Étude de la variabilité photométrique dans le vent des étoiles Wolf-Rayet

Ce mémoire présente une analyse de données photométriques d’étoiles de type Wolf-Rayet ayant pour objectif de caractériser les causes de la variabilité stochastique à court terme omniprésente chez ces étoiles, ainsi que la variabilité périodique due aux éclipses atmosphériques dans deux systèmes particuliers. À l’aide de données de la mission spatiale BRITE, les éclipses initialement supposées atmosphériques des systèmes WR22 et WR48 ont été investiguées avec un modèle analytique, suivi d’une modélisation numérique de l’éclipse pour WR22. Le taux de perte de masse de l’étoile Wolf-Rayet, la profondeur optique du vent et l’angle d’inclinaison du système ont été déterminés avec précision pour WR22, tandis que des valeurs-plafonds pour ces paramètres ont été obtenues avec un angle i tiré de la littérature pour WR48. L’étude de la variabilité stochastique dans le vent de 55 étoiles Wolf-Rayet galactiques a été conduite en utilisant 70 séries temporelles obtenues avec les satellites MOST, BRITE-Constellation et TESS. L’application d’un modèle semi-Lorentzien aux périodogrammes des observations a permis d’obtenir des paramètres caractéristiques : l’amplitude typique, la fréquence caractéristique et le taux d’augmentation de l’amplitude de variabilité. Des comparaisons entre les paramètres obtenus et ceux de la littérature nous ont permis de contextualiser le rôle de divers processus physiques dans l’origine de la variabilité, tel l’instabilité du processus d’entraînement du vent, d’une zone de convection subsurfacique engendrée par l’ionisation partielle des éléments du groupe du fer, ainsi qu’une possible contribution d’ondes de gravité générées à l’interface du coeur convectif et de la zone radiative.We present an analysis of photometric observations of Wolf-Rayet stars in an effort to characterize their ubiquitous short-term stochastic variability and the origin of the periodic variability due to atmospheric eclipses in two specific systems. With data from BRITE-Constellation, we investigated what we initially thought to be atmospheric eclipses in the WR22 and WR48 systems. Both stars’ datasets were first analyzed using an analytical atmospheric eclipse model, and then a fully numerical core and wind eclipse model was used to better characterize the ambiguous case of WR22. The mass-loss rate of the Wolf-Rayet component, the optical depth of the wind and the inclination angle of the system were accurately determined for WR22, whereas for WR48 an upper limit was found for those parameters using an inclination angle from the litterature. The study of the short-term stochastic variability in the winds of 55 Wolf-Rayet stars was obtained with 70 time-series produced by MOST, BRITE and TESS. The amplitude spectra of these stars were analyzed using a semi-Lorentzian function that allowed us to extract characteristic parameters. Comparisons between the values we obtained and those from the litterature were carried out and we discuss the possible contribution to the variability of various physical processes, namely the line de-shadowing instability, the presence of a subsurface iron-group driven convection zone and possibly of internal gravitational waves generated at the boundary between the convective core and the radiative layer to the observed variability

Flows of suspensions of particles in yield stress fluids

International audienceWe study the rheological behavior of suspensions of noncolloidal spheres in yield stress fluids (concentrated emulsions). These are good model systems for understanding, e.g., the rheology of fresh concrete or debris flows, and more generally, the behavior of particles dispersed in any nonlinear material. We use magnetic resonance imaging techniques to investigate the flows of these yield stress suspensions in a concentric-cylinder Couette geometry. We extend the theoretical approach of Chateau et al. [J. Rheol. 52, 489–506 (2008)], valid for isotropic suspensions, to describe suspensions in simple shear flows, in which an anisotropic spatial distribution of particles is induced by flow. Theory and experiments show that the suspensions can be modeled by a Herschel–Bulkley behavior of same index as their interstitial fluid. We characterize the increase of their consistency and their yield stress with the particle volume fraction / in the 0%–50% range. We observe a good agreement between the experimental variations of the consistency with / and the theoretical prediction. This shows that the average apparent viscosity of the sheared interstitial material is correctly estimated and taken into account. We also observe shear-induced migration with similar properties as in a Newtonian fluid, which we predict theoretically, suggesting that particle normal stresses are proportional to the shear stress. However, the yield stress at flow stoppage increases much less than predicted. We also show that new features emerge in the rheology of the yield stress fluid when adding particles. We predict and observe the emergence of a nonzero normal stress difference at the yielding transition. We observe that the yield stress at flow start can differ from the yield stress at flow stoppage, and depends on flow history. It is likely a signature of a shear-dependent microstructure, due to the nonlinear behavior of the interstitial fluid, which makes these materials different from suspensions in Newtonian media. This is confirmed by direct characterization of shear-rate-dependent pair distribution functions using X-ray microtomography. This last observation explains why the theory predictions for the consistency can be correct while failing to model the yield stress at flow stoppage: a unique microstructure was indeed assumed as a first approximation. More sophisticated theories accounting for a shear-dependent microstructure are thus needed

Couplage rhéologie - microstructure d'une suspension de particules dans un fluide à seuil

Une suspension concentrée de particules présente des propriétés rhéologiques complexes, telles que rhéofluidification, différence de contraintes normales, …, même lorsque la rhéologie du fluide suspendant est simplement Newtonienne et que les interactions entre particules sont purement hydrodynamiques et de contact. Certaines de ces propriétés macroscopiques non triviales peuvent être expliquées par le comportement de la suspension à l’échelle locale, en particulier par sa microstructure à l’échelle des particules. Sous cisaillement stationnaire, une suspension Newtonienne développe une microstructure anisotrope, mise en évidence par la fonction de distribution de paires [Parsi & Gadala-Maria 87, Blanc et al 13], qui peut notamment expliquer l’apparition de différences de contraintes normales [Sierou & Brady 02]. Que se passe-t-il dans le cas où la rhéologie du fluide suspendant est complexe ? Nous étudions le cas de sphères dures non Browniennes en suspension dans un fluide à seuil. Ce matériau, qui présente un comportement élasto-visco-plastique, est ici une émulsion concentrée. Nous étudions le couplage entre comportement macroscopique et microstructure de cette suspension en associant rhéométrie classique dans un dispositif de cisaillement plan entre deux plateaux rotatifs d’espacement variable et imagerie 3D par microtomographie X. L’étude de la fonction de distribution de paires à 3D en cisaillement simple stationnaire montre une anisotropie de la microstructure de la suspension dans le plan vitesse-cisaillement, pour des écoulements rotationnel et d’écrasement. Le régime instationnaire est également étudié suite à diverses préparations (i.e., diverses histoires de cisaillement). Les propriétés élasto-plastiques (contrainte seuil de plasticité, allongement plastique maximal, module élastique de cisaillement) du matériau montrent alors de remarquables propriétés d’écrouissage (durcissement, modification de la ductilité) et de variations du module élastique de cisaillement, qui peuvent être reliées à sa microstructure. Il est ainsi possible de contrôler les propriétés élasto-plastiques initiales d’une suspension de particules dans un fluide à seuil via l’histoire des sollicitations

Improving the treatment and remission of major depression in homeless people with severe mental illness: the multicentric French Housing First (FHF) program. French Housing First Study Group Running title: Major depression in homeless people with severe mental disorders

International audienceObjectiveThe aim of this study was to investigate the factors associated with violent behavior in a large multicenter sample of Homeless Schizophrenia (SZ) and Bipolar Disorder (BD) (HSB) subjects.MethodsThis multicenter study was conducted in 4 French cities: Lille, Marseille, Paris and Toulouse. Violent behavior was defined by at least one episode of verbal or physical violence in the last 6 months.ResultsOverall, 675 HSB patients, mean aged 38 years and 82.5% men were included, 458 SZ (68.4%) and 212 BD (31.6%). During the 6 months before evaluation, 213 (34.3%) committed at least one physical or verbal violence. In multivariate analysis, violence has been associated with younger age (aOR = 0.96[0.94–0.99], p = .001), number of nights in the street (aOR = 1.01[1.01–1.01]), BD diagnosis (aOR = 1.63[1.01–2.65], p = .04), higher current illness severity (CGI score) (aOR = 1.32[1.07–1.64], p = .01), higher rates of current manic episode (aOR = 2.24[1.32–3.81], p = .002), current alcohol use disorder (aOR = 2.05 [1.33–3.15], p = .001), antisocial personality disorder (aOR = 2.51[1.55–4.07], p < .001) and with antidepressant consumption (aOR = 2.01[1.01–4.04], p = .04). No specific antipsychotic or mood stabilizer has been associated with decreased rates of violent behavior, however clozapine, lithium and carbamazepine remained poorly prescribed.ConclusionIn case of violent behavior in HSB subjects, clinicians should focus in priority on the treatment of mania, antidepressant iatrogenic effect and alcohol use disorder by pharmacological and non-pharmacological treatments. Clozapine, lithium and carbamazepine should be chosen as the treatments of reference in this population but may be hard to manage in some cases.The current clinical trial number is NCT01570712

Biotic and abiotic drivers of intraspecific trait variation within plant populations of three herbaceous plant species along a latitudinal gradient

Background: The importance of intraspecific trait variation (ITV) is increasingly acknowledged among plant ecologists. However, our understanding of what drives ITV between individual plants (ITVBI) at the population level is still limited. Contrasting theoretical hypotheses state that ITVBI can be either suppressed (stress-reduced plasticity hypothesis) or enhanced (stress-induced variability hypothesis) under high abiotic stress. Similarly, other hypotheses predict either suppressed (niche packing hypothesis) or enhanced ITVBI (individual variation hypothesis) under high niche packing in species rich communities. In this study we assess the relative effects of both abiotic and biotic niche effects on ITVBI of four functional traits (leaf area, specific leaf area, plant height and seed mass), for three herbaceous plant species across a 2300 km long gradient in Europe. The study species were the slow colonizing Anemone nemorosa, a species with intermediate colonization rates, Milium effusum, and the fast colonizing, non-native Impatiens glandulifera. Results: Climatic stress consistently increased ITVBI across species and traits. Soil nutrient stress, on the other hand, reduced ITVBI for A. nemorosa and I. glandulifera, but had a reversed effect for M. effusum. We furthermore observed a reversed effect of high niche packing on ITVBI for the fast colonizing non-native I. glandulifera (increased ITVBI), as compared to the slow colonizing native A. nemorosa and M. effusum (reduced ITVBI). Additionally, ITVBI in the fast colonizing species tended to be highest for the vegetative traits plant height and leaf area, but lowest for the measured generative trait seed mass. Conclusions: This study shows that stress can both reduce and increase ITVBI, seemingly supporting both the stress-reduced plasticity and stress-induced variability hypotheses. Similarly, niche packing effects on ITVBI supported both the niche packing hypothesis and the individual variation hypothesis. These results clearly illustrates the importance of simultaneously evaluating both abiotic and biotic factors on ITVBI. This study adds to the growing realization that within-population trait variation should not be ignored and can provide valuable ecological insights

Hedging against biodiversity loss : forest herbs’ performance in hedgerows across temperate Europe

Questions: How do contrasting environmental conditions among forests and hedgerows affect the vegetative and reproductive performance of understorey forest herbs in both habitats? Can hedgerows support reproductive source populations of forest herbs, thus potentially allowing progressive dispersal of successive generations along these linear habitats? Location: Hedgerows and deciduous forest patches in agricultural landscapes across the European temperate biome. Methods: First, we assessed differences in environmental conditions among forests and hedgerows. Next, we quantified plant performance based on a set of functional life‐history traits for four forest herbs (Anemone nemorosa, Ficaria verna, Geum urbanum, Poa nemoralis) with contrasting flowering phenology and colonisation capacity in paired combinations of forests and hedgerows, and compared these traits among both habitats. Finally, we assessed relationships between plant performance and environmental conditions in both habitats. Results: All study species showed a higher above‐ground biomass in hedgerows than in forests. For Poa nemoralis and Geum urbanum, we also found a higher reproductive output in hedgerows, which was mainly correlated to the higher sub‐canopy temperatures therein. The “ancient forest herb” Anemone nemorosa, however, appeared to have a lower reproductive output in hedgerows than in forests, while for Ficaria verna no reproductive differences were found between the two habitats. Conclusions: This is the first study on such a broad geographical scale to provide evidence of reproductive source populations of forest herbs in hedgerows. Our findings provide key information on strategies by which forest plants grow, reproduce and disperse in hedgerow environments, which is imperative to better understand the dispersal corridor function of these wooded linear structures. Finally, we highlight the urgent need to develop guidelines for preserving, managing and establishing hedgerows in intensive agricultural landscapes, given their potential to contribute to the long‐term conservation and migration of forest herbs in the face of global environmental change

Plant hydraulic : new insights brought by HRCT

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