Radar observations of Itokawa in 2004 and improved shape estimation     

Steven J. Ostro  Lance A. M. Benner  Christopher Magri  Jon D. Giorgini  Randy Rose  Raymond F. Jurgens  Donald K. Yeomans  Alice A. Hine  Michael C. Nolan  Daniel J. Scheeres  Stephen B. Broschart  Mikko Kaasalainen  Jean‐Luc Margot 《Meteoritics & planetary science》2005,40(11):1563-1574

Abstract— We present June 2004 radar images of asteroid 25143 Itokawa (1998 SF36) that improve upon the longitude‐latitude coverage of images obtained in 2001 by Ostro et al. (2004) and use the 2001–2004 data to refine that paper's constraints on Itokawa's shape. The 2004 images, the first of the asteroid's southern side, look distinctly different from the 2001 images, revealing leading edges that are much more curved and rugged than the nearly convex leading edges seen at northern latitudes in 2001. Itokawa is shaped like a slightly asymmetrical, bent, lumpy ellipsoid with dimensions along the principal axes within 10% of 594 times 320 times 288 m. To illustrate the uncertainty space associated with shape reconstruction from images with suboptimal orientational coverage, we present two alternative three‐dimensional models of the object.  相似文献   

F stars, metallicity and the ages of red galaxies at z > 1     

L. A. Nolan  J. S. Dunlop  R. Jimenez  A. F. Heavens 《Monthly notices of the Royal Astronomical Society》2003,341(2):464-476

We explore whether the rest-frame near-ultraviolet spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by first testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-ultraviolet spectra of F stars of known (subsolar and supersolar) metallicity. F stars are of particular interest because the rest-frame near-ultraviolet spectra of the oldest known elliptical galaxies at   z > 1  appear to be dominated by F stars near to the main-sequence turn-off.
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the   z ≃ 1.5 mJy  radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range   z = 3–5  .  相似文献   

A geomagnetic induction anomaly from IMS data near Hudson Bay, and its relation to crustal electrical conductivity in central North America     

J. C. Gupta  R. D. Kurtz  P. A. Camfield  E. R. Niblett 《Geophysical Journal International》1985,81(1):33-46

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Classifying coastal waters: Current necessity and historical perspective     

J. C. Kurtz  N. D. Detenbeck  V. D. Engle  K. Ho  L. M. Smith  S. J. Jordan  D. Campbell 《Estuaries and Coasts》2006,29(1):107-123

Coastal ecosystems are ecologically and commercially valuable, productive habitats that are experiencing escalating compromises of their structural and functional integrity. The Clean Water Act (USC 1972) requires identification of impaired water bodies and determination of the causes of impairment. Classification simplifies these determinations, because estuaries within a class are more likely to respond similarly to particular stressors. We reviewed existing classification systems for their applicability to grouping coastal marine and Great Lakes water bodies based on their responses to aquatic stressors, including nutrients, toxic substances, suspended sediments, habitat alteration, and combinations of stressors. Classification research historically addressed terrestrial and freshwater habitats rather than coastal habitats. Few efforts focused on stressor response, although many well-researched classification frameworks provide information pertinent to stressor response. Early coastal classifications relied on physical and hydrological properties, including geomorphology, general circulation patterns, and salinity. More recent classifications sort ecosystems into a few broad types and may integrate physical and biological factors. Among current efforts are those designed for conservation of sensitive habitats based on ecological processes that support patterns of biological diversity. Physical factors, including freshwater inflow, residence time, and flushing rates, affect sensitivity to stressors. Biological factors, such as primary production, grazing rates, and mineral cycling, also need to be considered in classification. We evaluate each existing classification system with respect to objectives, defining factors, extent of spatial and temporal applicability, existing sources of data, and relevance to aquatic stressors. We also consider classification methods in a generic sense and discuss their strengths and weaknesses for our purposes. Although few existing classifications are based on responses to stressors, may well-researched paradigms provide important information for improving our capabilities for classification, as an investigative and predictive management tool.  相似文献   

The ins and outs of skateboarding and transgression in public space in Newcastle,Australia     

Nicholas Nolan 《The Australian geographer》2003,34(3):311-327

This study examines skateboarding as a transgressive activity in different inner Newcastle public spaces, highlighting the way certain places are constructed, and the values and meanings attached to them. Skateboarding has been sanctioned in some places, but is considered to be inappropriate in others, resulting in the implementation of skating restrictions in specific areas. Transgressive conduct is different from the norm and appears to be ‘out of place’. However, labels of ‘in’ and ‘out’ of place are too simplistic, because transgression is more nuanced and can simultaneously operate at multiple scales. Some skateboarding activities and locations are seen as more legitimate than others, and so a skater can be both ‘in place’ and ‘out of place’ at the same time. Problems and inconsistencies in the regulation of public space are revealed, because although skateboarding may be illegal in some places, the regulations are blunted by limited enforcement, justified by distinctions between ‘good’ and ‘bad’ skateboarding.  相似文献   

Differing controls on river‐ and lake‐water hydrogen and oxygen isotopic values in the western United States     

Anna King Henderson  Bryan Nolan Shuman 《水文研究》2010,24(26):3894-3906

Surface water oxygen and hydrogen isotopic values are commonly used as proxies of precipitation isotopic values to track modern hydrologic processes while proxies of water isotopic values preserved in lake and river sediments are used for paleoclimate and paleoaltimetry studies. Previous work has been able to explain variability in USA river‐water and meteoric‐precipitation oxygen isotope variability with geographic variables. These studies show that in the western United States, river‐water isotopic values are depleted relative to precipitation values. In comparison, the controls on lake‐water isotopic values are not well constrained. It has been documented that western United States lake‐water input values, unlike river water, reflect the monthly weighted mean isotopic value of precipitation. To understand the differing controls on lake‐ and river‐water isotopic values in the western United States, we examine the seasonal distribution of precipitation, evaporation and snowmelt across a range of seasonality regimes. We generate new predictive equations based on easily measured factors for western United States lake‐water, which are able to explain 69–63% of the variability in lake‐water hydrogen and oxygen isotopic values. In addition to the geographic factors that can explain river and precipitation values, lake‐water isotopic values need factors related to local hydrologic and climatic characteristics to explain variability. Study results suggest that the spring snowmelt runs off the landscape via rivers and streams, depleting river and stream‐water isotopic values. By contrast, lakes receive seasonal contributions of precipitation in proportion to the seasonal fraction of total annual precipitation within their watershed. Climate change may alter the ratio of snow to rain fall, affecting water resource partitioning between rivers and lakes and by implication of groundwater. Paleolimnological studies must account for the multiple drivers of water isotopic values; likewise, studies based on the isotopic composition of fossil material need to distinguish between species that are associated with rivers versus lakes. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

The conductivity of the crust and mantle beneath the Kapuskasing Uplift: electrical anisotropy in the upper mantle   总被引：1，自引：0，他引：1  

R. D. Kurtz  J. A. Craven  E. R. Niblett  R. A. Stevens 《Geophysical Journal International》1993,113(2):483-498

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Genetic differentiation of three key anadromous fish populations of the Hudson River     

John R. Waldman  Kathleen Nolan  John Hart 《Estuaries and Coasts》1996,19(4):759-768

Large, recreationally or commercially important populations of Atlantic sturgeon (Acipenser oxyrinchus), American shad (Alosa sapidissima), and striped bass (Morone saxatilis) occur in the Hudson River. Members of the Hudson River populations of these fishes also occur over a broad range along the Atlantic coast where they mix with conspecifics from other anadromous populations. For management purposes, it is imperative to be able to discriminate among individual stocks so that weak stocks may be protected and harvest may be allocated equitably. Because of their sensitivity and resistance to environmentally-induced temporal variation, molecular approaches have been increasingly employed in stock identification studies. However, post-Pleistocene recolonization of the Hudson River must have occurred less than 10,000 years ago—a relatively brief period for genetic divergence among populations. We tested whether various measures of DNA variation between Hudson River populations and adjacent populations of Atlantic sturgeon, American shad, and striped bass were sufficient to discriminate among their conspecific populations. American shad populations surveyed for mtDNA variation were highly diverse genotypically, but genotypic frequencies among the populations of the Connecticut, Hudson, and Delaware rivers were statistically homogenous (p>0.05). In contrast, Atlantic sturgeon (surveyed for mtDNA variation) and striped bass (surveyed for mtDNA and nuclear DNA variation) populations of the Hudson River were not genotypically diverse, but they were differentiated from northern and southern populations. Our results suggest higher gene flow (and lesser homing fidelity) among American shad populations in comparison with the two other species.  相似文献   

A model of lower crustal electrical anisotropy for the Pontiac Subprovince of the Canadian Shield     

R. L. Kellett  M. Mareschal  R. D. Kurtz 《Geophysical Journal International》1992,111(1):141-150

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Influence of image forces on combination in aerosols     

D. Keefe  P. J. Nolan 《Pure and Applied Geophysics》1961,50(1):155-160

Summary The effect of electrostatic image forces on the combination of small ions with uncharged aerosol nuclei and the self-combination of charged and uncharged nuclei is examined. In the ion-nucleus case a formula for the combination coefficient is derived for very small nuclei. This formula applies also to atmospheric nuclei and tends to the familiar 4ad form for large radii. For the case of the self-combination of nuclei the effect of the image force when one of the combining nuclei is charged, is shown to be small in the size range of atmospheric nuclei but not negligible if smaller sizes are considered; a table is included of the magnitude of the effect as a function of size.

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Zusammenfassung Es wird der Effekt der elektrostatischen Bildkräfte auf die Vereinigung von Kleinionen mit ungeladenen Aerosolkernen und die Selbstvereinigung von geladenen und ungeladenen Kernen untersucht. Im Falle der Ionen und Kerne wird eine Formel für den Vereinigungskoeffizienten von sehr kleinen Kernen abgeleitet. Diese Formel ist auch für atmosphärische Kerne anwendbar und strebt für grosse Radien dem bekannten Ausdruck 4ad zu. Für den Fall der Selbstvereinigung der Kerne wird gezeigt, dass der Effekt der Bildkraft im Grössenbereich der atmosphärischen Kerne klein ist, wenn einer der sich vereinigenden Kerne geladen ist. Der Einfluss ist aber nicht vernachlässigbar, wenn kleinere Grössen in Betracht gezogen werden; es wird eine Tabelle mitgeteilt, welche den Betrag des Effektes als Funktion der Grösse gibt.

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