Volume 517, July 2010
|Number of page(s)||5|
|Published online||30 July 2010|
Letter to the Editor
Observatoire de Paris-Meudon, 5 Place Jules Janssen, 92190 Meudon,
France e-mail: email@example.com
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
Accepted: 5 July 2010
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.
Key words: astrochemistry / line: identification / molecular processes / stars: AGB and post-AGB / circumstellar matter / stars: individual: IRC +10216
Based on observations carried out with the IRAM 30-m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).
Appendix is only available in electronic form at http://www.aanda.org
© ESO, 2010
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