全文获取类型
收费全文 | 60120篇 |
免费 | 563篇 |
国内免费 | 421篇 |
专业分类
测绘学 | 1213篇 |
大气科学 | 3499篇 |
地球物理 | 11700篇 |
地质学 | 22565篇 |
海洋学 | 5338篇 |
天文学 | 13950篇 |
综合类 | 143篇 |
自然地理 | 2696篇 |
出版年
2022年 | 557篇 |
2021年 | 813篇 |
2020年 | 906篇 |
2019年 | 990篇 |
2018年 | 2064篇 |
2017年 | 1884篇 |
2016年 | 2103篇 |
2015年 | 997篇 |
2014年 | 1918篇 |
2013年 | 3091篇 |
2012年 | 2218篇 |
2011年 | 2754篇 |
2010年 | 2511篇 |
2009年 | 3019篇 |
2008年 | 2645篇 |
2007年 | 2884篇 |
2006年 | 2556篇 |
2005年 | 1625篇 |
2004年 | 1569篇 |
2003年 | 1522篇 |
2002年 | 1397篇 |
2001年 | 1373篇 |
2000年 | 1210篇 |
1999年 | 945篇 |
1998年 | 963篇 |
1997年 | 991篇 |
1996年 | 751篇 |
1995年 | 744篇 |
1994年 | 685篇 |
1993年 | 589篇 |
1992年 | 573篇 |
1991年 | 552篇 |
1990年 | 607篇 |
1989年 | 531篇 |
1988年 | 479篇 |
1987年 | 562篇 |
1986年 | 432篇 |
1985年 | 633篇 |
1984年 | 669篇 |
1983年 | 643篇 |
1982年 | 596篇 |
1981年 | 549篇 |
1980年 | 548篇 |
1979年 | 472篇 |
1978年 | 544篇 |
1977年 | 453篇 |
1976年 | 420篇 |
1975年 | 443篇 |
1974年 | 396篇 |
1973年 | 426篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The properties of OH megamaser galaxies in the x-ray region are discussed. Observational data from the XMM-Newton x-ray satellite
are presented for three megamasers, IRAS 01418+1651, IRAS 11010+4107, and IRAS 13218+0552. Based on a sample of OH megamaser
galaxies it is shown that the x-ray and OH emission are closely coupled. Megamaser emission also correlates with the column
density of the medium and the OH emissivity increases as the square of the column density. The results of this study indicate
that in OH megamaser galaxies an active nucleus, x-ray heating of molecules, and saturation of the maser emission can play
an important role. In some megamasers, active star formation may be the dominant source.
__________
Translated from Astrofizika, Vol. 50, No. 2, pp. 171–182 (May 2007). 相似文献
992.
S. S. Kaisin A. V. Kasparova A. Yu. Knyazev I. D. Karachentsev 《Astronomy Letters》2007,33(5):283-291
We present our Hα observations of 11 isolated southern galaxies: SDIG, PGC 51659, E 222-010, E 272-025, E 137-018, IC 4662, Sag DIG, IC 5052, IC 5152, UGCA 438, and E 149-003, with distances from 1 to 7 Mpc. We have determined the total Hα fluxes from these galaxies. The star formation rates in these galaxies range from 10?1 (IC 4662) to 10?4 M ⊙ yr?1 (SDIG) and the gas depletion time at the observed star formation rates lies within the range from 1/6 to 24 Hubble times H 0 ?1 . 相似文献
993.
Accretion disks orbiting black holes power high-energy systems such as X-ray binaries and Active Galactic Nuclei. Observations
are providing increasingly detailed quantitative information about such systems. This data has been interpreted using standard
toy-models that rely on simplifying assumptions such as regular flow geometry and a parameterized stress. Global numerical
simulations offer a way to investigate the basic physical dynamics of accretion flows without these assumptions and, in principle,
lead to a genuinely predictive theory. In recent years we have developed a fully three-dimensional general relativistic magnetohydrodynamic
simulation code that evolves time-dependent inflows into Kerr black holes. Although the resulting global simulations of black
hole accretion are still somewhat simplified, they have brought to light a number of interesting results. These include the
formation of electro-magnetically dominated jets powered by the black hole’s rotation, and the presence of strong stresses
in the plunging region of the accretion flow. The observational consequences of these features are gradually being examined.
Increasing computer power and increasingly sophisticated algorithms promise a bright future for the computational approach
to black hole accretion. 相似文献
994.
Results from simultaneous spectral and photometric monitoring of the Herbig Ae star CQ Tau in the neighborhood of the Hα and
resonance sodium doublet Na I D lines are presented. It is shown that the inner structure of the accretion disk of CQ Tau
is nonuniform and consists of two regions with quite different kinematic characteristics. Region I is characterized by relative
stability and a smooth long-term variation in the velocity of the gas along the line of sight. Region II is distinguished
by the highest velocities and a variability in their maximum values over time scales from a few days to 700 days. The dust
clouds which produse the star’s brightness minima may also be the source of cold gas and contribute to the observed spectral
variability. We assume that region I of the disk coincides with the accretion disk of the star. The kinematic differences
in region II may be caused by dissipation of circumstellar dust clouds which, moving in elongated orbits, are able to approach
the star quite closely.
__________
Translated from Astrofizika, Vol. 50, No. 1, pp. 39–55 (February 2007). 相似文献
995.
V. K. Dubrovich 《Astronomy Letters》2007,33(10):645-648
Some problems of the two-photon decay of highly excited atomic hydrogen and helium levels are considered. The necessary accuracy of calculating the transition probabilities is shown to require abandoning the purely dipole approximation in the theory of atomic hydrogen radiation. The range of applicability of the perturbation theory to this process is discussed. We suggest a new mechanism of the Lyman photon redistribution due to very close locations of the 20P and 19P levels in the hydrogen and deuterium atoms, respectively (hydrogen-deuterium resonance). 相似文献
996.
997.
M. Fellhauer N. W. Evans V. Belokurov D. B. Zucker B. Yanny M. I. Wilkinson G. Gilmore M. J. Irwin D. M. Bramich S. Vidrih P. Hewett T. Beers 《Monthly notices of the Royal Astronomical Society》2007,375(4):1171-1179
Prominent in the 'Field of Streams'– the Sloan Digital Sky Survey map of substructure in the Galactic halo – is an 'Orphan Stream' without obvious progenitor. In this numerical study, we show a possible connection between the newly found dwarf satellite Ursa Major II (UMa II) and the Orphan Stream. We provide numerical simulations of the disruption of UMa II that match the observational data on the position, distance and morphology of the Orphan Stream. We predict the radial velocity of UMa II as −100 km s−1 , as well as the existence of strong velocity gradients along the Orphan Stream. The velocity dispersion of UMa II is expected to be high, though this can be caused both by a high dark matter content or by the presence of unbound stars in a disrupted remnant. However, the existence of a gradient in the mean radial velocity across UMa II provides a clear-cut distinction between these possibilities. The simulations support the idea that some of the anomalous, young halo globular clusters like Palomar 1 or Arp 2 or Ruprecht 106 may be physically associated with the Orphan Stream. 相似文献
998.
999.
Mark R. Calabretta Boudewijn F. Roukema 《Monthly notices of the Royal Astronomical Society》2007,381(2):865-872
The natural spherical projection associated with the Hierarchical Equal-Area and Isolatitude Pixelization (HEALPix) is described and shown to be one of a hybrid class that combines the cylindrical equal-area and Collignon projections, not previously documented in the cartographic literature. Projection equations are derived for the class in general and are used to investigate its properties. It is shown that the HEALPix projection suggests a simple method of (i) storing and (ii) visualizing data sampled on the grid of the HEALPix pixelization, and also suggests an extension of the pixelization that is better suited for these purposes. Potentially useful properties of other members of the class are described, and new triangular and hexagonal pixelizations are constructed from them. Finally, the standard formalism is defined for representing the celestial coordinate system for any member of the class in the FITS data format. 相似文献
1000.