Laboratory and Theoretical Results on Interstellar Molecule Production by Grains in Molecular Clouds

Abstract

Laboratory and theoretical studies have been made of the effects of ultraviolet photolysis of interstellar grain mantles. It has been shown that grain photolysis should be important even in dense clouds. A large number of molecules and radicals observed in the interstellar gas appear in the irradiated ices of CO, H2O, NH3 and CH4 which are deposited at 10 K. Energy released during warm-up is seen from visible and infrared luminescence and inferred from vapor pressure enhancement relative to unirradiated samples. Grains are pictured as a source as well as a sink (capture) of molecules. The photolysis of an individual grain provides the stored chemical energy which is sporadically released by relatively mild triggering events (such as low velocity grain-grain collisions in turbulent molecular clouds) to produce the impulsive heating needed to eject or evaporate a portion of the grain mantle. An extremely complex and non-volatile substance possessing the infrared signatures of amino and carboxylic acid groups and having a mass of 514 amu has been produced at a rate corresponding to a mass conversion rate of interstellar grains of between 2% and 20% in 107 years.