Hydrocarbon Anions in Interstellar Clouds and Circumstellar Envelopes

The recent detection of the hydrocarbon anion C sub(6)H super(-) in the interstellar medium has led us to investigate the synthesis of hydrocarbon anions in a variety of interstellar and drcumstellar environments. We find that the anion/neutral abundance ratio can be quite large, on the order of at least a few percent, once the neutral has more than five carbon atoms. Detailed modeling shows that the column densities of C sub(6)H super(-) observed in IRC +10 216 and TMC-1 can be reproduced. Our calculations also predict that other hydrocarbon anions, such as C sub(4)H super(-) and C sub(8)H super(-), are viable candidates for detection in IRC +10 216, TMC-1, and photon-dominated regions such as the Horsehead Nebula.