HAC-coated silicate grains and PAHs in diffuse clouds and low-velocity shocks

We investigate a model in which HAC-coated silicate grains subjected to low-velocity shocks release carbon to the gas in the form of atoms and PAHs. Both HAC and PAH are assumed to be subject to chemistry, including reactions with oxygen atoms, carbon atoms and carbon ions. The released species contribute to a conventional post-shock chemistry, during which the HAC and PAHs may be further eroded or restored. We find that the parameter of crucial importance is the reaction efficiency for atomic oxygen with both HAC and PAH. If this efficiency is less than 100 per cent, then HAC mantles are (at least) restored in a period of about 106 yr. PAHs are destroyed on time-scales of less than about 105 yr unless the oxidation efficiency is very small. (author)