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Paper: Laboratory Studies on Ion-Irradiation of Dust Analogs: Application to the Evolution of Interstellar Silicates
Volume: 309, Astrophysics of Dust
Page: 649
Authors: Demyk, K.; d'Hendecourt, L.; Jones, A.; Leroux, H.; Carrez, Ph.; Cordier, P.
Abstract: We present the results of experiments aimed at simulating the interaction of silicate grains with atoms ionized and accelerated during the propagation of supernova-generated shock waves. Such an interaction is a potentially important process for the silicate evolution in the diffuse interstellar medium (ISM). Olivine, enstatite and diopside were irradiated with low energy (4-50 keV) H+ and He+ ions with a maximum fluence of 1018 ions/cm2. The irradiated samples were analyzed by transmission electron microscopy, X-ray photoelectron spectroscopy and infrared micro-spectroscopy, allowing us to study changes in the mineral stoichiometry, structure and spectral characteristics induced by the irradiations. Irradiation of these three minerals leads to their total amorphization for ions fluences < 1018 ions/cm2. This amorphization is accompanied by the implantation into the target mineral of incident ions which form bubbles and modify the compactness of the mineral. Furthermore at 4 and 10 keV, stoichiometric changes were observed in the studied sample, olivine, which is deficient in oxygen and magnesium after the irradiation. These physico-chemical modifications are translated into important changes of the infrared spectral characteristics of the minerals, which, after the irradiation, consist of two broad and structureless bands at ~ 9.8 and 18 μm, characteristic of amorphous materials and qualitatively similar with the spectrum of the interstellar medium. Since the experimental conditions used in this study are compatible with the interstellar case, the interaction of grains with ions accelerated in high velocity supernovae-generated shock waves appears to be an important process for the dust evolution and may be responsible for the absence of crystalline silicates in the ISM.
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