Letter to the Editor

Astronomical detection of C, the second interstellar anion^*

¹ Dpt. Molecular and Infrared Astrophysics, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006, Spain e-mail: cerni@damir.iem.csic.es
² IRAM, Domaine Universitaire, 300 rue de la piscine, 38400 St Martin d'Hères, France e-mail: guelin@iram.fr
³ Department of Chemistry, Faculty of Science, Okayama University, Tsushimanaka, Okayama 700-8530, Japan
⁴ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA

Received: 6 March 2007
Accepted: 30 March 2007

Abstract

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.