全文获取类型
收费全文 | 3452篇 |
免费 | 205篇 |
国内免费 | 465篇 |
专业分类
测绘学 | 45篇 |
大气科学 | 108篇 |
地球物理 | 539篇 |
地质学 | 969篇 |
海洋学 | 751篇 |
天文学 | 1422篇 |
综合类 | 77篇 |
自然地理 | 211篇 |
出版年
2024年 | 18篇 |
2023年 | 53篇 |
2022年 | 67篇 |
2021年 | 81篇 |
2020年 | 71篇 |
2019年 | 96篇 |
2018年 | 71篇 |
2017年 | 80篇 |
2016年 | 82篇 |
2015年 | 95篇 |
2014年 | 107篇 |
2013年 | 102篇 |
2012年 | 96篇 |
2011年 | 141篇 |
2010年 | 117篇 |
2009年 | 276篇 |
2008年 | 256篇 |
2007年 | 269篇 |
2006年 | 269篇 |
2005年 | 212篇 |
2004年 | 221篇 |
2003年 | 185篇 |
2002年 | 170篇 |
2001年 | 161篇 |
2000年 | 154篇 |
1999年 | 144篇 |
1998年 | 145篇 |
1997年 | 45篇 |
1996年 | 47篇 |
1995年 | 44篇 |
1994年 | 42篇 |
1993年 | 21篇 |
1992年 | 18篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 27篇 |
1988年 | 8篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 22篇 |
1984年 | 16篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 10篇 |
1978年 | 3篇 |
1977年 | 1篇 |
排序方式: 共有4122条查询结果,搜索用时 15 毫秒
41.
J. P. Phillips 《Monthly notices of the Royal Astronomical Society》2007,376(3):1120-1132
We present Two Micron All Sky Survey photometry for 216 symbiotic and possible symbiotic stars. This represents the most comprehensive near-infrared photometry for these sources which has been published to date. The results are used to define the statistics of S- and D-type stars; to investigate the photometric properties of Magellanic S-type symbiotics; and to define the possible evolution of D-type symbiotics within the J − H / H − K S colour plane. It appears that the colours of D-type symbiotics are consistent with differing mass-loss rates d M /d t , provided that the grains have a silicate-like composition, and maximum temperatures of the order of ∼800 K. 相似文献
42.
Shude Mao Hans J. Witt Leon V. E. Koopmans 《Monthly notices of the Royal Astronomical Society》2001,323(2):301-307
Recent observations indicate that many if not all galaxies host massive central black holes. In this paper we explore the influence of black holes on the lensing properties. We model the lens as an isothermal ellipsoid with a finite core radius plus a central black hole. We show that the presence of the black hole substantially changes the critical curves and caustics. If the black hole mass is above a critical value, then it will completely suppress the central images for all source positions. Realistic central black holes are likely to have masses below this critical value. Even in such subcritical cases, the black hole can suppress the central image when the source is inside a zone of influence, which depends on the core radius and black hole mass. In the subcritical cases, an additional image may be created by the black hole in some regions, which for some radio lenses may be detectable with high-resolution and large dynamic range VLBI maps. The presence of central black holes should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses. 相似文献
43.
44.
45.
云南金顶铅锌矿床是世界上成矿时代最新且唯一的陆相沉积岩容矿的超大型铅锌矿床。矿区存在2类有机质——原生型和迁移型,与矿石共生。生物标志化合物研究结果表明:正构烷烃碳数分布范围较宽,有2种峰型;萜烷化合物的相对丰度五环三萜烷>三环萜烷>四环萜烷;规则甾烷C27~C28~C29呈不对称V字型分布;有机质的母质来源以低等水生生源的海藻类为主,伴随有一定量的陆源高等植物组分;有机质沉积于弱氧化—弱还原环境;成矿热液的热力作用使样品遭受了一定程度的热解,部分样品还遭受了生物降解作用;生物标志物多参数对比表明有机质来源于三叠纪三合洞组碳酸盐岩地层。原生型有机质的成矿作用主要体现在对成矿金属元素的预富集上,而迁移型有机质的成矿作用主要是对成矿金属元素的活化运移。 相似文献
46.
We have mapped the high-mass star-forming region W49A at 450, 800, and 1100 microns with the JCMT. Spectral index measurements suggest an increase in temperature towards the emission peaks, consistent with previous data. We derive the gas masses associated with the central and extended emission from each of the three components, and find a deficit of gas around W49SW. The mass found for the core of W49N is in good agreement with the value previously derived from C34S (5-4) maps (Serabynet al., 1993), and similar morphologies are found in the line and continuum maps. 相似文献
47.
48.
M. Lyutikov 《Monthly notices of the Royal Astronomical Society》2006,373(1):73-78
Sharp fronts observed by the Chandra satellite between dense cool cluster cores moving with near-sonic velocity through the hotter intergalactic gas, require strong suppression of thermal conductivity across the boundary. This may be due to magnetic fields tangential to the contact surface separating the two plasma components. We point out that a super-Alfvenic motion of a plasma cloud (a core of a merging galaxy) through a weakly magnetized intercluster medium leads to 'magnetic draping', formation of a thin, strongly magnetized boundary layer with a tangential magnetic field. For supersonic cloud motion, M s ≥ 1 , magnetic field inside the layer reaches near-equipartition values with thermal pressure. Typical thickness of the layer is ∼ L / M 2 A ≪ L , where L is the size of the obstacle (plasma cloud) moving with Alfvén Mach number M A ≫ 1 . To a various degree, magnetic draping occurs for both subsonic and supersonic flows, random and ordered magnetic fields and it does not require plasma compressibility. The strongly magnetized layer will thermally isolate the two media and may contribute to the Kelvin–Helmholtz stability of the interface. Similar effects occur for radio bubbles, quasi-spherical expanding cavities blown up by active galactic nucleus jets; in this case, the thickness of the external magnetized layer is smaller, ∼ L / M 3 A ≪ L . 相似文献
49.
50.
Most main sequence stars are binaries or higher multiplicity Systems and it appears that at birth most stars have circumstellar
disks. It is commonly accepted that planetary systems arise from the material of these disks; consequently, binary and multiple
systems may have a main role in planet formation. In this paper, we study the stage of planetary formation during which the
particulate material is still dispersed as centimetre-to-metre sized primordial aggregates. We investigate the response of
the particles, in a protoplanetary disk with radius RD = 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300–1600
AU) orbit. For this purpose, we have carried out a series of simulations of coplanar hierarchical configurations using a direct
integration code that models gravitational and viscous forces. The massive protoplanetary disk is around one of the components
of the binary. The evolution in time of the dust sub-disk depends mainly on the nature (prograde or retrograde) of the relative
revolution of the stellar companion, and on the temperature and mass of the circumstellar disk. Our results show that for
binary companions near the limit of tidal truncation of the disk, the perturbation leads to an enhanced accretion rate onto
the primary, decreasing the lifetime of the particles in the protoplanetary disk with respect to the case of a single star.
As a consequence of an enhanced accretion rate the mass of the disk decreases faster, which leads to a longer resultant lifetime
for particles in the disk. On the other hand, binary companions may induce tidal arms in the dust phase of protoplanetary
disks. Spiral perturbations with m = 1 may increase in a factor 10 or more the dust surface density in the neighbourhood of
the arm, facilitating the growth of the particles. Moreover, in a massive disk (0.01M⊙) the survival time of particles is
significantly shorter than in a less massive nebula (0.001M⊙) and the temperature of the disk severely influences the spiral-in
time of particles. The rapid evolution of the dust component found in post T Tauri stars can be explained as a result of their
binary nature. Binarity may also influence the evolution of circumpulsar disks.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献