Letter to the Editor

Astronomical identification of CN^-, the smallest observed molecular anion ^*,**

M. Agúndez¹, J. Cernicharo², M. Guélin³, C. Kahane⁴, E. Roueff¹, J. Kłos⁵, F. J. Aoiz⁶, F. Lique⁷, N. Marcelino², J. R. Goicoechea², M. González García⁸, C. A. Gottlieb⁹, M. C. McCarthy⁹ and P. Thaddeus⁹

¹ LUTH, Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92190 Meudon, France e-mail: marcelino.agundez@obspm.fr
² Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. de Torrejón a Ajalvir km 4, 28850 Madrid, Spain
³ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Héres, France
⁴ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, 38041 Grenoble, France
⁵ Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA
⁶ Departamento de Química Física, Facultad de Química, Universidad Complutense, 28040 Madrid, Spain
⁷ LOMC FRE 3102, CNRS Université du Havre, 25 rue Philippe Lebon, BP 540, 76058 Le Havre, France
⁸ Instituto de Radioastronomía Milimétrica, Av Divina Pastora 7, Local 20, 18012 Granada, Spain
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 9 June 2010
Accepted: 5 July 2010

Abstract

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN^-, and quantum mechanical calculations of the excitation of this anion by means of collisions with para-H₂. The anion CN^- is identified by observing the J = 2–1 and J = 3–2 rotational transitions in the C-star envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles indicate that CN^-, like the large anion C₆H^-, is formed in the outer regions of the envelope. Chemical and excitation model calculations suggest that this species forms from the reaction of large carbon anions with N atoms, rather than from the radiative attachment of an electron to CN, as is the case for large molecular anions. The unexpectedly high abundance derived for CN^-, 0.25% relative to CN, indicates that its detection in other astronomical sources is likely. A parallel search for the small anion C₂H^- remains inconclusive, despite the previous tentative identification of the J = 1–0 rotational transition. The abundance of C₂H^- in IRC +10216 is found to be vanishingly small, <0.0014% relative to C₂H.