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^-. CN^- is identified through the observation of 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 C_6H^-, 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 large abundance derived for CN^-, 0.2 % relative to CN, makes likely its detection in other astronomical sources. A parallel search for the small anion C_2H^- remains so far unconclusive, despite the previous tentative identification of the J = 1-0 rotational transition. The abundance of C_2H^- in IRC +10216 is found to be vanishingly small, < 0.0014 % relative to C_2H.
- Publication:
-
The Molecular Universe
- Pub Date:
- May 2011
- Bibcode:
- 2011IAUS..280P..77A