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951.
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954.
Han-PingLiu JinSun 《中国天文和天体物理学报》2002,2(1):51-58
It is recognized that the interstellar methanol-107 GHz masers and OH-4.765 GHz masers towards Class Ⅱ sources are associated with each other and coexist towards ultracompact HⅡ regions. Therefore we suggest a new pumping mechanism-methanol masers without population inversion. It can explain the formation of 107GHz methanol masers, with the 4.765GHz OH masers acting as a driving coherent microwave field. It is argued that this mechanism is compatible with the astronomical conditions. 相似文献
955.
Ji Yang Jie-Long Zhang Zhi-Yong Cai Deng-Rong Lu You-Heng Tan Purple Mountain Observatory Chinese Academy of Sciences Nanjing Key Laboratory of Particle Astrophysics Institute of High-Energy Physics Chinese Academy of Sciences Beijing National Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2006,6(2):210-216
The distribution of dense molecular gas around the supernova remnant G40.5-0.5 has been investigated by radio spectroscopic observations in the CO (J = 1-0) transition. The molecular gas is found to extend over the entire region of G40.5-0.5. A molecular shell, with a diameter of - 26', coincides with the ionized gas as revealed by the cm-radio observations. This coincidence, along with the velocity discontinuity following the shell, provides direct evidence for interaction between the ionized gas and the dense molecular gas. No clear evidence for cosmic-ray acceleration can be identified from this SNR as previously suggested, due to positional uncertainty in relating the SNR shell defined by CO to the EGRET gamma-ray sources, GRO J1904 06, from the gamma-ray observations. 相似文献
956.
A systematic study of the linear thermal instability of a self-gravitating magnetic molecular cloud is carried out for the
case when the unperturbed background is subject to local expansion or contraction. We consider the ambipolar diffusion, or
ion-neutral friction on the perturbed states. In this way, we obtain a non-dimensional characteristic equation that reduces
to the prior characteristic equation in the non-gravitating stationary background. By parametric manipulation of this characteristic
equation, we conclude that there are, not only oblate condensation forming solutions, but also prolate solutions according
to local expansion or contraction of the background. We obtain the conditions for existence of the Field lengths that thermal
instability in the molecular clouds can occur. If these conditions establish, small-scale condensations in the form of spherical,
oblate, or prolate shape may be produced via thermal instability. 相似文献
957.
Dr Serena Viti PhD Thomas W. Hartquist Philip C. Myers 《Astrophysics and Space Science》2006,302(1-4):109-115
We present models for the chemistry in gas moving towards the ionization front of an HII region. When it is far from the ionization
front, the gas is highly depleted of elements more massive than helium. However, as it approaches the ionization front, ices
are destroyed and species formed on the grain surfaces are injected into the gas phase. Photodissociation removes gas phase
molecular species as the gas flows towards the ionization front. We identify models for which the OH column densities are
comparable to those measured in observations undertaken to study the magnetic fields in star forming regions and give results
for the column densities of other species that should be abundant if the observed OH arises through a combination of the liberation
of H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S. Observations of these other species may help establish the nature of the OH spatial distribution in the clouds, which is
important for the interpretation of the magnetic field results. 相似文献
958.
Thierry Montmerle Philippe Claeys Muriel Gargaud Purificatión López-García Hervé Martin Robert Pascal Jacques Reisse Franck Selsis 《Earth, Moon, and Planets》2006,98(1-4):299-312
This concluding chapter is divided into two main parts. The first part is a summary of the main facts and events which constitute the present body of knowledge of the chronology of life in the solar system, in the form of “highlights” in astronomy, geology, chemistry and biology. The second part raises the interrogation “Is life universal?”, and tries to provide answers based on these facts and events. These answers turn out to differ widely among the various disciplines, depending on how far they feel able to extrapolate their current knowledge. 相似文献
959.
960.
A. Pedlar T. W. B. Muxlow M. A. Garrett P. Diamond K. A. Wills P. N. Wilkinson W. Alef 《Monthly notices of the Royal Astronomical Society》1999,307(4):761-768
We have used the European VLBI Network (EVN) at 18 cm to study five of the more compact radio sources in the starburst galaxy M82. The angular resolution of the observations is 15 mas, corresponding to 0.2 pc at the distance of M82. The observations reveal shells ranging in diameter from 40 to 90 mas (0.6 to 1.4 pc), although the strongest source (41.95+575) is only marginally resolved by these measurements (∼20×10 mas2 ).
We have found clear evidence for expansion in one of the shell sources (43.31+592) by re-analysing, in wide-field mode, EVN data taken in 1986. Between 1986 and 1997 this source has increased its diameter by 13.6±2 mas, corresponding to an average expansion velocity of 9850±1500 km s−1 . If we assume that the remnant is in free expansion, this is consistent with a supernova event in the early 1960s. Hence this remnant is almost certainly younger than the strongest, most compact source (41.95+575) which was known to be present in the 1960s. 41.95+575 shows no clear evidence for expansion (<4000 km s−1 ), consistent with a greater age; this is further evidence of its anomalous status. Comparison of the EVN images with earlier MERLIN data is also consistent with expansion in at least two more of the sources. We discuss the flux density variability of the compact sources in M82 and conclude that, with the exception of 41.95+575 and two transient sources, there is little evidence for significant changes in flux density of most of the remnants since the early 1980s. 相似文献
We have found clear evidence for expansion in one of the shell sources (43.31+592) by re-analysing, in wide-field mode, EVN data taken in 1986. Between 1986 and 1997 this source has increased its diameter by 13.6±2 mas, corresponding to an average expansion velocity of 9850±1500 km s