Hydrocarbon Anions in Interstellar Clouds and Circumstellar Envelopes

The recent detection of the hydrocarbon anion C 6 H - in the interstellar medium has led us to investigate the synthesis of hydrocarbon anions in a variety of interstellar and circumstellar 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 6 H - observed in IRC +10 216 and TMC-1 can be reproduced. Our calculations also predict that other hydrocarbon anions, such as C 4 H - and C 8 H - , are viable candidates for detection in IRC +10 216, TMC-1, and photon-dominated regions such as the Horsehead Nebula.