Aims: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.
Methods: 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.
Results: The abundance of C4H- in IRC+10216 is 1/6 of the abundance of C6H- and 1/4200 of that of C4H.
Conclusions: 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.