Rugate filters for OH-suppressed imaging at near-infrared wavelengths     

Alison R. Offer  & Joss Bland-Hawthorn 《Monthly notices of the Royal Astronomical Society》1998,299(1):176-188

Ground-based observations at near-infrared wavelengths are severely affected by atmospheric OH bands. Many authors have recognized the potential gains in sensitivity from suppressing these features. Dispersive instruments show some promise but are both expensive and complicated to build. OH suppression filters using single or periodic notches have the advantage of simplicity but significant gains have not yet been realized.   Rugate filters (with graded index inhomogeneous coatings) offer key advantages for astronomical imaging. It is possible to produce a transmission profile comprising a series of irregular and sharply defined bandpasses. We demonstrate through numerical simulation of rugate filters that it should be possible to achieve 95 per cent suppression of the OH features in the J photometric band, while retaining roughly half of the spectral coverage. This would lead to extraordinary gains in sensitivity even for observations of continuum sources. In addition, these filters allow longer exposures before the detector saturates on the sky background. I - and z -band filters can also be envisaged.   In 1-arcsec seeing, a J -band rugate filter used in conjunction with a 4-m telescope would detect a J  = 23 continuum source at 5.0σ in a single 10-min exposure. In comparison, a conventional J filter requires multiple exposures for a 10-min integration time and achieves only a 2.5σ detection. For emission-line sources, the rugate filter has an even bigger advantage over conventional filters, with a fourfold increase in signal-to-noise ratio possible in certain instances.   Astrophysical studies that could benefit from rugate filters are searches for very low-mass stars and galaxy evolution out to z  = 3.  相似文献   

The high-pressure stability of Mg-sursassite in a model hydrous peridotite: a possible mechanism for the deep subduction of significant volumes of H<Subscript>2</Subscript>O     

Geoffrey D. Bromiley  Alison R. Pawley 《Contributions to Mineralogy and Petrology》2002,142(6):714-723

The stability of the high-pressure phase Mg-sursassite, previously MgMgAl-pumpellyite, in ultramafic compositions has been determined in experiments in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). The breakdown of Mg-sursassite + forsterite + enstatite to pyrope + vapour with increasing temperature was bracketed at 6.0 and 7.0 GPa. Below 6.0 GPa, Mg-sursassite + forsterite + vapour reacts to chlorite + enstatite. This reaction provides a mechanism for transfer of water from chlorite- to Mg-sursassite-bearing assemblages. At pressures of 7.0 GPa and above, the assemblage Mg-sursassite + phase A + enstatite was found. Phase relations involving Mg-sursassite and phase A are considered. For bulk compositions with a low water content, the vapour-absent reaction Mg-sursassite + forsterite = pyrope + phase A + enstatite determines the upper-pressure stability of Mg-sursassite, and provides a mechanism for the complete transfer of water from Mg-sursassite to phase A-bearing assemblages. Mg-sursassite plays an important role in peridotite compositions in the subducting slab because, at temperatures below 700 °C, it can transfer water from hydrous phases such as antigorite and chlorite to high-pressure stable phases such as phase A.  相似文献   

A Biota Associated with Matuyama-Age Sediments in West-Central Illinois     

Barry B. Miller  Russell W. Graham  Alan V. Morgan  Norton G. Miller  William D. McCoy  Donald F. Palmer  Alison J. Smith  J. J. Pilny 《Quaternary Research》1994,41(3)

A fossil assemblage containing molluscs, mammals, insects, ostracodes, and plants has been recovered from a silt-filled depression near Lima, in west-central Illinois. The reversed remanent magnetic signature of the sediments and the temporal ranges of two mammals, Microtus paroperarius and Lasiopodomys deceitensis, constrain the age of the assemblage to between 730,000 and 830,000 yr B.P. The extent of isoleucine epimerization in the molluscan shell is consistent with this age interpretation. The fauna includes at least 43 taxa of beetles from 11 families, 35 nominal species of molluscs, and two genera of ostracodes. The mammals include two shrews, three rodents, and a rabbit. The plant macrofossils (no pollen recovered) include 25 species of seed plants and four kinds of terrestrial or wetland mosses. Most of the plant species identified still occur in the upper Midwest, although a few of the taxa are found mainly to the north of the site. The fauna is characterized by an almost total absence of true aquatic taxa. The association of both boreal and thermophilous faunal and floral elements suggest that summer temperatures were not greatly different from present ones, but cooler, moist areas must have been available to support the boreal elements. Local conditions were probably similar to those now found in northeastern Iowa, where rains blocks, fissures, and joints in carbonate bedrock serve as traps for debris accumulations, provide shade, and are kept cool and moist during the hot summer months by cold-air drainage and groundwater seepage. Summer mean temperature in these microhabitats was probably between 18 and 20°C, similar to temperatures that now occur near the northern hardwood spruce-fir transition in the eastern United States.  相似文献   

Fecundity and spawning of the Atlantic horseshoe crab, Limulus polyphemus, in Pleasant Bay, Cape Cod, Massachusetts, USA     

Alison S. Leschen  Sara P. Grady  & Ivan Valiela 《Marine Ecology》2006,27(1):54-65

This study provided the first comprehensive analysis of Atlantic horseshoe crab (Limulus polyphemus) fecundity. Limulus appear to be a determinate spawner, maturing all eggs for the breeding season before spawning begins. On average, larger females held a larger number of eggs (63,500) than smaller females (14,500). By the end of the breeding season there was an average of 11,600 mature eggs per female left undeposited, regardless of female size. Larger females laid a higher percentage of the eggs they contained. Thus they not only contain more eggs, but are more effective at laying them as well. Size of spawning females ranged from about 185–300 mm prosomal width, with by far the highest concentration in the mid‐size ranges. Although on an individual basis large females carry and lay the greatest number of eggs, mid‐size crabs as a group contributed more to the horseshoe crab population in Pleasant Bay because they were more plentiful (net fecundity was highest for mid‐size crabs). These results have implications for the management of this important species, which is harvested for bait, scientific, and biomedical uses. Incorporation of these results into models and other management tools can help predict growth rates, effects of size‐selective harvest, reproductive value, and stable stage distribution of populations.  相似文献   

A towed near-surface optical reflectance meter for measuring ocean colour in support of remote sensing     

Ian S. Robinson  Alison R. Weeks  Bruce A. Booty 《Deep Sea Research Part I: Oceanographic Research Papers》1995,42(11-12)

An underwater optical instrument, the Lighfish, measures near-surface irradiance reflectance in six spectral channels while it is towed from a research vessel at a depth of a few metres. The detectors are configured such that all downward-looking detectors observe the same instrument shadow. Effects of self-shading are negligible for waters and wavelengths having absorption coefficients less than 0.3 m⁻¹. Variance in the derived spectral reflectance ratios is shown to be significantly reduced compared with that in single channel irradiance measurements. The instrument can be used to develop remote sensing algorithms as well as for other oceanographic applications.  相似文献   

Trans-Tasman migration: New Zealanders' explanations for their move     

Alison E. Green  Mary R. Power  Deannah M. Jang 《New Zealand geographer》2008,64(1):34-45

Abstract:  This study investigates why New Zealanders move to Australia. A survey of New Zealanders living in Australia found 'pull' factors, notably greater opportunities and better climate, were the most common reasons. While desire for change or sense of adventure was important, dissatisfaction with life in New Zealand was also a factor. Results indicated a form of transnationalism as migrants felt at home in both countries. They maintained links with New Zealand evident in support for New Zealand sporting teams, expressions of New Zealand nationality, emotional connection and regular contact with New Zealand.  相似文献   

Zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys   总被引：3，自引：0，他引：3  

Seth G. John  Olivier J. Rouxel  Paul R. Craddock  Alison M. Engwall  Edward A. Boyle   《Earth and Planetary Science Letters》2008,269(1-2):17-28

Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ⁶⁶Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ⁶⁶Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ⁶⁶Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ⁶⁶Zn of chimney minerals was lighter or similar to the fluid δ⁶⁶Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ⁶⁶Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ⁶⁶Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ⁶⁶Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes.  相似文献   

Latest Pleistocene and Holocene dune construction at the north-eastern edge of the Rub Al Khali, United Arab Emirates   总被引：4，自引：0，他引：4  

Andrew S. Goudie  Alison Colls  Stephen Stokes  Adrian Parker  Kevin White  & Asma Al-Farraj 《Sedimentology》2000,47(5):1011-1021

Optical dating of two dune profiles developed in linear dunes in Ras Al Khaimah, United Arab Emirates, has been used to establish the timing and rate of dune accretion. One section at Awafi was over 17 m high, while that at Idhn was over 40 m high. The Awafi dune appears to have accumulated very rapidly ≈10 000 years ago, with a vertical accumulation rate of about 3·3 m ka⁻¹. The Idhn dune appears to have accumulated over the past 1000 years, with 20 m of sediment accumulating in a time period of about 270 years. The Awafi dune may have accumulated in response to the transgression of the Persian Gulf by rising sea levels in late Pleistocene and Holocene times. The Idhn dune may have accumulated rapidly because of intensified human activity, a short-lived climatic event, or because it may periodically be reactivated after erosion by fluvial action at its base.  相似文献   

On the structure of tidal tails     

Andreas H. W. Küpper  rew Macleod  Douglas C. Heggie 《Monthly notices of the Royal Astronomical Society》2008,387(3):1248-1252

We examine the longitudinal distribution of the stars escaping from a cluster along tidal tails. Using both theory and simulations, we show that, even in the case of a star cluster in a circular galactic orbit, when the tide is steady, the distribution exhibits maxima at a distance of many tidal radii from the cluster.  相似文献   

The formation of vesicular cylinders in pahoehoe lava flows     

A.C. Fowler  Alison C. Rust  M. Vynnycky 《地球物理与天体物理流体动力学》2015,109(1):39-61

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