• Godard, Marie

Abstract

The hydrogenated amorphous carbons (a-C:H or HAC) are an important component of interstellar dust. These hydrocarbon grains are observed through IR absorption bands at 3.4, 6.9 and 7.3 μm, due to aliphatic C-H bond vibrations. Their spectral signatures are detected in the diffuse interstellar medium along several sight lines in the Milky Way and in many external galaxies. This thesis deals with the study of such interstellar a-C:H, both through their astrophysical observation and the synthesis and characterisation of laboratory analogues. A first part of this PhD work concerns the 3.4 μm band observation in the galactic diffuse interstellar medium in the direction of IRAS 18511+0146 source. The C-H stretching mode absorption band detected toward several lines of sight of this cluster has the strongest optical depths observed in the Milky Way outside the galactic center. Different interpretations for this deep absorption band in this direction are discussed. Analogues of the amorphous carbonaceous dust component have been produced in the laboratory with a plasma source. Their IR spectra are in excellent agreement with the absorption bands observed in the diffuse interstellar medium. The samples have been characterised by UV-visible and IR absorption spectroscopy. Since a-C:H emit a visible radiation when they absorb UV or visible photons, a systematic study of this photoluminescence is performed. For the first time, the absolute and intrinsic photoluminescence yield of a-C:H is measured for a broad range of excitation wavelengths. The photoluminescence properties of a-C:H is compared to observations of the Extended Red Emission, a large interstellar emission band whose carriers are not identified. To infer the influence of cosmic rays on this carbonaceous dust, the produced analogues have been irradiated by different swift ions, similar to interstellar cosmic rays. The induced modifications have been monitored by their IR spectrum. The a-C:H dehydrogenation is observed through the progressive disappearance of the aliphatic C-H bands. Its analysis allows us to deduce the evolution of the a-C:H dust and its spectral signatures under cosmic ray exposition. The destruction due to cosmic rays is compared to the effects induced by exposure to UV photons and hydrogen atoms, in order to interpret the evolution of the absorption band at 3.4 μm observed in the diffuse interstellar medium, but not in dense clouds....

Topics

• impedance spectroscopy
• photoluminescence
• cluster
• amorphous
• Carbon
• grain
• Hydrogen