Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Context. Several exoplanet direct imaging instruments will soon be in operation. They use an extreme adaptive optics (XAO) system to correct the atmospheric turbulence and provide a highly-corrected beam to a near-infrared (NIR) coronagraph for starlight suppression. The performance of the coronagraph is however limited by the non-common path aberrations (NCPA) due to the differential wavefront errors existing between the visible XAO sensing path and the NIR science path, leading to residual speckles in the coronagraphic image. Aims. Several approaches have been developed in the past few years to accurately calibrate the NCPA, correct the quasi-static speckles and allow the observation of exoplanets at least 1e6 fainter than their host star. We propose an approach based on the Zernike phase-contrast method for the measurements of the NCPA between the optical path seen by the visible XAO wavefront sensor and that seen by the near-IR coronagraph. Methods. This approach uses a focal plane phase mask of size {\lambda}/D, where {\lambda} and D denote the wavelength and the telescope aperture diameter, respectively, to measure the quasi-static aberrations in the upstream pupil plane by encoding them into intensity variations in the downstream pupil image. We develop a rigorous formalism, leading to highly accurate measurement of the NCPA, in a quasi-linear way during the observation. Results. For a static phase map of standard deviation 44 nm rms at {\lambda} = 1.625 {\mu}m (0.026 {\lambda}), we estimate a possible reduction of the chromatic NCPA by a factor ranging from 3 to 10 in the presence of AO residuals compared with the expected performance of a typical current-generation system. This would allow a reduction of the level of quasi-static speckles in the detected images by a factor 10 to 100 hence, correspondingly improving the capacity to observe exoplanets.Comment: 11 pages, 14 figures, A&A accepted, 2nd version after language-editor correction

Two Dimensional Ir-Cluster Lattices on Moir\'e of Graphene with Ir(111)

Lattices of Ir clusters have been grown by vapor phase deposition on graphene moir\'{e}s on Ir(111). The clusters are highly ordered, spatially and thermally stable below 500K. Their narrow size distribution is tunable from 4 to about 130 atoms. A model for cluster binding to the graphene is presented based on scanning tunneling microscopy and density functional theory. The proposed binding mechanism suggests that similar cluster lattices might be grown of materials other than Ir.Comment: Submitted to PRL on 27Apr0

Supplementation of Cattle with Rock Phosphate and Urea Treated Straw to Improve Manure Quality and Crop Yields in the Sahel Zone of Senegal

Mineral deficiencies are a major constraint in improving animal production and crop yield in the Sahel zone (Cissé et al., 1996). Millet (Pennisetum glaucum) and groundnut (Arachis hypogaea,) are two major food and cash crops in this zone. The purpose of this study was to assess effects of supplementing grazing cattle with rock phosphate and nitrogen enriched diets on animal performances, and the effects of the application of their manure on crop yield in a pearl millet-groundnut rotational system located in N Senegal

Two phase residence time distribution in a modified twin screw extruder

Biomass fractionation is performed with a modified Clextral twin-screw extruder used as a thermo-mechano-chemical reactor. This new process is firstly analyzed. Visual observations, residence time distributions, and global mass balances are used to obtain information about the process phenomena and their coupling. Residence time distributions (RTD) classical models are adopted to represent the experimental plots. The influence of continuous and discrete process parameters upon the RTD of the solid and liquid phases is analyzed

Luffin, Xavier. Les fils d’Antara. Représentations des Africains dans la fiction arabe contemporaine (1914-2011)

Luffin, Xavier. — Les fils d’Antara. Représentations des Africains dans la fiction arabe contemporaine (1914-2011). Bruxelles, Éditions Safran, 2012, 180 p., bibl. C’est à un sujet intéressant et plus ou moins polémique que s’attaque Xavier Luffin dans Les fils d’Antara : la manière dont les Africains noirs, de 1914 à 2011, furent imaginés, pensés, décrits, racontés, mis en scène dans leurs textes par les écrivains arabes. Si l’ouvrage, par un retour rapide à l’histoire, permet de lister une ..

Luffin, Xavier. Les fils d’Antara. Représentations des Africains dans la fiction arabe contemporaine (1914-2011)

Luffin, Xavier. — Les fils d’Antara. Représentations des Africains dans la fiction arabe contemporaine (1914-2011). Bruxelles, Éditions Safran, 2012, 180 p., bibl. C’est à un sujet intéressant et plus ou moins polémique que s’attaque Xavier Luffin dans Les fils d’Antara : la manière dont les Africains noirs, de 1914 à 2011, furent imaginés, pensés, décrits, racontés, mis en scène dans leurs textes par les écrivains arabes. Si l’ouvrage, par un retour rapide à l’histoire, permet de lister une ..

Mémoire et réconciliation en Mauritanie

RésuméDu milieu des années 1980 au début des années 1990, la Mauritanie fut le théâtre d’une vaste répression contre une catégorie de la composante négro-africaine de sa population, les Halpulareen. Le régime militaire du colonel Ould Taya se rendit coupable de la disparition de plusieurs centaines de militaires, de fonctionnaires et de civils issus de cette ethnie. Plusieurs milliers de Mauritaniens également issus de ce même groupe furent expulsés vers le Sénégal voisin. En 2007, deux ans après le coup d’État contre le président Ould Taya et suite à l’élection du nouveau chef d’État, Sidi Ould Cheikh Abdellahi, le débat sur la réconciliation post-conflictuelle s’ouvrit. Mais si le principe de cette réconciliation fut favorablement accueilli, l’amorce des discussions généra cependant entre divers groupes, des luttes visant à établir une vérité sur le passé. Le débat mémoriel apparut alors entre ces groupes et parfois au sein même de ces groupes comme une opportunité dont les acteurs se saisirent pour se réapproprier en l’ajustant, la mémoire des années de violence, l’objectif étant aussi d’investir des positions politiques de choix sur l’échiquier politique.AbstractFrom the mid-1980s to early 1990s, Mauritania was the scene of a broad crackdown against a class of Negro-African component of its population, Halpulareen.  The military regime of Colonel Ould Taya was guilty of the disappearance of hundreds of military officers and civilians from this ethnic group.  Several thousand Mauritanian also from this same group were expelled to neighbouring Senegal.  In 2007, two years after the coup against President Ould Taya and following the election of the new head of state, Sidi Ould Cheikh Abdellah, the debate on post-conflict reconciliation opened.  But if the principle of reconciliation was welcomed, the initiation of discussions created between various groups, struggles to establish a truth around the past.  The debate around the memory appeared between these groups and sometimes even within these groups as an opportunity which the will take to reclaim the memory of years of violence, the aim being also to invest political choice in the political space