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Search Results: 1 - 10 of 189894 matches for " G. Cordier "
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Het conflict van de Westelijke Sahara: een muur van zand en een muur van stilte
G. Cordier
Afrika Focus , 1986,
Abstract: For more than a decade the Saharawi people have been fighting for their independence, first against the Spanish colonialists then against Morocco and Mauretania. Although Mauretania has withdrawn from the conflict, the POLISARIO-front, the liberation organisation of the Saharawi people, remains at war with Morocco.The Western Sahara, thus still forms a pole of tension in Northern Africa but the European Press does not pay it much attention. However committees to support the Saharawi people have been trying to break down this wall of silence. An "intergroupe de solidarité avec le peuple sahraoui", set up in the European parliament in May 1986, may constitute a major step towards mobilizing public opinion on the Saharawi case. This article, which gives a chronology of the conflict in its regional context, seeks to contribute to a better understanding of the problems of the Saharawi.
Contribution à l’étude préhistorique de la vallée de l’Indre et de ses vallées tributaires Contribution of the prehistoric study of the Indre valley and its tributaries. Note 20: The site of Morillon at Azay-sur-Indre (Indre-et-Loire)
Gérard Cordier
Revue Archéologique du Centre de la France , 2008,
Abstract: On expose ici les découvertes faites anciennement sur un site du confluent de l’Indrois et de l’Indre, la plus importante étant celle d’un petit dép t du Bronze moyen. Shown here are the discoveries made in former times on a site at the confluence of the Indrois and the Indre, the most important being that of a small deposit from the Middle Bronze Age.
Photophoretic transport of hot minerals in the solar nebula
A. Moudens,O. Mousis,J. -M. Petit,G. Wurm,D. Cordier,S. Charnoz
Physics , 2011, DOI: 10.1051/0004-6361/201116476
Abstract: Hot temperature minerals have been detected in a large number of comets and were also identified in the samples of Comet Wild 2 that were returned by the Stardust mission. Meanwhile, observations of the distribution of hot minerals in young stellar systems suggest that these materials were produced in the inner part of the primordial nebula and have been transported outward in the formation zone of comets. We investigate the possibility that photophoresis provides a viable mechanism to transport high-temperature materials from the inner solar system to the regions in which the comets were forming. We use a grid of time-dependent disk models of the solar nebula to quantify the distance range at which hot minerals can be transported from the inner part of the disk toward its outer regions as a function of their size and density. The particles considered here are in the form of aggregates that presumably were assembled from hot mineral individual grains ranging down to submicron sizes and formed by condensation within the hottest portion of the solar nebula. Our particle-transport model includes the photophoresis, radiation pressure, and gas drag. Depending on the postulated disk parameters and the density of particles, 10-2 to 10-1 m aggregates can reach heliocentric distances up to 35 AU in the primordial nebula over very short timescales (no more than a few hundred thousand years). 10-3 m particles follow the same trajectory as the larger ones but their maximum migration distance does not exceed 26 AU and is reached at later epochs in the disks. On the other hand, 10-5 to 10-4 m aggregates are continuously pushed outward during the evolution of the solar nebula. Our simulations suggest that irrespective of the employed solar nebula model, photophoresis is a mechanism that can explain the presence of hot temperature minerals in the formation region of comets.
PDR Model Mapping of Physical Conditions via Spitzer/IRS Spectroscopy of H2: Theoretical Success toward NGC 2023-South
Y. Sheffer,M. G. Wolfire,D. J. Hollenbach,M. J. Kaufman,M. Cordier
Physics , 2011, DOI: 10.1088/0004-637X/741/1/45
Abstract: We use the IRS on Spitzer to observe the southern part of the reflection nebula NGC 2023, including the Southern Ridge, which is a photodissociation region par excellence excited by HD 37903. Five pure-rotational H2 emission lines are detected and mapped over and around the Southern Ridge in order to compare with predicted level column densities from theoretical PDR models. We find very good agreement between PDR model predictions and emission line intensities and ratios measured with Spitzer, leading us to conclude that grain photoelectric heating sufficiently warms the gas to produce the observed H2 line emission via collisional excitation. On the Southern Ridge, we infer a hydrogen nucleus density n_H approx 2x10^5 cm^-3 and radiation field strength \chi\ approx 10^4 relative to the local Galactic interstellar radiation field. This high value for \chi\ independently predicts a distance toward HD 37903 of 300 pc, and is consistent with the most recent HIPPARCOS results. Over the map we find that both n_H and \chi\ vary by a factor of sim 3. Such 2-D variations provide clues about the underlying 3-D structure of the Southern Ridge field, which appears to be the tip of a molecular cloud. We also map variations in excitation temperature and the ortho-to-para ratio, the latter attaining values of sim 1.5---2.0 on the Southern Ridge, and find that PDR modeling can readily reproduce observed ortho-to-para ratios that are < 3 for rotational excitation dominated by collisional processes. Last, the stars Sellgren C and G are discovered to be resolved on archival HST images into two point sources each, with separations of lesssim 0.5".
Simulations of GRB detections with the ECLAIRs telescope onboard the future SVOM mission
S. Antier,S. Schanne,B. Cordier,A. Gros,D. G?tz,C. Lachaud
Physics , 2015,
Abstract: The soft gamma-ray telescope ECLAIRs with its Scientific Trigger Unit is in charge of detecting Gamma-Ray Bursts (GRBs) on-board the future SVOM satellite. Using the "scientific software model" (SSM), we study the efficiency of both implemented trigger algorithms, the Count-Rate Trigger for time-scales below 20s and the Image Trigger for larger ones. The SMM provides a simulation of ECLAIRs with photon projection through the coded-mask onto the detection plane. We developed an input GRB database for the SSM based on GRBs light curves detected by the Fermi GBM instrument. We extrapolated the GRB spectra into the ECLAIRs band (4-120 keV) and projected them onto the detection plane, superimposed with cosmic extragalactic background photons (CXB). Several simulations were performed by varying the GRB properties (fluxes and positions in the field of view). We present first results of this study in this paper.
Measurement of Thermo-Elastic Deformation of an Optic using a Polarization Based Shearing Interferometer
Peter Beyersdorf,Mark Cordier
Physics , 2012, DOI: 10.1364/AO.51.007426
Abstract: A shearing interferometer is presented which uses polarization control to shear the wavefront and to modulate the interference pattern. The shear is generated by spatial walk-off in a birefringent crystal. By adjusting the orientation of the birefringent crystal, the components of the wavefront gradient can be independently measured to allow determination of the full wavefront vector gradient as well as reconstruction of the wavefront. Further, the monolithic nature of the crystal used for shearing allows the interferometer to be setup without need for precise alignment of any components. An algorithm incorporating homodyne detection is presented which analyzes the modulated interferograms to determine the components of the wavefront gradient, from which the wavefront is reconstructed. The thermal deformation of a mirror subject to heating from absorption of a Gaussian pump beam was accurately observed with a sensitivity better than \lambda/160. We show that this sensitivity is scale invariant, and present a method to account for the non-uniform spatial frequency response of the interferometer.
A measure of similarity between scientific journals and of diversity of a list of publications
Stéphane Cordier
Computer Science , 2012,
Abstract: The aim of this note is to propose a definition of the scientific diversity and corollarly, a measure of the "interdisciplinarity" of collaborations. With respect to previous studies, the proposed approach consists of 2 steps : first, the definition of similarity between journals and second, these similarities are used to characterize the homogeneity (or, on the contrary the diversity) of a publication list (that can be for one individual or a team).
About the possible role of hydrocarbon lakes in the origin of Titan's noble gas atmospheric depletion
D. Cordier,O. Mousis,J. I. Lunine,S. Lebonnois,P. Lavvas,L. Q. Lobo,A. G. M. Ferreira
Physics , 2010, DOI: 10.1088/2041-8205/721/2/L117
Abstract: An unexpected feature of Titan's atmosphere is the strong depletion in primordial noble gases revealed by the Gas Chromatograph Mass Spectrometer aboard the Huygens probe during its descent on 2005 January 14. Although several plausible explanations have already been formulated, no definitive response to this issue has been yet found. Here, we investigate the possible sequestration of these noble gases in the liquid contained in lakes and wet terrains on Titan and the consequences for their atmospheric abundances. Considering the atmosphere and the liquid existing on the soil as a whole system, we compute the abundance of each noble gas relative to nitrogen. To do so, we make the assumption of thermodynamic equilibrium between the liquid and the atmosphere, the abundances of the different constituents being determined via {\bf } regular solution theory. We find that xenon's atmospheric depletion can be explained by its dissolution at ambient temperature in the liquid presumably present on Titan's soil. In the cases of argon and krypton, we find that the fractions incorporated in the liquid are negligible, implying that an alternative mechanism must be invoked to explain their atmospheric depletion.
A-STAR: The All-Sky Transient Astrophysics Reporter
J. P. Osborne,P. O'Brien,P. Evans,G. W. Fraser,A. Martindale,J. -L. Atteia,B. Cordier,S. Mereghetti
Physics , 2013, DOI: 10.1051/eas/1361099
Abstract: The small mission A-STAR (All-Sky Transient Astrophysics Reporter) aims to locate the X-ray counterparts to ALIGO and other gravitational wave detector sources, to study the poorly-understood low luminosity gamma-ray bursts, and to find a wide variety of transient high-energy source types, A-STAR will survey the entire available sky twice per 24 hours. The payload consists of a coded mask instrument, Owl, operating in the novel low energy band 4-150 keV, and a sensitive wide-field focussing soft X-ray instrument, Lobster, working over 0.15-5 keV. A-STAR will trigger on ~100 GRBs/yr, rapidly distributing their locations.
Titan's lakes chemical composition: sources of uncertainties and variability
D. Cordier,O. Mousis,J. I. Lunine,S. Lebonnois,P. Rannou,P. Lavvas,L. Q. Lobo,A. G. M. Ferreira
Physics , 2011, DOI: 10.1016/j.pss.2011.05.009
Abstract: Between 2004 and 2007 the instruments of the CASSINI spacecraft discovered hydrocarbon lakes in the polar regions of Titan. We have developed a lake-atmosphere equilibrium model allowing the determination of the chemical composition of these liquid areas. The model is based on uncertain thermodynamic data and precipitation rates of organic species predicted to be present in the lakes and seas that are subject to spatial and temporal variations. Here we explore and discuss the influence of these uncertainties and variations. The errors and uncertainties relevant to thermodynamic data are simulated via Monte-Carlo simulations. Global Circulation Models (GCM) are also employed in order to investigate the possibility of chemical asymmetry between the south and the north poles, due to differences in precipitation rates. We find that mole fractions of compounds in the liquid phase have a high sensitivity to thermodynamic data used as inputs, in particular molar volumes and enthalpies of vaporization. When we combine all considered uncertainties, the ranges of obtained mole fractions are rather large (up to ~8500%) but the distributions of values are narrow. The relative standard deviations remain between 10% and ~300% depending on the compound considered. Compared to other sources of uncertainties and variability, deviation caused by surface pressure variations are clearly negligible, remaining of the order of a few percent up to ~20%. Moreover no significant difference is found between the composition of lakes located in north and south poles. Because the theory of regular solutions employed here is sensitive to thermodynamic data and is not suitable for polar molecules such as HCN and CH3CN, our work strongly underlines the need for experimental simulations and the improvement of Titan's atmospheric models.
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