The study of Supernova Remnants (SNRs) is fundamental to understanding the chemical enrichment and magnetism in galaxies, including our own Milky Way. In an effort to understand the connection between the morphology of SNRs and the Galactic Magnetic Field (GMF), we have examined the radio images of all known SNRs in our Galaxy and compiled a large sample that have an axisymmetric morphology, which we define to mean SNRs with a bilateral or barrel-shaped morphology, in addition to one-sided shells. We selected the cleanest examples and model each of these at their appropriate Galactic position using two GMF models, one of which includes a vertical halo component, and another that is oriented entirely parallel to the plane. Since the magnitude and relative orientation of the magnetic field changes with distance from the Sun, we analyze a range of distances, from 0.5 to 10 kpc in each case. Using a physically motivated model of an SNR expanding into an ambient GMF that includes a vertical halo component, we find it is possible to reproduce observed morphologies of many SNRs in our sample. These results strongly support the presence of an off-plane, vertical component to the GMF, and the importance of the Galactic field on SNR morphology. Our approach also provides a potentially new method for determining distances to SNRs, or conversely, distances to features in the large-scale GMF if SNR distances are known. The mechanism for acceleration of cosmic rays in SNRs is another outstanding question in the field. To investigate this, the same sample of axisymmetric SNRs was again modelled, but this time using two competing, and physically motivated, Cosmic Ray Electron (CRE) acceleration cases: quasi-perpendicular and quasi-parallel. We find that the quasi-perpendicular CRE acceleration case is much more consistent with the data than the quasi-parallel CRE acceleration case, with G327.6+14.6 (SN1006) being a notable exception. We propose that SN1006 may be an example of a case where both quasi-parallel and quasi-perpendicular acceleration is simultaneously at play in a single SNR. / October 2016

When molecules are excited by photons or energetic particles, they will cool through the emission of photons, electrons, or by fragmenting. Such processes are often thermal as they occur after the excitation energy has been redistributed across all degrees-of-freedom in the system. Collisions with atoms or ions may also lead to ultrafast fragmentation in Rutherford-like scattering processes, where one or several atoms can literally be knocked out of the molecule by the incoming projectile before the energy can be completely redistributed. The resulting fragmentation pathways can in such knockout processes be very different from those in thermal processes. This thesis covers extensive studies of collisions between ions/atoms and isolated Polycyclic Aromatic Hydrocarbon (PAH) molecules, isolated fullerene molecules, or clusters of these. The high stabilities and distinct fragmentation channels make these types of molecules excellent test cases for characterizing knockout-driven fragmentation and the reactions that these processes can lead to. I will present experimental measurements for a wide range of energies and compare them with my own molecular dynamics simulations and quantum chemical calculations. In this thesis, I present an in-depth study of the role of knockout in the energetic processing of molecules and clusters. The competition between knockout and thermally driven fragmentation is discussed in detail. Knockout-driven fragmentation is shown to result in exotic fragments that are far more reactive than the intact parent molecules or fragments from thermal processes. When such reactive species are formed within molecular clusters efficient molecular growth can take place on sub-picosecond timescales. The cluster environments are crucial here because they protect the newly formed molecules by absorbing excess energy. This is a possible pathway for the growth of large PAHs, fullerenes, and similar carbonaceous complexes found in, for instance, the interstellar medium. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 1: Submitted.</p>

Os compostos de fósforo apresentam várias implicações em processos atmosféricos e interestelares. Nesta tese, será realizado um estudo ab initio em três diferentes sistemas. (i) Um destes sistemas é o radical PCO. No capítulo três, foi analisado os aspectos conformacionais do radical PCO, sendo avaliado as propriedades moleculares e termodinâmicas da espécie cíclica e das espécies lineares. Entre os isômeros do radical PCO, a espécie i3 (C&infin;V, 2&pi;) é a mais estável delas, com uma entalpia de formação a 0K de 22.33&plusmn;2.10 kcal mol-1. Considerando as barreiras de energia, que convergem em direção a espécie i3, elas apresentam valores próximos de 10 kcal mol-1, ao passo que a energia necessária para a isomerização da molécula i3 para outros isômeros é acima de 60 kcal mol-1. Estes valores de energia explicam o fato que, em condições experimentais, foi detectado apenas a espécie i3 (C&infin;V, 2&pi;). (ii) No capítulo quatro, foi elucidado o mecanismo da reação PH3 +F, assim como foi também caracterizado as propriedades eletrônicas dos intermediários observados nesta reação. Na reação PH3 +F, pode ser observado que, diferente da literatura, dois caminhos são possíveis para esta reação: (a) a reação de abstração de hidrogênio, com a formação dos produtos PH2 e HF, e a reação de adição-eliminação, com a formação das espécies PH2F e H. Analisando a energia eletrônica, é possível observar que a reação de adição-eliminação apresenta uma barreira de energia de 23 kcal mol-1 abaixo da mesma observada para a reação de abstração de hidrogênio. Considerando os intermediários formados na reação PH3 + F, são observados dois tipos de simetria: uma C1 e duas Cs. Além disso, a entalpia de formação em 298K dos intermediários variou de -22.99 to -29.74 kcal mol-1. (iii) No capítulo cinco, foi avaliada a interação da fosfina com diferentes aglomerados de água. Neste capítulo, foi realizada uma análise nos aglomerados PH3-(H2O)n (n=1-8,12,20) e avaliado a estabilidade dos potenciais isômeros, assim como as propriedades moleculares e espectroscópicas dos diferentes isômeros. Nas estruturas PH3-(H2O)n (n=1-8) foi possível observar que os modos vibracionais &delta;(PH3) e &delta;sym(PH3) apresentaram um deslocamento para o vermelho, ao passo que as freqüências de estiramento simétrico e assimétrico da fosfina apresentaram um deslocamento para o azul. No caso da estrutura PH3(H2O)20, as variações nos modos vibracionais foram similares embora que mais intensas, sendo o mesmo também observado para as respectivas intensidades. Além disso, a energia necessária para o confinamento da fosfina no interior de um aglomerado de água dodecaedro hexagonal variou de -1.81 to -6.38 kcal mol-1. / The phosphorus compounds show several implications to atmospheric and interstellar processes. In this thesis, was performed an ab initio study on three different systems. (i) One of these systems is the PCO radical. In chapter three, was studied the conformation aspects of PCO radicals, which was assessed the molecular and thermodynamic properties of the cyclic and linear species. In PCO species, it was observed that the linear specie i3 (C&infin;V, 2&pi;) is the most stable one, with the enthalpy formation at 0K of 22.33&plusmn;2.10 kcal mol-1. Considering the barrier heights, which converge into the direction of i3, they are close to 10 kcal mol-1, while the energy necessary to the isomerization of i3 to other conformations are high than 60 kcal mol-1. These energy values explain the fact that, in experimental conditions, was detected only the i3 (C&infin;V, 2&pi;) specie. (ii) In chapter four, it was elucidated the mechanism of the PH3 + F reaction, and it was also described the electronic properties of the intermediates observed in this reaction. In the PH3 + F reaction can be seen that, different from the literature, two possible pathways can be observed: (a) the hydrogen abstraction, with the formation of PH2 and HF products, (b) and the addition-elimination route, with the formation of PH2F and H species. Analyzing the electronic energy, can be observed that the barrier energy of the addition-elimination process is 23 kcal mol-1 bellow the hydrogen abstraction route. Considering the intermediates, two different point groups were detected: a C1 and two Cs. In addition, the enthalpy formation at 298K of the intermediates, in the PH3 + F reaction, range from -22.99 to -29.74 kcal mol-1. (iii) In chapter five, it was studied the interaction of phosphine with different water clusters. In this chapter was performed an analysis of the PH3-(H2O)n (n=1-8,12,20) clusters, and evaluated the stability of the potential isomers as also the molecular and spectroscopy aspects of the different species. In the PH3(H2O)n (n=1-8) structures are seen a redshift in the &delta;(PH3) and &delta;sym(PH3) vibrational modes, while is observed a blueshift from the asymmetric and symmetric stretching frequencies of phosphine. In the case of PH3(H2O)20 structure, the variations in the vibrational modes were similar however more intense, as also in their respective intensities. In addition, the necessary energy to trap phosphine in the interior of an hexagonal dodecahedron water clusters range from -1.81 to -6.38 kcal mol-1.

Neste trabalho primeiramente estudamos os efeitos na propagação de ondas de Alfvén em plasmas empoeirados encontrados em inúmeros ambientes astrofísicos. A relação de dispersão da onda é modificada apresentando novos mecanismos de amortecimento. Há na literatura dados observacionais que indicam a existência de condições para crescimento de partículas de poeira na base da atmosfera de estrelas gigantes e supergigantes frias. Construímos um modelo de perda de massa para essas estrelas no qual mostramos que um fluxo de ondas de Alfvén, amortecido pela presença de poeira, pode contribuir para a geração de um vento de baixa velocidade e alta taxa de perda de massa, de acordo com as observações. Já no caso de estrelas quentes, mostramos como é possível obter as condições ideais para formação e crescimento destas partículas em um modelo de colisão de ventos em sistemas binários. A partir deste modelo, é possível explicar as altas emissões em raios-X observadas, além do crescimento de grãos no pós-choque. Aplicando a idéia ao sistema binário de eta Carinae, o modelo permite a determinação dos parâmetros orbitais do sistema. A poeira contida nos ventos estelares é então ejetada para o meio interestelar. Através de um cálculo semi-empírico determinamos a importância de cada intervalo de massa estelar, em cada etapa evolutiva, no retorno de material sólido ao MI. Em regiões de formação estelar investigamos como as ondas de Alfvén, amortecidas pela presença de poeira, influenciam a estabilidade de nuvens moleculares. Em oposição às teorias encontradas na literatura, mostramos que uma nuvem molecular anã, suportada apenas por pressão magnética, não pode ser dinamicamente estável. / In this work, we firstly discuss the propagation of Alfvén waves in dusty plasmas found in several astrophysical environments. The wave dispersion relation is modified giving rise to new damping mechanisms. There are in the literature observational data indicating the presence of dust near the surface of cool giant and supergiant stars. We developed a stellar mass loss model where we show that a flux of Alfvén waves, damped by the dust presence,can generate a low velocity and high mass loss rate wind, in agreement with the observations. In the case of hot stars we show how it is possible to obtain the special conditions for dust growth in a wind collision model of massive binary systems. For this model it is possible to explain both, the high X-rays emissions and dust growth at the post-shock phase. Applying the idea to the $eta$ Carinae binary system, the model allows the determination of the system orbital and the stellar wind parameters. The stellar wind´s dust is then ejected to the interstellar medium. In a semi-empirical calculation we showed the importance of each stellar mass, at each evolutionary phase, on the dust feedback of the ISM. For star formation regions, we investigate the role of Alfvén waves, damped by the dust presence, on the stability of the molecular clouds. In spite of the works found in the literature, we showed that a dwarf molecular cloud, magnetically supported only, is unstable.

La dernière génération de récepteurs radio, dotés à la fois d'une grande bande passante et d'une haute résolution, fait de toute observation radio-astronomique une étude spectroscopique. Dans le cas de l'imagerie à grand champ du milieu interstellaire, une telle abondance de données fournit de nouveaux outils de diagnostic, mais pose aussi de nouveaux défis en termes de traitement et d'analyse des données. L'objectif du projet ORION-B est d'observer 5 degrés carrés du nuage moléculaire OB, soit près de la moitité de la surface du nuage, dans toute la bande à 3mm. L'émission de dizaines de traceurs moléculaires à été cartographiée, ce qui inclut CO et ses isotopologues, HCO, HCN, HNC, N$_2$H$^+$, le méthanol, SO, CN...L'accès à des cartes résolues spatialement pour de nombreuse espèces chimiques nous permet d'identifier les meilleurs traceurs de la densité du gaz et de son illumination. Ces cartes ont aussi été soumises à des méthodes d'apprentissage automatique, afin de segmenter le nuage moléculaire en régions caractérisées par une émission moléculaire similaire, et de quantifier les corrélations les plus importantes entre différents traceurs moléculaires, et entre les traceurs et des quantités physiques telles que la densité ou la température des poussières.La grande surface observée, combinée à une haute résolution spatiale et spectrale, permet aussi de caractériser statistiquement la cinématique et la dynamique du gaz. La fraction de quantité de mouvement dans les modes compressifs et solénoïdaux (rotationels) de la turbulence peut être calculée, ce qui montre que le nuage est dominé par des mouvements solenoidaux, tandis que les mouvements compressifs sont concentrés dans deux régions de formation stellaire. Ce résultat est cohérent avec l a très faible efficacité de formation stellaire de ce nuage, et souligne l'importance du forçage compressif pour la formation des étoiles.Les nombreux filaments identifiés dans ce nuage moléculaire ont par ailleurs des densités relativement faibles, et sont très stables vis à vis de l'effondrement gravitationnel. La plupart des filaments sont dépourvus d'étoiles jeunes, mais ils montrent des signes de fragmentation radiale et longitudinale, ce qui indique que de la formation stellaire pourrait à l'avenir y avoir lieu. / The new generation of wide-bandwidth high-resolution receivers turns almost any radio observation into a spectral survey. In the case of wide-field imaging of the interstellar medium, such a wealth of data provides new diagnostic tools, but also poses new challenges in terms of data processing and analysis. The ORION-B project aims at observing 5 square degrees of the OB molecular cloud, or about half of the cloud's surface, over the entire 3mm band. The emission of tens of molecular tracers has been mapped, including CO isotopologues, HCO, HCN, HNC, N$_2$H$^+$, methanol, SO, CN...Having access to spatially resolved maps from many molecular species enables us to identify the best tracers of the gas density and illumination. Machine learning techniques have also been applied to these maps, in order to segment the molecular cloud into typical regions based on their molecular emission, and to quantify the most meaningful correlations of different molecular tracers with each other and with physical quantities such as density or dust temperature.The wide-field coverage, together with the spatial and spectral resolution, also allows to characterize statistically the kinematics and dynamics of the gas. The amount of momentum in the compressive and solenoidal (rotational) modes of turbulence are retrieved, showing that the cloud is dominated by solenoidal motions, with the compressive modes being concentrated in two star-forming regions. This result is in line with the overall very low star formation efficiency of the cloud, and highlights the role of compressive forcing in the star formation process.The numerous filaments identified in the molecular cloud also prove to have rather low densities, and are very stable against gravitational collapse. Most filaments are starless, but they show signs of longitudinal and radial fragmentation, which indicates that star formation might occur later on.

Within the currently well-established ΛCDM cosmological framework we still lack a satisfactory un- derstanding of the processes that trigger, regulate and eventually quench star formation on galactic scales. Gas flows (including inflows from the cosmic web and supernovae-driven outflows) are con- sidered to act as self-regulatory mechanisms, generating the scaling relations between stellar mass, star formation rate and metallicity observed in the local Universe by large spectroscopic surveys. These surveys, however, have so far been limited by the availability of only one spectrum per galaxy. The aim of this dissertation is to expand the study of star formation and chemical abundances to resolved scales within galaxies by using integral field spectroscopy (IFS) data, mostly from the ongoing SDSS- IV MaNGA survey. In the first part of this thesis I demonstrate the ubiquitous presence of extended low ionisation emission-line regions (LIERs) in both late- and early-type galaxies. By studying the Hα equivalent width and diagnostic line ratios radial profiles, together with tracers of the underlying stellar popula- tion, I show that LIERs are not due to a central point source but to hot evolved (post-asymptotic giant branch) stars. In light of this, I suggest a new classification scheme for galaxies based on their line emission. By analysing the colours, star formation rates, morphologies, gas and stellar kinematics and environmental properties of galaxies with substantial LIER emission, I identify two distinct popula- tions. Galaxies where the central regions are LIER-like, but show star formation at larger radii are late types in which star formation is slowly quenched inside-out. This transformation is associated with massive bulges. Galaxies dominated by LIER emission at all radii, on the other hand, are red-sequence galaxies harbouring a residual cold gas component, acquired mostly via external accretion. Quiescent galaxies devoid of line emission reside in denser environments, which suggests environmental effects as a likely cause for the existence of line-less galaxies on the red sequence. In the second part of this dissertation I focus on the study of resolved chemical abundances by characterising the gas phase oxygen and nitrogen abundance gradients in a large sample of star forming galaxies. I analyse the deviations from an exponential profile at small and large radii and the dependence of the gradients on stellar mass. These findings are interpreted in the context of the inside-out paradigm of disc growth. I then demonstrate the necessity of gas flows, which are responsible for the observed flattening of the metallicity and N/O ratio gradients at large radii. Finally, I present a case study based on one nearby galaxy (NGC 628), in which I combine IFS and cold gas data to derive a spatially resolved metal budget and estimate the mass of metals lost by the galaxy throughout its life- time. By using simple physically-motivated models of chemical evolution I infer the average outflow loading factor to be of order unity.

La dynamique du milieu interstellaire (MIS) est au cœur de notre compréhension de la formation des étoiles. La structuration du MIS est régiée par l’interaction entre gravité, mouvements turbulents et champs magnétiques. Si les deux premiers acteurs sont bien caractérisés par les observations, le r ˆ ole du champ magnétique Galactique (CMG), ainsi que son interaction avec la matière interstellaire, sont encore peu connus. La cartographie complète du ciel dans le domaine submillimétrique réalisée par le satellite Planck nous permet d’étudier la structure du CMG, projetée sur le plan du ciel, à partir de la polarisationde l’émission de la poussière interstellaire. Dans cette thèse, nous présentons une analyse des cartes en polarisation à 353 GHz, observées par l’instrument à hautes fréquences HFI de Planck. Nous étudions la structure du CMG et son interaction avec la turbulence interstellaire, de l’ensemble du voisinage Solaire aux échelles de la structure en filaments de la matière, dans le milieu diffus et les nuages moléculaires. La première étude statistique de la corrélation entre la géométrie du CMG et la structure de matière interstellaire est réaliséeen utilisant une méthode originale pour identifier les structures et mesurer leur orientation sur l’ensemble du ciel, puis quantifier les effets de projections. Les résultats sont interprétés en référence aux hypothèses théoriques et aux résultats obtenus à partir de simulationsnumériques magnétohydrodynamiques. Nous concluons que la turbulence tend à former des structures parallèles au champ magnétique dans le milieu diffus, et la gravité des structures perpendiculaires au champ dans les nuages moléculaires. Enfin, nous présentons et discutons deux perspectives liées à notre travail: la modélisation du ciel en polarisation aux hautes latitudes Galactiques comme avant-plan du fond cosmologique, et la relation entre cinématique du gaz et champ magnétique dans les structures en filaments du MIS. / The study of the dynamics of the interstellar medium (ISM) is essential to understand star formation. The ISM is a turbulent and magnetized medium governed by the interplay of self-gravity, turbulent motions and magnetic fields. While the former two are well characterized byexisting models and observations, today, the role of the Galactic magnetic field (GMF), and its interaction with interstellar matter, represent the most difficult aspects to probe. Thanks to its high sensitivity and resolution, full-sky coverage, and imaging at sub-mm wavelengths,the Planck satellite represents an enormous step forward in the characterization of the statistical properties of the GMF, as traced by dust polarization. In this thesis we present an analysis of the 353 GHz Stokes parameters maps (I, Q, U) observed with the High Frequency Instrument (HFI) of Planck. we aim at studying the structure of the GMF, and its interplay with interstellar turbulence, from the largest scales in the Solar neighborhood to those of the filamentary structures in the diffuse ISM and molecular clouds. We also develop a dedicated methodology to produce the first statistical study on the correlation betweenthe geometry of the GMF, projected on the plane of the sky, and the structure of interstellar matter. We interpret the results of the data analysis in light of theoretical models and Magneto-Hydrodynamic (MHD) numerical simulations, and conclude that my findings supporta scenario of formation of structures in the ISM where turbulence organizes matter parallel to the magnetic field in the diffuse medium, and the gas self-gravity produces perpendicular structures in the densest and magnetically dominated regions. We propose and discuss several perspectives of research related to our results, from the modeling of the high-latitude sky in polarization, for the investigation of dust properties and cosmological foregrounds, to the comparison of magnetic fields with the kinematics of the filamentary structures in the SM.

Ce travail est une étude expérimentale de la spectroscopie électronique des cations hydrocarbures aromatiques polycycliques (PAH) et de leurs agrégats dans des conditions simulant celles régnant dans le milieu interstellaire, froids et totalement isolés. L’objectif principal est d’obtenir des spectres au laboratoire qui puissent être comparés aux spetres des Bandes Diffuses Interstellaires et d’explorer les propriétés des agrégats de PAH. / This work is mainly an experimental study of the electronic spectroscopy of the polycyclic aromatic hydrocarbon cations and their clusters in conditions close to those of the interstellar medium. The aim of this study is to obtain data that can be compared with the spectrum of the diffuse interstellar bands and to explore the properties of PAH clusters.

Les glaces du milieu interstellaire sont invoquées comme l'une des origines de la formation de molécules organiques complexes dans l'espace. En effet, elles constituent un support catalytique pour des réactions chimiques et pourraient ainsi expliquer la formation de molécules d'intérêt prébiotique. Toutefois, en raison de la faible température des milieux considérés, la vitesse de la réaction est contrainte par le déplacement des différentes réactifs l'un vers l'autre. L'objectif de cette thèse est donc de traiter la réactivité et la diffusion de molécules simples dans les glaces interstellaires. Je présente dans cette thèse les résultats de l'étude de la réaction entre le dioxyde de carbone et l'ammoniac dans les glaces interstellaires. Cette étude a été effectuée au Laboratoire Univers et Particules de Montpellier ; elle combine différentes méthodes de la chimie théorique et confronte les résultats avec ceux issus d'expériences que j'ai réalisées au Laboratoire de Physique des interactions Ioniques et Moléculaires de l'Université d'Aix-Marseille. Dans une première partie, des simulations de dynamique moléculaire classiques sont employées pour simuler un modèle de glace amorphe basse densité. Ce modèle est utilisé pour simuler la trajectoire de petites molécules (NH3, CO, CO2, H2CO) dans cette glace et en déduire des coefficients de diffusion à différentes températures. Ces résultats sont comparés à des résultats expérimentaux de diffusion du dioxyde de carbone ce qui valide la méthode théorique utilisée et permet de suggérer un mécanisme pour ce processus de diffusion. Dans une deuxième partie, la réaction entre le dioxyde de carbone et l'ammoniac est traitée dans le cadre de la théorie de la fonctionnelle densité par une approche « super-molécule ». Dans cette approche, le profil d'énergie et le mécanisme de la réaction dans des complexes moléculaires xNH3:CO2:yH2O sont étudiés. Deux produits de la réaction sont localisés : le carbamate d'ammonium et l'acide carbamique. La barrière d'énergie de la réaction obtenue est similaire à celle obtenue expérimentalement, et le carbamate d'ammonium est confirmé comme produit majoritaire de la réaction. Le profil d'énergie obtenu par cette approche « super-molécule » est ensuite étudié par dynamique moléculaire ab initio contrainte et le profil d'énergie libre est calculé par la méthode d'Intégration Thermodynamique. Cette approche confirme la forme générale du profil d'énergie et met en évidence un fort effet entropique du réseau d'eau. Je dresse finalement des conclusions sur les plans méthodologiques et astrochimiques permettant d'inscrire ma thèse dans des perspectives futures, notamment en incluant les barrières d'énergie de diffusion et de réaction, dans des modèles astrochimiques prenant en compte directement les réactions chimiques dans les glaces interstellaires. / It is postulated nowadays that complex organic molecules in space form on the surface and in the volume of interstellar ices. These ices can catalyse chemical reactions what could explain the formation of prebiotic molecules. However, because of the low temperatures, the diffusion of the reactants one towards another is slow, limiting their reactivity. The objectif of this thesis is to treat the reactivity and the diffusion of simple molecules in interstellar ices. I present in this thesis the results of the study of the chemical reaction between carbon dioxide and ammonia in interstellar ices. This study was conducted in the Laboratoire Univers et Particules de Montpellier : it combines various theoretical chemistry methods and confront the results to experimental ones I participated in at the Laboratoire de Physique des Interactions Ioniques etMoléculaires of the Aix-Marseille university. In a first part, classical molecular dynamic simulations are used to simulate a low-density amorphous ice model and to calculate the diffusion coefficients at various temperatures of a series of small molecules (NH3, CO, CO2, H2CO). These results are compared to the experimental diffusion coeficients of the carbon dioxide validating the theoretical approach used and allowing to porpose a mechanism for the diffusion process. In a second part, the reaction between carbon dioxide and ammonia is studied in the frame of Density-Functional Theory using a « super-molecular » approach. In this approach, the reaction energy profile for the molecular complexes xNH3:CO2:yH2O, is studied. Two reaction products are localised : the ammonium carbamate and the carbamic acid. The reaction energy barrier obtained by these calculations is similar to the one obtained experimentally, and the ammonium carbamate is confirmed as the major reaction product. The obtained energy profile is therfore investigated by constrained ab initio molecular dynamics and the free energy profile is computed with the Thermodynamics Integration method. These calculations confirme the general form of the previous energy profile and enlight the strong entropic effect of the water network. Finally, conclusions are drawn, and perspectives on methodological as well as on astrochemical aspects, as the inclusion of the reaction and diffusion energy barriers I calculated in astrochemical models, are given.

Orientador: Carmen Maria Andreazza / Banca: Edson Denis Leonel / Banca: José Williams dos Santos Vilas Boas / Resumo: De acordo com os resultados observacionais de condensações de nuvens moleculares escuras, grandes variações na razão 13CO/C18O são observadas quando se comparam os resultados obtidos nas condensações situadas dentro da mesma nuvem, bem como de nuvem para nuvem. O valor médio dessa razão na condensação principal de Ophiuchus é inferior a 5. Por outro lado, o valor encontrado nas condensações que estão situadas ao norte de Oph é maior que 10. Grandes diferenças também são encontradas quando se comparam os resultados observacionais de diferentes nuvens escuras, tais como Ophiuchus e Taurus, onde são observados também um decréscimo da razão C18O/C17O com o aumento da densidade. Os processos químicos e físicos que governam essas variações ainda não estão claros. Nesse sentido, o objetivo da presente proposta é analisar a influência do colapso gravitacional de condensações de nuvens moleculares escuras na síntese das moléculas CO, C17O, C18O, 13CO, 13C17O e 13C18O. Tal análise é feita com base em comparações entre modelos que consideram diferentes condições entre si, tais como, tamanho da cadeia química, velocidade de colapso, densidade inicial e processos de congelamento de espécies químicas na superfície de grãos de poeira. Os resultados obtidos mostram que o tamanho da cadeia química tem influência nas razões 13CO/C18O e C18O/C17O, mas não tanto quanto a densidade inicial e a velocidade do colapso. Além disso, o congelamento das espécies químicas nos grãos é mais significativo nos estágios mais avançados da evolução da condensação. Os modelos de condensações escuras que sofrem colapso gravitacional lento e em queda livre reproduzem satisfatoriamente as razões 13CO/C18O e C18O/C17O observadas, o que permite concluir que o colapso gravitacional pode ter um importante efeito nas referidas razões. / Abstract: According to the observational results of dark molecular clouds condensations, large variations in the ratio 13CO/C18O are observed when comparing the results obtained in the condensations located within the same cloud and cloud to cloud. The average value of this ratio in the main condensation of Ophiuchus is below 5. On the other hand, the value found in the condensations that are located north of Oph is larger than 10. Large differences are also found when comparing the observational results of different dark clouds such as Ophiuchus and Taurus, in which are also found a decrease of the C18O/C17O ratio with increasing density. The chemical and physical processes that govern these variations are still unclear. In this sense, the objective of this proposal is to analyze the influence of the gravitational collapse of centrally condensed clumps of dense molecular gas in the synthesis of the CO, C17O, C18O, 13CO, 13C17O and 13C18O molecules. This analysis is based on comparisons among models that consider different condition, such as, chemical chain, initial density, speed of collapse and freezing processes of the chemical species on the surface of dust grains. The results show that the size of the chemical chain has influence on the 13CO/C18O and C18O/C17O ratios, but they are not as important as the initial density and the speed of the collapse. Furthermore, the freezing of chemical species on the grains occurs at later times of the collapse. The models of a gravitational free-fall collapsing core and of slowly contracting core with higher initial density are consistent with observations. These results indicate that the gravitational collapse of molecular cores can have an important effect in the 13CO/C18O and C18O/C17O ratios. / Mestre