Du bricolage chez Picasso? : Étude materielle et technique de trois carnets de l’epoque cubiste

Este artículo efectúa un análisis material y técnico de tres cuadernos insólitos de Picasso que datan de la época cubista: MP 1859 del invierno 1906-1907, MP 1862 de mayo-junio de 1907 y MP 1990-98 de 1915. Estos tres cuadernos, conservados en el museo nacional Picasso de París parecen, en efecto, haber sido realizados a mano por un aficionado, puede ser incluso que por Picasso mismo. Se trata, así, gracias al peritaje de dos conservadoras-restauradoras, de determinar los procesos de fabricación de estos cuadernos para volver a situarlos en el corpus general de los cuadernos de Picasso, y la materialidad de estos objetos de trabajo. Este estudio se esfuerza así por demostrar la importancia de la historia material y técnica en la historia del arte, tanto para comprender las obras como para inscribirlas en su contexto histórico de fabricación, marcado por las prácticas creativas corrientes. En efecto, el último tercio del siglo XIX vio desarrollarse múltiples hobbies creativos de aficionados que han dejado una huella sobre el cubismo, la invención de los papiers collés y de las construcciones.Cet article effectue une analyse matérielle et technique de trois carnets insolites de Picasso datant de l’époque cubiste: MP 1859 de l’hiver 1906-1907, MP 1862 de mai-juin 1907, et MP 1990-98 de 1915. Ces trois carnets, conservés au musée national Picasso de Paris, semblent en effet avoir été réalisés à la main par un amateur, peut-être même par Picasso lui-même. Il s’agit ainsi, grâce à l’expertise de deux conservatrices-restauratrices, de déterminer les procédés de fabrication de ces carnets pour les replacer dans le corpus plus général des carnets de Picasso et la matérialité de ces objets de travail. Cette étude s’attache ainsi à démontrer l’importance de l’histoire matérielle et technique en histoire de l’art, tant pour comprendre les œuvres que pour les inscrire dans leur contexte historique de fabrication marqué par les pratiques créatives ordinaires. En effet, le dernier tiers du XIXe siècle voit se développer de multiples loisirs créatifs amateurs qui ont laissé leur empreinte sur le cubisme, l’invention des papiers collés et des construction

A New and Simple Approach to Determine the Abundance of Hydrogen Molecules on Interstellar Ice Mantles

Water is usually the main component of ice mantles, which cover the cores of dust grains in cold portions of dense interstellar clouds. When molecular hydrogen is adsorbed onto an icy mantle through physisorption, a common assumption in gas-grain rate equation models is to use an adsorption energy for molecular hydrogen on a pure water substrate. However, at high density and low temperature, when H2 is efficiently adsorbed onto the mantle, its surface abundance can be strongly overestimated if this assumption is still used. Unfortunately, the more detailed microscopic Monte Carlo treatment cannot be used to study the abundance of H2 in ice mantles if a full gas-grain network is utilized. We present a numerical method adapted for rate-equation models that takes into account the possibility that an H2 molecule can, while diffusing on the surface, find itself bound to another hydrogen molecule, with a far weaker bond than the H2-water bond, which can lead to more efficient desorption. We label the ensuing desorption "encounter desorption". The method is implemented first in a simple system consisting only of hydrogen molecules at steady state between gas and dust using the rate-equation approach and comparing the results with the results of a microscopic Monte Carlo calculation. We then discuss the use of the rate-equation approach with encounter desorption embedded in a complete gas-grain chemical network. For both systems, the rate-equation model with encounter desorption reproduces the H2 granular coverage computed by the microscopic Monte Carlo model. The method is especially useful for dense and cold environments, and for time-dependent physical conditions, such as occur in the collapse of dense cores and the formation of protoplanetary disks. It is not significantly CPU time consuming, so can be used for example with complex 3D chemical-hydrodynamical simulations.Comment: 8 pages, 7 figures, language editing, accepted for publication in Astronomy & Astrophysic

Modeling Of Astrochemistry During Star Formation

Interstellar matter is not inert, but is constantly evolving. On the one hand, its physical characteristics such as its density and its temperature, and on the other hand, its chemical characteristics such as the abundances of the species and their distribution, can change drastically. The phases of this evolution spread over different timescales, and this matter evolves to create very different objects such as molecular clouds ($\rm T \sim 10~K$, $\rm n \sim 10^4~cm^{-3}$, $\rm t \sim 10^6~years$), collACSing prestellar cores (inner core : $\rm T \sim 1000~K$, $\rm n \sim 10^{16}~cm^{-3}$, $\rm t \sim 10^4~years$), protostellar cores (inner core : $\rm T \sim 10^5~K$, $\rm n \sim 10^{24}~cm^{-3}$, $\rm t \sim 10^6~years$), or protoplanetary disks ($\rm T \sim 10-1000~K$, $\rm n \sim 10^{9}-10^{12}~cm^{-3}$, $\rm t \sim 10^7~years$). These objects are the stages of the star formation process. Starting from the diffuse cloud, matter evolves to form molecular clouds. Then, matter can condense to form prestellar cores, which can collACSe to form a protostar surrounded by a protoplanetary disk. The protostar can evolve in a star, and planets and comets can be formed in the disk. Thus, modeling of astrochemistry during star formation should consider chemical and physical evolution in parallel. We present a new gas-grain chemical network involving deuterated species, which takes into account ortho, para, and meta states of H$_2$, D$_2$, H$_3^+$, H$_2$D$^+$, D$_2$H$^+$, and D$_3^+$. It includes high temperature gas phase reactions, and some ternary reactions for high density, so that it should be able to simulate media with temperature equal to [$10;800$]~K and density equal to [$\sim10^4;\sim10^{12}$]~cm$^{-3}$. We apply this network to the modeling of low-mass and high-mass star formation, using a gas-grain chemical code coupled to a time dependent physical structure. Comparisons with observational constraints, such as the HDO/H$_2$O ratio in high mass star forming region, give good agreement which is promising. Besides, high density conditions have highlighted some limitations of our grain surface modeling. We present a numerical technique to model in a more realistic way H$_2$ diffusion and desorption in high density conditions

Caractérisation physico-chimique des premières phases de formation des disques protoplanétaires

Les étoiles de type solaire se forment par l'effondrement d'un nuage moléculaire, durant lequel la matière s'organise autour de l'étoile en formation sous la forme d'un disque, appelé disque protoplanétaire. Dans ce disque se forment les planètes, comètes et autres objets du système stellaire. La nature de ces objets peut donc avoir un lien avec l'histoire de la matière du disque.J'ai étudié l'évolution chimique et physique de cette matière, du nuage au disque, à l'aide du code de chimie gaz-grain Nautilus.Une étude de sensibilité à divers paramètres du modèle (comme les abondances élémentaires et les paramètres de chimie de surface) a été réalisée. Notamment, la mise à jour des constantes de vitesse et des rapports de branchement des réactions de notre réseau chimique s'est avérée influente sur de nombreux points, comme les abondances de certaines espèces chimiques, et la sensibilité du modèle à ses autres paramètres.Plusieurs modèles physiques d'effondrement ont également été considérés. L'approche la plus complexe et la plus consistante a été d'interfacer notre code de chimie avec le code radiatif magnétohydrodynamique de formation stellaire RAMSES, pour modéliser en trois dimensions l'évolution physique et chimique de la formation d'un jeune disque. Notre étude a démontré que le disque garde une trace de l'histoire passée de la matière, et sa composition chimique est donc sensible aux conditions initiales.Low mass stars, like our Sun, are born from the collapse of a molecular cloud. The matter falls in the center of the cloud, creating a protoplanetary disk surrounding a protostar. Planets and other solar system bodies will be formed in the disk.The chemical composition of the interstellar matter and its evolution during the formation of the disk are important to better understand the formation process of these objects.I studied the chemical and physical evolution of this matter, from the cloud to the disk, using the chemical gas-grain code Nautilus.A sensitivity study to some parameters of the code (such as elemental abundances and parameters of grain surface chemistry) has been done. More particularly, the updates of rate coefficients and branching ratios of the reactions of our chemical network showed their importance, such as on the abundances of some chemical species, and on the code sensitivity to others parameters.Several physical models of collapsing dense core have also been considered. The more complex and solid approach has been to interface our chemical code with the radiation-magneto-hydrodynamic model of stellar formation RAMSES, in order to model in three dimensions the physical and chemical evolution of a young disk formation. Our study showed that the disk keeps imprints of the past history of the matter, and so its chemical composition is sensitive to the initial conditions.BORDEAUX1-Bib.electronique (335229901) / SudocBORDEAUX1-Observatoire (331672201) / SudocSudocFranceF

A Novel Bacterial 6-Phytase Improves Productive Performance, Precaecal Digestibility of Phosphorus, and Bone Mineralization in Laying Hens Fed a Corn-Soybean Meal Diet Low in Calcium and Available Phosphorus

Exogenous phytases are commonly added to low-phosphorus and low-calcium diets to improve P availability and reduce P excretion by poultry. This study investigated the effect of supplementation with a novel bacterial 6-phytase on egg production, egg quality, bone mineralization, and precaecal digestibility of P in laying hens fed corn-soybean meal-based diets. A total of 576 Hy-Line brown laying hens were used in a completely randomized block design at 25–45 weeks of age (woa). The three treatments included a positive control (PC) adequate-nutrient diet with 2840 kcal metabolizable energy/kg, 0.77% digestible lysine, 3.5% Ca, and 0.30% available P (avP); a negative control (NC) diet with 0.16% points less Ca and avP; and an NC diet supplemented with a novel bacterial 6-phytase at 300 phytase units/kg diet. Hen performance and the percentage of damaged eggs were measured every 4 weeks. Body weight, precaecal digestibility of P, and bone parameters at 45 woa were also measured. The reduction in avP and Ca in the NC diet did not compromise performance or egg quality. However, it decreased (P < 0.001) body weight, tibial dry matter, tibial ash and P content, and precaecal digestibility of P. Importantly, all these parameters were significantly improved (P < 0.001) and essentially restored to the levels measured in PC diet-fed hens upon supplementation with phytase. In summary, the present study demonstrates that the new bacterial 6-phytase could effectively counteract the negative effects of P and Ca deficiencies on body weight, bone mineralization, and P availability, thereby supporting high productivity without compromising the welfare of laying hens.info:eu-repo/semantics/publishedVersio

Elemental nitrogen partitioning in dense interstellar clouds

Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N2, with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N2 is difficult to detect spectroscopically through infrared or millimetre-wavelength transitions so its abundance is often inferred indirectly through its reaction product N2H+. Two main formation mechanisms each involving two radical-radical reactions are the source of N2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N + CN reaction down to 56 K. The effect of the measured rate constants for this reaction and those recently determined for two other reactions implicated in N2 formation are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N2 formation are inefficient, we argue that N2 does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.Comment: Accepted for publication in the Proceedings of the National Academy of Sciences of the United States of America - published online since June 11, 201

A Novel Bacterial 6-Phytase Improves Growth Performance, Tibia Mineralization and Precaecal Digestibility of Phosphorus in Broilers: Data from Four Independent Performance Trials

A series of four broiler performance studies were conducted in different facilities to investigate the efficacy of a novel bacterial 6-phytase added at 500 FTU/kg diet on growth performance, bone mineralization and precaecal digestibility of phosphorus (pcdP) in broilers fed diets deficient in available P (avP) and calcium (Ca). The experimental design was the same for all studies, with each having three treatments: positive control (PC) diet formulated to meet or exceed the requirements of birds, negative control (NC) diet similarly reduced by 0.15% points in avP and Ca compared to the PC diet, and the NC diet supplemented with phytase (PHY) at 500 FTU/kg diet from 1 to 35 days of age. Body weight (BW) and feed intake were measured at 21 and 35 days of age, and average daily gain (ADG), average daily feed intake (ADFI), feed conversion ratio (FCR), BW gain-corrected-FCR (cFCR), mortality and European performance efficiency factor (EPEF) were calculated. Tibia dry matter, tibia ash content and pcd of P were measured at 21 days of age in all experiments. The analysis of the data from the four experiments showed that compared with birds fed the adequate-nutrient diet, birds fed the NC diet resulted in a decrease (p < 0.05) in BW, ADG, ADFI and EPEF by 6.4, 6.3, 5.9 and 7.1%, respectively, and an increase in (p = 0.02) cFCR by 2.0%. The tibia dry matter and tibia ash content of these birds were also reduced (p < 0.001) by 3.8 and 4.0% points, respectively. PHY diets improved (p < 0.05) BW, ADG, ADFI, EPEF and cFCR by 8.0, 8.3, 7.3, 10.6 and 2.8%, respectively. Phytase addition at 500 FTU/kg diet also increased (p < 0.001) the tibia dry matter and tibia ash content by 3.5 and 4.2% points, respectively. The pcd of P was improved (p < 0.001) by 11.1 and 11.3% points, in comparison with NC and PC diets, respectively, when phytase was added. These performance parameters and tibia mineralization obtained with a diet supplemented with phytase were comparable to or better than the PC diet. The results demonstrated that avP and Ca could be lowered similarly by 0.15% points in broilers diets by using the new bacterial 6-phytase at 500 FTU/kg diet.info:eu-repo/semantics/publishedVersio