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Évolution physico-chimique des hydrocarbures aromatiques polycycliques dans les régions de photodissociation

Abstract : Polycyclic aromatic hydrocarbons (PAHs) play a major role in the physics and chemistry of photodissociation regions (PDRs) in our galaxy. In these environments, the physical conditions and in particular the UV radiation field drive the evolution of PAHs. It was proposed that very small dust grains are also related to this evolution. We propose here an investigation of these evolution scenarios by combining chemical and physical studies with astrophysical studies of these species in PDRs. In this work, I present my contribution to the development of PDR modeling tools, and their application to the analysis of data obtained with the Spitzer infrared space telescope and the Herschel space observatory. New constraints are provided concerning the morphology and energetics of the reflection nebula NGC 7023. The need for a good description of PAH evolution in PDR models is emphasized. In addition, I developed a model dedicated to the study of the charge and hydrogenation states of PAHs in PDRs. It was applied to three PAHs of different sizes and showed that species containing up to 54 carbon atoms quickly loose their hydrogen atoms to form pure carbon clusters. I conclude that theoretical and experimental studies are needed to quantify the reactivity of neutral PAHs with hydrogen, the recombination of PAH cations with electrons and the dissociation of superhydrogenated species. The last part of this work is dedicated to the study of evaporating very small carbonaceous grains (eVSGs) observed in PDRs. A fitting tool for the analysis of PAH and eVSG emissions in the mid-infrared spectral domain is presented and used to connect the evaporation of eVSGs and the local UV radiation field in several PDRs. Considering PAH clusters as models for eVSGs, I computed their evaporation properties using theoretical methods based on statistical physics. I used these properties to model their evolution in astrophysical environments and showed that the properties of PAH clusters are consistent with the observational constraints. Guidelines are proposed for a simpler modeling of these species in the perspective of their inclusion in PDR models. Thanks to the Herschel observatory and to the future facilities like the JWST and SPICA space missions or the ALMA interferometer, data with unprecedented sensitivity and spatial resolution will provide new observational constraints. Their analysis will require to further understand the physical and chemical evolution of PAH species..
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Julien Montillaud. Évolution physico-chimique des hydrocarbures aromatiques polycycliques dans les régions de photodissociation. Planétologie et astrophysique de la terre [astro-ph.EP]. Université Paul Sabatier - Toulouse III, 2011. Français. ⟨tel-00697363⟩

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