全文获取类型
收费全文 | 3081篇 |
免费 | 182篇 |
国内免费 | 379篇 |
专业分类
测绘学 | 33篇 |
大气科学 | 86篇 |
地球物理 | 363篇 |
地质学 | 869篇 |
海洋学 | 634篇 |
天文学 | 1421篇 |
综合类 | 60篇 |
自然地理 | 176篇 |
出版年
2024年 | 14篇 |
2023年 | 34篇 |
2022年 | 47篇 |
2021年 | 58篇 |
2020年 | 60篇 |
2019年 | 80篇 |
2018年 | 56篇 |
2017年 | 67篇 |
2016年 | 69篇 |
2015年 | 84篇 |
2014年 | 94篇 |
2013年 | 88篇 |
2012年 | 82篇 |
2011年 | 123篇 |
2010年 | 101篇 |
2009年 | 248篇 |
2008年 | 232篇 |
2007年 | 246篇 |
2006年 | 257篇 |
2005年 | 190篇 |
2004年 | 209篇 |
2003年 | 172篇 |
2002年 | 159篇 |
2001年 | 146篇 |
2000年 | 143篇 |
1999年 | 133篇 |
1998年 | 136篇 |
1997年 | 42篇 |
1996年 | 41篇 |
1995年 | 38篇 |
1994年 | 35篇 |
1993年 | 16篇 |
1992年 | 10篇 |
1991年 | 13篇 |
1990年 | 18篇 |
1989年 | 20篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1978年 | 2篇 |
排序方式: 共有3642条查询结果,搜索用时 500 毫秒
261.
We study the effect of contamination by interlopers in kinematic samples of galaxy clusters. We demonstrate that without the proper removal of interlopers the inferred parameters of the mass distribution in the cluster are strongly biased towards higher mass and lower concentration. The interlopers are removed using two procedures previously shown to work most efficiently on simulated data. One is based on using the virial mass estimator and calculating the maximum velocity available to cluster members and the other relies on the ratio of the virial and projected mass estimators. We illustrate the performance of the methods in detail using the example of A576, a cluster with a strong uniform background contamination, and compare the case of A576 to 15 other clusters with different degree of contamination. We model the velocity dispersion and kurtosis profiles obtained for the cleaned data samples of these clusters solving the Jeans equations to estimate the mass, concentration and anisotropy parameter. We present the mass–concentration relation for the total sample of 22 clusters. 相似文献
262.
We present near-infrared polarimetric images of the dusty circumstellar envelope (CSE) of IRAS 19306+1407, acquired at the United Kingdom Infrared Telescope (UKIRT) using the UKIRT 1–5 μm Imager Spectrometer (UIST) in conjunction with the half-waveplate module IRPOL2. We present additional 450- and 850-μm photometry data obtained with the Submillimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), as well as archived Hubble Space Telescope ( HST ) F606W - and F814W -filter images. The CSE structure in polarized flux at J and K bands shows an elongation north of north-east and south of south-west with two bright scattering shoulders north-west and south-east. These features are not perpendicular to each other and could signify a recent 'twist' in the outflow axis. We model the CSE using an axisymmetric light scattering ( als ) code to investigate the polarization produced by the CSE, and an axisymmetric radiation transport ( dart ) code to fit the spectral energy distribution. A good fit was achieved with the als and dart models using silicate grains, 0.1–0.4 μm with a power-law size distribution of a −3.5 , and an axisymmetric shell geometry with an equator-to-pole ratio of 7:1. The spectral type of the central star is determined to be B1 i supporting previous suggestions that the object is an early planetary nebula. We have constrained the CSE and interstellar extinction as 2.0 and 4.2 mag, respectively, and have estimated a distance of 2.7 kpc. At this distance, the stellar luminosity is ∼4500 L⊙ and the mass of the CSE is ∼0.2 M⊙ . We also determine that the mass loss lasted for ∼5300 yr with a mass-loss rate of ∼3.4 × 10−5 M⊙ yr−1 . 相似文献
263.
264.
265.
Andreas Faltenbacher William G. Mathews 《Monthly notices of the Royal Astronomical Society》2007,375(1):313-323
Based on results from cold dark matter N -body simulations, we develop a dynamical model for the evolution of subhaloes within group-sized host haloes. Only subhaloes more massive than 5 × 108 M⊙ are considered, because they are massive enough to possibly host luminous galaxies. On their orbits within a growing host potential the subhaloes are subject to tidal stripping and dynamical friction. At the present time ( z = 0) , all model hosts have equal mass ( M vir = 3.9 × 1013 M⊙ ) but different concentrations associated with different formation times. We investigate the variation of subhalo (or satellite galaxy) velocity dispersion with host concentration and/or formation time. In agreement with the Jeans equation, the velocity dispersion of subhaloes increases with the host concentration. Between concentrations of ∼5 and ∼20, the subhalo velocity dispersions increase by a factor of ∼1.25. By applying a simplified tidal disruption criterion, that is, rejection of all subhaloes with a tidal truncation radius below 3 kpc at z = 0 , the central velocity dispersion of the 'surviving' subhalo sample increases substantially for all concentrations. The enhanced central velocity dispersions in the surviving subhalo samples are caused by a lack of slow tangential motions. Additionally, we present a fitting formula for the anisotropy parameter which does not depend on concentration if the group-centric distances are scaled by r s , the characteristic radius of the Navarro, Frenk & White profile. Since the expected loss of subhaloes and galaxies due to tidal disruption increases the velocity dispersion of surviving galaxies, the observed galaxy velocity dispersion can substantially overestimate the virial mass. 相似文献
266.
267.
268.
269.
G. S. Khadekar Vaishali Kamdi Anirudh Pradhan Saeed Otarod 《Astrophysics and Space Science》2007,310(1-2):141-147
We assume the four dimensional induced matter of the 5D Ricci flat bouncing cosmological solution contains a perfect fluid.
The big bounce singularity of simple 5D cosmological model is studied with the cosmological term Λ=α
ρ and Λ=β
H
2 where α and β are constants and ρ and H are respectively energy density and Hubble parameter. This big bounce singularity is found to be an event horizon at which
the scale factor and mass density of the universe are finite, while the pressure is infinite.
相似文献
270.
C. J. Clarke 《Monthly notices of the Royal Astronomical Society》2007,376(3):1350-1356
We present models in which the photoevaporation of discs around young stars by an external ultraviolet source (as computed by Adams et al.) is coupled with the internal viscous evolution of the discs. These models are applied to the case of the Orion Nebula Cluster (ONC), where the presence of a strong ultraviolet field from the central OB stars, together with a detailed census of circumstellar discs and photoevaporative flows, is well established. In particular we investigate the constraints that are placed on the initial disc properties in the ONC by the twin requirement that most stars possess a disc on a scale of a few astronomical unit (au), but that only a minority (<20 per cent) are resolved by Hubble Space Telescope ( HST ) at a scale of 50 au. We find that these requirements place very weak constraints on the initial radius distribution of circumstellar discs: the resulting size distribution readily forgets the initial radius distribution, owing to the strong positive dependence of the photoevaporation rate on disc radius. Instead, the scarcity of large discs reflects the relative scarcity of initially massive discs (with mass >0.1 M⊙ ). The ubiquity of discs on a small scale, on the other hand, mainly constrains the time-span over which the discs have been exposed to the ultraviolet field (<2 Myr). We argue that the discs that are resolved by HST represent a population of discs in which self-gravity was important at the time that the dominant central OB star switched on, but that, according to our models, self-gravity is unlikely to be important in these discs at the present time. We discuss the implications of our results for the so-called proplyd lifetime problem. 相似文献