Observations of Interstellar Carbon Compounds

Infrared absorption and emission features observed spectroscopically in our Galaxy allow to probe the composition of solid dust grains, their evolution and thus follow the cycling of matter in the Galaxy. Many observables do reveal the presence of large amounts of carbonaceous particles in space, other than the PAH-like emission lines. The carbonaceous materials observed include amorphous carbons, diamondoids showing in emission for a few specific sources, and the recently detected fullerenes. An important hydrogenated amorphous carbon component (HAC or a-C:H), traced by the 2940 cm−1 structured absorption feature is observed against Galactic background sources. Since the discovery of this feature in the early eighties (Allen 1981), the observation of a-C:H has been extended to the mid-infrared by space observatories, giving insight into additional associated features. They are also observed in external galaxies, showing the ubiquitous nature of these components. We will focus on astronomical observations of organic matter other than PAHs, amorphous carbons and associated laboratory dust analogues relevant to astrophysical applications.