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101.
D. G. Turner †‡ D. Forbes † P. J. T. Leonard M. Abdel-Sabour Abdel-Latif D. J. Majaess L. N. Berdnikov ‡ 《Monthly notices of the Royal Astronomical Society》2009,397(2):1046-1052
Photoelectric UBV photometry and star counts are presented for the previously unstudied open cluster Collinder 236, supplemented by observations for stars near the Cepheid WZ Car. Collinder 236 is typical of groups associated with Cepheids, with an evolutionary age of (3.2 ± 1.1) × 107 yr , but it is 1944 ± 71 pc distant, only half the predicted distance to WZ Car. The cluster is reddened by E ( B − V ) ≃ 0.26 , and has nuclear and coronal radii of r n ≃ 2 arcmin (1.1 pc) and R c ≃ 8 arcmin (4.5 pc) , respectively. The Cepheid is not a member of Collinder 236 on the basis of location beyond the cluster tidal radius and implied distance, but its space reddening can be established as E ( B − V ) = 0.268 ± 0.006 s.e. from five adjacent stars. Period changes in WZ Car studied with the aid of archival data are revised. The period of WZ Car is increasing, its rate of +8.27 ± 0.19 s yr−1 being consistent with a third crossing of the instability strip. 相似文献
102.
Andrés E. Piatti Juan J. Clariá Andrea V. Ahumada 《Monthly notices of the Royal Astronomical Society》2009,397(2):1073-1083
We present CCD photometry in the Johnson U , B and V and Kron–Cousins I passbands for the open cluster NGC 2587. The sample consists of 4406 stars reaching down to V ∼ 21.0 . We developed a new method to clean statistically the colour–magnitude diagrams. NGC 2587 appears to be a sparse, relatively bright open cluster, with a few tens of members projected on to a populous star field. The comparatively bright F7/8 II type star HD 70927, located close to the cluster centre, seems not to be a member. Our analysis suggests that NGC 2587 is slightly younger than the Hyades and probably of solar metallicity. A cluster radius of roughly 8 arcmin was estimated from the radial stellar density profile. From 18 probable cluster members with measured proper motions, we derive the following mean values for NGC 2587: μα =−4.3 ± 3.6 mas yr−1 and μδ =−2.5 ± 3.4 mas yr−1 . Adopting the theoretical metal content Z = 0.02 , which provides the best global fit, we derive a cluster age of 500+60 −50 . Simultaneously, colour excesses E ( B − V ) = 0.10 and E ( V − I ) = 0.15 and an apparent distance modulus of V − MV = 12.50 are obtained. The interstellar extinction in the cluster direction is found to follow the normal law. NGC 2587 is located at a distance of (2.70 ± 0.70) kpc from the Sun and ∼9.8 kpc from the Galactic centre. 相似文献
103.
G. A. J. Hussain A. Collier Cameron M. M. Jardine N. Dunstone J. Ramirez Velez H. C. Stempels J.-F. Donati M. Semel G. Aulanier T. Harries J. Bouvier C. Dougados J. Ferreira B. D. Carter W. A. Lawson 《Monthly notices of the Royal Astronomical Society》2009,398(1):189-200
We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G- and K-type T Tauri stars in the Chamaeleon I star-forming cloud with ages of 3–5 Myr. Our magnetic field maps show evidence for strong, complex multipolar fields similar to those obtained for young rapidly rotating main-sequence stars. Brightness maps indicate the presence of dark polar caps and low-latitude spots – these brightness maps are very similar to those obtained for other pre-main-sequence and rapidly rotating main-sequence stars.
Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence. 相似文献
Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence. 相似文献
104.
We present a photometric and spectroscopic study of the poorly investigated open cluster Trumpler 3. Basic parameters such as the age of 70 ± 10 Myr, the color excess E (B – V) = 0.30 ± 0.02 mag, the distance of 0.69 ± 0.03 kpc and the limiting radius of 12′ were redetermined and compared with previous preliminary studies. The distance of 0.65 ± 0.09 kpc was determined independently by spectral parallaxes. Simultaneously, our analysis allowed us to estimate a total number of members to be Ntot = 570 ± 90 and a total mass of the cluster to be Mtot = 270 ± 40 M⊙. We also determined a state of cluster's dynamical evolution. We conclude that Trumpler 3 is a young low‐massive stellar ensemble with a typical mass function slope, located near to the outer edge of the Galaxy's Orion Spur. As a result of a wide‐field search for short period variable stars, 24 variables were discovered in the cluster's area. Only one of them – a variable of the γ ‐Dor type – was found to be a likely cluster member (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
105.
106.
The degree of polarization of light from stars in 44 O B associations as a function of interstellar absorption is investigated. It is shown that the character of the dependence of P on A
V for stars in associations and stars not in associations depends on the value of A
V: for A
V 2
m
.5 it has a linear character and is the same for both groups of stars. For A
V > 2
m
.5 the dependence of P on A
V for stars in and not in associations departs from linearity and for A
V = 4
m
.5 it reaches P
ass = 1.8% and P
nonass = 1%, respectively. Such a difference is explained by the additional depolarization in stellar associations. Such strong depolarization in associations may be due to the overall magnetic field of the Galaxy and to physical peculiarities in the association itself. 相似文献
107.
Ian A. Bonnell Matthew R. Bate & Hans Zinnecker 《Monthly notices of the Royal Astronomical Society》1998,298(1):93-102
We present a model for the formation of massive ( M ≳10 M⊙) stars through accretion-induced collisions in the cores of embedded dense stellar clusters. This model circumvents the problem of accreting on to a star whose luminosity is sufficient to reverse the infall of gas. Instead, the central core of the cluster accretes from the surrounding gas, thereby decreasing its radius until collisions between individual components become sufficient. These components are, in general, intermediate-mass stars that have formed through accretion on to low-mass protostars. Once a sufficiently massive star has formed to expel the remaining gas, the cluster expands in accordance with this loss of mass, halting further collisions. This process implies a critical stellar density for the formation of massive stars, and a high rate of binaries formed by tidal capture. 相似文献
108.
Onno R. Pols Klaus-Peter Schro¨der Jarrod R. Hurley Christopher A. Tout & Peter P. Eggleton 《Monthly notices of the Royal Astronomical Society》1998,298(2):525-536
We have calculated a grid of empirically well tested evolutionary tracks with masses M between 0.5 and 50 M⊙, spaced by approximately 0.1 in log M , and with metallicities Z = 0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02 and 0.03. We use a robust and fast evolution code with a self-adaptive non-Lagrangian mesh, which employs the mixing-length theory but treats convective mixing as a diffusion process, solving simultaneously for the structure and the chemical composition. The hydrogen and helium abundances are chosen as functions of the metallicity: X = 0.76 − 3.0 Z Y = 0.24 + 2.0 Z . Two sets of models were computed, one without and one with a certain amount of enhanced mixing or 'overshooting'. This amount has been empirically chosen by means of various sensitive tests for overshooting: (1) the luminosity of core helium burning (blue loop) giants of well-known mass, (2) the width of the main sequence as defined by double-lined eclipsing binaries with well-measured masses and radii, and (3) the shape and implied stellar distribution of isochrones of various open clusters. The first two tests have been the subject of previous papers, the third test is discussed in this paper. On the basis of these tests, we recommend the use of the overshooting models for masses above about 1.5M ⊙. We describe here the characteristics of the models, the procedure for constructing isochrones for arbitrary age and metallicity from the models, and the performance of these isochrones for several intermediate-age and old open clusters. All original models are available in electronic form and we describe the means by which they may be obtained. 相似文献
109.
R. J. Sylvester C. J. Skinner & M. J. Barlow 《Monthly notices of the Royal Astronomical Society》1998,301(4):1083-1094
Following our discovery of unidentified infrared (UIR) band emission in a number of M supergiants in h and χ Per, we have obtained 10-μm spectra of a sample of 60 galactic M supergiants. Only three new sources, V1749 Cyg, UW Aql and IRC+40 427, appear to show the UIR bands; the others show the expected silicate emission or a featureless continuum. The occurrence of UIR-band emission in M supergiants is therefore much higher in the h and χ Per cluster than in the Galaxy as a whole. Possible explanations for the origin and distribution of UIR bands in oxygen-rich supergiants are discussed. We use our spectra to derive mass-loss rates ranging from 10−8 to 10−4 M⊙ yr−1 for the new sample, based on the power emitted in the silicate feature. The relationship between mass-loss rate and luminosity for M supergiants is discussed, and correlations are explored between their mid-infrared emission properties. 相似文献