鲕粒灰岩中发现了三种微生物矿化组构:空腔状微生物矿化组构、显微瘤状微生物矿化组构和纤状微生物矿化组构;通过沉积学、岩石学及地球化学的综合分析,认为显微瘤状微生物矿化组构是微生物新陈代谢时形成的纳米球状云质颗粒,纤状微生物矿化组构是微生物的胞外聚合物,而空腔状微生物矿化组构则属于微生物个体。这些微生物矿化组构的发现,为早三叠世微生物大爆发提供了一定的直观证据;推测大量的微生物活动是早三叠世碳酸盐快速沉积现象的直接诱因,并导致于二叠纪末发育的克拉通内裂陷——开江—梁平海槽在早三叠世被快速填平。 相似文献