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W. H. Jefferys G. F. Benedict P. D. Hemenway P. J. Shelus R. L. Duncombe 《Celestial Mechanics and Dynamical Astronomy》1985,37(3):299-305
The Hubble Space Telescope (HST), a large optical telescope having an aperture of 2.4 meters and a length of 8.8 meters, is being developed by the National Aeronautics and Space Administration. This telescope will be placed into earth orbit by the space shuttle. Astrometric observations with the HST are made using a Fine Guidance Sensor which is capable of measuring the position of one object relative to another with an accuracy of ±0.002 arcseconds. The astrometric user of HST will be provided with an Astrometric Data Reduction Software package (ADRS). The variety of astrometric problems to be investigated with HST is discussed. 相似文献
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Benedict W. Ritchie Peter A. Thomas 《Monthly notices of the Royal Astronomical Society》2002,329(3):675-688
We present the results of high-resolution AP3 M+SPH simulations of merging clusters of galaxies. We find that the compression and shocking of the core gas during a merger can lead to large increases in bolometric X-ray luminosities and emission-weighted temperatures of clusters. Cooling flows are completely disrupted during equal-mass mergers, with the mass deposition rate dropping to zero as the cores of the clusters collide. The large increase in the cooling time of the core gas strongly suggests that cooling flows will not recover from such a merger within a Hubble time. Mergers with subclumps having one eighth of the mass of the main cluster are also found to disrupt a cooling flow if the merger is head-on. However, in this case the entropy injected into the core gas is rapidly radiated away and the cooling flow restarts within a few Gyr of the merger. Mergers in which the subcluster has an impact parameter of 500 kpc do not disrupt the cooling flow, although the mass deposition rate is reduced by ∼30 per cent. Finally, we find that equal mass, off-centre mergers can effectively mix gas in the cores of clusters, while head on mergers lead to very little mixing. Gas stripped from the outer layers of subclumps results in parts of the outer layers of the main cluster being well mixed, although they have little effect on the gas in the core of the cluster. None of the mergers examined here resulted in the intracluster medium being well mixed globally. 相似文献
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Water samples from 26 Oregon beaches were analyzed for Escherichia coli (E. coli) and enterococci concentrations by the Oregon Department of Human Services (ODHS) Public Health Laboratory. Nine Oregon beaches exceeded US Environmental Protection Agency's (USEPA) single sample maximum density of 104 enterococci colony forming units (cfu) per 100 mL with levels ranging from 121 to 4325 most probable number (MPN)/100 milliliters (mL). Otter Rock at South Cove had the highest enterococci concentration at 4352 MPN/100 mL. The results from two Oregon beaches exceeded Oregon Department of Environmental Quality's (ODEQ) estuarine E. coli standard of 406 organisms/100 mL. Otter Rock at South Cove had the highest E. coli concentration at 1850 MPN/100 mL while Road's End had an E. coli density of 771 MPN/100 mL. Results of this study suggest that adopting USEPA's marine enterococci standard in lieu of the ODEQ's estuarine standard will lead to increased Oregon beach water standard failures. 相似文献
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Bioavailable contaminant concentrations are an important component in assessing environmental effects as they directly affect ecosystem health. Shellfish contaminant monitoring programmes have traditionally filled this requirement but are being phased out in some jurisdictions. Passive sampling devices (PSDs) have the potential to replace shellfish monitoring; however, there are still knowledge gaps to address before this can occur. This study assessed the suitability of three different PSDs in providing the required information to replace shellfish monitoring. PSDs were deployed at three historic mussel monitoring sites with different levels of urban influence in the Waitemata Harbour, Auckland, New Zealand. Contaminants of interest were urban heavy metals, plus current and emerging organic contaminants. PSDs provided extremely low detection limits and, for some contaminants, very strong correlations to shellfish. PSDs can currently complement shellfish in monitoring, but it is premature to make conclusions as to the suitability of PSDs in replacing shellfish monitoring until more information is available. 相似文献
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This paper describes an effort to develop a predictive tool for the design of a new promising marine transport — the WIG craft. The presented mathematical model of the WIG craft is capable of modeling the aerodynamics of a WIG system including the ground effect, the hydrodynamics of a stepped planing hull with a hydrofoil and also the simulation of motion for the craft. Based on extensive experience using the model, it is shown that the most important and necessary features of WIG aero- and hydrodynamics are taken into account. The results of simulations have been validated through comparison with other theoretical approaches and also with model experiments. The mathematical model is applied to investigate the dynamics of the small manually piloted WIG craft: “Hydrowing VT01”. The numerical study resulted in recommendations allowing the pilot to overcome the pitch-up tendency and also to perform the take-off manoeuvre smoothly. The stability of the WIG with a hydrofoil and also the dynamic properties of anti-collision manoeuvres have been studied and are presented. 相似文献
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James B. Benedict 《Quaternary Research》1976,6(1):55-76
Frost creep and gelifluction are the cold-climate representatives of mass-wasting processes that occur in a broad range of environments. Neither process requires permafrost, and frost creep can be inhibited by its presence at shallow depth. Acting in various combinations, frost creep and gelifluction produce distinctive lobate and terrace-like landforms, which are easy to recognize while fresh and active, but difficult to distinguish from mudflow lobes, earthslides, and similar deposits after they have been modified by other processes. Large frost creep and gelifluction features are currently active in many tundra environments that experience only deep seasonal freezing; thus they are not generally considered to be indicators of permafrost. Most radio-carbon-dated lobes and terraces, however, seem to have originated at times when permafrost was more widespread than it is today. This is true in the Colorado Front Range, where the formation of lobes and terraces appears to have been initiated by rapid melting of ice-enriched permafrost during the warming phases of frost-heave cycles that were centuries or millennia in duration. There is growing evidence that lobes and terraces developed in many parts of the world between about 3000 and 2500 BP; the climatic significance of their formation during this interval is open to several interpretations. Long-term average rates of frontal advance, calculated for deposits in Colorado, Australia, Greenland, Yukon Territory, Alaska, Scotland, and Norway, range from 0.6 to 3.5 mm per calendar year, significantly slower than maximum rates of movement measured on the surfaces of active lobes and terraces in comparable environments; the features are clearly not as effective at transporting debris as was previously supposed. Variations in past rates of downslope soil movement, estimated from close-interval dating of buried humus horizons or plant remains overrun by the advancing fronts of lobes and terraces, provide a sensitive record of climatic change. The dated humus layers are also suitable for detailed pollen analyses and soil chronosequence studies. 相似文献
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