Hydrocarbon anions in interstellar clouds and circumstellar envelopes

The recent detection of the hydrocarbon anion C6H − 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 C6H − observed in IRC+10216 and TMC-1 can be reproduced. Our calculations also predict that other hydrocarbon anions, such as C4H − and C8H −, are viable candidates for detection in IRC+10216, TMC-1 and photon-dominated regions such as the Horsehead Nebula. Subject headings: astrochemistry — ISM: abundances — ISM: molecules — ISM: clouds — stars: carbon