全文获取类型
收费全文 | 6628篇 |
免费 | 1259篇 |
国内免费 | 1817篇 |
专业分类
测绘学 | 94篇 |
大气科学 | 48篇 |
地球物理 | 629篇 |
地质学 | 6175篇 |
海洋学 | 570篇 |
天文学 | 1053篇 |
综合类 | 373篇 |
自然地理 | 762篇 |
出版年
2024年 | 90篇 |
2023年 | 266篇 |
2022年 | 362篇 |
2021年 | 353篇 |
2020年 | 279篇 |
2019年 | 320篇 |
2018年 | 210篇 |
2017年 | 234篇 |
2016年 | 236篇 |
2015年 | 270篇 |
2014年 | 369篇 |
2013年 | 316篇 |
2012年 | 357篇 |
2011年 | 368篇 |
2010年 | 319篇 |
2009年 | 338篇 |
2008年 | 351篇 |
2007年 | 252篇 |
2006年 | 287篇 |
2005年 | 266篇 |
2004年 | 265篇 |
2003年 | 243篇 |
2002年 | 220篇 |
2001年 | 267篇 |
2000年 | 273篇 |
1999年 | 308篇 |
1998年 | 312篇 |
1997年 | 325篇 |
1996年 | 285篇 |
1995年 | 279篇 |
1994年 | 234篇 |
1993年 | 167篇 |
1992年 | 211篇 |
1991年 | 166篇 |
1990年 | 113篇 |
1989年 | 103篇 |
1988年 | 26篇 |
1987年 | 26篇 |
1986年 | 12篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1978年 | 2篇 |
1975年 | 2篇 |
1954年 | 2篇 |
1950年 | 3篇 |
1949年 | 2篇 |
1948年 | 1篇 |
1944年 | 1篇 |
排序方式: 共有9704条查询结果,搜索用时 93 毫秒
991.
992.
993.
该文总结了近年来椭圆星系中尘埃起源研究中的一些关键问题,包括尘埃大小的分布及其组成、椭圆星系恒星质量损失速率、尘埃的两种加热机制(恒星辐射加热和电子-尘埃碰撞加热),以及由此导致的尘埃温度分布;尘埃的总质量估算;由热离子导致的尘埃破坏速率,以及与此相关联的椭圆星系热气体分布,等等.综合考虑这些因素,作者认为热气体分布对于椭圆星系中尘埃存在和破坏的影响不可忽视,椭圆星系中星系内部恒星质量损失产生尘埃的起源可能占据着非常重要的地位. 相似文献
994.
Shan-Jie Qian 《中国天文和天体物理学报》2008,8(2):219-229
Correlated radio-optical variations on intraday timescales have been observed (e.g. In BLO 0716 714) and such radio intraday variability is suggested to have an intrinsic ori- gin. Recently, multi-wavelength observations, simultaneous at radio, mm-submm, optical and hard X-rays, of 0716 714, show that during a period of intraday/interday variations at ra- dio and mm wavelengths, the apparent brightness temperature of the source exceeded the Compton-limit (~1012 K) by 2--4 orders of magnitude, but no Compton catastrophe (or no high luminosity of inverse-Compton radiation) was detected. It is also found that the intra- day/interday variations at mm-submm wavelengths are consistent with the evolutionary be- havior of a standard synchrotron source and for the intraday/interday variations at centimeter wavelengths opacity effects can play a significant role, which is consistent with the interpreta- tion suggested previously by Qian et al. Thus the apparent high brightness temperatures may probably be explained in terms of Doppler boosting effects due to bulk relativistic motion of the source. We will argue a scenario to simulate the correlations between the radio and optical variations on intraday timescales observed in BLO 0716 714 in terms of a relativistic shock propagating through a jet with a dual structure. 相似文献
995.
We investigate the effect of bulk motion on the detection of molecular outflows in the sources S 146, GGD27, and IRAS 22566 5830. The traditional techniques do allow for bulk motions or systematic VLSR shifts of the core emissions, which may cause contamination of the high velocity gas emissions, and outflows may either fail to be detected or have their properties miscalculated. We used a program to follow the systematic shift of VLSR and better results have been obtained. 相似文献
996.
It is well accepted that feedback from active galactic nuclei (AGNs) plays an important role in the coevolution of the supermassive black hole (SMBH) and its host galaxy,but the concrete mechanism of feedback remains unclear.A considerable body of evidence suggests that AGN feedback suppresses star formation in the host galaxy.We assemble a sample of Seyfert 2 galaxies with recent observational data of compact nuclear starbursts and estimate the gas surface density as a function of column density to illuminate the relation between feedback and AGN properties.Although there are some uncertainties,our data still imply the deviation from the star formation law (Kennicutt-Schmidt law).Further,they indicate that:(1) Feedback correlates with the Eddington ratio,rather than with the mass of SMBH,as a result of decreasing star formation efficiency.(2) The SMBH and the torus are probably undergoing coevolution.Conclusions presented here can be refined through future high resolution CO or HCN observations. 相似文献
997.
Feedback from supernovae (SNe) and from active galactic nuclei (AGN) accom-panies the history of star formation and galaxy evolution. We present an analytic model to explain how and when the SNe and AGN exert their feedback effects on the star formation and galaxy evolution processes. By using SNe and AGN kinetic feedback mechanisms based on the Lambda Cold Dark Matter (LCDM) model, we explore how these feedback mecha-nisms affect the star formation history (SFH), the Near-Infrared Background (NIRB) flux and the cosmological reionization. We find the values of the feedback strengths, ∈AGN =1.0+0.50.3and ∈SN=0.04+0.02-0.02, can provide a reasonable explanation of most of the observational re-suits, and that the AGN feedback effect on star formation history is quite different from the SNe feedback at high redshifts. Our conclusions manifest quantitatively that these feedback effects decrease star formation rate density (SFRD) and the NIRB flux (in 1.4 - 4.0μm), and postpone the time of completion of the cosmological reionization. 相似文献
998.
Li-Ming Dou Wei-Min Yuan 《中国天文和天体物理学报》2008,8(6)
We report on our results of X-ray spectral analysis for a sample of radio-loud quasars covering a wide range of the radio core-dominance parameter, R, from core-dominated to lobe-dominated objects, using data obtained mostly with the XMM-Newton Observatory. We find that the spectral shape of the underlying power-law continuum is flat even for the lobe-dominated objects (average photon index ~ 1.5), indistinguishable from that of core-dominated quasars. For lobe-dominated objects, contribution of X-rays from the jets is expected to be very small based on previous unification schemes, more than one order of magnitude lower than the observed X-ray luminosities. Assuming that radio-loud quasars follow the same X-ray-UV/optical luminosity relation for the disk-corona emission as found for radio-quiet quasars, we estimate the X-ray flux contributed by the disk-corona component from the optical/UV continuum. We find that neither the luminosity, nor the spectral shape, of the disk-corona X-ray emission can account for the bulk of the observed X-ray properties. Thus in lobe-dominated quasars, either the disk-corona X-ray emission is much enhanced in strength and flatter in spectral shape (photon index~1.5) compared to normal radio-quiet quasars, or their jet X-ray emission is much enhanced compared to their weak radio core-jet emission. If the latter is the case, our result may imply that the jet emission in X-rays is less Doppler beamed than that in the radio. As a demonstrating example, we test this hypothesis by using a specific model in which the X-ray jet has a larger opening angle than the radio jet. 相似文献
999.
We present a study of the nearby Seyfert galaxy NGC 3982 using optical,infrared and X-ray data acquired by SDSS,Spitzer and Chandra.Our main results are as follows:(1) A simple stellar population synthesis on the nuclear and circumnuclear SDSS spectra gives unambiguous evidence of young stellar components in both the nuclear and circumnuclear regions.(2) The Spitzer Infrared Spectrograph (IRS) spectrum of the central region (~3") shows a power-law continuum,a silicate emission feature at 9.7 μm,and significant PAH emission features at 7.7,8.6,11.3 and 12.7/zm,suggesting the coexistence of AGN and starburst activities in the central region of NGC 3982.(3) We estimate the star formation rate (SFR) of the circumnuclear (~5"-20") region from the Ha luminosity to be for the active nucleus of NGC 3982 from radio to X-ray,and obtain a bolometric luminosity of Lbol=4.5×1042 erg s-1,corresponding to an Eddington ratio (Lbol/LEdd) of 0.014.The HST image of NGC 3982 shows a nuclear mini-spiral between the circumnuclear starforming region and the nucleus,which could be the channel through which gas is transported to the supermassive black hole from the circumnuclear star-forming region. 相似文献
1000.
南华北地区的构造演化与含油气性 总被引:16,自引:2,他引:14
本文对南华北盆地进行了深入细致的研究后认为其经历了12个演化阶段:变质结晶基底的形成(Ar-Pt1)、拗拉槽-裂谷盆地阶段(长城纪Ch)、克拉通—裂谷盆地阶段(蓟县纪Jx-新元古代青白口纪Qb)、克拉通-被动大陆边缘盆地阶段(Z-O2)、整体隆升-弧后盆地阶段(O3-D2)、克拉通-(弧后)前陆盆地阶段(D3-T1)、印支运动阶段(T2-T3)、类前陆盆地阶段(T3-J2)、压扭背景下的挤压冲断-走滑拉分盆地阶段(J3-K11)、冲断抬升剥蚀阶段(K21-K2)、裂陷盆地阶段(E)及坳陷盆地阶段(N-Q)。南华北盆地石炭-二叠系煤系地层是南华北盆地的主要烃源岩。石炭-二叠系煤系地层经历了沉积→埋深→抬升剥蚀改造→再埋深的过程,在再次埋深过程中具备二次生烃的条件。 相似文献