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991.
Enrico Vesperini Steve McMillan Simon Portegies Zwart 《Astrophysics and Space Science》2009,324(2-4):277-282
A significant degree of mass segregation inconsistent with the effects of standard two-body relaxation has been observed in a number of young star clusters. In this paper we present the results of a survey of N-body simulations aimed at exploring the origin and the dynamical evolution of young mass-segregated star clusters. Our simulations show that large segregated clusters can form from the merger of small clumps that are either initially segregated or in which segregation is produced before the merger is complete; the large cluster produced at the end of the merger process inherits the progenitor clumps’ segregation. We show that, in a young mass-segregated cluster, the effect of early mass loss associated with stellar evolution is, in general, more destructive than for an unsegregated cluster with the same density profile, and leads to shorter lifetimes, a faster initial evolution towards less-concentrated structure and a faster flattening of the stellar initial mass function. 相似文献
992.
Jill M. Rathborne James M. Jackson Robert Simon Qizhou Zhang 《Astrophysics and Space Science》2009,324(2-4):155-162
Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveals many compact (<0.5 pc) and massive (10–2100 M⊙) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters. 相似文献
993.
N.J.T. Edberg A.I. Eriksson U. Auster S. Barabash A. Bößwetter C.M. Carr S.W.H. Cowley E. Cupido M. Fränz K.-H. Glassmeier R. Goldstein M. Lester R. Lundin R. Modolo H. Nilsson I. Richter M. Samara J.G. Trotignon 《Planetary and Space Science》2009,57(8-9):1085-1096
We present the first two-spacecraft near-simultaneous observations of the Martian bow shock (BS), magnetic pileup boundary (MPB) and photo-electron boundary (PEB) obtained by the plasma instruments onboard Rosetta and Mars Express during the Rosetta Mars flyby on February 25, 2007. Our observations are compared with shape models for the BS and MPB derived from previous statistical studies. The MPB is found at its expected position but the BS for this event is found significantly closer to the planet than expected for the rather slow and moderately dense solar wind. Cross-calibration of the density measurements on the two spacecraft gives a density profile through the magnetosheath, indicating an increasing solar wind flux during the Rosetta passage which is consistent with the multiple BS crossings at the Rosetta exit. 相似文献
994.
Richard J. Parker Simon P. Goodwin Pavel Kroupa M. B. N. Kouwenhoven 《Monthly notices of the Royal Astronomical Society》2009,397(3):1577-1586
We examine the dynamical destruction of binary systems in star clusters of different densities. We find that at high densities (104 – 105 M⊙ pc−3 ) almost all binaries with separations >103 au are destroyed after a few crossing times. At low densities [ ], many binaries with separations >103 au are destroyed, and no binaries with separations >104 au survive after a few crossing times. Therefore, the binary separations in clusters can be used as a tracer of the dynamical age and past density of a cluster.
We argue that the central region of the Orion nebula cluster was ∼100 times denser in the past with a half-mass radius of only 0.1–0.2 pc as (i) it is expanding, (ii) it has very few binaries with separations >103 au and (iii) it is well mixed and therefore dynamically old.
We also examine the origin of the field binary population. Binaries with separations <102 au are not significantly modified in any cluster, therefore at these separations the field reflects the sum of all star formation. Binaries with separations in the range 102 – 104 au are progressively more and more heavily affected by dynamical disruption in increasingly dense clusters. If most star formation is clustered, these binaries must be overproduced relative to the field. Finally, no binary with a separation >104 au can survive in any cluster and so must be produced by isolated star formation, but only if all isolated star formation produces extremely wide binaries. 相似文献
We argue that the central region of the Orion nebula cluster was ∼100 times denser in the past with a half-mass radius of only 0.1–0.2 pc as (i) it is expanding, (ii) it has very few binaries with separations >10
We also examine the origin of the field binary population. Binaries with separations <10
995.
996.
Asteroids can be considered as sources of contamination of point sources and also sources of confusion noise, depending whether their presence is detected in the image or their flux is under the detection limit. We estimate that at low ecliptic latitudes, ≈10,000–20,000 asteroids/sq. degree will be detected with an E-ELT like telescope, while by the end of Spitzer and Herschel missions, infrared space observatories will provide ≈100,000 serendipitous asteroid detections. The detection and identification of asteroids is therefore an important step in survey astronomy. 相似文献
997.
Based on the Sloan Digital Sky Survey DR6 (SDSS) and the Millennium Simulation (MS), we investigate the alignment between galaxies and large-scale struc-ture. For this purpose, we develop two new statistical tools, namely the alignment cor-relation function and the cos(20)-statistic. The former is a two-dimensional extension of the traditional two-point correlation function and the latter is related to the ellipticity correlation function used for cosmic shear measurements. Both are based on the cross correlation between a sample of galaxies with orientations and a reference sample which represents the large-scale structure. We apply the new statistics to the SDSS galaxy cat-alog. The alignment correlation function reveals an overabundance of reference galaxies along the major axes of red, luminous (L L*) galaxies out to projected separations of 60 h-1Mpc. The signal increases with central galaxy luminosity. No alignment signal is detected for blue galaxies. The cos(2θ)-statistic yields very similar results. Starting from a MS semi-analytic galaxy catalog, we assign an orientation to each red, luminous and central galaxy, based on that of the central region of the host halo (with size similar to that of the stellar galaxy). As an alternative, we use the orientation of the host halo itself. We find a mean projected misalignment between a halo and its central region of ~ 25°. The misalignment decreases slightly with increasing luminosity of the central galaxy. Using the orientations and luminosities of the semi-analytic galaxies, we repeat our alignment analysis on mock surveys of the MS. Agreement with the SDSS results is good if the central orientations are used. Predictions using the halo orientations as proxies for cen-tral galaxy orientations overestimate the observed alignment by more than a factor of 2. Finally, the large volume of the MS allows us to generate a two-dimensional map of the alignment correlation function, which shows the reference galaxy distribution to be flat-tened parallel to the orientations of red luminous galaxies with axis ratios of ~ 0.5 and ~ 0.75 for halo and central orientations, respectively. These ratios are almost independent of scale out to 60h-1Mpc. 相似文献
998.
Silvia Bonoli Federico Marulli Volker Springel Simon D. M. White Enzo Branchini Lauro Moscardini 《Monthly notices of the Royal Astronomical Society》2009,396(1):423-438
We use semi-analytic modelling on top of the Millennium simulation to study the joint formation of galaxies and their embedded supermassive black holes. Our goal is to test scenarios in which black hole accretion and quasar activity are triggered by galaxy mergers, and to constrain different models for the light curves associated with individual quasar events. In the present work, we focus on studying the spatial distribution of simulated quasars. At all luminosities, we find that the simulated quasar two-point correlation function is fit well by a single power law in the range 0.5 ≲ r ≲ 20 h −1 Mpc , but its normalization is a strong function of redshift. When we select only quasars with luminosities within the range typically accessible by today's quasar surveys, their clustering strength depends only weakly on luminosity, in agreement with observations. This holds independently of the assumed light-curve model, since bright quasars are black holes accreting close to the Eddington limit, and are hosted by dark matter haloes with a narrow mass range of a few 1012 h −1 M⊙ . Therefore, the clustering of bright quasars cannot be used to disentangle light-curve models, but such a discrimination would become possible if the observational samples can be pushed to significantly fainter limits. Overall, our clustering results for the simulated quasar population agree rather well with observations, lending support to the conjecture that galaxy mergers could be the main physical process responsible for triggering black hole accretion and quasar activity. 相似文献
999.
Axel Brandenburg Simon Candelaresi Piyali Chatterjee 《Monthly notices of the Royal Astronomical Society》2009,398(3):1414-1422
Using mean-field models with a dynamical quenching formalism, we show that in finite domains magnetic helicity fluxes associated with small-scale magnetic fields are able to alleviate catastrophic quenching. We consider fluxes that result from advection by a mean flow, the turbulent mixing down the gradient of mean small-scale magnetic helicity density or the explicit removal which may be associated with the effects of coronal mass ejections in the Sun. In the absence of shear, all the small-scale magnetic helicity fluxes are found to be equally strong for both large- and small-scale fields. In the presence of shear, there is also an additional magnetic helicity flux associated with the mean field, but this flux does not alleviate catastrophic quenching. Outside the dynamo-active region, there are neither sources nor sinks of magnetic helicity, so in a steady state this flux must be constant. It is shown that unphysical behaviour emerges if the small-scale magnetic helicity flux is forced to vanish within the computational domain. 相似文献
1000.
Corinne A. Locke Simon A. Johnson John Cassidy Jeffrey L. Mauk 《Journal of Geochemical Exploration》1999,65(2):91
The Puhipuhi epithermal area, which occurs in a region of graywacke basement partially covered by basalt and lake-bed deposits, is characterized by both large-scale and small-scale geophysical anomalies. Known occurrences of locally intense alteration or silicification are typically associated with strong gravity, resistivity or IP anomalies. Gravity data define a complex negative residual anomaly (up to −50 gu) which has been used to identify and delineate a large area (about 20 km2) of low-density, presumably clay-altered, graywacke basement rocks. This zone, modeled as extending to a few kilometers depth, encompasses, but is more extensive than, the known areas of alteration and has a close spatial association with the basalt cover rocks. Short-wavelength gravity minima and maxima, which indicate that the most intense alteration of the basement rocks occurs below the basalt, correlate, in part, with the inferred location of hydrothermal upflow zones. The control on the location of these zones and their relationship to the location of the basalts is not well known; however, if the basalts acted as a cap rock to the geothermal system, then these areas merit further exploration. High (≥100 ohm-m) and low (≤10 ohm-m) resistivity and high (≥30 mS) IP anomalies occur in association with known silicification, clay alteration and sulfide mineralisation, respectively. In addition, magnetic data help constrain the relative timing of hydrothermal alteration and basaltic volcanism and indicate that mineralisation was broadly synchronous with volcanism. 相似文献