Recombination line profile shapes are derived for ionized spherical stellar winds at radio wavelengths. It is assumed that the wind is optically thick owing to free-free opacity. Emission lines of arbitrary optical depth are obtained assuming that the free-free photosphere forms in the outer, constant expansion portion of the wind. Previous works have derived analytic results for isothermal winds when the line and continuum source functions are equal. Here, semi-analytic results are derived for unequal source functions to reveal that line shapes can be asymmetric about line center. A parameter study is presented and applications discussed.

Detailed knowledge of the hydrogen population structure is necessary for the interpretation of hydrogen recombination line (HRL) observations. Calculations of hydrogen departure coefficients using a capture-collision-cascade type model with the angular momentum quantum levels resolved that includes the effects of external radiation fields are presented. The stimulating processes are important at radio frequencies and can influence level populations. Updated atomic rates and new numerical techniques with a solid mathematical basis have been incorporated into the model to ensure convergence of the solution. My results differ from previous results by up to 20 per cent. The effects on departure coefficients of continuum radiation from dust, the cosmic microwave background, the stellar ionising radiation, and free-free radiation are quantified. Atomic hydrogen masers occur in recombination plasmas in sufficiently dense HII regions. These HRL masers have been observed in a handful of objects to date and the analysis of the atomic physics involved has been rudimentary. A new model of HRL masers is presented which uses an nl-model to describe the atomic populations interacting with free-free radiation from the plasma, and an escape probability framework to deal with radiative transfer effects. The importance of including the collisions between angular momentum quantum states and the free-free emission in models of HRL masers is demonstrated. The model is used to describe the general behaviour of radiative transfer of HRLs and to investigate the conditions under which HRL masers form. The model results show good agreement with observations collected over a broad range of frequencies. Theoretical predictions are made regarding the ratio of recombination lines from the same upper quantum level for these objects. / Physics / Ph. D. (Astronomy)

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Un peu plus de 150 molécules ont été détectées dans le milieu interstellaire et circumstellaire. Parmi elles, nous dénombrons une soixantaine de molécules complexes composées d'au moins 6 atomes. La chimie du milieu interstellaire, synthétisant des molécules plus ou moins complexes à la surface des grains ou en phase gazeuse, est très différente de celle connue sur Terre. À ce jour, seules l'observation et l'analyse de l'émission des différentes espèces moléculaires permettent de contraindre les modèles de chimie interstellaire.Au cours de cette thèse, j'ai recherché des molécules complexes au sein de la nébuleuse d'Orion Kleinmann-Low qui est la région de formation d'étoiles massives la plus proche de nous. De nombreuses étoiles de faible masse s'y forment également. Je me suis intéressée en particulier à la molécule du formiate de méthyle HCOOCH3 qui est une molécule complexe abondante et qui s'est révélée être un traceur de température et de structure de l'ensemble de la région étudiée. Grâce à des observations millimétriques de hautes résolutions spatiales et spectrales (respectivement de 7’’ à 2’’ et de 2.3 km/s à 0.4 km/s), obtenues avec l'interféromètre du Plateau de Bure de l'IRAM, j'ai réalisé une étude détaillée de l'émission cette molécule oxygénée dans la région du Compact Ridge. Notre étude montre que cette région particulière semble être chauffée par des mécanismes externes tels des chocs. De plus, nos observations en direction du Compact Ridge et de son voisinage tendent à confirmer la désorption suite à un choc du formiate de méthyle, ou d'un de ses précurseurs, formé à la surface des grains interstellaires.J'ai également recherché les deux isomères de formule [C2H4O2] du formiate de méthyle : le glycolaldéhyde et l'acide acétique. Leur étude a montré la difficulté de détecter des molécules peu abondantes dans Orion K-L en raison d'une confusion spectrale importante, mettant ainsi en évidence la nécessité d'observations de hautes résolutions aussi bien spatiale que spectrale pour la recherche de molécules comme le permettra l'interféromètre ALMA. Les limites supérieures de densité de colonne déduites de nos données pour le pré-sucre glycolaldehyde (CH2OHCHO, détecté dans SgrB2) sont très contraignantes pour les modèles de chimie. Nos résultats pourraient permettre une avancée dans la compréhension de l'origine de cette espèce moléculaire. / Over 150 different molecular species have been detected in the interstellar and circumstellar media. Among these, approximatively 60 are complex molecules and contain 6 or more atoms. The interstellar chemical processes that form more or less complex molecules, either on the surface of dust grains or in gas phase, are different from the processes we know on Earth. The only way to constrain chemical models relies on the observation and the analysis of the emission coming from various molecular species.The main goal of my PhD is to look for complex molecules in the nearest star forming region with both high and low mass stars, the Orion Kleinmann-Low nebula. I specially studied the emission of the methyl formate molecule (HCOOCH3) which appeared to be an abundant molecule and a good probe of the temperature and structure of Orion K-L.Using high spectral and spatial resolution millimetre observations (from 7’’ to 2’’ and from 2.3 km/s to 0.4 km/s, respectively) from the IRAM Plateau de Bure Interferometer, I carried out a detailed study of the emission of this O-bearing molecule towards the Compact Ridge component. Our study shows that this region seems to be heated by external mechanisms (e.g. shocks).Moreover, our observations toward the Compact Ridge region and its surroundings tend to confirm that methyl formate or a precursor seems to be formed on grain surfaces and is subsequently desorbed due to shocks.I also looked for the two isomers of methyl formate [C2H4O2] : glycolaldehyde and acetic acid. Owing to strong spectral confusion in the region, it is very difficult to detect low abundance molecules such as these two isomers. In order to lower the confusion level, higher spatial as well as spectral resolutions must be achieved which ALMA will soon allow.We derived upper limits for the column density of glycolaldehyde, a precursor of sugar (CH2OHCHO that has been detected towards SgrB2), these limits provide strong constraints for chemical models.