CARBON DUST FORMATION ON INTERSTELLAR GRAINS

A set of laboratory experiments is presented that study the evolution of solid organic matter at low temperature as a result of exposure to photons and ions, with the aim of simulating the formation and evolution of organic refractory mantles on interstellar grains. Organic residue is a product of UV photolysed ice mantles in dense clouds and consists of 2-3 C atom containing compounds. From laboratory experiments, the efficiency of organic residue formation is given as a function of UV radiation dose. The yield of organic residue is so high that it probably is the major source of solid carbonaceous matter in the interstellar medium. The processing into more complex carbonaceous matter has been simulated by irradiation with 10eV UV photons and bombardment with 3keV He+ ions. The latter have a stopping power similar to cosmic ray 1 MeV protons. The subsequent evolution of the residue is characterized by two processes: the loss of H, O, and N (carbonization) and the formation of polyaromatic clusters in the polymeric solid (polymerization). Both processes are shown to occur, but notably polymerization by cosmic ray ions is found to be about 10(3) times more efficient than that by UV photons per unit energy absorbed. UV photons, however, will dominate the polymerization and carbonization, because in the diffuse medium UV photons deposit as much as 10(5) times more energy in the organic refractory grain mantles than cosmic rays. After a typical lifetime in the diffuse medium, the processed organic refractory grain mantle becomes an hydrogenated amorphous carbon with a relatively large optical gap of about 0.61. 1 eV, with small but significant amounts of H, O, and N, and graphitic clusters up to sizes of 30-60 rings. This makes interstellar grain mantles a potential source of PAHs.