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51.
C D Bennett 《Nature》1974,252(5483):511-512
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Effect of gastrin on isolated smooth muscle preparation 总被引:10,自引:0,他引:10
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C Alcock R A Allsman D R Alves T S Axelrod A C Becker D P Bennett K H Cook A J Drake K C Freeman M Geha K Griest S C Keller M J Lehner S L Marshall D Minniti C A Nelson B A Peterson P Popowski M R Pratt P J Quinn C W Stubbs W Sutherland A B Tomaney T Vandehei D Welch 《Nature》2001,414(6864):617-619
The nature of dark matter remains mysterious, with luminous material accounting for at most approximately 25 per cent of the baryons in the Universe. We accordingly undertook a survey looking for the microlensing of stars in the Large Magellanic Cloud (LMC) to determine the fraction of Galactic dark matter contained in massive compact halo objects (MACHOs). The presence of the dark matter would be revealed by gravitational lensing of the light from an LMC star as the foreground dark matter moves across the line of sight. The duration of the lensing event is the key observable parameter, but gives non-unique solutions when attempting to estimate the mass, distance and transverse velocity of the lens. The survey results to date indicate that between 8 and 50 per cent of the baryonic mass of the Galactic halo is in the form of MACHOs (ref. 3), but removing the degeneracy by identifying a lensing object would tighten the constraints on the mass in MACHOs. Here we report a direct image of a microlens, revealing it to be a nearby low-mass star in the disk of the Milky Way. This is consistent with the expected frequency of nearby stars acting as lenses, and demonstrates a direct determination of a lens mass from a microlensing event. Complete solutions such as this for halo microlensing events will probe directly the nature of the MACHOs. 相似文献
56.
We compared the ability of global positioning system (GPS) radio collars deployed on elk ( Cervus elaphus nelsoni ) to obtain valid positions (position acquisition rate [PAR]) in seasonal home ranges with differing vegetation and topographical characteristics. We also compared GPS collar PARs under varying levels of cloud cover and within differing daily time periods. We recorded a mean PAR of 69% ( n = 10 elk, s = 14%) for collared elk. Multiple regression analysis of seasonal home range characteristics indicated that vegetation cover type and slope, either as individual variables or in combination with one another, were not significant predictors of GPS collar PARs. We did not observe statistical differences in position acquisition rates between cloud cover classes or varying cloud base heights. PAR was significantly higher between 1600 h and 2000 h (mountain standard time) compared to 0000 h-1200 h, which may have been due to elk behavior. We believe using GPS collars is a more effective and efficient method of tracking elk in our study area than using very-high-frequency (VHF) collars since GPS collars can be programmed to obtain fixes automatically, have fewer logistical problems, and are more economical with long-term data collection efforts. 相似文献
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Solutions for a cultivated planet 总被引:61,自引:0,他引:61
Foley JA Ramankutty N Brauman KA Cassidy ES Gerber JS Johnston M Mueller ND O'Connell C Ray DK West PC Balzer C Bennett EM Carpenter SR Hill J Monfreda C Polasky S Rockström J Sheehan J Siebert S Tilman D Zaks DP 《Nature》2011,478(7369):337-342
Increasing population and consumption are placing unprecedented demands on agriculture and natural resources. Today, approximately a billion people are chronically malnourished while our agricultural systems are concurrently degrading land, water, biodiversity and climate on a global scale. To meet the world's future food security and sustainability needs, food production must grow substantially while, at the same time, agriculture's environmental footprint must shrink dramatically. Here we analyse solutions to this dilemma, showing that tremendous progress could be made by halting agricultural expansion, closing 'yield gaps' on underperforming lands, increasing cropping efficiency, shifting diets and reducing waste. Together, these strategies could double food production while greatly reducing the environmental impacts of agriculture. 相似文献
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Sensitivity loss in odd-symmetric mechanisms and phase anomalies in peripheral vision 总被引:1,自引:0,他引:1
The ability to detect, discriminate and identify spatial stimuli is much poorer in the peripheral than in the central visual field. Some deficits are eliminated by scaling stimulus size. For example, grating detectibility is roughly constant across the visual field when spatial frequency and target extent are scaled appropriately. Other deficits persist despite scaling. For instance, some readily detectable patterns are more difficult to identify peripherally than in the fovea. This deficit is caused, at least partially, by a reduced ability to encode spatial phase (or relative position). To specify the properties of foveal and peripheral phase-encoding mechanisms, we measured discrimination thresholds for compound gratings at several eccentricities. Our observations are consistent with a two-channel model of phase encoding based on even- and odd-symmetric mechanisms (see Fig. 1), but the sensitivity of the odd-symmetric mechanisms decreases dramatically with eccentricity. Thus, the loss of sensitivity in one type of mechanism may underlie the reduced ability to encode spatial phase peripherally. 相似文献
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D Pennica W E Holmes W J Kohr R N Harkins G A Vehar C A Ward W F Bennett E Yelverton P H Seeburg H L Heyneker D V Goeddel D Collen 《Nature》1983,301(5897):214-221
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