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Search Results: 1 - 10 of 397575 matches for " J. -F. Legrand "
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Lossy chaotic electromagnetic reverberation chambers: Universal statistical behavior of the vectorial field
J. -B. Gros,U. Kuhl,O. Legrand,F. Mortessagne
Physics , 2015,
Abstract: The effective Hamiltonian formalism is extended to vectorial electromagnetic waves in order to describe statistical properties of the field in reverberation chambers. The latter are commonly used in electromagnetic compatibility tests. As a first step, the distribution of wave intensities in chaotic systems with varying opening in the weak coupling limit for scalar quantum waves is derived by means of random matrix theory. In this limit the only parameters are the modal overlap and the number of open channels. Using the extended effective Hamiltonian, we describe the intensity statistics of the vectorial electromagnetic eigenmodes of lossy reverberation chambers. Finally, the typical quantity of interest in such chambers, namely, the distribution of the electromagnetic response, is discussed. By determining the distribution of the phase rigidity - describing the coupling to the environment - using random matrix numerical data, we find good agreement between the theoretical prediction and numerical calculations of the response.
Abundance profile and stellar content of IZw18
F. Legrand
Physics , 1997,
Abstract: New spectroscopic observations of the metal poor galaxy IZw 18 are discussed. Wolf-Rayet stars of WC type have been detected in the NW-HII region contrary to evolutionary synthesis model predictions. Implications on the mass loss rate and on the formation processes of WR stars are discussed. A very homogeneous metal abundance is observed within the HII region. This emphasizes the problem of the dispersal and mixing of new synthesized element in a starburst. Different scenarios are discussed, showing that metals remain most likely hidden in a hot phase and that the observed present metallicity is the result of a previous star formation event.
On the star formation history of IZw 18
F. Legrand
Physics , 1999,
Abstract: It has been suggested that a continuous low star formation rate has been the dominant regime in IZw 18 and in dwarf galaxies for the lifetime of these objects (Legrand et al. 1999). Here, we discuss and model various star-forming histories for IZw 18. Particularly, we show that if the metallicity observed in IZw 18 results from starburst events only, the observed colors constrain the fraction of the metals ejected from the galaxy to be less than 50-70 %. We demonstrate that the continuous star formation scenario reproduces the observed parameters of IZw 18. A continuous star formation rate (SFR) of about 10E-4 Msol/yr during 14 Gyr reproduces precisely the observed abundances. This SFR is comparable with the lowest SFR observed in low surface brightness galaxies (van Zee et al. 1997). Generalized to all galaxies, the low continuous SFR scenario accounts for various facts: the presence of star formation in quiescent dwarfs and LSBG, the metallicity increase with time in the most underabundant DLA systems, and the metal content extrapolations to the outskirts of spiral galaxies. Also the apparent absence of galaxies with a metallicity lower than IZw 18, the apparent absence of HI clouds without optical counterparts, and the homogeneity of abundances in dwarfs galaxies are natural outcomes of the scenario. This implies that, even if starbursts are strong and important events in the life of galaxies, their more subdued but continuous star formation regime cannot be ignored when accounting for their chemical evolution.
Detection of WR stars in the metal-poor starburst galaxy IZw 18
F. Legrand,D. Kunth,J. -R. Roy,J. M. Mas-Hesse,J. R. Walsh
Physics , 1997,
Abstract: Wolf-Rayet stars (WR) have been detected in the NW region of the metal-poor starburst galaxy IZw 18. The integrated luminosity and FWHM of the bumps at 4650 A and 5808 A are consistent with the presence of a few individual stars of WC4 or WC5 type. Evolutionary synthesis models predict few WRs in this galaxy, but only of WN type. The presence of WC stars at such low metallicity could however be explained by high mass loss rates, which would constrain the IMF upper mass cut-off in IZw 18 to be higher than 80 Msol or alternatively favor a binary channel for WR formation. WC stars could also explain the strong and narrow HeII 4686 A emission line which peaks co-spatially with the WR bump emission, as suggested by Schaerer (1996). This detection shows that WR stars, even of WC type, are formed at metallicities below 1/40th solar.
A continuous low star formation rate in IZw 18 ?
F. Legrand,D. Kunth,J. -R. Roy,J. M. Mas-Hesse,J. R. Walsh
Physics , 2000,
Abstract: Deep long-slit spectroscopic observations of the blue compact galaxy IZw 18 obtained with the CFH 3.6 m Telescope are presented. The very low value of oxygen abundance previously reported is confirmed and a very homogeneous abundance distribution is found (no variation larger than 0.05 dex) over the whole ionized region. We concur with Tenorio-Tagle (1996) and Devost et al. (1997) that the observed abundance level cannot result from the material ejected by the stars formed in the current burst, and propose that the observed metals were formed in a previous star formation episode. Metals ejected in the current burst of star formation remain most probably hidden in a hot phase and are undetectable using optical spectroscopy. We discuss different scenarios of star formation in IZw 18. Combining various observational facts, for instance the faint star formation rate observed in low surface brightness galaxies van Zee et al. (1997), it is proposed that a low and continuous rate of star formation occurring during quiescent phases between bursts could be a significant source of metal enrichment of the interstellar medium.
Evidences for an expanding shell in the Blue Compact Dwarf Galaxy Haro 2
F. Legrand,D. Kunth,J. M. Mas-Hesse,J. Lequeux
Physics , 1997,
Abstract: Long-slit observations of the blue compact galaxy Haro 2 have been performed to detect the Halpha emission originating in the partially ionized wind outflowing at 200 km/s, that had been previously detected with the Hubble Space Telescope. A shallow broadening of the Halpha line wings has been observed, consistent with the existence of an expanding shell. The rotation curve shows two dips at the same systemic velocity as the nucleus. At the positions of the dips the Halpha line is clearly broadened with respect to the central core. This broadening is produced by the outer layers of the expanding shell. From the position of these dips we estimate the size of the shell to be around 20'' in diameter, with a corresponding kinematical age between 5 and 6 Myr. A comparison of the Halpha and Ly_alpha profiles shows that Ly_alpha is significantly broader than Halpha, with an additional emission in the red wing. We interpret this redshifted source of Ly_alpha emission as line photons backscattered by the receding part of the expanding shell. These observations outline the extremely high sensitivity of the Ly_alpha line to the structure and kinematics of the interstellar medium.
What Role for Angiogenesis in Childhood Acute Lymphoblastic Leukaemia?
P. Schneider,I. Dubus,F. Gouel,E. Legrand,J. P. Vannier,M. Vasse
Advances in Hematology , 2011, DOI: 10.1155/2011/274628
Abstract: The role of angiogenesis in acute leukaemia has been discussed since the cloning of the gene of vascular endothelial growth factor (VEGF) from the acute myelogenous leukemia cell line (HL60) and, thereafter, when the first studies reported increased bone marrow vascularity and elevation of angiogenic cytokines in acute lymphoblastic leukaemia (ALL). VEGF and basic fibroblast growth factor (bFGF) are the major proangiogenic cytokines that have been studied, and evaluation of their prognostic impact in childhood ALL has been reported in several studies, though with controversial results. The antiangiogenic response, contributing to the angiogenic balance, has scarcely been reported. The origin of the factors, their prognostic value, and their relevance as good markers of what really happens in the bone marrow are discussed in this paper. The place of antiangiogenic drugs in ALL has to be defined in the global treatment strategy. 1. Angiogenesis in Hematological Malignancies Angiogenesis is a highly regulated process balanced by inhibitors and stimulators of endothelial cell proliferation, endothelial cell migration, and capillary formation molecules. Cancer cells begin to promote angiogenesis early in tumorigenesis. This angiogenic switch is promoted by oncogene-driven tumor expression of proangiogenic proteins. The key mediator of angiogenesis is a proangiogenic factor, the vascular endothelial growth factor (VEGF). VEGF is a homodimeric glycoprotein which stimulates angiogenesis and vascular permeability by interacting with the tyrosine kinase receptor 2 (VEGFR-2 or KDR/Flk-1) and 1 (VEGFR-1 or Flt-1) [1]. These receptors belong to the superfamily of receptor tyrosine kinases (RTKs) and share a common structure with a single transmembrane domain. The central role of the VEGF-R2 in angiogenesis in general and in the development of solid tumors in particular has been widely demonstrated, and it is now considered as the main mediator of VEGFA effect on endothelial cells. Its interaction with VEGF is enhanced by neuropilin-1, which, thereby, has been proposed as a potential target to inhibit VEGF-driven angiogenesis [2]. One other major factor involved is the basic fibroblast growth factor (bFGF), a member of the FGF family of proteins, which interacts with tyrosine kinase receptors (FGFR-1 or FGFR-2) to exert mitogenic effects on endothelial, ectoderm-, and mesoderm-derived cells. Beyond its mitogenic activity, bFGF acts synergistically with VEGF in the angiogenic switch [3]. VEGF-A and bFGF are both major stimulators of angiogenesis that are commonly
On the long term change in the geomagnetic activity during the 20th century
F. Ouattara, C. Amory-Mazaudier, M. Menvielle, P. Simon, ,J.-P. Legrand
Annales Geophysicae (ANGEO) , 2009,
Abstract: The analysis of the aa index series presented in this paper clearly shows that during the last century (1900 to 2000) the number of quiet days (Aa<20 nT) drastically diminished from a mean annual value greater than 270 days per year at the end of the nineteenth century to a mean value of 160 quiet days per year one hundred years later. This decrease is mainly due to the decrease of the number of very quiet days (Aa<13 nT). We show that the so-evidenced decrease in the number of quiet days cannot be accounted for by drift in the aa baseline resulting in a systematic underestimation of aa during the first quarter of the century: a 2–3 nT overestimation in the aa increase during the 20th century would lead to a 20–40% overestimation in the decrease of the number of quiet days during the same period. The quiet days and very quiet days correspond to periods during which the Earth encounters slow solar wind streams flowing in the heliosheet during the period where the solar magnetic field has a dipolar geometry. Therefore, the observed change in the number of quiet days is the signature of a long term evolution of the solar coronal field topology. It may be interpreted in terms of an increase in the magnitude of the solar dipole, the associated decrease of the heliosheet thickness accounting for the observed decrease in the number of quiet days.
Statistics of the electromagnetic response of a chaotic reverberation chamber
J. -B. Gros,U. Kuhl,O. Legrand,F. Mortessagne,O. Picon,E. Richalot
Physics , 2014,
Abstract: This article presents a study of the electromagnetic response of a chaotic reverberation chamber (RC) in the presence of losses. By means of simulations and of experiments, the fluctuations in the maxima of the field obtained in a conventional mode-stirred RC are compared with those in a chaotic RC in the neighborhood of the Lowest Useable Frequency (LUF). The present work illustrates that the universal spectral and spatial statistical properties of chaotic RCs allow to meet more adequately the criteria required by the Standard IEC 61000-4-21 to perform tests of electromagnetic compatibility.
Experimental Width Shift Distribution: A Test of Nonorthogonality for Local and Global Perturbations
J. -B. Gros,U. Kuhl,O. Legrand,F. Mortessagne,E. Richalot,D. V. Savin
Physics , 2014, DOI: 10.1103/PhysRevLett.113.224101
Abstract: The change of resonance widths in an open system under a perturbation of its interior has been recently introduced by Fyodorov and Savin [Phys. Rev. Lett. 108, 184101 (2012)] as a sensitive indicator of the nonorthogonality of resonance states. We experimentally study universal statistics of this quantity in weakly open two-dimensional microwave cavities and reverberation chambers realizing scalar and electromagnetic vector fields, respectively. We consider global as well as local perturbations, and also extend the theory to treat the latter case. The influence of the perturbation type on the width shift distribution is more pronounced for many-channel systems. We compare the theory to experimental results for one and two attached antennas and to numerical simulations with higher channel numbers, observing a good agreement in all cases.
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