Chemical Evolution of Carbonaceous Material in Interstellar Clouds

The evolution of the composition of carbonaceous material in interstellar and circumstellar environments is discussed in terms of the effect of thermal, radiative, and chemical processing of a prototypical solid such as hydrogenated amorphous carbon (HAC). A complex chemistry is suggested in which HAC reacts with atomic H to yield a variety of volatile hydrocarbons including CH4. However, an important chemical effect is the development and modification of bridging groups between aromatic components. These structures are most stable if they contain C=C bonds, suggesting the evolution of a chemically and photochemically resistant composition in which polycyclic aromatic hydrocarbon (PAH) components are joined via conjugated double bonded carbon side groups. Dissolution of this material in photodissociation regions will liberate these (C=C)n fragments as well as PAH molecules. IR spectral bands characteristic of these materials are discussed and it is shown that molecules such as the allenes, dienes, etc., could contribute to the aromatic emission bands observed in the 6.2 and 11-12 μm regions. A possible contribution of C=C in interstellar graphite particles to the 6.2 μm band is also discussed. The appearance of a particular mixture of carbonaceous compounds in a given source may be an indicator of the evolutionary status of that object extending from cool post-asymptotic giant branch carbon stars showing only IR absorption by saturated hydrocarbons, to evolved planetary nebulae that exhibit the full range of aromatic hydrocarbon spectral features in emission. Infrared spectra in the 3.3-3.4 μm range would seem to define the evolutionary status of such objects.