全文获取类型
收费全文 | 73878篇 |
免费 | 931篇 |
国内免费 | 411篇 |
专业分类
测绘学 | 1709篇 |
大气科学 | 4776篇 |
地球物理 | 13756篇 |
地质学 | 29474篇 |
海洋学 | 6102篇 |
天文学 | 15614篇 |
综合类 | 261篇 |
自然地理 | 3528篇 |
出版年
2022年 | 439篇 |
2021年 | 734篇 |
2020年 | 766篇 |
2019年 | 906篇 |
2018年 | 3965篇 |
2017年 | 3680篇 |
2016年 | 2958篇 |
2015年 | 965篇 |
2014年 | 1630篇 |
2013年 | 2871篇 |
2012年 | 2707篇 |
2011年 | 4565篇 |
2010年 | 4108篇 |
2009年 | 4716篇 |
2008年 | 3936篇 |
2007年 | 4551篇 |
2006年 | 2410篇 |
2005年 | 1852篇 |
2004年 | 1797篇 |
2003年 | 1919篇 |
2002年 | 1671篇 |
2001年 | 1318篇 |
2000年 | 1149篇 |
1999年 | 1000篇 |
1998年 | 978篇 |
1997年 | 987篇 |
1996年 | 803篇 |
1995年 | 794篇 |
1994年 | 708篇 |
1993年 | 606篇 |
1992年 | 583篇 |
1991年 | 575篇 |
1990年 | 597篇 |
1989年 | 555篇 |
1988年 | 506篇 |
1987年 | 557篇 |
1986年 | 572篇 |
1985年 | 675篇 |
1984年 | 719篇 |
1983年 | 710篇 |
1982年 | 652篇 |
1981年 | 614篇 |
1980年 | 586篇 |
1979年 | 566篇 |
1978年 | 512篇 |
1977年 | 510篇 |
1976年 | 465篇 |
1975年 | 471篇 |
1974年 | 447篇 |
1973年 | 514篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
261.
Andrés E. Piatti Juan J. Clariá Andrea V. Ahumada 《Monthly notices of the Royal Astronomical Society》2003,346(2):390-402
Charge-coupled device (CCD) photometry in the Johnson V , Kron–Cousins I and Washington CMT 1 systems is presented in the field of the poorly known open cluster NGC 2627. Four independent Washington abundance indices yield a mean cluster metallicity of [Fe/H]=−0.12 ± 0.08 , which is compatible with the existence of a radial gradient in the Galactic disc. The resultant colour–magnitude diagrams indicate that the cluster is an intermediate-age object of 1.4 Gyr. Based on the best fits of the Geneva group's isochrones to the ( V , V − I ) and ( T 1 , C − T 1 ) diagrams, we estimate E ( V − I ) = 0.25 ± 0.05 and V − MV = 11.80 ± 0.25 for log t = 9.15 , and E ( C − T 1 ) = 0.23 ± 0.07 and T 1 − M T 1 = 11.85 ± 0.25 for log t = 9.10 , respectively, assuming solar metal content. The derived reddening value E ( C − T 1 ) implies E ( B − V ) = 0.12 ± 0.07 and a distance from the Sun of 2.0 ± 0.4 kpc . Using the WEBDA data base and the available literature, we re-examined the overall properties of all the open clusters with ages between 0.6 and 2.5 Gyr. We identified peaks of cluster formation at 0.7–0.8, 1.0–1.1, 1.6–1.7 and 2.0–2.1 Gyr, separated by relative quiescent epochs of ∼0.2–0.3 Gyr. We also estimated a radial abundance gradient of −0.08 ± 0.02 , which is consistent with the most recent determinations for the Galactic disc, but no clear evidence for a gradient perpendicular to the Galactic plane is found. 相似文献
262.
The host galaxies of active galactic nuclei 总被引:2,自引:0,他引:2
Guinevere Kauffmann Timothy M. Heckman Christy Tremonti Jarle Brinchmann Stéphane Charlot Simon D. M. White Susan E. Ridgway Jon Brinkmann Masataka Fukugita Patrick B. Hall eljko Ivezi Gordon T. Richards Donald P. Schneider 《Monthly notices of the Royal Astronomical Society》2003,346(4):1055-1077
We examine the properties of the host galaxies of 22 623 narrow-line active galactic nuclei (AGN) with 0.02 < z < 0.3 selected from a complete sample of 122 808 galaxies from the Sloan Digital Sky Survey. We focus on the luminosity of the [O iii ]λ5007 emission line as a tracer of the strength of activity in the nucleus. We study how AGN host properties compare with those of normal galaxies and how they depend on L [O iii ]. We find that AGN of all luminosities reside almost exclusively in massive galaxies and have distributions of sizes, stellar surface mass densities and concentrations that are similar to those of ordinary early-type galaxies in our sample. The host galaxies of low-luminosity AGN have stellar populations similar to normal early types. The hosts of high-luminosity AGN have much younger mean stellar ages. The young stars are not preferentially located near the nucleus of the galaxy, but are spread out over scales of at least several kiloparsecs. A significant fraction of high-luminosity AGN have strong Hδ absorption-line equivalent widths, indicating that they experienced a burst of star formation in the recent past. We have also examined the stellar populations of the host galaxies of a sample of broad-line AGN. We conclude that there is no significant difference in stellar content between type 2 Seyfert hosts and quasars (QSOs) with the same [O iii ] luminosity and redshift. This establishes that a young stellar population is a general property of AGN with high [O iii ] luminosities. 相似文献
263.
264.
265.
North-South asymmetry of the daily interplanetary magnetic field spiral during the period: 1965–1990
I. Sabbah 《Earth, Moon, and Planets》1995,70(1-3):173-178
We have extended our earlier study of the dependance of interplanetary magnetic field (IMF) spiral on the magnetic polarity to cover the 26-year period 1965–1990. Our analysis reveals that: 1. The spiral angle north of the current sheet is higher than south of it. 2. During both of negative solar polarity epochs the IMF spiral is stable; it shows more variation during positive polarity epoch. 3. The included angle is lower than 180° during negative polarity epochs and higher than 180° during positive polarity epoch. 4. The Earth spent more time north of the current sheet during our period of analysis. 相似文献
266.
B. R. Johnson M. E. Abroe P. Ade J. Bock J. Borrill J. S. Collins P. Ferreira S. Hanany A. H. Jaffe T. Jones A. T. Lee L. Levinson T. Matsumura B. Rabii T. Renbarger P. L. Richards G. F. Smoot R. Stompor H. T. Tran C. D. Winant 《New Astronomy Reviews》2003,47(11-12):1067
We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10′ to 2°. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary dataset was collected during a 26 h turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress. 相似文献
267.
The internal structure of stellar jets arising from young stellar objects is characterized by a series of `knots' or condensations which have highly supersonic proper motions. These structures, we believe, are the result of a variable ejection from the source, which leads to the formation of internal working surfaces or small bowshock within the jet beam. In this paper we present a long-slit spectrum of Herbig-Haro object HH 111 obtained with STIS and an interpretation of this observation in terms of an ejection velocity variability model. 相似文献
268.
Nonstationary hydrodynamic models of a viscous accretion disk around a central compact object were constructed. Two different numerical methods (TVD and SPH) are used to study the dynamics of dissipatively unstable acoustic perturbations at the nonlinear stage in terms of the standard α-disk model. The standard disk accretion in the Shakura-Sunyaev model is unstable against acoustic waves for various parameters of the system. If the α parameter, which specifies the level of turbulent viscosity, exceeds α?0.03, then a complex nonstationary system of small-scale weak shock waves is formed. The growth rate of the perturbations is higher in the central disk region. For α?0.2, the relative shock amplitude can exceed 50% of the equilibrium disk parameters. The reflection of waves from the disk boundaries and their nonlinear interaction are important factors that can produce unsteady accretion. The luminosity of such a disk undergoes quasi-periodic oscillations at a level of several percent (?5%) of the equilibrium level. 相似文献
269.
The effect of particle-particle interaction on the adiabatic index γ for an electron-positron plasma is considered. An improved method for numerically calculating the Hartree-Fock exchange integral is presented and its relativistic asymptotics is determined. An approximation formula is derived for the correlation part of the interaction in the low-density limit. This formula includes degeneracy and the positron component. 相似文献
270.