全文获取类型
收费全文 | 861篇 |
免费 | 66篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 57篇 |
地球物理 | 286篇 |
地质学 | 313篇 |
海洋学 | 105篇 |
天文学 | 93篇 |
综合类 | 5篇 |
自然地理 | 66篇 |
出版年
2024年 | 5篇 |
2023年 | 8篇 |
2022年 | 7篇 |
2021年 | 24篇 |
2020年 | 33篇 |
2019年 | 35篇 |
2018年 | 35篇 |
2017年 | 39篇 |
2016年 | 47篇 |
2015年 | 39篇 |
2014年 | 46篇 |
2013年 | 50篇 |
2012年 | 50篇 |
2011年 | 59篇 |
2010年 | 59篇 |
2009年 | 50篇 |
2008年 | 42篇 |
2007年 | 40篇 |
2006年 | 29篇 |
2005年 | 30篇 |
2004年 | 35篇 |
2003年 | 22篇 |
2002年 | 24篇 |
2001年 | 16篇 |
2000年 | 17篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有939条查询结果,搜索用时 488 毫秒
781.
Luca Casagrande Laura Portinari Chris Flynn 《Monthly notices of the Royal Astronomical Society》2006,373(1):13-44
We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the Infrared Flux Method to multiband photometry. Our study is based on 104 stars for which we have excellent BV ( RI )C JHK S photometry, excellent parallaxes and good metallicities.
Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement.
A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology.
Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices.
We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem. 相似文献
Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement.
A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology.
Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices.
We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem. 相似文献
782.
783.
784.
785.
Jesper Sommer-Larsen Martin Götz Laura Portinari 《Astrophysics and Space Science》2002,281(1-2):519-524
We have performed TreeSPH simulations of galaxy formation in a standard ΛCDM cosmology, including effects of star formation,
energetic stellar feedback processes and a meta-galactic UV field, and obtain a mix of disk, lenticular and elliptical galaxies.
The disk galaxies are deficient in angular momentum by only about a factor of two compared to observed disk galaxies. The
stellar disks have approximately exponential surface density profiles, and those of the bulges range from exponential to r
1/4, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations and likewise are their integrated
B-V colours, which have been calculated using stellar population synthesis techniques. Furthermore, we can match the observed
I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is (M/L
I) ≃ 0.6–0.7. The ellipticals and lenticulars have approximately r
1/4 stellar surface density profiles, are dominated by non-disklike kinematics and flattened due to non-isotropic stellar velocity
distributions, again consistent with observations.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
786.
Laura Portinari Jesper Sommer-Larsen Rosaria Tantalo 《Astrophysics and Space Science》2003,284(2):723-726
A low mass-to-light ratio for the baryonic component of spiral galaxies is advocated by a number of dynamical studies and
by cosmological simulations of galaxy formation. We discuss the possibility of obtaining low mass-to-light ratios for the
stellar component in discs, by changing the Initial Mass Function and the Star Formation History.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
787.
John C. Ayers Steven Goodbred Gregory George David Fry Laura Benneyworth George Hornberger Kushal Roy Md. Rezaul Karim Farjana Akter 《Geochemical transactions》2016,17(1):4
Background
High salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012–2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy.Results
Drill cuttings, exposures in pits and regional studies reveal a >5 m thick surface mud cap overlying a ~30 m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60 m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of ~2 cm/year through the early Holocene, with local subsidence or compaction rates of 1–3 mm/year. Most tubewells are screened at depths of 15–52 m in sediments deposited 8000–9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na–Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11–57 mg/L and As 2–258 ug/L, and low sulfur (S) 2–33 mg/L.Conclusions
Groundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO4) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured 14C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46 % for As and 100 % for salinity.788.
789.
Oceanographic controls on shallow‐water temperate carbonate sedimentation: Spencer Gulf,South Australia 下载免费PDF全文
Laura G. O'Connell Noel P. James Mark Doubell John F. Middleton John Luick David R. Currie Yvonne Bone 《Sedimentology》2016,63(1):105-135
Spencer Gulf is a large (ca 22 000 km2), shallow (<60 m water depth) embayment with active heterozoan carbonate sedimentation. Gulf waters are metahaline (salinities 39 to 47‰) and warm‐temperate (ca 12 to ?28°C) with inverse estuarine circulation. The integrated approach of facies analysis paired with high‐resolution, monthly oceanographic data sets is used to pinpoint controls on sedimentation patterns with more confidence than heretofore possible for temperate systems. Biofragments – mainly bivalves, benthic foraminifera, bryozoans, coralline algae and echinoids – accumulate in five benthic environments: luxuriant seagrass meadows, patchy seagrass sand flats, rhodolith pavements, open gravel/sand plains and muddy seafloors. The biotic diversity of Spencer Gulf is remarkably high, considering the elevated seawater salinities. Echinoids and coralline algae (traditionally considered stenohaline organisms) are ubiquitous. Euphotic zone depth is interpreted as the primary control on environmental distribution, whereas seawater salinity, temperature, hydrodynamics and nutrient availability are viewed as secondary controls. Luxuriant seagrass meadows with carbonate muddy sands dominate brightly lit seafloors where waters have relatively low nutrient concentrations (ca 0 to 1 mg Chl‐a m?3). Low‐diversity bivalve‐dominated deposits occur in meadows with highest seawater salinities and temperatures (43 to 47‰, up to 28°C). Patchy seagrass sand flats cover less‐illuminated seafloors. Open gravel/sand plains contain coarse bivalve–bryozoan sediments, interpreted as subphotic deposits, in waters with near normal marine salinities and moderate trophic resources (0·5 to 1·6 mg Chl‐a m?3) to support diverse suspension feeders. Rhodolith pavements (coralline algal gravels) form where seagrass growth is arrested, either because of decreased water clarity due to elevated nutrients and associated phytoplankton growth (0·6 to 2 mg Chl‐a m?3), or bottom waters that are too energetic for seagrasses (currents up to 2 m sec?1). Muddy seafloors occur in low‐energy areas below the euphotic zone. The relationships between oceanographic influences and depositional patterns outlined in Spencer Gulf are valuable for environmental interpretations of other recent and ancient (particularly Neogene) high‐salinity and temperate carbonate systems worldwide. 相似文献
790.