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931.
932.
933.
C. S. Crawford P. Gandhi A. C. Fabian R. J. Wilman R. M. Johnstone A. J. Barger † L. L. Cowie ‡ 《Monthly notices of the Royal Astronomical Society》2002,333(4):809-824
We present optical spectra and near-infrared imaging of a sample of 31 serendipitous X-ray sources detected in the field of Chandra observations of the A 2390 cluster of galaxies. The sources have 0.5–7 keV fluxes of (0.6–8)×10-14 erg cm-2 s-1 and lie around the break in the 2–10 keV source counts. They are therefore typical of sources dominating the X-ray Background in that band. 12 of the 15 targets for which we have optical spectra show emission lines at a range of line luminosities, and half of these show broad lines. These active galaxies and quasars have soft X-ray spectra. Including photometric redshifts and published spectra, we have redshifts for 17 of the sources, ranging from z ∼0.2 up to z ∼3 , with a peak between z =1–2 . 10 of our sources have hard X-ray spectra indicating a spectral slope flatter than that of a typical unabsorbed quasar. Two hard sources that are gravitationally lensed by the foreground cluster are obscured quasars, with intrinsic 2–10 keV luminosities of (0.2–3)×1045 erg s-1 , and absorbing columns of N H >1023 cm-2 . Both of these sources were detected in the mid-infrared by ISOCAM on the Infrared Space Observatory , which when combined with radiative transfer modelling leads to the prediction that the bulk of the reprocessed flux emerges at ∼100 μm. 相似文献
934.
935.
A. Alonso-Herrero S. D. Ryder J. H. Knapen † 《Monthly notices of the Royal Astronomical Society》2001,322(4):757-769
We present high-resolution near-infrared imaging obtained using adaptive optics and HST /NICMOS, and ground-based spectroscopy of the hotspot galaxy NGC 2903. Our near-infrared resolution imaging enables us to resolve the infrared hotspots into individual young stellar clusters or groups of these. The spatial distribution of the stellar clusters is not coincident with that of the bright H ii regions, as revealed by the HST /NICMOS Pa α image. Overall, the circumnuclear star formation in NGC 2903 shows a ring-like morphology with an approximate diameter of 625 pc.
The star formation properties of the stellar clusters and H ii regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4–7×106 yr after the peak of star formation, or absolute ages 6.5–9.5×106 yr (for the assumed short-duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar clusters accounts for some 7–12 per cent of the total stellar mass in the central 625 pc of NGC 2903. The H ii regions in the ring of star formation have luminosities close to that of the supergiant H ii region 30 Doradus, they are younger than the stellar clusters, and they will probably evolve into bright infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC 2903 is higher than in normal galaxies, approaching the lower end of infrared luminous galaxies. 相似文献
The star formation properties of the stellar clusters and H ii regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4–7×10
936.
Sungho Lee Soojong Pak Christopher J. Davis Robeson M. Herrnstein T. R. Geballe Paul T. P. Ho J. Craig Wheeler 《Monthly notices of the Royal Astronomical Society》2003,341(2):509-516
We have detected the v = 1 → 0 S(1) (λ= 2.1218 μm) and v = 2 → 1 S(1) (λ= 2.2477 μm) lines of H2 in the Galactic Centre, in a 90 × 27 arcsec2 region between the north-eastern boundary of the non-thermal source Sgr A East, and the giant molecular cloud (GMC) M−0.02 − 0.07 . The detected H2 v = 1 → 0 S(1) emission has an intensity of 1.6–21 × 10−18 W m−2 arcsec−2 and is present over most of the region. Along with the high intensity, the large linewidths (FWHM = 40–70 km s−1 ) and the H2 v = 2 → 1 S(1) to v = 1 → 0 S(1) line ratios (0.3–0.5) can be best explained by a combination of C-type shocks and fluorescence. The detection of shocked H2 is clear evidence that Sgr A East is driving material into the surrounding adjacent cool molecular gas. The H2 emission lines have two velocity components at ∼+50 and ∼0 km s−1 , which are also present in the NH3 (3, 3) emission mapped by McGary, Coil & Ho. This two-velocity structure can be explained if Sgr A East is driving C-type shocks into both the GMC M−0.02 − 0.07 and the northern ridge of McGary et al. 相似文献
937.
Patricia Whitelock Freddy Marang 《Monthly notices of the Royal Astronomical Society》2001,323(1):L13-L16
The binary companion to the peculiar F supergiant HD 172481 is shown to be a Mira variable with a pulsation period of 312 d. Its characteristics are within the normal range found for solitary Miras of that period, although its pulsation amplitude and mass-loss rate ̇ ∼3×10−6 M⊙ yr−1 are higher than average. Reasons are given for suspecting that the F supergiant, which has L ∼104 L⊙ , is a white dwarf burning hydrogen accreted from its companion. 相似文献
938.
徐好民 《地球信息科学学报》2002,4(2):50-54
地震预报工作在我国已取得了明显的进展。对此 ,我们相信在地震预报的研究中 ,只要坚持百花齐放、百家争鸣的方针 ,博采众长 ,宏观微观结合 ,对人口稠密地区 6级以上强烈破坏性地震 ,漏报、虚报的可能性都会大大降低 ,唐山一类的地震惨害有可能避免、减轻 (人员伤亡 )。所以普及地震知识 ,提高全民族科学精神对防震减灾是至关重要的。 相似文献
939.
R.J. Beswick A. Pedlar C.G. Mundell J.F. Gallimore 《Monthly notices of the Royal Astronomical Society》2001,325(1):151-158
We present neutral hydrogen absorption observations of the luminous infrared merger NGC 6240 using MERLIN with a resolution of 0.2 arcsec. Broad absorption (a few hundred km s−1 ) has been found against two compact radio sources within the central kpc providing dynamical information about the neutral gas components in front of these sources. A narrow absorption component is also detected superimposed upon this broad absorption and additionally against some of the extended L -band continuum. From these results we deduce that the broad component is a result of absorption by a highly disturbed disc-like structure of neutral gas aligned along the position angle of the two compact radio sources, similar to the model previously proposed by Tacconi et al. at the end of the last century based on spectral CO emission data. The narrow component is likely to arise from absorption by less disturbed neutral gas at much larger scales within the system.
Continuum observations presented here at 1.4 and 5 GHz support the view that NGC 6240 contains a double nucleus resulting from a galactic merger event and show these as two compact radio sources separated by 1.52 arcsec. We have also applied luminosity and morphological considerations to the continuum results to determine the most feasible source of radio emission for this luminous merger galaxy. We conclude that the most likely source of the radio flux found in NGC 6240 is a combination of starburst emission from radio luminous supernova remnants, similar to those found in Arp 220, and emission from a weak AGN probably triggered by a merger event. 相似文献
Continuum observations presented here at 1.4 and 5 GHz support the view that NGC 6240 contains a double nucleus resulting from a galactic merger event and show these as two compact radio sources separated by 1.52 arcsec. We have also applied luminosity and morphological considerations to the continuum results to determine the most feasible source of radio emission for this luminous merger galaxy. We conclude that the most likely source of the radio flux found in NGC 6240 is a combination of starburst emission from radio luminous supernova remnants, similar to those found in Arp 220, and emission from a weak AGN probably triggered by a merger event. 相似文献
940.
Near-infrared template spectra of normal galaxies: k-corrections, galaxy models and stellar populations 总被引:2,自引:0,他引:2
F. Mannucci F. Basile B.M. Poggianti A. Cimatti E. Daddi L. Pozzetti L. Vanzi 《Monthly notices of the Royal Astronomical Society》2001,326(2):745-758
We have observed 28 local galaxies in the wavelength range between 1 and 2.4 μm in order to define template spectra of the normal galaxies along the Hubble sequence. Five galaxies per morphological type were observed in most cases, and the resulting rms spread of the normalized spectra of each class, including both intrinsic differences and observational uncertainties, is about 1 per cent in K , 2 per cent in H and 3 per cent in J . Many absorption features can be accurately measured. The target galaxies and the spectroscopic aperture (7×53 arcsec2 ) were chosen to be similar to those used by Kinney et al. to define template UV and optical spectra. The two data sets are matched in order to build representative spectra between 0.1 and 2.4 μm. The continuum shape of the optical spectra and the relative normalization of the near-IR ones were set to fit the average effective colours of the galaxies of the various Hubble classes. The resulting spectra are used to compute the k -corrections of the normal galaxies in the near-IR bands, and to check the predictions of various spectral synthesis models: while the shape of the continuum is generally well predicted, large discrepancies are found in the absorption lines. Among the other possible applications, here we also show how these spectra can be used to place constraints on the dominant stellar population in local galaxies. Spectra and k -corrections are publicly available and can be downloaded from the web site http://www.arcetri.astro.it/~filippo/spectra . 相似文献