  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.
