An ion-molecule scheme for the synthesis of hydrocarbon-chain and organonitrogen molecules in dense interstellar clouds

Abstract

A general scheme is proposed for the synthesis of large molecules in dense interstellar clouds based on rates of ion-molecule reactions measured in our laboratory and elsewhere. C-C bond formation is dominated by reactions involving the must abundant ions C +, CH3 +, C2H2 +, and C2H3 + and the most abundant neutrals C2H2 and CHx, where 1 < x < 4. As a particular example, we show that the scheme can account for the observation of methylacetylene, methyl cyanide, and the cyanoacetylenes HC3N, HC5N, HC7N, and HC9N and is consistent with their observed relative abundance. However, the scheme does require high concentrations of C2H2 and C2H2 + ions to account for the absolute abundances observed. Subject headings: interstellar: molecules - molecular processes