Improving the seeing with wide-field adaptive optics in the near-infrared     

M. Le Louarn  N. Hubin 《Monthly notices of the Royal Astronomical Society》2006,365(4):1324-1332

In this paper, we present simulation results of a ground-layer correction adaptive optics system (GLAO), based on four laser guide stars and a single deformable mirror. The goal is to achieve a seeing improvement over an 8-arcmin field of view, in the near-infrared (from 1.06 to 2.2 μm). We show results on the scaling of this system (number of subapertures, frame rates), and the required number of tip-tilt stars. We investigate the use for GLAO of both sodium and Rayleigh guide stars. We also show that if the lasers can be repositioned, the performance of the adaptive optics can be tailored to the astronomical observations.  相似文献   

Gamma–hadron separation methods for the VERITAS array of four imaging atmospheric Cherenkov telescopes     

H. Krawczynski  D.A. Carter-Lewis  C. Duke  J. Holder  G. Maier  S. Le Bohec  G. Sembroski 《Astroparticle Physics》2006,25(6):380-390

Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged as the most sensitive γ-ray detectors in the energy range of about 100 GeV and above. The strengths of these arrays are a very large effective collection area on the order of 10⁵ m², combined with excellent single photon angular and energy resolutions. The sensitivity of such detectors is limited by statistical fluctuations in the number of Cosmic-ray initiated air showers that resemble γ-ray air showers in many ways. In this paper, we study the performance of simple event reconstruction methods when applied to simulated data of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) experiment. We review methods for reconstructing the arrival direction and the energy of the primary photons, and examine means to improve on their performance. For a software threshold energy of 300 GeV (100 GeV), the methods achieve point source angular and energy resolutions of σ_63% = 0.1° (0.2°) and σ_68% = 15% (22%), respectively. The main emphasis of the paper is the discussion of γ–hadron separation methods for the VERITAS experiment. We find that the information from several methods can be combined based on a likelihood ratio approach and the resulting algorithm achieves a γ–hadron suppression with a quality factor that is substantially higher than that achieved with the standard methods used so far.  相似文献   

The Zurich Extragalactic Bayesian Redshift Analyzer and its first application: COSMOS     

R. Feldmann  C. M. Carollo  C. Porciani  S. J. Lilly  P. Capak  Y. Taniguchi  O. Le Fèvre  A. Renzini  N. Scoville  M. Ajiki  H. Aussel  T. Contini  H. McCracken  B. Mobasher  T. Murayama  D. Sanders  S. Sasaki  C. Scarlata  M. Scodeggio  Y. Shioya  J. Silverman  M. Takahashi  D. Thompson  G. Zamorani 《Monthly notices of the Royal Astronomical Society》2006,372(2):565-577

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Shock melting of ordinary chondrite powders and implications for asteroidal regoliths     

Friedrich Hrz  Mark J. Cintala  Thomas H. See  Loan Le 《Meteoritics & planetary science》2005,40(9-10):1329-1346

Abstract— A series of 59 impacts in the laboratory reduced a coherent 460 g piece of the L6 ordinary chondrite ALH 85017 to a coarse‐grained “regolith.” We then subjected the 125–250 μm fines from this sample to reverberation shock stresses of 14.5–67 GPa in order to delineate the melting behavior of porous, unconsolidated, chondritic asteroid surfaces during meteorite impact. The initial pore space (40–50%) was completely closed at 14.5 GPa and a dense aggregate of interlocking grains resulted. Grain‐boundary melting commenced at <27 GPa and ?50% of the total charge was molten at 67 GPa; this stress corresponds to typical asteroid impacts at ?5 km/sec. Melting of the entire sample most likely mandates >80 GPa, which is associated with impact velocities >8 km/sec. The Fe‐Ni and troilite clasts of the original meteorite melted with particular ease, forming immiscible melts that are finely disseminated throughout the silicate glass. These metal droplets are highly variable in size, extending to <100 nm and most likely to superparamagnetic domains; such opaques are also observed in the natural melt veins of ordinary chondrites. It follows that melting and dissemination of pre‐existing, Fe‐rich phases may substantially affect the optical properties of asteroidal surfaces. It seems unnecessary to invoke reduction of Fe²⁺ (or Fe³⁺) by sputtering or impact‐processes—in analogy to the lunar surface—to produce “space weathering” effects on S‐type asteroids. We note that HED meteorites contain ample FeO (comparable to that in lunar basalts) for reduction processes to take place, yet their probable parent object(s), Vesta and its collisional fragments, display substantially unweathered surfaces. Howardites, eucrites, and diogenites (HEDs), however, contain little native metal (typically <0.5%), in contrast to ordinary chondrites (commonly 10–15%) and their S‐type parent objects. These considerations suggest that the modal content of native metal and sulfides is more important for space weathering on asteroids than total FeO.  相似文献   

Sea Level Rise Affecting the Vietnamese Mekong Delta: Water Elevation in the Flood Season and Implications for Rice Production   总被引：3，自引：0，他引：3  

Reiner Wassmann  Nguyen Xuan Hien  Chu Thai Hoanh  To Phuc Tuong 《Climatic change》2004,66(1-2):89-107

In this study, we assessed the impact of sea level rise, one of the most ascertained consequences of global climate change, for water levels in the Vietnamese Mekong Delta (VMD). We used a hydraulic model to compute water levels from August to November - when flooding is presently critical- under sea level rise scenarios of 20 cm (=Δ 20) and 45 cm (=Δ 45), respectively. The outputs show that the contour lines of water levels will be shifted up to 25 km (Δ 20) and 50 km (Δ 45) towards the sea due to higher sea levels. At the onset of the flood season (August), the average increment in water levels in the Delta is 14.1 cm (Δ 20) and 32.2 cm (Δ 45), respectively. At the peak of the flood season (October), high discharge from upstream attenuates the increment in water level, but average water level rise of 11.9 cm (Δ 20) and 27.4 cm (Δ 45), respectively, still imply a substantial aggravation of flooding problems in the VMD. GIS techniques were used to delineate areas with different levels of vulnerability, i.e., area with high (2.3 mio ha =60% of the VMD), medium (0.6 mio ha = 15%) and low (1 mio ha =25%) vulnerability due to sea level rise. Rice production will be affected through excessive flooding in the tidally inundated areas and longer flooding periods in the central part of the VMD. These adverse impacts could affect all three cropping seasons, Mua (main rainfed crop), Dong Xuan (Winter-Spring) and He Thu (Summer-Autumn) in the VMD unless preventive measures are taken.  相似文献   

Classical and Mg ii-selected damped Lyman α absorbers: impact on Ω_H i at z < 1.7     

Céline Péroux †  Jean-Michel Deharveng  Vincent Le Brun  Stefano Cristiani 《Monthly notices of the Royal Astronomical Society》2004,352(4):1291-1301

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A general purpose autonomous transmitter and receiver system forunderwater acoustic experiments     

Nguyen  H.B. DeFerrari  H.A. Williams  N.J. 《Oceanic Engineering, IEEE Journal of》1996,21(1):85-93

A versatile multifrequency, multi-purpose acoustic transmitter and receiver system has been developed and used to measure low frequency acoustic backscatter from the ocean surface. The instrumentation is general purpose in that all signal and sample parameters are selected by software and a variety of transmission, scattering and reverberation experiments are possible without hardware modification. The system is battery-powered and self-contained for remote and unattended operation for periods of months, depending upon the experimental parameters that affect power and tape consumption. The main features of the system are: a high-powered broadband set of acoustic sources-with center frequencies of 100, 200, 400, and 800 Hz, programmable output signal types, a 64-element vertical hydrophone array, fifty gigabytes of data storage capacity, dynamically-tuneable band pass filters and variable sampling rates, high power microprocessors for in situ signal processing and independent subsystems for recording mooring depth and inclination. The system has been used in surface backscattering experiments at sea and results that demonstrate the capability of the system are presented and discussed  相似文献   

Core-mantle boundary shape: constraints inferred from the pressure torque acting between the core and the mantle   总被引：1，自引：0，他引：1  

D. Jault  J.-L. Le Mouël 《Geophysical Journal International》1990,101(1):233-241

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Horticultural Reform in the European Union and New Zealand: Further developments towards a global fresh fruit and vegetable complex     

David J. Hayward  Richard B. Le Heron 《The Australian geographer》2002,33(1):9-27

The coincidental reregulations of horticultural industries in the European Union (EU) and New Zealand (NZ) are the context for this study of the development possibilities for the global fresh fruit and vegetable (FF&V) complex. Both regions developed specific post-war regulatory frameworks that have provided considerable periods of market and organisational stability. Presently, the FF&V industries in both the EU and NZ are experiencing regulatory restarts, in which the uniformities of the domestic commodity chains are being removed and new growth opportunities are being afforded. These opportunities privilege different nodes within the commodity chains, leading to direct and - more profoundly - indirect impacts. The analysis is based upon interviews with key informants in the EU and NZ, supported by secondary data. At the time of this analysis the reregulations are in their formative stages and so the paper attempts to expose the origins and developments thus far, and to sketch the potential outcomes in a broad sense. In both regions FF&V marketing organisations are privileged and effectively relieved of their former grower-community responsibilities. They are thus able to exploit new value-adding opportunities and pursue strategies focused upon fruit quality rather than quantity, the spread of risk, and brand marketing.  相似文献   

Application of thermodynamics to the global modelling of shallow foundations on frictional material     

Y. Le Pape  J.‐G. Sieffert 《国际地质力学数值与分析法杂志》2001,25(14):1377-1408

Soil–shallow foundation interaction has been theoretically analysed within the framework of thermomechanics. The design of a global interaction model has been achieved with an original treatment of the Clausius–Duhem inequality. The role of the gravity volume forces is emphasized. The paper is focused on a strip footing based on dense sand and subjected to time‐independent plastic processes. The theoretical approach has confirmed that an associated global flow rule cannot be expected to hold true. The analysis of the sources of dissipation has led to the development of a soil–footing interface model and a complete interaction model accounting for the interface constraints and the intrinsic frictional properties of the soil. Finally, the abilities of the complete model are checked by comparisons with experimental results found in the literature. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献