Spectroscopy of Interstellar Carbonaceous Dust

Abstract

Carbonaceous grains make a large part of cosmic dust and contain a significant fraction of the carbon available in the interstellar medium (ISM). Observational evidence for the presence of carbonaceous grains comes mostly from IR absorption spectra. The most intense IR absorption is the 3.4 μm band which is ubiquitous in the diffuse ISM and is also present in other galaxies. It is attributed to CH stretching vibrations of CH2 and CH3 groups; weaker absorptions at 6.85 and 7.27 μm are due to aliphatic bendings, and a broad, weak absorption between 6 and 6.4 μm is assigned to C=C stretching. These observations, together with laboratory work on dust analogs, point to amorphous hydrogenated carbon (abbreviated a-C:H or HAC), as the most likely carrier of these IR bands, and thus as the predominant component of carbonaceous grains; however, the H/C ratio and the proportion of sp2 and sp3 carbon atoms are presently debated. Emission IR bands at 3.3, 6.2, 7.7, 8.6 and 11.2 μm are also seen at many places in the ISM. They have been associated with stretching and bending vibrations of aromatic material and are often referred to as aromatic IR bands (AIB). They are mostly attributed to polycyclic aromatic hydrocarbons (PAHs). Refinements to the original PAH hypothesis including the presence of heteroatoms, aliphatic chains or non-planar defective π domains have been introduced to account for the presence of aliphatic emissions and for the observed band shapes. It is conjectured that the PAHs or PAHs-related molecules responsible for the IR emissions are related with the a-C:H structures responsible for dust absorption either as formation precursors or as destruction products. IR spectroscopy provides also clues on grain evolution. It is generally assumed that carbonaceous grains are formed as nanoparticles in the outflows of carbon-rich AGB stars and evolve from this point. When they reach the diffuse ISM, the grains are subjected to H atom bombardment, which leads to the formation of aliphatic structures, and to UV photon and cosmic ray irradiation that destroy aliphatic structures and lead to graphitization. A core/mantel structure is often assumed for the grains in the diffuse ISM, but models differ on whether the aliphatic chains responsible for the 3.4 μm band are located in the core or in the mantle and present day observations cannot decide on that point. In dense clouds the 3.4 μm band disappears. It is believed that the aliphatic carriers of this band are destroyed somewhere in the transition from the diffuse to the dense ISM, since in the interior of dense clouds UV and cosmic ray fluxes are too weak.