Magnetically driven accretion in protoplanetary discs

We characterize magnetically driven accretion at radii between 1 au and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary discs might lie close to the borderline between laminar and turbulent behaviour. Given the number of important control parameters that have already been identified in MHD models, quantitative predictions for disc structure in terms of only radius and accretion rate appear to be difficult. Instead, we identify robust qualitative tests of magnetically driven accretion. These include the presence of turbulence in the outer disc, independent of the orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in turbulent properties extending from the region of thermal ionization to 10 au.Comment: accepted to MNRAS after very minor revision

Anion selective membrane

Experimental anion permselective membranes were prepared and tested for their suitability as cell separators in a chemical redox power storage system being developed at NASA-Lewis Research Center. The goals of long-term (1000 hr) oxidative and thermal stability at 80 C in FeCl3 and CrCl3 electrolytes were met by most of the weak base and strong base amino exchange groups considered in the program. Good stability is exhibited by several of the membrane substrate resins. These are 'styrene' divinylbenzene copolymer and PVC film. At least four membrane systems produce strong flexible films with electrochemical properties (resistivity, cation transfer) superior to those of the 103QZL, the most promising commercial membrane. The physical and chemical properties of the resins are listed

CARACTERISATION D'UN LASER Nd : YAG CONTINU A MODES COUPLES

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Vortex migration in protoplanetary disks

We consider the radial migration of vortices in two-dimensional isothermal gaseous disks. We find that a vortex core, orbiting at the local gas velocity, induces velocity perturbations that propagate away from the vortex as density waves. The resulting spiral wave pattern is reminiscent of an embedded planet. There are two main causes for asymmetries in these wakes: geometrical effects tend to favor the outer wave, while a radial vortensity gradient leads to an asymmetric vortex core, which favors the wave at the side that has the lowest density. In the case of asymmetric waves, which we always find except for a disk of constant pressure, there is a net exchange of angular momentum between the vortex and the surrounding disk, which leads to orbital migration of the vortex. Numerical hydrodynamical simulations show that this migration can be very rapid, on a time scale of a few thousand orbits, for vortices with a size comparable to the scale height of the disk. We discuss the possible effects of vortex migration on planet formation scenarios.Comment: 13 pages, 13 figures, accepted for publication in Ap

Formation of Zn–Ca phyllomanganate nanoparticles in grass roots

International audienceIt is now well established that a number of terrestrial and aquatic microorganisms have the capacity to oxidize and precipitate Mn as phyllomanganate. However, this biomineralization has never been shown to occur in plant tissues, nor has the structure of a natural Mn(IV) biooxide been characterized in detail. We show that the graminaceous plant Festuca rubra (red fescue) produces a Zn-rich phyllomanganate with constant Zn:Mn and Ca:Mn atomic ratios (0.46 and 0.38, respectively) when grown on a contaminated sediment. This new phase is so far the Zn-richest manganate known to form in nature (chalcophanite has a Zn:Mn ratio of 0.33) and has no synthetic equivalent. Visual examination of root fragments under a microscope shows black precipitates about ten to several tens of microns in size, and their imaging with backscattered and secondary electrons demonstrates that they are located in the root epidermis. In situ measurements by Mn and Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) with a micro-focused beam can be quantitatively described by a single-phase model consisting of Mn(IV) octahedral layers with 22% vacant sites capped with tetrahedral and octahedral Zn in proportions of 3:1. The layer charge deficit is also partly balanced by interlayer Mn and Ca. Diffracting crystallites have a domain radius of 33 Å in the ab plane and contain only 1.2 layers (not, vert, similar8.6 Å) on average. Since this biogenic Mn oxide consists mostly of isolated layers, basal 00l reflections are essentially absent despite its lamellar structure. Individual Mn layers are probably held together in the Mn–Zn precipitates by stabilizing organic molecules. Zinc biomineralization by plants likely is a defense mechanism against toxicity induced by excess concentrations of this metal in the rhizosphere

Prévalence des lombalgies et de leurs facteurs de risque professionnels chez les travailleurs de l'agriculture en France en 2010

Introduction La phase pilote du programme Coset-MSA a permis d’évaluer la prévalence des lombalgies et de leurs facteurs de risque professionnels chez les travailleurs de l’agriculture, et notamment les agriculteurs exploitants et les ouvriers agricoles. Méthodes Les travailleurs ont été recrutés, dans cinq départements, par tirage au sort dans les bases de données de la MSA et invités à remplir un auto-questionnaire portant sur les expositions et les lombalgies au cours des 12 derniers mois. L’échantillon d’analyse était constitué de 1130 travailleurs, âgés de 18 à 65 ans, exerçant dans le secteur agriculture, sylviculture et pêche, et en activité au moment de l’enquête. Résultats La prévalence des lombalgies au cours des 12 derniers mois était élevée, chez les hommes (65 %) comme chez les femmes (64 %). Chez ces dernières, la prévalence diminuait significativement avec l’âge (de 95 % chez les moins de 30 ans à 56 % chez les 50 ans et plus, p < 0,01). Des douleurs quotidiennes étaient rapportées par 10 % des hommes et 11 % des femmes, avec une prévalence qui augmentait avec l’âge, de façon significative chez les hommes (p < 0,001). La prévalence ne différait pas significativement selon la catégorie socioprofessionnelle (CSP). Cependant, soulignons la forte prévalence des lombalgies parmi les hommes artisans, commerçants et chefs d’entreprise (surtout des jardiniers, paysagistes…) malgré des effectifs faibles. Chez les femmes, les agricultrices exploitantes et les ouvrières agricoles étaient plus nombreuses à souffrir de lombalgies, quelle que soit la durée des symptômes. Le port de charges de plus de 25 kg plus de 2 heures par jour concernait 16 % des hommes et 5 % des femmes, se pencher en avant ou sur le côté plus de 4 heures par jour 15 % et 16 % respectivement et la conduite plus de 4 heures par jour 27 % et 10 %. Chez les hommes, les ouvriers non agricoles étaient les plus exposés aux trois facteurs (17 % au port de charges [p < 0,05], 22 % au fait de se pencher [p < 0,01], 32 % à la conduite [NS]). Chez les femmes, les ouvrières agricoles rapportaient les plus fortes expositions : 29 % se penchaient plus de 4 h/j (p < 0,001) et 16 % conduisaient plus de 4 h/j (NS). Conclusions La prévalence des lombalgies est particulièrement élevée dans l’agriculture, comparativement à des études portant sur tous secteurs d’activité (par exemple, l’étude Cosali menée auprès de salariés des Pays de la Loire). De plus, cette étude permet d’identifier les CSP les plus exposées aux facteurs de risque professionnels de lombalgie, qui devraient être ciblées prioritairement par les actions de prévention