2010A&A...517L...2A

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.