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排序方式: 共有255条查询结果,搜索用时 31 毫秒
91.
92.
Stephen B. Foulkes Carole A. Haswell James R. Murray 《Monthly notices of the Royal Astronomical Society》2006,366(4):1399-1409
We present three-dimensional smoothed particle hydrodynamics calculations of warped accretion discs in X-ray binary systems. Geometrically thin, optically thick accretion discs are illuminated by a central radiation source. This illumination exerts a non-axisymmetric radiation pressure on the surface of the disc, resulting in a torque that acts on the disc to induce a twist or warp. Initially planar discs are unstable to warping driven by the radiation torque and, in general, the warps also precess in a retrograde direction relative to the orbital flow. We simulate a number of X-ray binary systems which have different mass ratios, using a number of different luminosities for each. Radiation-driven warping occurs for all systems simulated. For mass ratios q ∼ 0.1 a moderate warp occurs in the inner disc while the outer disc remains in the orbital plane (cf. X 1916−053). For less extreme mass ratios, the entire disc tilts out of the orbital plane (cf. Her X–1). For discs that are tilted out of the orbital plane in which the outer edge material of the disc is precessing in a prograde direction, we obtain both positive and negative superhumps simultaneously in the dissipation light curve (cf. V603 Aql). 相似文献
93.
Giant landslides on volcanic islands represent the largest formations which can be created in a single geological moment. Such landslides are distributed across the globe and have attracted a significant amount of research interest. Yet, no coherent attempts have been made to rationalise this information into a single online resource. This report summarises information about the structure of the recently created database of giant landslides on volcanic islands and presents some observations regarding the uncertainties inherent in the inventories. The database is being prepared over a 3-year period: the first year of the project has focused on rationalising information about giant landslides around the Atlantic Ocean while the second and third years will focus on rationalising information about such landslides from the Pacific Ocean and Indian Ocean, respectively. Using this database, it should be possible to interrogate the spatial and temporal patterns of land sliding and landslide reactivation as well as to better assess the hazard and potential risks posed by giant landslides on volcanic islands. It will be particularly interesting to see if any evidence can be found for global triggers, such as eustatic or climatic changes, instead of the more commonly expounded local triggers. Ultimately, it is hoped that the database will benefit both the geoscientific community and those agencies responsible for civil defence. This work is part of the activities of the International Consortium on Landslides, namely its International Programme on Landslides (Project n. 212). The database is available from the giant landslides project webpage: https://www.irsm.cas.cz/ext/giantlandslides. 相似文献
94.
Sanaz Vahidinia Jeffrey N. Cuzzi Matt Hedman Bruce Draine Roger N. Clark Ted Roush Gianrico Filacchione Philip D. Nicholson Robert H. Brown Bonnie Buratti Christophe Sotin 《Icarus》2011,215(2):682-694
We present models of the near-infrared (1-5 μm) spectra of Saturn’s F ring obtained by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) at ultra-high phase angles (177.4-178.5°). Modeling this spectrum constrains the size distribution, composition, and structure of F ring particles in the 0.1-100 μm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2 μm absorption bands, and the expected 3 μm water ice primary absorption appears as an unusually narrow dip at 2.87 μm. We have modeled these data using multiple approaches. First, we use a simple Mie scattering model to constrain the size distribution and composition of the particles. The Mie model allows us to understand the overall shapes of the spectra in terms of dominance by diffraction at these ultra-high phase angles, and also to demonstrate that the 2.87 μm dip is associated with the Christiansen frequency of water ice (where the real refractive index passes unity). Second, we use a combination of Mie scattering with Effective Medium Theory to probe the effect of porous (but structureless) particles on the overall shape of the spectrum and depth of the 2.87 μm band. Such simple models are not able to capture the shape of this absorption feature well. Finally, we model each particle as an aggregate of discrete monomers, using the Discrete Dipole Approximation (DDA) model, and find a better fit for the depth of the 2.87 μm feature. The DDA models imply a slightly different overall size distribution. We present a simple heuristic model which explains the differences between the Mie and DDA model results. We conclude that the F ring contains aggregate particles with a size distribution that is distinctly narrower than a typical power law, and that the particles are predominantly crystalline water ice. 相似文献
95.
Three‐dimensional architecture and hydrostratigraphy of cross‐cutting buried valleys using airborne electromagnetics,glaciated Central Lowlands,Nebraska, USA 下载免费PDF全文
Buried valleys are characteristic features of glaciated landscapes, and their deposits host important aquifers worldwide. Understanding the stratigraphic architecture of these deposits is essential for protecting groundwater and interpreting sedimentary processes in subglacial and ice‐marginal environments. The relationships between depositional architecture, topography and hydrostratigraphy in dissected, pre‐Illinoian till sheets is poorly understood. Boreholes alone are inadequate to characterize the complex geology of buried valleys, but airborne electromagnetic surveys have proven useful for this purpose. A key question is whether the sedimentary architecture of buried valleys can be interpreted from airborne electromagnetic profiles. This study employs airborne electromagnetic resistivity profiles to interpret the three‐dimensional sedimentary architecture of cross‐cutting buried valleys in a ca 400 km2 area along the western margin of Laurentide glaciation in North America. A progenitor bedrock valley is succeeded by at least five generations of tunnel valleys that become progressively younger northward. Tunnel‐valley infills are highly variable, reflecting under‐filled and over‐filled conditions. Under‐filled tunnel valleys are expressed on the modern landscape and contain fine sediments that act as hydraulic barriers. Over‐filled tunnel valleys are not recognized in the modern landscape, but where they are present they form hydraulic windows between deep aquifer units and the land surface. The interpretation of tunnel‐valley genesis herein provides evidence of the relationships between depositional processes and glacial landforms in a dissected, pre‐Illinoian till sheet, and contributes to the understanding of the complex physical hydrology of glacial aquifers in general. 相似文献
96.
Hazard research has made significant strides over the last several decades, answering critical questions surrounding vulnerability and recovery. Recently, resilience has come to the forefront of scholarly debates and practitioner strategies, yet there remain challenges implementing resilience in practice, the result of a complex web of research that spread across numerous fields of study. As a result, there is a need to analyze and reflect on the current state of resilience literature. We reviewed 241 journal articles from the Web of Science and Engineering Village databases from 1990 to 2015 to analyze research trends in geographic location of studies, methods employed, units of analysis, and resilience dimensions studied, as well as correlations between each of these categories. The majority of the studies analyzed were conducted in North America, used quantitative methods, focused on infrastructure and community units of analysis, and studied governance, infrastructure, and economic dimensions of resilience. This analysis points to the need to: (1) conduct studies in developing country contexts, where resilience is particularly important; (2) employ mixed-methods for additional depth to quantitative studies; (3) connect units of analysis, such as infrastructure and community; and (4) expand on the measurement and study of environmental and social dimensions of resilience. 相似文献
97.
M. Guainazzi G. Matt L. A. Antonelli L. Bassani A. C. Fabian R. Maiolino A. Marconi F. Fiore K. Iwasawa L. Piro 《Monthly notices of the Royal Astronomical Society》1999,310(1):10-20
We present an X-ray spectroscopic study of the bright Compton-thick Seyfert 2 galaxies NGC 1068 and the Circinus Galaxy, performed with BeppoSAX . Matt et al. interpreted the spectrum above 4 keV as the superposition of Compton reflection and warm plasma scattering of the nuclear radiation. When this continuum is extrapolated downwards to 0.1 keV, further components arise. The NGC 1068 spectrum is rich in emission lines, mainly owing to K α transitions of He-like elements from oxygen to iron, plus a K α fluorescent line from neutral iron. If the ionized lines originate in the warm scatterer, its thermal and ionization structure must be complex. From the continuum and line properties, we estimate a column density, N warm , of the warm scatterer less than a few×1021 cm−2 . In the Circinus Galaxy, the absence of highly ionized iron is consistent with a scattering medium with U X ≲5 and N warm ∼ a few×1022 cm−2 . In both cases the neutral iron line is most naturally explained as fluorescence in the medium responsible for the Compton reflection continuum. In NGC 1068 an optically thin plasma emission with kT ≃500 eV and strongly sub-solar metallicity is required, while such a component is only marginal in the Circinus Galaxy. We tentatively identify this component as emission of diffuse hot gas in the nuclear starbursts. Possible causes for the metal depletion are discussed. 相似文献
98.
The release of dissolved actinium to the ocean: A global comparison of different end-members 总被引:1,自引:1,他引:0
Walter Geibert Matt Charette Guebuem Kim Willard S. Moore Joseph Street Megan Young Adina Paytan 《Marine Chemistry》2008,109(3-4):409
The measurement of short-lived 223Ra often involves a second measurement for supported activities, which represents 227Ac in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of 227Ac, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles 227Ac data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of 227Ac near its potential sources.The average 227Ac activities of nearshore marine end-members range from 0.4 dpm m− 3 at the Gulf of Mexico to 3.0 dpm m− 3 in the coastal waters of the Korean Strait. In analogy to 228Ra, we find the extension of adjacent shelf regions to play a substantial role for 227Ac activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean 227Ac inventory of 1.35 * 1018 dpm 227Acex in the ocean, which corresponds to 37 moles, or 8.4 kg. This implies a flux of 127 dpm m−2 y− 1 from the deep-sea floor. For the shelf regions, we obtain a global inventory of 227Ac of 4.5 * 1015 dpm, which cannot be converted directly into a flux value, as the regional loss term of 227Ac to the open ocean would have to be included.Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to 231Paex. Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in 227Ac/228Ra and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including 227Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution. 相似文献
99.
100.