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AbstractAbstract
[en] The interstellar medium (ISM) is a highly complex system. It corresponds to an intermediate scale between stars and galaxies. The interstellar gas is present throughout the galaxy, filling the volume between stars. A wide variety of coupled processes, such as gravity, magnetic fields, turbulence and chemistry, participate in its evolution, making the modeling of the ISM a challenging problem. A correct description of the ISM requires a good treatment of the magnetohydrodynamics (MHD) equations, gravity, thermal balance, and chemical evolution within the molecular clouds. This thesis work aims at a better understanding of the formation and evolution of molecular clouds, especially how they become 'molecular', paying particular attention to the transition HI-to-H2. We have performed ideal MHD simulations of the formation of molecular clouds and the formation of molecular hydrogen under the influence of gravity and turbulence, using accurate estimates for the shielding effects from dust and the self-shielding for H2, calculated with a Tree-based method, able to provide fast estimates of column densities. We find that H2 is formed faster than predicted by the usual estimates due to local density enhancements created by the gas turbulent motions. Molecular hydrogen, formed at higher densities, could then migrate toward low density warmer regions. Total H2 column densities show that the HI-to-H2 transition occurs at total column densities of a few 1020 cm-2. We have calculated the populations of rotational levels of H2 at thermal equilibrium, and integrated along several lines of sight. These two results reproduce quite well the values observed by Copernicus and FUSE, suggesting that the observed transition and the excited populations could arise as a consequence of the multi-phase structure of molecular clouds. As H2 formation is prior to further molecule formation, warm H2 could possibly allow the development of a warm chemistry, and eventually explain some aspects of the molecular richness observed in the ISM. (author)
[fr]
Le milieu interstellaire (MIS) est un systeme extremement complexe. Il correspond a une echelle intermediaire entre les etoiles et les galaxies. Le gaz interstellaire est present dans toute la galaxie, remplissant l'espace entre les etoiles. Une grande diversite de processus couples, comme la gravite, le champs magnetiques, la turbulence et la chimie, participe a son evolution, faisant de la modelisation du MIS un probleme ardu. Une description correcte du MIS necessite un bon traitement des equations de la magnetohydrodynamique (MHD), de la gravite, du bilan thermique et de l'evolution chimique a l'interieur du nuage moleculaire. L'objectif de ce travail de these est une meilleure comprehension de la formation et de l'evolution des nuages moleculaires, et plus particulierement de la transition du gaz atomique en gaz moleculaire. Nous avons realise des simulations numeriques de la formation des nuages moleculaires et de la formation de l'hydrogene moleculaire sous l'influence de la gravite et de la turbulence MHD, en utilisant des estimations precises de l'ecrantage par les poussieres et de l'auto-ecrantage par la molecule H2. Ceci a ete calcule grace a une methode en arbre, a meme de fournir une rapide estimation des densites de colonne.Nous avons trouve que l'hydrogene moleculaire se forme plus rapidement que prevu par les estimations classiques du fait de l'augmentation de densite locale provoquee par les fluctuations turbulentes du gaz. L'hydrogene moleculaire, forme a des densites plus elevees, peut alors migrer vers les regions plus chaudes et moins denses.Les densites de colonne totale d'hydrogene moleculaire montrent que la transition HI-H2 se produit a des densites de colonne de quelques 1020 cm-2. Nous avons calcule les populations des niveaux rotationnels de H2 a l'equilibre thermique et integre le long de plusieurs lignes de visee. Ces resultats reproduisent bien les valeurs observees par Copernicus et FUSE, suggerant que la transition observee et les populations excitees pourraient etre une consequence de la structure multi-phasique des nuages moleculaires. Comme la formation de H2 precede la formation des autres molecules, le H2 chaud pourrait permettre le developpement d'especes endothermiques et eventuellement expliquer certains aspects de la richesse moleculaire observee dans l'ISM. (auteur)Original Title
Impact du transfert radiatif et de la chimie sur la formation des nuages moleculaires
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24 Sep 2015; 173 p; [220 refs.]; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses; Thesis Doctor of Philosophy
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Miscellaneous
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Thesis/Dissertation
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COMPUTER CODES, DIAGRAMS, ELECTROMAGNETIC RADIATION, ELEMENTS, ENERGY LEVELS, ENERGY TRANSFER, EXCITED STATES, FLUID MECHANICS, FLUIDS, FUNCTIONS, GASES, HEAT TRANSFER, HYDRODYNAMICS, INFORMATION, LINE BROADENING, MECHANICS, NONMETALS, NUCLEAR REACTIONS, PHYSICAL PROPERTIES, RADIATIONS, SIMULATION, SPACE
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