1,720,965 research outputs found
Excitation collisionnelle dans l'espace : du milieu interstellaire aux atmosphères planétaires
Full text linkOur perception and understanding of many astrochemical environments, such as the interstellar medium (ISM), have been transformed over the last approximately 50 years. In the last decades, the synergism between astronomical observations and laboratory studies (increasingly based on the interplay of experiment and theory) has led to the discovery of an ever growing number of molecules, with the observed spectra becoming more and more detailed. However, in space, physical conditions pose severe constraints to the chemical processes it hosts, which greatly differ from those occurring in terrestrial environments. For instance, in the ISM, the molecular energy level populations are rarely at the local thermodynamic equilibrium (LTE) since the density is usually so low (~10^2-10^6 cm^-3) that collisions compete with radiative processes. Under such conditions, deriving molecular abundances from spectral lines requires the knowledge of the collisional rate coefficients of the molecule under consideration for the most abundant perturbing species. The nature of the latter depends on the investigated astronomical environment. For instance, in the interstellar clouds the most important perturber is molecular hydrogen (H2), with He also considered as approximation of H2; otherwise, if we aim to constrain molecular abundances in cometary comas, collisional excitation is generally dominated by H2O (and/or electrons) and, for comets at large heliocentric distances, by CO; in the planetary atmosphere, moreover, the molecular composition could further change: in the thermosphere of Titan, for example, N2 and CH4 are the most abundant collisional partners.Given the extreme conditions of those environments, which are difficult to be reproduced in the laboratory, the knowledge of the collisional rate coefficients heavily relies on theoretical calculations. However, broadening measurements are frequently employed to provide experimental validation of the computational procedure. During my PhD, I have mainly focused my research on the theoretical and experimental determination of the relevant collisional parameters for molecules of astrochemical interest. The aim of this research is to gain more information regarding their population distribution through rotational levels in order to accurately determine their abundance and excitation temperature via radiative transfer models.Notre perception et notre compréhension de nombreux environnements astro-chimiques, tels que le milieu interstellaire (ISM), se sont profondément transformées au cours des quelque cinquante dernières années. Au cours des dernières décennies, le synergisme entre les observations astronomiques et les études de laboratoire (de plus en plus fondées sur l’interaction entre l’expérience et la théorie) a conduit à la découverte d’un nombre toujours croissant de molécules, tandis que les spectres observés devenaient de plus en plus détaillés.Cependant, dans l’espace, les conditions physiques imposent de fortes contraintes aux processus chimiques qui s’y déroulent, lesquels diffèrent largement de ceux qui ont lieu dans les environnements terrestres. Par exemple, dans le ISM, les populations des niveaux d’énergie moléculaires sont rarement à l’équilibre thermodynamique local (LTE), car la densité est généralement si faible (~10^2–10^6 cm^-3) que les collisions sont en compétition avec les processus radiatifs. Dans de telles conditions, la détermination des abondances moléculaires à partir des raies spectrales nécessite la connaissance des coefficients de taux collisionnels de la molécule considérée avec les espèces perturbatrices les plus abondantes. La nature de ces dernières dépend de l’environnement astronomique étudié.Ainsi, dans les nuages interstellaires, le principal pertur-bateur est l’hydrogène moléculaire (H2), l’hélium (He) étant également utilisé comme approximation de H2 ; en revanche, si l’on cherche à contraindre les abondances moléculaires dans les comas cométaires, l’excitation collisionnelle est généralement dominée par H2O (et/ou les électrons) et, pour les comètes à grande distance héliocentrique, par le CO. Dans les atmosphères planétaires, la composition moléculaire peut encore varier : dans la thermosphère de Titan, par exemple, N2 et CH4 sont les partenaires collisionnels les plus abondants.Compte tenu des conditions extrêmes de ces environnements, difficiles à reproduire en laboratoire, la connaissance des coefficients de taux collisionnels repose largement sur des calculs théoriques. Néanmoins, des mesures d’élargissement de raies sont fréquemment utilisées pour fournir une validation expérimentale des procédures de calcul. Au cours de ma thèse de doctorat, j’ai principalement orienté mes recherches vers la détermination théorique et expérimentale des paramètres collisionnels pertinents pour des molécules d’intérêt astro-chimique. L’objectif de ces travaux est d’obtenir davantage d’informations sur la distribution de population des niveaux rotationnels afin de déterminer avec précision leur abondance et leur température d’excitation au moyen de modèles de transfert radiatif
Hyperfine resolved rate coefficients of HC17O+ with H2 (j = 0)
Full text linkThe formyl cation (HCO+) is one of the most abundant ions in molecular clouds and plays a major role in the interstellar chemistry. For this reason, accurate collisional rate coefficients for the rotational excitation of HCO+ and its isotopes due to the most abundant perturbing species in interstellar environments are crucial for non-local thermal equilibrium models and deserve special attention. In this work, we determined the first hyperfine resolved rate coefficients of HC17O+ in collision with H2 (j = 0). Indeed, despite no scattering calculations on its collisional parameters have been performed so far, the HC17O+ isotope assumes a prominent role for astrophysical modelling applications. Computations are based on a new four dimensional (4D) potential energy surface obtained at the CCSD(T)-F12a/aug-cc-pVQZ level of theory. A test on the corresponding cross-section values pointed out that, to a good approximation, the influence of the coupling between rotational levels of H2 can be ignored. For this reason, the H2 collider has been treated as a spherical body and an average of the potential based on five orientations of H2 has been employed for scattering calculations. State-to-state rate coefficients resolved for the HC17O+ hyperfine structure for temperature ranging from 5 to 100K have been computed using recoupling techniques. This study provides the first determination of HC17O+–H2 inelastic rate coefficients directly computed from full quantum close-coupling equations, thus supporting the reliability of future radiative transfer modellings of HC17O+ in interstellar environments
The Quest for a Plausible Formation Route of Formyl Cyanide in the Interstellar Medium: A State-of-the-art Quantum-chemical and Kinetic Approach
Full text linkInterstellar complex organic molecules are assumed to be mainly formed on dust-grain surfaces. However, neutral gas-phase reactions in the interstellar medium can play an important role. In this paper, by investigating the reaction between aldehydes and the cyano radical, we show that both formaldehyde (CH2O) and acetaldehyde (CH3CHO) can lead to the formation of formyl cyanide (HCOCN). Owing to accurate quantum-chemical computations followed by rate constant evaluations, we have been able to suggest and validate an effective mechanism for the formation of HCOCN, one of the molecules observed in the ISM. Quite interestingly, the mechanism starting from CH2O is very effective at a low temperature, while that involving CH3CHO becomes more efficient at temperatures above 200 K
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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