Astronomical identification of CN-, the smallest observed molecular anion
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.
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- Publication:
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Astronomy and Astrophysics
- Pub Date:
- July 2010
- DOI:
- 10.1051/0004-6361/201015186
- arXiv:
- arXiv:1007.0662
- Bibcode:
- 2010A&A...517L...2A
- Keywords:
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- astrochemistry;
- line: identification;
- molecular processes;
- stars: AGB and post-AGB;
- circumstellar matter;
- stars: individual: IRC +10216;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 5 pages, 4 figures