The Second International Hydrographic Technical Conference: Plymouth, 3–7 September 1984     

P.B. Beazley 《Marine Policy》1985,9(1):78-79

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Contrasting the chemical evolution of the Milky Way and Andromeda     

Agostino Renda  Daisuke Kawata  Yeshe Fenner  Brad K. Gibson 《Monthly notices of the Royal Astronomical Society》2005,356(3):1071-1078

The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass.  相似文献   

Compaction-induced inclination shallowing of the post-depositional remanent magnetization in a synthetic sediment     

G. L. Anson  K. P. Kodama 《Geophysical Journal International》1987,88(3):673-692

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SID flare production and Mt. Wilson magnetic class: An alternative interpretation     

Constance Sawyer  A. Achong  P. A. Stahl 《Solar physics》1985,98(1):193-194

The statistical results presented by Achong and Stahl (1984) may alternatively be interpreted as demonstrating a strong dependence of SID flare production on Mt. Wilson magnetic class of the parent sunspot group.  相似文献   

The origin of selected lunar geochemical anomalies: Implications for early volcanism and the formation of light plains     

Bernard Ray Hawke  Paul D. Spudis  P. E. Clark 《Earth, Moon, and Planets》1985,32(3):257-273

The Apollo orbital geochemistry, photogeologic, and other remote sensing data sets were used to identify and characterize geochemical anomalies on the eastern limb and farside of the Moon and to investigate the processes responsible for their formation. The anomalies are located in the following regions: (1) Balmer basin, (2) terrain northeast of Mare Smythii, (3) near Langemak crater, (4) Pasteur crater, (5) terrain northwest of Milne basin, (6) northeast of Mendeleev basin, (7) north and northeast of Korolev basin, (8) terrain north of Taruntius crater, and (9) terrain north of Orientale basin. The anomalies are commonly associated with Imbrian- or Nectarian-aged light plains units which exhibit dark-haloed impact craters. The results of recent spectral reflectance studies of dark-haloed impact craters plus consideration of the surface chemistry of the anomalies strongly indicate that those geochemical anomalies associated with light plains deposits which display dark-haloed impact craters result from the presence of basaltic units that are either covered by varying thickness of highland debris or have a surface contaminated with significant amounts of highlands material. The burial or contamination of ancient volcanic surfaces by varying amounts of highland material appears to have been an important (though not the dominant) process in the formation of lunar light plains. Basaltic volcanism on the eastern limb and farside of the Moon was more extensive in both space and time than has been accepted.  相似文献   

The intracluster iron distribution around 4C+55.16     

K. Iwasawa  A.C. Fabian  S.W. Allen  S. Ettori 《Monthly notices of the Royal Astronomical Society》2001,328(1):L5-L10

We report on the metal distribution in the intracluster medium around the radio galaxy 4C+55.16     observed with the Chandra X-ray Observatory . The radial metallicity profile shows a dramatic change at 10 arcsec (∼50 kpc) in radius from half solar to twice solar at inner radii. Also found was a plume-like feature located at ∼3 arcsec to the south-west of the centre of the galaxy, which is mostly accounted for by a strong enhancement of iron L emission. The X-ray spectrum of the plume is characterized by the metal abundance pattern of Type Ia supernovae (SNeIa), i.e. large ratios of Fe to α elements, with the iron metallicity being unusually high at     solar (90 per cent error). How the plume has been formed is not entirely clear. The inhomogeneous iron distribution suggested in this cluster provides important clues to understanding the metal enrichment process of the cluster medium.  相似文献   

Porous interstellar grains     

M. A. Iatì  C. Cecchi-Pestellini  D. A. Williams  F. Borghese  P. Denti  R. Saija  S. Aiello 《Monthly notices of the Royal Astronomical Society》2001,322(4):749-756

Recent observations of stellar composition suggest that elements in the Sun are significantly more abundant than in other stars. The reduction in the available element budget implies a drastic revision in current models of interstellar dust. Theoretical models are therefore exploring fluffy, porous physical structure for the grain material. Since a detailed exact treatment of extinction cross-sections is mandatory for a correct understanding of the nature of interstellar dust, we present a technique based on the multipole expansions of the electromagnetic field, which has proven to be general, flexible and powerful in treating scattering of light by porous, composite, arbitrarily shaped particles. The results of this study speak in favour of core–mantle structures characterized by the presence of porosities.  相似文献   

Deforestation and leaching of nitrogen as nitrates into underground water in intertropical zones: The example of Côte d'lvoire     

J. P. Faillat  A. Rambaud 《Environmental Geology》1991,17(2):133-140

High nitrate contents (up to 200 mgl) have been observed in wells drilled into fractured aquifers lying beneath layers of weathered and decayed rock in the humid tropics where annual rainfall is over 1000 mm and where there is no notable pollution. The source of the nitrates is linked mainly with localized deforestation by man.  相似文献   

Dissolution and depletion of ferromagnesian minerals from Holocene tephra layers in an acid bog,New Zealand,and implications for tephra correlation     

A. P. W. Hodder  P. J. De Lange  D. J. Lowe 《第四纪科学杂志》1991,6(3):195-208

This study examines the depletion of ferromagnesian silicate minerals from a sequence of thin, distal, mainly rhyolitic tephra layers of Holocene age preserved in an acid peat bog (Kopouatai), North Island, New Zealand. The rate of such depletion has been fast, as indicated by the complete loss of biotite from one tephra layer (Kaharoa Tephra), in which it is normally dominant, in only ca. 770 yr. Chemical dissolution is advocated as the likely cause for the depletion, with amphiboles and other mineral grains commonly showing etch pits, microcaves, and other characteristic surface solution features. Theoretical thermodynamic and kinetic models show a marked increase in the rate of dissolution of all ferromagnesian minerals under conditions of low pH (< 4), but that where silica concentrations in solution are high the relative proportions of minerals remaining are unaffected. However, where concentrations of dissolved silica are low, as in most bog environments, the relative proportions of ferromagnesian minerals are affected as well as absolute amounts being decreased. Amphiboles are depleted relative to pyroxenes, consistent with kinetic studies. The results show that the identification and correlation of tephras on the basis of relative abundances of ferromagnesian minerals alone may be unreliable, and emphasise the need to use multiple criteria in such studies.  相似文献   

Response of circular footings and anchor plates in non-homogeneous elastic soils     

R. K. N. D. Rajapakse  A. P. S. Selvadurai 《国际地质力学数值与分析法杂志》1991,15(7):457-470

The axisymmetric elastic response of circular footings and anchor plates in a linearly non-homogeneous elastic soil is analysed. It is assumed that footings/anchors are flexible and subjected to axisymmetric vertical loads. The response of the footing/anchor is modelled by using the classical Poisson–Kirchhoff thin plate theory. A variational technique is used to analyse the interaction problem. A representation for the contact stress is established by using a fundamental solution corresponding to a unit vertical pressure acting over an annular region in the interior of the non-homogeneous soil. The fundamental solution can be derived by using rigorous analytical procedures. The influence of the footing flexibility and the degree of soil non-homogeneity on the displacements, bending moments and contact stresses of a surface footing is examined over a wide range of governing parameters. In the case of anchor plates the influence of depth of embedment, degree of soil non-homogeneity and anchor flexibility on the anchor displacement is investigated.  相似文献