9,177 research outputs found
A search for clusters and groups of galaxies on the line of sight towards 8 lensed quasars
In this paper we present new ESO/VLT FORS1 and ISAAC images of the fields
around eight gravitationally lensed quasars: CTQ414, HE0230-2130,
LBQS1009-0252, B1030+074, HE1104-1805, B1359+154, H1413+117 and HE2149-2745.
When available and deep enough, HST/WFPC2 data were also used to infer the
photometric redshifts of the galaxies around the quasars. The search of galaxy
overdensities in space and redshift, as well as a weak-shear analysis and a
mass reconstruction are presented in this paper. We find that there are most
probably galaxy groups towards CTQ414, HE0230-2130, B1359+154, H1413+117 and
HE2149-2745, with a mass ~ 4x10^14 M_sol h^-1. Considering its photometric
redshift, the galaxy group discovered in the field around HE1104-1805 is
associated with the quasar rather than with the lensing potential.Comment: 14 pages, 11 figures(.jpg
Upper bound on the density of Ruelle resonances for Anosov flows
Using a semiclassical approach we show that the spectrum of a smooth Anosov
vector field V on a compact manifold is discrete (in suitable anisotropic
Sobolev spaces) and then we provide an upper bound for the density of
eigenvalues of the operator (-i)V, called Ruelle resonances, close to the real
axis and for large real parts.Comment: 57 page
Environmental protection of titanium alloys in centrifugal compressors at 500°C in saline atmosphere
The use of the titanium alloy Ti-6246 (Ti–6Al–2Sn–4Zr–6Mo, wt-%) for gas turbine compressors allows an increase in working temperature and stress level. Under severe service conditions, the material experiences combined high temperature and high mechanical stress and, in saline atmospheres, stress corrosion cracking (SCC) can occur, leading to catastrophic mechanical failure. The present study was performed to evaluate the potential of several surface treatments to protect Ti-6246 alloy, after salt deposit, from hot salt SCC at temperatures ?500°C and 500 MPa static mechanical stress conditions. Shot peening, thermal oxidation and metal–ceramic coatings were investigated. Experimental results confirm the existence of brittle stress corrosion phenomena marked by a low residual elongation of test samples and the presence of oxides on the fracture surfaces. Both shot peening and metal–ceramic coatings increase the hot salt SCC resistance of the alloy. Times to rupture were improved by a factor of 3 for shot peening and by a factor of 10 for metal–ceramic coatings. Inversely, the time to rupture of preoxidised alloys has been halved compared with uncoated alloys. As well as these interesting quantitative results, structural studies of metal–ceramic coatings showed that they are mechanically and chemically compatible with the titanium alloy substructure and should work under severe thermomechanical stresses and aggressive atmospheres
Plant sphingolipids: their importance in cellular organization and adaption
Sphingolipids and their phosphorylated derivatives are ubiquitous bio-active components of cells. They are structural elements in the lipid bilayer and contribute to the dynamic nature of the membrane. They have been implicated in many cellular processes in yeast and animal cells, including aspects of signaling, apoptosis, and senescence. Although sphingolipids have a better defined role in animal systems, they have been shown to be central to many essential processes in plants including but not limited to, pollen development, signal transduction and in the response to biotic and abiotic stress. A fuller understanding of the roles of sphingolipids within plants has been facilitated by classical biochemical studies and the identification of mutants of model species. Recently the development of powerful mass spectrometry techniques hailed the advent of the emerging field of lipidomics enabling more accurate sphingolipid detection and quantitation. This review will consider plant sphingolipid biosynthesis and function in the context of these new developments. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner
Semi-classical study of the Quantum Hall conductivity
The semi-classical study of the integer Quantum Hall conductivity is
investigated for electrons in a bi-periodic potential .
The Hall conductivity is due to the tunnelling effect and we concentrate our
study to potentials having three wells in a periodic cell. A non-zero
topological conductivity requires special conditions for the positions, and
shapes of the wells. The results are derived analytically and well confirmed by
numerical calculations.Comment: 23 pages, 13 figure
High-energy acceleration phenomena in extreme radiation-plasma interactions
We simulate, using a particle-in-cell code, the chain of acceleration
processes at work during the Compton-based interaction of a dilute electron-ion
plasma with an extreme-intensity, incoherent gamma-ray flux with a photon
density several orders of magnitude above the particle density. The plasma
electrons are initially accelerated in the radiative flux direction through
Compton scattering. In turn, the charge-separation field from the induced
current drives forward the plasma ions to near-relativistic speed and
accelerates backwards the non-scattered electrons to energies easily exceeding
those of the driving photons. The dynamics of those energized electrons is
determined by the interplay of electrostatic acceleration, bulk plasma motion,
inverse Compton scattering and deflections off the mobile magnetic fluctuations
generated by a Weibel-type instability. The latter Fermi-like effect notably
gives rise to a forward-directed suprathermal electron tail. We provide simple
analytical descriptions for most of those phenomena and examine numerically
their sensitivity to the parameters of the problem
Topological properties of quantum periodic Hamiltonians
We consider periodic quantum Hamiltonians on the torus phase space
(Harper-like Hamiltonians). We calculate the topological Chern index which
characterizes each spectral band in the generic case. This calculation is made
by a semi-classical approach with use of quasi-modes. As a result, the Chern
index is equal to the homotopy of the path of these quasi-modes on phase space
as the Floquet parameter (\theta) of the band is varied. It is quite
interesting that the Chern indices, defined as topological quantum numbers, can
be expressed from simple properties of the classical trajectories.Comment: 27 pages, 14 figure
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