全文获取类型
收费全文 | 19482篇 |
免费 | 520篇 |
国内免费 | 181篇 |
专业分类
测绘学 | 422篇 |
大气科学 | 197篇 |
地球物理 | 521篇 |
地质学 | 1802篇 |
海洋学 | 224篇 |
天文学 | 16460篇 |
综合类 | 77篇 |
自然地理 | 480篇 |
出版年
2024年 | 41篇 |
2023年 | 73篇 |
2022年 | 141篇 |
2021年 | 89篇 |
2020年 | 88篇 |
2019年 | 152篇 |
2018年 | 54篇 |
2017年 | 44篇 |
2016年 | 68篇 |
2015年 | 164篇 |
2014年 | 133篇 |
2013年 | 124篇 |
2012年 | 195篇 |
2011年 | 213篇 |
2010年 | 262篇 |
2009年 | 1561篇 |
2008年 | 1497篇 |
2007年 | 1760篇 |
2006年 | 1774篇 |
2005年 | 1587篇 |
2004年 | 1727篇 |
2003年 | 1471篇 |
2002年 | 1288篇 |
2001年 | 1137篇 |
2000年 | 916篇 |
1999年 | 879篇 |
1998年 | 1054篇 |
1997年 | 213篇 |
1996年 | 96篇 |
1995年 | 256篇 |
1994年 | 282篇 |
1993年 | 120篇 |
1992年 | 68篇 |
1991年 | 84篇 |
1990年 | 75篇 |
1989年 | 130篇 |
1988年 | 81篇 |
1987年 | 81篇 |
1986年 | 74篇 |
1985年 | 37篇 |
1984年 | 26篇 |
1983年 | 22篇 |
1982年 | 3篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1977年 | 6篇 |
1905年 | 3篇 |
1900年 | 3篇 |
1897年 | 7篇 |
1877年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 111 毫秒
991.
Gavin Ramsay & Peter J. Wheatley 《Monthly notices of the Royal Astronomical Society》1998,301(1):95-100
We present low–medium resolution optical spectroscopy of the eclipsing AM Her system MN Hya (RX J0929–24). We determine the magnetic field strength at the primary accretion region of the white dwarf to be 42 MG from the spacing of cyclotron features visible during π ∼ 0.4–0.7. From spectra taken during the eclipse we find that the secondary has an M3–4 spectral type. Combined with the eclipse photometry of Sekiguchi, Nakada &38; Bassett and an estimate of the interstellar extinction we find a distance of ∼300–700 pc. We find unusual line variations at π ∼ 0.9: Hα is seen in absorption and emission. This is at the same point in the orbital phase at which a prominent absorption dip is seen in soft X-rays. 相似文献
992.
P. E. J. Nulsen X. Barcons & A. C. Fabian 《Monthly notices of the Royal Astronomical Society》1998,301(1):168-174
It is argued that the formation of a dwarf galaxy causes a massive burst of star formation, resulting in the ejection of most of the available gas from the galaxy as a weakly collimated wind. The ejected gas can give rise to a damped Lyα absorber (DLA). Weakly collimated outflows naturally explain the asymmetric profiles seen in low-ionization absorption lines caused by heavy elements associated with DLAs, where absorption is strongest at one edge of the absorption feature. The shape of the distribution of column densities in the model agrees reasonably well with observations. In particular, the break in slope is caused by external photoionization of the wind. A semi-analytical model for galaxy formation is used to show that, for currently acceptable cosmological parameters, dwarf galaxy outflows can account for the majority of DLA systems and their distribution with redshift. This model also predicts a correlation between velocity structure and metallicity of DLA systems, in qualitative agreement with observations. DLAs do not require many large, rapidly rotating disc galaxies to have formed early on, as in other models for their origin. 相似文献
993.
E. Rizza J. O. Burns M. J. Ledlow F. N. Owen W. Voges & M. Bliton 《Monthly notices of the Royal Astronomical Society》1998,301(2):328-342
We present results from a ROSAT HRI study of 11 distant ( z ∼ 0.2–0.3) Abell clusters. We have performed a morphological analysis to search for and quantify substructure in the clusters. About 70 per cent of the sample shows significant evidence of substructure in the form of centroid shift or obvious X-ray clumps. We examine the clusters for the presence of cooling flows, and determine the physical properties of the ICM by deprojecting the HRI data. Nine of the clusters have central cooling times less than the age of the system, in agreement with fractions determined from nearby, X-ray-bright samples. Additional PSPC results are presented for four clusters in the sample, and ASCA results for six clusters. The temperatures and metallicities for these distant clusters appear to be consistent with nearby clusters of similar richness. 相似文献
994.
Martin G. Haehnelt Priyamvada Natarajan & Martin J. Rees 《Monthly notices of the Royal Astronomical Society》1998,300(3):817-827
We demonstrate that the luminosity function of the recently detected population of actively star-forming galaxies at redshift z = 3 and the B -band luminosity function of quasi-stellar objects (QSOs) at the same redshift can both be matched with the mass function of dark matter haloes predicted by standard variants of hierarchical cosmogonies for lifetimes of optically bright QSOs anywhere in the range 106 to 108 yr. There is a strong correlation between the lifetime and the required degree of non-linearity in the relation between black hole and halo mass. We suggest that the mass of supermassive black holes may be limited by the back-reaction of the emitted energy on the accretion flow in a self-gravitating disc. This would imply a relation of black hole to halo mass of the form M bh ∝ v 5 halo ∝ M 5/3 halo and a typical duration of the optically bright QSO phase of a few times 107 yr. The high integrated mass density of black holes inferred from recent black hole mass estimates in nearby galaxies may indicate that the overall efficiency of supermassive black holes for producing blue light is smaller than previously assumed. We discuss three possible accretion modes with low optical emission efficiency: (i) accretion at far above the Eddington rate, (ii) accretion obscured by dust, and (iii) accretion below the critical rate leading to an advection-dominated accretion flow lasting for a Hubble time. We further argue that accretion with low optical efficiency might be closely related to the origin of the hard X-ray background and that the ionizing background might be progressively dominated by stars rather than QSOs at higher redshift. 相似文献
995.
Raman K. Prinja & Paul A. Crowther 《Monthly notices of the Royal Astronomical Society》1998,300(3):828-836
Archival HST FOS and GHRS data sets have been used to collect ultraviolet evidence for large- and small-scale stellar wind structure in extragalactic Local Group OB stars (i.e. SMC, LMC including R136, M31, M33 and NGC 6822). By comparison with previous studies of Galactic OB stars, wind activity is principally diagnosed in individual spectrograms via the presence of 'narrow absorption components' and saturated 'black' absorption troughs in the resonance line doublets. Their characteristics broadly suggest that these stars share the same physical mechanisms for perturbing the winds as those that act in Galactic stars. Both of these spectral indicators are also used to provide reliable measures of wind terminal velocities. These velocities are directly compared with previously published Galactic values, without reliance on model profile fitting. Relative to Galactic OB stars, the most discrepant terminal velocities (and wind line profiles) result from main-sequence early O-type stars in the SMC. 相似文献
996.
Mock 2dF and SDSS galaxy redshift surveys 总被引:1,自引:0,他引:1
997.
T. Shahbaz R. M. Bandyopadhyay P. A. Charles R. M. Wagner P. Muhli P. Hakala J. Casares & J. Greenhill 《Monthly notices of the Royal Astronomical Society》1998,300(4):1035-1040
We present optical spectroscopy and optical and infrared photometry of the neutron star soft X-ray transient Aql X–1 during its X-ray outburst of 1997 August. By modelling the X-ray, optical and IR light curves, we find a 3-d delay between the IR and X-ray rise times, analogous to the UV–optical delay seen in dwarf novae outbursts and black hole X-ray transients. We interpret this delay as the signature of an 'outside-in' outburst, in which a thermal instability in the outer disc propagates inward. This outburst is the first of this type definitively identified in a neutron star X-ray transient. 相似文献
998.
J. I. Davies P. Alton S. Bianchi & M. Trewhella 《Monthly notices of the Royal Astronomical Society》1998,300(4):1006-1014
We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 109 yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium. 相似文献
999.
Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》1998,300(4):1057-1070
A model of the gravitationally evolved dark matter distribution, in the Eulerian space, is developed. It is a simple extension of the excursion set model that is commonly used to estimate the mass function of collapsed dark matter haloes. In addition to describing the evolution of the Eulerian space distribution of the haloes, the model allows one to describe the evolution of the dark matter itself. It can also be used to describe density profiles, on scales larger than the virial radius of these haloes, and to quantify the way in which matter flows in and out of Eulerian cells. When the initial Lagrangian space distribution is white noise Gaussian, the model suggests that the Inverse Gaussian distribution should provide a reasonably good approximation to the evolved Eulerian density field, in agreement with numerical simulations. Application of this model to clustering from more general Gaussian initial conditions is discussed at the end. 相似文献
1000.
Neal Jackson Clive Tadhunter & William B. Sparks 《Monthly notices of the Royal Astronomical Society》1998,301(1):131-141
We present new Hubble Space Telescope ( HST ) continuum and spectral line images of the radio galaxy Cygnus A. The images show much complex structure in the central kpc2 . Continuum images show the central dust lane in detail, allowing detailed maps of E ( B − V ) to be constructed; the dust appears to follow a roughly Galactic extinction law. The emission-line components are resolved in the line images and investigated in detail. A clear 'opening cone' morphology is found, especially in the lines of Hα and [O i ]. Blue condensations are seen in the south-eastern emission component and surrounding the central region. These are almost certainly due to star formation, which began <1 Gyr ago as deduced from the colour of the regions. More extended blue continuum is also seen and corresponds to the blue polarized component detected by other recent spectropolarimetric observations. 相似文献