全文获取类型
收费全文 | 5411篇 |
免费 | 797篇 |
国内免费 | 879篇 |
专业分类
测绘学 | 71篇 |
大气科学 | 226篇 |
地球物理 | 323篇 |
地质学 | 3425篇 |
海洋学 | 257篇 |
天文学 | 2306篇 |
综合类 | 201篇 |
自然地理 | 278篇 |
出版年
2024年 | 23篇 |
2023年 | 65篇 |
2022年 | 124篇 |
2021年 | 143篇 |
2020年 | 139篇 |
2019年 | 152篇 |
2018年 | 131篇 |
2017年 | 154篇 |
2016年 | 149篇 |
2015年 | 184篇 |
2014年 | 228篇 |
2013年 | 231篇 |
2012年 | 258篇 |
2011年 | 250篇 |
2010年 | 218篇 |
2009年 | 443篇 |
2008年 | 356篇 |
2007年 | 464篇 |
2006年 | 436篇 |
2005年 | 417篇 |
2004年 | 373篇 |
2003年 | 359篇 |
2002年 | 304篇 |
2001年 | 254篇 |
2000年 | 231篇 |
1999年 | 205篇 |
1998年 | 204篇 |
1997年 | 100篇 |
1996年 | 65篇 |
1995年 | 84篇 |
1994年 | 66篇 |
1993年 | 54篇 |
1992年 | 66篇 |
1991年 | 27篇 |
1990年 | 31篇 |
1989年 | 43篇 |
1988年 | 20篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有7087条查询结果,搜索用时 10 毫秒
21.
Davide Lazzati Rosalba Perna Gabriele Ghisellini 《Monthly notices of the Royal Astronomical Society》2001,325(4):L19-L23
The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor. 相似文献
22.
Hong Lin Zhang Douglas H. Sampson 《Monthly notices of the Royal Astronomical Society》2001,322(3):433-437
Calculations are made of the resonance contribution to electron-impact excitation of H-like 13 C and Li-like 23 Na, 25 Mg, 27 Al and 29 Si to the upper hyperfine levels that produce millimetre (mm) lines of likely astrophysical interest. The resonance contribution is found to be very important for these Li-like ions, considerably more important than for Li-like 57 Fe considered previously. However, resonances are found to be rather unimportant for H-like 13 C. The effect of radiative decay on the resonance contribution is found to be insignificant in all of the present calculations. 相似文献
23.
Francesca Matteucci Antonio Pipino 《Monthly notices of the Royal Astronomical Society》2005,357(2):489-496
We have studied the effects of a hypothetical initial generation containing very massive stars [ M > 100 M⊙ , pair-creation supernovae] on the chemical and photometric evolution of elliptical galaxies. To this purpose, we have computed the evolution of a typical elliptical galaxy with luminous mass of the order of 1011 M⊙ and adopted chemical evolution models already tested to reproduce the main features of ellipticals. We have tested several sets of yields for very massive zero-metallicity stars: these stars should produce quite different amounts of heavy elements than lower-mass stars. We found that the effects of Population III stars on the chemical enrichment is negligible if only one or two generations of such stars occurred, whereas they produce quite different results from the standard models if they continuously formed for a period not shorter than 0.1 Gyr. In this case, the results are at variance with the main observational constraints of ellipticals such as the average [〈α/ Fe〉* ] ratio in stars and the integrated colours. Therefore, we conclude that if Population III stars ever existed they must have been present for a very short period of time and their effects on the following evolution of the parent galaxy must have been negligible. This effect is minimum if a more realistic model with initial infall of gas rather than the classic monolithic model is adopted. Ultimately, we conclude that there is no need to invoke a generation of very massive stars in ellipticals to explain their chemical and photometric properties. 相似文献
24.
S. C. Trager Guy Worthey S. M. Faber Alan Dressler 《Monthly notices of the Royal Astronomical Society》2005,362(1):2-8
We present numerical investigations into the formation of massive stars from turbulent cores of density structure ρ∝ r −1.5 . The results of five hydrodynamical simulations are described, following the collapse of the core, fragmentation and the formation of small clusters of protostars. We generate two different initial turbulent velocity fields corresponding to power-law spectra P ∝ k −4 and P ∝ k −3.5 , and we apply two different initial core radii. Calculations are included for both completely isothermal collapse, and a non-isothermal equation of state above a critical density (10−14 g cm−3 ) . Our calculations reveal the preference of fragmentation over monolithic star formation in turbulent cores. Fragmentation was prevalent in all the isothermal cases. Although disc fragmentation was largely suppressed in the non-isothermal runs due to the small dynamic range between the initial density and the critical density, our results show that some fragmentation still persisted. This is inconsistent with previous suggestions that turbulent cores result in the formation of a single massive star. We conclude that turbulence cannot be measured as an isotropic pressure term. 相似文献
25.
26.
27.
28.
29.
30.
M. A. Smith 《第四纪科学杂志》2009,24(7):747-760
Puritjarra rock shelter provides a long record of late Quaternary vegetation in the Australian arid zone. Analysis of the sedimentary history of this rock shelter is combined with reanalysis of charcoal and phytolith records to provide a first‐order picture of changing landscapes in western Central Australia. These show a landscape responding to increasing aridity from 45 ka with deflation of clay‐rich red palaeosols (<45 ka) and sharp declines in grassland and other vegetation at 40–36 ka, and at the beginning of the Last Glacial Maximum (LGM) (24 ka). Vegetation in the catchment of the rock shelter recovered after 15 ka with expansion of both acacia woodland and spinifex grasslands, registering stronger summer rainfall in the interior of the continent. By 8.3 ka re‐vegetation of local palaeosols and dunes had choked off sediment supply to the rock shelter and the character of the sediments changed abruptly. Poaceae values peaked at 5.8 ka, suggesting the early–mid Holocene climatic optimum in Central Australia is bracketed between 8.3 and 5.8 ka. Local vegetation was disrupted in the late Holocene with a sharp decline in Poaceae at 3.8 ka, coinciding with an abrupt intensification of ENSO. Local grasslands recovered over the next two millennia and by 1.5 ka the modern vegetation appears to have become established. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献