Les silicates interstellaires : composition physico-chimique et évolution

Abstract : This PhD work is devoted to the study of the structure, composition and evolution of the physico-chemical properties of oxygen-rich interstellar dust (silicates and oxydes). This work is based on infrared spectroscopic data (2-200 µm) obtained from the Infrared Space Observatory (ISO). The interpretation of the data is carried out by the modelling of radiative transfer of the studied objects and by an experimental approach aimed to simulate the cosmic dust and the various physical processes to which it is submitted in the Interstellar Medium (MIS). The silicate dust is formed in the circumstellar shells around evolved oxygen-rich stars (AGB stars) and injected into the ISM via stellar winds. After passage through the diffuse medium where they may undergo various structural modifications, it is destroyed or incorporated into the regions around protostars and into (proto) planetary systems by the collapse of molecular clouds. The detailed study of the newly formed silicate dust, around evolved stars, and of the older dust, around protostars, shows that its composition and structure vary at the beginning and at the end of its evolutionary cycle. Indeed, the newly formed silicate grains are mainly composed of amorphous olivine and 20 to 25 % of crystalline silicates, whereas around protostars, the silicate grains are porous and composed of pyroxenes and aluminosilicates (more than 90 %) with inclusions of oxides. The results of laboratory experiments of irradiation on crystalline olivine with 4 and 10 keV He+ ions, aimed to simulate ion implantation into interstellar dust grains in shocks in the ISM, show that this irradiation mechanism may explain the observed disappearance of the crystalline silicates and the chemical change of the amorphous silicates from olivine to pyroxene in the ISM.
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Karine Demyk. Les silicates interstellaires : composition physico-chimique et évolution. Astrophysique [astro-ph]. Université Paris Sud - Paris XI, 2000. Français. ⟨tel-00010953⟩

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