Microtextures and crystal chemistry in P21/c pigeonites     

M. Tribaudino  D. Pasqual  G. Molin  L. Secco 《Mineralogy and Petrology》2003,77(3-4):161-176

Summary ?A single-crystal X-ray investigation was performed on crystals of P2₁/c natural pigeonite with varying Ca and Fe^* ( = Fe²⁺ + Mn²⁺) contents, in order to verify the effect of microtextural disorder on structure refinements and to constrain the crystal chemistry of pigeonite. Antiphase domains and exsolution lamellae affect differently the refinement results. In a crystal free of exsolution the structure obtained after refinement with all reflections is an average of that of the antiphase domains and of their boundaries, whereas in an exsolved crystal it represents only the structure of the prevailing pigeonite lamellae. The refinement using only h + k odd reflections seems to give the structure of the Ca-free pigeonite characteristic of the antiphase domains rather than that of Ca-rich domain walls. The ratio of the scale factors in refinements with all reflections and with only h + k odd reflections allows the ratios of the exsolved augite and pigeonite phases to be estimated. The crystal chemistry of the investigated samples follows the trends outlined by data on Ca-free and Fe-free synthetic samples. In particular, it is shown that Ca and Fe^* substitution for Mg induce similar changes in the average structure, i.e. both induce an expansion in the M1 polyhedron and decrease the difference between the M2–O3 distances. Received October 18, 2001; revised version accepted February 15, 2002  相似文献   

Probing the nuclear activity with supermassive black holes     

V.D. Ivanov  A. Alonso-Herrero 《Astrophysics and Space Science》2003,284(2):565-568

We report three new or updated techniques for probing the parameters of active galaxies based on the masses of their central black holes M_BH). First, we derived a near-IR analog of the bulge luminosity versus M_BH relationship. The low scatter makes it a promising new tool to study the black hole demographics. Next, we present relations between M_BH and the10 μm and 2-10 keV nuclear luminosity. They may help to study the M_BH evolution over wide redshift ranges. Finally, we measured M_BH in quasars from z ∼ 3.4 to z ∼ 0.3 to search directly for M_BH growth. Surprisingly, we found no evidence for growth implying that the majority of quasar host galaxies have undergone their last major merger at z ≥ 3. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

A model of polarized OH maser emission in W75N     

M. D. Gray  B. Hutawarakorn  R. J. Cohen 《Monthly notices of the Royal Astronomical Society》2003,343(4):1067-1080

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The discovery of two distant, massive clusters of galaxies in the ROSAT All-Sky Survey     

A. C. Edge  H. Ebeling  †  M. Bremer  H. Röttgering  M. P. van Haarlem  R. Rengelink  N. J. D. Courtney 《Monthly notices of the Royal Astronomical Society》2003,339(4):913-924

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The 2dF QSO Redshift Survey – X. Lensing of background QSOs by galaxy groups     

A. D. Myers  P. J. Outram  T. Shanks  B. J. Boyle  S. M. Croom  N. S. Loaring  L. Miller  R. J. Smith 《Monthly notices of the Royal Astronomical Society》2003,342(2):467-482

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Speckle interferometry observations of asteroids at tng     

A. Cellino  E. Diolaiti  D. Hestroffer  A. Ghedina 《Icarus》2003,162(2):278-284

The Speckle camera of the 3.5 m Telescopio Nazionale Galileo (TNG) has been used to measure apparent sizes and shapes of a number of main belt asteroids. The average size measurements are in a generally good agreement with the results of indirect IRAS-based radiometric techniques. The measured shapes are compared with predictions based on previously derived spin axis directions and lightcurve photometry of some of the observed objects. Also in this case the agreement is reasonable and the speckle observations allow us to discriminate in some cases between the two pole solutions usually found for each object. No clear evidence of binaries was found. The results show that the TNG speckle camera can be a powerful tool to resolve relatively large main belt asteroids and to calibrate the results of the IRAS survey.  相似文献   

Mesoscale linear streaks on Mars: environments of dust entrainment     

P.C Thomas  P Gierasch  D.S Miller  B Cantor 《Icarus》2003,162(2):242-258

Variable surface albedo features on Mars are likely caused by the entrainment and deposition of dust by the wind. Most discrete markings are associated with topographic forms or with regional slopes that serve to alter the effective wind shear stress on the surface. Some of the largest variable features, here termed mesoscale linear streaks, are up to 400 km in length and repeatedly occur in one of the smoothest regions of Mars: Amazonis Planitia. Their orientations and apparent season of variability as observed by Viking and Mars Orbiter cameras indicate linear streak formation by enhanced surface wind stresses during regional or local dust storms and during the initial stages of global dust storms. They provide an example of the ability of large-scale winds, without significant local enhancement, to initiate dust motion on Mars. The sizes and spacing of the linear streaks may be controlled by boundary layer rolls. The repetitive formation of these streaks, over a span of more than 11 Mars years, gives one measure of the stability of Mars’ eolian processes.  相似文献   

Size and shape evolution of pores in a viscoplastic matrix under compression     

M. Berli  M. L. Accorsi  D. Or 《国际地质力学数值与分析法杂志》2006,30(12):1259-1281

The frequent use of soils and earth materials for hydraulic capping and for geo‐environmental waste containment motivated our interest in detailed modelling of changes in size and shape of macro‐pores to establish links between soil mechanical behaviour and concurrent changes in hydraulic and transport properties. The objective of this study was to use finite element analysis (FEA) to test and extend previous analytical solutions proposed by the authors describing deformation of a single macro‐pore embedded in linear viscoplastic soil material subjected to anisotropic remote stress. The FEA enables to consider more complex pore geometries and provides a detailed picture of matrix yield behaviour to explain shortcomings of approximate analytical solutions. Finite element and analytical calculations agreed very well for linear viscous as well as for viscoplastic materials, only limited for the case of isotropic remote stress due to the simplifications of the analytical model related to patterns and onset of matrix‐yielding behaviour. FEA calculations were compared with experimental data obtained from a compaction experiment in which pore deformation within a uniform modelling clay sample was monitored using CAT scanning. FEA predictions based on independently measured material properties and initial pore geometry provided an excellent match with experimentally determined evolution of pore size and shape hence lending credence to the potential use of FEA for more complex pore geometries and eventually connect macro‐pore deformation with hydraulic properties. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Search for groups of extragalactic radio sources     

A. G. Gubanov  A. I. Kopylov  A. S. Potapov 《Astronomy Letters》2003,29(4):224-235

We explore the possibility of searching for groups of radio sources from the FIRST catalog on angular scales 1′–5′. We developed an efficient method of searching for such groups that takes into account the need for combining the components of extended sources represented in the catalog by separate objects. We found 31 groups of radio sources with angular sizes <5′ that contain no fewer than five sources with flux densities ≥3 mJy. This number is at least triple the expected number of such groups for a random Poisson distribution of radio sources in the sky. The prospects for using groups of radio sources to detect and study distant systems of galaxies are discussed.  相似文献   

The shape of the rotation curves of edge-on galaxies     

A. V. Zasov  A. V. Khoperskov 《Astronomy Letters》2003,29(7):437-446

We consider the effects of projection, internal absorption, and gas-or stellar-velocity dispersion on the measured rotation curves of galaxies with edge-on disks. Axisymmetric disk models clearly show that the rotational velocity in the inner galaxy is highly underestimated. As a result, an extended portion that imitates nearly rigid rotation appears. At galactocentric distances where the absorption is low (i.e., it does not exceed 0.3–0.5^m kpc^?1), the line profiles can have two peaks, and a rotation curve with minimum distortions can be obtained by estimating the position of the peak that corresponds to a higher rotational velocity. However, the high-velocity peak disappears in high-absorption regions and the actual shape of the rotation curve cannot be reproduced from line-of-sight velocity estimates. In general, the optical rotation curves for edge-on galaxies are of little use in reconstructing the mass distribution in the inner regions, particularly for galaxies with a steep velocity gradient in the central region. In this case, estimating the rotation velocities for outer (transparent) disk regions yields correct results.  相似文献