1,721,221 research outputs found
Study of the early phases of star formation
No full textLes coeurs denses sont la première étape de la formation des étoiles, possédants des densités moyennes (entre 1e4 et 1e6 cm-3), de basses températures (< 15 K) et avec une irradiation par les UV faible. Ces environnements abritent une chimie riche, se produisant en grande partie à la surface des grains de poussière. Pour comprendre les processus physico-chimiques en jeu, il est essentiel de retracer l'origine des molécules dont la formation se produit en majorité à la surface des grains. Cette chimie est encore mal contrainte mais grâce aux nouveaux instruments, dont le JWST, les caractéristiques des glaces et la formation de ces molécules à la surface vont pouvoir être étudiées à grande échelle et avec une résolution sans précédent. Nous présentons ici l'étude de deux coeurs denses, L429-C et L694, tous deux observés avec le radiotélescope 30m de l'IRAM. Les espèces visées en phase gazeuse sont le CO (et ses isotopologues) et CH3OH, espèces clefs précurseurs des molécules organiques complexes. Une méthode unique est appliquée sur chaque coeur afin d'obtenir les abondances moléculaires de la phase gazeuse. Cette méthode se base sur les paramètres physiques du nuage et permet d'obtenir une densité de colonne grâce à un code de transfert radiatif dans un milieu qui n'est pas à l'équilibre thermodynamique local. Ces observations nous ont permis d'étudier les distributions d'abondance moléculaire en fonction de l'extinction visuelle et la densité.Les deux coeurs présentent des observations de glace (précédent le JWST) et montrent d'intéressantes tendances dans leur rapport gaz-sur-glace. L694 étant plus avancé dynamiquement, nous pouvons ainsi tracer l'évolution de la composition de la glace et du gaz à travers le temps. Couplés à ces observations, nous utilisons des modèles chimiques afin d'ajouter des contraintes sur les mécanismes de désorption non-thermique, responsables de la présence d’espèces formées sur les glaces dans la phase gazeuse. En parallèle de cette étude, nous présentons un outil de création de spectres synthétiques de glace pour les observations du JWST, afin d’aider à l’écriture des demandes de temps d’observation mais également d’aider à l’analyse des observations attendues.Cold cores are the first step of star formation, characterized by medium densities (between 1e4 and 1e6 cm-3), low temperatures (< 15 K) and low UV radiations. In these environments, a rich chemistry takes place on the surface of the dust grains. Understanding the physico-chemical processes at play in these cores is essential to trace the origin of molecules predominantly formed via reactions on dust grain surfaces. With new instruments, like JWST, and unpredeced resolution and power, the ice phase will be uncovered and studied in depth. We present a study of two cold cores, L429-C and L694, both observed with the IRAM 30m single dish radiotelescope. Observed species count CO (and its isotopologues) and CH3OH, key species precursors of more complex organic molecules. The same method is applied to each core in order to obtain the gas-phase abundance by deriving the column densities of detected species from physical parameters, with a non-local thermal equilibrium radiative transfer model. These observations allowed us to probe the molecular abundances as a function of density and visual extinction.Both the cores present ice observations (prior to JWST) and show interesting trends in their gas-to-ice ratio. L694 being at more advanced state (infalling), we try to understand the evolution of both ice and gas abundances in the cold core evolution, as well as the ice composition evolving through time. Coupled to these observations are chemical models trying to uncover the non-thermal mechanisms responsible for the desorption of ice in the gas-phase. In parallel to these studies, we present a synthetic ice spectra generator tool to match JWST observations, in order to support proposals and help analyzing real observations
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Chemical and physical characterization of the first stages of protoplanetary disk formation
No full textLes 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
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Nahoon: Time-dependent gas-phase chemical model
No full text2014ascl.soft09009W- Astrophysics Source Code Library, record ascl:1409.009Nahoon is a gas-phase chemical model that computes the chemical evolution in a 1D temperature and density structure. It uses chemical networks downloaded from the KInetic Database for Astrochemistry (KIDA) but the model can be adapted to any network. The program is written in Fortran 90 and uses the DLSODES (double precision) solver from the ODEPACK package to solve the coupled stiff differential equations. The solver computes the chemical evolution of gas-phase species at a fixed temperature and density and can be used in one dimension (1D) if a grid of temperature, density, and visual extinction is provided. Grains, both neutral and negatively charged, and electrons are considered as chemical species and their concentrations are computed at the same time as those of the other species. Nahoon contains a test to check the temperature range of the validity of the rate coefficients and avoid extrapolations outside this range. A test is also included to check for duplication of chemical reactions, defined over complementary ranges of temperature
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