2007A&A...467L..37C

Following the recent detection of C₆H^– in the laboratory and in space we have succeeded in studying the microwave spectrum of C₄H^–. We report here the first detection in space of this negative ion. We have observed in the envelope of the carbon star IRC +10216 five lines corresponding to the J=9-8, 11-10, 12-11, 14-13 and 15-14 rotational transitions of C₄H^–. The C₄H^– lines have a cusped shape, denoting that this ion is formed in the outer part of the envelope, like its neutral counterpart C₄H. The abundance of C₄H^– in IRC+10216 is 1/6 of the abundance of C₆H^– and 1/4200 of that of C₄H. The detection of C₄H^–, after that of C₆H^–, confirms the theoretical prediction that C-chain anions are abundant in interstellar clouds and yields a first measurement of the electron radiative attachment rates.