Formation, Destruction and Excitation of Carbon Grains and PAH Molecules

Abstract

The nature of grains in diffuse clouds is reviewed with particular emphasis on very small grains. An attempt is made to correlate the properties of very small grains and large molecules of the PAH type. It is found that the transition from particle to molecule likely occurs for systems containing about 100 carbon atoms and involves no discontinuity in physical parameters. Possible formation and destruction routes for PAH under interstellar conditions are also discussed. It appears that the lifetime of PAH in clouds will be limited by photo-oxidation type reactions. The most plausible formation route for PAH seems to be via grain disruption in interstellar shocks. This implies that gaseous PAH molecules may exist only in localized regions of high excitation. Hydrogenated amorphous carbon (aC:H or HAC) grains are the dominant carbon condensate in interstellar clouds and their excitation can lead to both broad band and narrow-line luminescent emission in interstellar sources. In addition, IR absorption by HAC grains provides a good simulation of the 3.4pm feature seen in spectra of the galactic centre. The relation between PAH molecules and HAC is discussed in an evolutionary model of dust grains.