全文获取类型
收费全文 | 71977篇 |
免费 | 1721篇 |
国内免费 | 1183篇 |
专业分类
测绘学 | 1621篇 |
大气科学 | 5221篇 |
地球物理 | 14464篇 |
地质学 | 26667篇 |
海洋学 | 6475篇 |
天文学 | 15486篇 |
综合类 | 378篇 |
自然地理 | 4569篇 |
出版年
2022年 | 524篇 |
2021年 | 847篇 |
2020年 | 910篇 |
2019年 | 1031篇 |
2018年 | 1876篇 |
2017年 | 1828篇 |
2016年 | 2076篇 |
2015年 | 1249篇 |
2014年 | 2042篇 |
2013年 | 3614篇 |
2012年 | 2520篇 |
2011年 | 3275篇 |
2010年 | 2908篇 |
2009年 | 3599篇 |
2008年 | 3171篇 |
2007年 | 3253篇 |
2006年 | 3079篇 |
2005年 | 2145篇 |
2004年 | 2059篇 |
2003年 | 1944篇 |
2002年 | 1847篇 |
2001年 | 1586篇 |
2000年 | 1504篇 |
1999年 | 1286篇 |
1998年 | 1301篇 |
1997年 | 1293篇 |
1996年 | 1093篇 |
1995年 | 1061篇 |
1994年 | 913篇 |
1993年 | 811篇 |
1992年 | 779篇 |
1991年 | 759篇 |
1990年 | 832篇 |
1989年 | 712篇 |
1988年 | 672篇 |
1987年 | 777篇 |
1986年 | 717篇 |
1985年 | 894篇 |
1984年 | 1027篇 |
1983年 | 934篇 |
1982年 | 890篇 |
1981年 | 798篇 |
1980年 | 794篇 |
1979年 | 710篇 |
1978年 | 676篇 |
1977年 | 695篇 |
1976年 | 613篇 |
1975年 | 610篇 |
1974年 | 609篇 |
1973年 | 667篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
M. Bellazzini F.R. Ferraro E. Pancino † 《Monthly notices of the Royal Astronomical Society》2001,327(1):L15-L20
We present wide-field multiband ( BVI ) CCD photometry (down to of the very low surface brightness dwarf spheroidal galaxy Sextans. In the derived colour–magnitude diagrams we find evidence suggesting the presence of multiple stellar populations in this dwarf spheroidal. In particular, we discover (i) a blue horizontal branch tail that appears to lie on a brighter sequence with respect to the prominent red horizontal branch and the RR Lyrae stars, very similar to what was found by Majewski et al. for the Sculptor dwarf spheroidal, (ii) hints of a bimodal distribution in colour of the red giant branch stars, (iii) a double red giant branch bump. All of these features suggest that (at least) two components are present in the old stellar population of this galaxy: the main one with and a minor component around . The similarity to the Sculptor case may indicate that multiple star formation episodes are also common in the most nearby dwarf spheroidals that ceased their star formation activity at very early epochs. 相似文献
172.
173.
Abstract— Measurements of He isotopes in cluster interplanetary dust particles (IDPs) from stratospheric dust collector L2009 reveal anomalous 3He/4He ratios comparable to those seen earlier, up to ~40x the solar wind ratio, in particles from the companion collector L2011. These overabundances of 3He in the L2009 samples are masked by much higher 4He contents compared to the L2011 particles, and are visible only in minor gas fractions evolved by stepwise heating at high temperatures. Cosmic‐ray induced spallogenic reactions are efficient producers of 3He. The majority of this paper is devoted to a detailed assessment of the possible role of spallation in generating the 3He excesses in these and other cluster IDPs. A model of collisional erosion and fragmentation during inward transit through the interplanetary dust environment is used to estimate space lifetimes of particles from asteroidal and Edgeworth–Kuiper Belt sources. Results of the modeling indicate that Poynting–Robertson orbital evolution timescales of IDPs small enough to elude destruction on their way to Earth from either location are far shorter than the cosmic‐ray exposure ages required to account for observed 3He overabundances. Grains large enough to have sufficiently long space residence times are fragmented close to their sources. An alternative to long in‐space exposure could be prolonged irradiation of particles buried in parent body regoliths prior to their ejection as IDPs. A qualitative calculation suggests, however, that collisional erosion of asteroidal upper‐regolith materials is likely to occur on timescales shorter than the > 1 Ga burial times needed for accumulation of spallogenic 3He to the levels seen in several cluster particles. In contrast, regoliths on Edgeworth–Kuiper Belt objects may be stable enough to account for the 3He excesses, and delivery of heavily pre‐irradiated IDPs to the inner solar system by short‐period Edgeworth–Kuiper Belt comets remains a possibility. A potential problem is that the expected associated abundances of spallation‐produced 21Ne appear to be absent, although here the present IDP data base is too sparse and for the most part too imprecise to rule out a spallogenic origin. Relatively short periods of pre‐ejection residence in asteroidal regoliths may be responsible for the curiously broad exposure age distributions reported for micrometeorites extracted from Greenland and sea‐floor sediments. 相似文献
174.
C.J. Mooney W.R.J. Rolleston F.P. Keenan P.L. Dufton D.L. Pollacco H.R. Magee 《Monthly notices of the Royal Astronomical Society》2001,326(3):1101-1109
We present a model-atmosphere analysis for the bright ( V ∼13) star ZNG-1, in the globular cluster M10. From high-resolution ( R ∼40 000) optical spectra we confirm ZNG-1 to be a post-asymptotic giant branch (post-AGB) star. The derived atmospheric parameters are T eff =26 500±1000 K and log g =3.6±0.2 dex . A differential abundance analysis reveals a chemical composition typical of hot post-AGB objects, with ZNG-1 being generally metal poor, although helium is approximately solar. The most interesting feature is the large carbon underabundance of more than 1.3 dex. This carbon deficiency, along with an observed nitrogen enhancement relative to other elements, may suggest that ZNG-1 evolved off the AGB before the third dredge-up occurred. Also, iron depletions observed in other similar stars suggest that gas–dust fractionation in the AGB progenitor could be responsible for the observed composition of these objects. However, we need not invoke either scenario since the chemical composition of ZNG-1 is in good agreement with abundances found for a Population II star of the same metallicity. 相似文献
175.
K. C. WELTEN K. NISHIIZUMI J. MASARIK M. W. CAFFEE A. J. T. JULL S. E. KLANDRUD R. WIELER 《Meteoritics & planetary science》2001,36(2):301-317
Abstract— We measured the concentrations of 10Be, 26Al, 36Cl, 41Ca and 14C in the metal and/or stone fractions of 27 Antarctic chondrites from Frontier Mountain (FRO), including two large H‐chondrite showers. To estimate the pre‐atmospheric size of the two showers, we determined the contribution of neutron‐capture produced 36Cl (half‐life = 3.01 times 105 years) and 41Ca (1.04 times 105 years) in the stone fraction. The measured activities of neutron‐capture 36Cl and 41Ca, as well as spallation produced 10Be and 26Al, were compared with Monte Carlo‐based model calculations. The largest shower, FRO 90174, includes eight fragments with an average terrestrial age of (100 ± 30) × 103 years; the neutron‐capture saturation activities extend to 27 dpm/kg stone for 36Cl and 19 dpm/kg stone for 41Ca. The concentrations of spallation produced 10Be, 26Al and 36Cl constrain the radius (R) to 80–100 cm, while the neutron‐capture 41Ca activities indicate that the samples originated from the outer 25 cm. With a pre‐atmospheric radius of 80–100 cm, FRO 90174 is among the largest of the Antarctic stony meteorites. The large pre‐atmospheric size supports our hypothesis that at least 50 of the ~150 classified H5/H6‐chondrites from the Frontier Mountain stranding area belong to this single fall; this hypothesis does not entirely account for the high H/L ratio at Frontier Mountain. The smaller shower, FRO 90001, includes four fragments with an average terrestrial age of (40 ± 10) × 103 years; they contain small contributions of neutron‐capture 36Cl, but no excess of 41Ca. FRO 90001 experienced a complex exposure history with high shielding conditions in the first stage (150 < R < 300 cm) and much lower shielding in the second stage (R < 30 cm), the latter starting ~1.0 million years (Ma) ago. Based on the measured 10Be/21Ne and 26Al/21Ne ratios, the cosmic‐ray exposure ages of the two showers are 7.2 ± 0.5 Ma for FRO 90174 and 8 ± 1 Ma for FRO 90001. These ages coincide with the well‐established H‐chondrite peak and corroborate the observation that the exposure age distribution of FRO H‐chondrites is similar to that of non‐Antarctic falls. In addition, we found that corrections for neutron‐capture 36Ar (from decay of 36Cl) result in concordant 21Ne and 38Ar exposure ages. 相似文献
176.
R. T. Schilizzi W. Aldrich B. Anderson A. Bos R. M. Campbell J. Canaris R. Cappallo J. L. Casse A. Cattani J. Goodman H. J. van Langevelde A. Maccafferri R. Millenaar R. G. Noble F. Olnon S. M. Parsley C. Phillips S. V. Pogrebenko D. Smythe A. Szomoru H. Verkouter A. R. Whitney 《Experimental Astronomy》2001,12(1):49-67
177.
178.
Profiles of spectral lines emitted from an accretion ring around an object with strong gravitational field should be affected by Doppler shift, gravitational redshift, and deflection of light. Taking these effects into account, precise line profile of a Keplerian ring around a Kerr black hole for a distant observer is obtained by solving the kinetic equation of photons. 相似文献
179.
E. I. Timoshkova 《Celestial Mechanics and Dynamical Astronomy》1985,36(2):105-121
The first integrals of motion of the restricted planar circular problem of three bodies are constructed as the formal power series in r1/2, r being the distance of a moving particle from the primary. It is shown that the coefficients of these series are trigonometric polynomials of an angular variable. Some particular solutions have been found in a closed form. The proposed method for constructing the formal integrals can be generalized to a spatial problem of three bodies. 相似文献
180.
K.A. MAASCH P.A. MAYEWSKI E.J. ROHLING J.C. STAGER W. KARLEN L.D. MEEKER E.A. MEYERSON 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):7-15
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14 C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale. 相似文献