Astronomical detection of C4H-, the second interstellar anion

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.