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941.
942.
Ross P. Church Johann Dischler Melvyn B. Davies Christopher. A. Tout Tim Adams Martin E. Beer 《Monthly notices of the Royal Astronomical Society》2009,395(2):1127-1134
To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a 0.6 M⊙ donor star and a 1 M⊙ white dwarf; such binaries are thought to form cataclysmic variable (CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of q = 0.6 and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity. 相似文献
943.
Martin White Nikhil Padmanabhan 《Monthly notices of the Royal Astronomical Society》2009,395(4):2381-2384
We show that a suitably defined marked correlation function can be used to break degeneracies in halo-occupation distribution modelling. The statistic can be computed on both three-dimensional and two-dimensional data sets and should be applicable to all upcoming galaxy surveys. A proof of principle, using mock catalogues created from N -body simulations, is given. 相似文献
944.
945.
Martin Harwit 《Experimental Astronomy》2009,26(1-3):163-177
In early 1984, the astronomical community’s plans to launch a series of powerful telescopes able to study celestial sources at almost any wavelength were in trouble. The President of the United States had just declared his priority for a Space Station that was bound to be expensive, and Congress could not understand why yet another set of space observatories was needed when others were already being funded. To realize their aims, astronomers would have to advocate their needs much more effectively than in the past. 相似文献
946.
The 1.07?Myr old Bosumtwi impact structure (Ghana), excavated in 2.1-2.2?Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U-Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145?±?82?Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb-Sr and Sm-Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb-Sr and Sm-Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n?=?29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846?±?160?Ma, with an initial Sr isotope ratio of 0.703?±?0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible "metamorphic age" of ~?1.8-1.9?Ga tentatively derived from our U-Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb-Sr isotope systematics has not basically been reset on the whole rock scale during the Bosumtwi impact event, or even reflect another unknown geologic event. 相似文献
947.
Laboratory testing of rectangular beams using a synthetic rock was used to investigate the onset of dilation and spalling.
The beams are axially compressed and subjected to 4-point bending to provide non-uniform compressive stresses. The maximum
tangential stress occurs at the top of the beam and rapidly decreases with distance from the top of the beam. This stress
distribution was used to simulate the maximum tangential stress distribution found around circular excavations. The results
showed using this beam test configuration that the onset of dilation based on beam displacement and visually observed spalling
began at approximately the same stress level. Discrete element numerical analyses (particle flow code) were used to evaluate
the stress path at various locations in the beams. The analyses revealed that spalling and dilation in the beams occurred
well below the peak strength failure envelope determined from conventional laboratory tests. The findings suggest that the
onset of dilation in laboratory tests appears to be a good indicator for assessing the stress magnitudes required to initiate
spalling. 相似文献
948.
949.
Martin Held 《Computational Geosciences》2010,14(2):311-317
We analyze the geometric set-up of Lindquist’s “The Geometry of Primary Drainage” (J Colloid Interface Sci 296(2):655–668,
2006) and show that his main equation, which models the arc menisci configuration under primary drainage in capillary tube cross
sections, can be solved analytically for the special case of a convex cross section and the wetting angle being assumed 0.
Our approach relies on the Voronoi diagram of the cross section and runs in O(n logn) time for an n-segment cross section. It requires only the solution of n − 2 second-degree polynomial equations. 相似文献
950.
Surface roughness and geological mapping at subhectometer scale from the High Resolution Stereo Camera onboard Mars Express 总被引:2,自引:0,他引:2
Aurélien Cord David Baratoux Patrick Martin Ronald Greeley Philippe Masson Gerhard Neukum 《Icarus》2007,191(1):38-51
The quantitative measurement of surface roughness of planetary surfaces at all scales provides insights into geological processes. A characterization of roughness variations at the scale of a few tens of meters is proposed that complements the analysis of local topographic data of the martian surface at kilometer scale, as achieved from the Mars Orbiter Laser Altimeter (MOLA) data, and at the subcentimeter scale using photometric properties derived from multi-angular observations. Relying on a Gabor filtering process, an algorithm developed in the context of image classification for the purpose of texture analysis has been adapted to handle data from the High Resolution Stereo Camera (HRSC). The derivation of roughness within a wavelength range of tens of meters, combined with analyses at even longer wavelengths, gives an original view of the martian surface. The potential of this approach is evaluated for different examples for which the geological processes are identified and the geological units are mapped and characterized in terms of roughness. 相似文献