INVESTIGATION OF CARBONACEOUS INTERSTELLAR DUST ANALOGUES BY INFRARED SPECTROSCOPY: EFFECTS OF ENERGETIC PROCESSING

Abstract

Carbonaceous compounds, both solids and gas-phase molecules, are found in very diverse astronomical mediafootnote{A.G.G.M. Tielens. Rev. Mod. Phys., 85, 1021 (2013)}. A significant amount of the elemental carbon is found in small dust grains. This carbonaceous dust, mostly formed in the last stages of evolution of C-rich stars, is the carrier of characteristic IR absorption bands revealing the presence of aliphatic, aromatic and olefinic functional groups in variable proportionsfootnote{J.E. Chiar, A.G.G.M. Tielens, A.J. Adamson and A. Ricca. Astrophys. J., 770, 78 (2013)}. Among the various candidate materials investigated as possible carriers of these bands, hydrogenated amorphous carbon (a-C:H) has led to the best agreement with the observations. Carbonaceous grains are processed by H atoms, UV radiation, cosmic rays and interstellar shocks in their passage from asymptotic giant branch stars to planetary nebulae and to the diffuse interstellar medium. The mechanisms of a-C:H production and evolution in astronomical media are presently a subject of intensive investigation. In this work we present a study of the stability of carbonaceous dust analogues generated in chem{He}+chem{CH_4} radiofrequency discharges. In order to simulate the processing of dust in the interstellar environments, the samples have been subjected to electron bombardment, UV irradiation, and both chem{He} and chem{H_2} plasma processing. IR spectroscopy is employed to monitor the changes in the structure and composition of the carbonaceous films.