全文获取类型
收费全文 | 7884篇 |
免费 | 1024篇 |
国内免费 | 2044篇 |
专业分类
测绘学 | 308篇 |
大气科学 | 3365篇 |
地球物理 | 630篇 |
地质学 | 924篇 |
海洋学 | 836篇 |
天文学 | 4084篇 |
综合类 | 343篇 |
自然地理 | 462篇 |
出版年
2024年 | 49篇 |
2023年 | 70篇 |
2022年 | 127篇 |
2021年 | 188篇 |
2020年 | 197篇 |
2019年 | 227篇 |
2018年 | 170篇 |
2017年 | 209篇 |
2016年 | 194篇 |
2015年 | 281篇 |
2014年 | 306篇 |
2013年 | 346篇 |
2012年 | 354篇 |
2011年 | 330篇 |
2010年 | 293篇 |
2009年 | 660篇 |
2008年 | 607篇 |
2007年 | 674篇 |
2006年 | 662篇 |
2005年 | 674篇 |
2004年 | 652篇 |
2003年 | 583篇 |
2002年 | 449篇 |
2001年 | 463篇 |
2000年 | 446篇 |
1999年 | 388篇 |
1998年 | 484篇 |
1997年 | 145篇 |
1996年 | 123篇 |
1995年 | 118篇 |
1994年 | 117篇 |
1993年 | 68篇 |
1992年 | 65篇 |
1991年 | 62篇 |
1990年 | 27篇 |
1989年 | 21篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 11篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 6篇 |
1981年 | 13篇 |
1980年 | 5篇 |
1977年 | 6篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1971年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 258 毫秒
991.
Although Type Ia supernovae(SNe Ia) play an important role in the study of cosmology, their progenitors are still poorly understood. Thermonuclear explosions from the helium double-detonation sub-Chandrasekhar mass model have been considered as an alternative method for producing SNe Ia. By adopting the assumption that a double detonation occurs when a He layer with a critical ignition mass accumulates on the surface of a carbon–oxygen white dwarf(CO WD), we perform detailed binary evolution calculations for the He double-detonation model, in which a He layer from a He star accumulates on a CO WD. According to these calculations, we obtain the initial parameter spaces for SNe Ia in the orbital period and secondary mass plane for various initial WD masses. We implement these results into a detailed binary population synthesis approach to calculate SN Ia birthrates and delay times. From this model,the SN Ia birthrate in our Galaxy is ~0.4- 1.6 × 10-3yr-1. This indicates that the double-detonation model only produces part of the SNe Ia. The delay times from this model are ~ 70- 710 Myr, which contribute to the young population of SNe Ia in the observations. We found that the CO WD + sdB star system CD-30 11223 could produce an SN Ia via the double-detonation model in its future evolution. 相似文献
992.
Adaptive optics (AO), which provides diffraction limited imaging over a field-of-view (FOV), is a powerful technique for solar observation. In the tomographic approach, each wavefront sensor (WFS) is looking at a single reference that acts as a guide star. This allows a 3D reconstruction of the distorted wavefront to be made. The correction is applied by one or more deformable mirrors (DMs). This technique benefits from information about atmospheric turbulence at different layers, which can be used to reconstruct the wavefront extremely well. With the assistance of the MAOS software package, we consider the tomography errors and WFS aliasing errors, and focus on how the performance of a solar telescope (pointing toward zenith) is related to atmospheric anisoplanatism. We theoretically quantify the performance of the to- mographic solar AO system. The results indicate that the tomographic AO system can improve the average Strehl ratio of a solar telescope in a 10" - 80" diameter FOV by only employing one DM conjugated to the telescope pupil. Furthermore, we discuss the effects of DM conjugate altitude on the correction achievable by the AO system by selecting two atmospheric models that differ mainly in terms of atmospheric prop- erties at ground level, and present the optimum DM conjugate altitudes for different observation sites. 相似文献
993.
994.
T. S. R. Babbedge M. Rowan-Robinson M. Vaccari J. A. Surace C. J. Lonsdale D. L. Clements F. Fang D. Farrah A. Franceschini E. Gonzalez-Solares E. Hatziminaoglou C. G. Lacey S. Oliver N. Onyett I. Pérez-Fournon M. Polletta F. Pozzi G. Rodighiero D. L. Shupe B. Siana H. E. Smith 《Monthly notices of the Royal Astronomical Society》2006,370(3):1159-1180
995.
We have identified a binary companion to (42355) 2002 CR46 in our ongoing deep survey using the Hubble Space Telescope's High Resolution Camera. It is the first companion to be found around an object in a non-resonant orbit that crosses the orbits of giant planets. Objects in orbits of this kind, the Centaurs, have experienced repeated strong scattering with one or more giant planets and therefore the survival of binaries in this transient population has been in question. Monte Carlo simulations suggest, however, that binaries in (42355) 2002 CR46-like heliocentric orbits have a high probability of survival for reasonable estimates of the binary's still-unknown system mass and separation. Because Centaurs are thought to be precursors to short period comets, the question of the existence of binary comets naturally arises; none has yet been definitively identified. The discovery of one binary in a sample of eight observed by HST suggests that binaries in this population may not be uncommon. 相似文献
996.
利用一种非定常的恒星非局部对流理论,对球状星团中低光度的红巨星进行了线性非绝热脉动理论计算.结果表明,对所有温度高于约5400 K模型的基音到4阶泛音都是脉动稳定的.随着恒星光度的增大,低阶泛音也变得脉动不稳定.对中低光度的红巨星,脉动稳定性非常低,接近中性稳定.因此他们将是不变星或非常小振幅的短周期变星(P<2天). 相似文献
997.
The Hill stability of a binary or planetary system during encounters with a third inclined body 总被引:2,自引:0,他引:2
J. R. Donnison 《Monthly notices of the Royal Astronomical Society》2006,369(3):1267-1280
The dynamical interaction of a binary or planetary system and a third body moving on a parabolic orbit inclined to the system is discussed in terms of Hill stability for the full three-body problem. The situation arises in binary star disruption and exchange, in extrasolar planetary system disruption, exchange and capture. It is found that increasing the inclination of the third body decreases the Hill regions of stability. This makes exchange or disruption of the component masses more likely as does increasing the eccentricity of the binary.
The stability criteria are applied to determine possible disruption and capture distances for currently known extrasolar planetary systems. 相似文献
The stability criteria are applied to determine possible disruption and capture distances for currently known extrasolar planetary systems. 相似文献
998.
D. R. Lorimer A. J. Faulkner A. G. Lyne R. N. Manchester M. Kramer M. A. McLaughlin G. Hobbs A. Possenti I. H. Stairs F. Camilo M. Burgay N. D'Amico A. Corongiu F. Crawford 《Monthly notices of the Royal Astronomical Society》2006,372(2):777-800
We present the discovery and follow-up observations of 142 pulsars found in the Parkes 20-cm multibeam pulsar survey of the Galactic plane. These new discoveries bring the total number of pulsars found by the survey to 742. In addition to tabulating spin and astrometric parameters, along with pulse width and flux density information, we present orbital characteristics for 13 binary pulsars which form part of the new sample. Combining these results from another recent Parkes multibeam survey at high Galactic latitudes, we have a sample of 1008 normal pulsars which we use to carry out a determination of their Galactic distribution and birth rate. We infer a total Galactic population of 30 000 ± 1100 potentially detectable pulsars (i.e. those beaming towards us) having 1.4-GHz luminosities above 0.1 mJy kpc2 . Adopting the Tauris & Manchester beaming model, this translates to a total of 155 000 ± 6000 active radio pulsars in the Galaxy above this luminosity limit. Using a pulsar current analysis, we derive the birth rate of this population to be 1.4 ± 0.2 pulsars per century. An important conclusion from our work is that the inferred radial density function of pulsars depends strongly on the assumed distribution of free electrons in the Galaxy. As a result, any analyses using the most recent electron model of Cordes & Lazio predict a dearth of pulsars in the inner Galaxy. We show that this model can also bias the inferred pulsar scaleheight with respect to the Galactic plane. Combining our results with other Parkes multibeam surveys we find that the population is best described by an exponential distribution with a scaleheight of 330 pc. Surveys underway at Parkes and Arecibo are expected to improve the knowledge of the radial distribution outside the solar circle, and to discover several hundred new pulsars in the inner Galaxy. 相似文献
999.
1000.
Similar to the case of pulsars the magnetic axis and the spin axis of gamma-ray burst sources may not lie on the same line. This may cause the formation of a ring-like jet due to collimation of the processing magnetic axis. We analyze the tail emission from such a jet, and find that it has a shallow decay phase with a temporal index of -1/2 if the Lorentz factor of the ejecta is not very high, which is consistent with the shallow decay phase of some early X-ray afterglow detected by Swift. The ring-like jet has a tail cusp with sharp rising and very sharp decay. This effect can provide an explanation for the re-brightening and sharp decay of the X-ray afterglow of GRB 050709. 相似文献