Dispersion mécanique de l'or dans les matériaux de surface : exemple du site aurifère de Piéla (Burkina-Faso)

A Piéla, l'or primaire est associé à des filons de quartz, dans une zone de cisaillement des formations volcano-sédimentaires birimiennes (Protérozoïque inférieur). La présence d'or a également été mise en évidence dans les matériaux superficiels latéritiques bien que les roches sous-jacentes et leurs altérites soient en général stériles. L'étude de la répartition de l'or sur les différentes surfaces aplanies montre que l'or est concentré par des transferts mécaniques dans les cailloutis et graviers de cuirasse ferrugineuse et de quartz de la plaine alluviale et dans des portions du moyen-glacis cuirassé assimilables à une ancienne terrasse. Ailleurs sur les surfaces cuirassées (haut-glacis, moyen-glacis), l'or est dispersé à faible teneur ou absent. Les caractères morphologiques et chimiques des particules d'or évoluent depuis la zone minéralisée jusqu'à la plaine alluviale distale ainsi que sur les glacis. Cette évolution se marque par une augmentation de l'émoussé, par des traits morphologiques spécifiques du transport mécanique (stries, bordures repliées, aplatissement) et par un lessivage de l'argent jusqu'au coeur des particules. Dans les matériaux superficiels du site de Piéla, les particules d'or présentent des degrés d'usure différents et ne montrent pas l'évolution progressive avec diminution de la taille des particules et lessivage de l'argent, souvent décrite de bas en haut des profils cuirassés sur roche mère minéralisée. (Résumé d'auteur

Détermination du début de la période thermosensible pour le déterminisme du sexe chez le tilapia (Oreochromis niloticus L.)

L’objectif de la présente étude a été de préciser davantage le “timing” de la période thermosensible ainsi que l’effet du moment d’application de la température de “masculinisation” sur la sex-ratio chez le tilapia du Nil Oreochromis niloticus. L’analyse des résultats a montré que la réduction de la période d’exposition aux hautes températures (21 au lieu de 28 jours) n’a pas d’effet négatif sur la sex-ratio. Dans tous les lots exposés à 37°C entre 0 et 11 jours post-résorption vitelline, les populations sont presque exclusivement monosexes mâles (> 97%), alors que dans les lots qui ont été soumis le plus tard au traitement thermique, la proportion des mâles est plus faible (82%) et une forte proportion d’individus intersexués (18%) a été observée. Ainsi, la limite inférieure de thermosensibilité d’Oreochromis niloticus pourrait se situer entre le 11ème et le 14ème jour après la résorption de la vésicule vitelline

Spurious diffusion in particle simulations of the Kolmogorov flow

Particle simulations of the Kolmogorov flow are analyzed by the Landau-Lifshitz fluctuating hydrodynamics. It is shown that a spurious diffusion of the center of mass corrupts the statistical properties of the flow. The analytical expression for the corresponding diffusion coefficient is derived.Comment: 10 pages, no figure

Behaviour and passage of European silver eels (Anguilla anguilla) at a small hydropower plant during their downstream migration

Between 2004 and 2007, 116 downstream migrant silver eels (Anguilla anguilla) were monitored at a hydropower plant on the Gave de Pau river in South-West France using radio and PIT telemetry. The objectives of the study were: (i) to determine the environmental conditions when eels arrived and passed the facility; (ii) to determine the rate of eel escapement (passage other than via the turbines); (iii) to describe the behaviour of eels faced with the intake structure and the permeability of the intake trashracks for the different sizes of eel; and (iv) to determine whether surface bypasses originally designed for salmon could be effective for eels. Five types of behaviour of silver eels in the forebay and at the plant intakes were identified. The study showed the key factor influencing both eel behaviour and the route taken through the plant was variation in river discharge. Escapement rate was related to eel length and the spill flow to river flow ratio, which could be described by a logistic regression model. The surface bypasses originally designed for salmon were found to aid downstream eel migration significantly. At velocities < 0.40 m·s−1, no eels, even the largest, for which the racks are a physical barrier, were found impinged on the trashracks

Lifespan theorem for constrained surface diffusion flows

We consider closed immersed hypersurfaces in $\R^{3}$ and $\R^4$ evolving by a class of constrained surface diffusion flows. Our result, similar to earlier results for the Willmore flow, gives both a positive lower bound on the time for which a smooth solution exists, and a small upper bound on a power of the total curvature during this time. By phrasing the theorem in terms of the concentration of curvature in the initial surface, our result holds for very general initial data and has applications to further development in asymptotic analysis for these flows.Comment: 29 pages. arXiv admin note: substantial text overlap with arXiv:1201.657

Advanced Orbiting Systems Data Generator/Simulator: A Functional Description of the Software (Version 3)

The Advanced Orbiting System (AOS) Data Generator/Simulator is a software implementation of the transmitter (data generation) section of the CCSDS Recommendation 701.0-B-2 for Advanced Orbiting Systems: Networks and Data Links. An object-oriented approach to the simulation of a complex, high-performance communication protocol, it makes full use of the concepts of data-encapsulation and inheritance to ease implementation. The backbone of the software is a general-purpose packet description and generation module that may be used as part of any packet- based simulation software. The user-interface to the program is in the form of a command-language, designed to ease the process of generation of large, multiple data-streams. The output of the program may be configured for interpretation by a graphical user interface (for visual inspection of the data), or as a bit-stream suitable for further processing. This paper consists of three sections. The first two sections provide a brief, yet comprehensive description of the above CCSDS Recommendation. The various kinds and qualities of user-services, data units involved, and data-paths defined by the protocol are discussed. The different qualities of service (in terms or data reliability) available to the user (and the error-control schemes used to provide them) are also discussed. The last section describes the structure and user-interfaces of the AOS Data Generator/Simulator

Advanced Orbiting Systems Data Generator/Simulator: A Functional Description of the Software (Version 3)

The Advanced Orbiting System (AOS) Data Generator/Simulator is a software implementation of the transmitter (data generation) section of the CCSDS Recommendation 701.0-B-2 for Advanced Orbiting Systems: Networks and Data Links. An object-oriented approach to the simulation of a complex, high-performance communication protocol, it makes full use of the concepts of data-encapsulation and inheritance to ease implementation. The backbone of the software is a general-purpose packet description and generation module that may be used as part of any packet- based simulation software. The user-interface to the program is in the form of a command-language, designed to ease the process of generation of large, multiple data-streams. The output of the program may be configured for interpretation by a graphical user interface (for visual inspection of the data), or as a bit-stream suitable for further processing. This paper consists of three sections. The first two sections provide a brief, yet comprehensive description of the above CCSDS Recommendation. The various kinds and qualities of user-services, data units involved, and data-paths defined by the protocol are discussed. The different qualities of service (in terms or data reliability) available to the user (and the error-control schemes used to provide them) are also discussed. The last section describes the structure and user-interfaces of the AOS Data Generator/Simulator

Analysis of the Reaction Rate Coefficients for Slow Bimolecular Chemical Reactions

Simple bimolecular reactions $A_1+A_2\rightleftharpoons A_3+A_4$ are analyzed within the framework of the Boltzmann equation in the initial stage of a chemical reaction with the system far from chemical equilibrium. The Chapman-Enskog methodology is applied to determine the coefficients of the expansion of the distribution functions in terms of Sonine polynomials for peculiar molecular velocities. The results are applied to the reaction $H_2+Cl\rightleftharpoons HCl+H$, and the influence of the non-Maxwellian distribution and of the activation-energy dependent reactive cross sections upon the forward and reverse reaction rate coefficients are discussed.Comment: 11 pages, 5 figures, to appear in vol.42 of the Brazilian Journal of Physic

Gas phase Elemental abundances in Molecular cloudS (GEMS) VI. A sulphur journey across star-forming regions: study of thioformaldehyde emission

In the context of the IRAM 30m Large Program GEMS, we present a study of thioformaldehyde in several starless cores located in star-forming filaments of Taurus, Perseus, and Orion. We investigate the influence of the environmental conditions on the abundances of these molecules in the cores, and the effect of time evolution. We have modelled the observed lines of H2CS, HDCS, and D2CS using the radiative transfer code RADEX. We have also used the chemical code Nautilus to model the evolution of these species depending on the characteristics of the starless cores. We derive column densities and abundances for all the cores. We also derive deuterium fractionation ratios, Dfrac, to determine and compare the evolutionary stage between different parts of each star-forming region. Our results indicate that the north region of the B213 filament in Taurus is more evolved than the south, while the north-eastern part of Perseus presents an earlier evolutionary stage than the south-western zone. Model results also show that Dfrac decreases with the cosmic-ray ionisation rate, while it increases with density and with the degree of sulphur depletion. In particular, we only reproduce the observations when the initial sulphur abundance in the starless cores is at least one order of magnitude lower than the solar elemental sulphur abundance. The progressive increase in HDCS/H2CS and D2CS/H2CS with time makes these ratios powerful tools for deriving the chemical evolutionary stage of starless cores. However, they cannot be used to derive the temperature of these regions, since both ratios present a similar evolution at two different temperature ranges (7-11 K and 15-19 K). Regarding chemistry, (deuterated) thioformaldehyde is mainly formed through gas-phase reactions (double-replacement and neutral-neutral displacement reactions), while surface chemistry plays an important role as a destruction mechanism.Comment: 31 pages, 26 figure