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<sup>-<sup/>, and quantum mechanical calculations of the excitation of this anion by means of collisions with para-H<sub>2<sub/>. The anion CN<sup>-<sup/> is identified by observing the <i>J<i/> = 2–1 and <i>J<i/> = 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<sup>-<sup/>, like the large anion C<sub>6<sub/>H<sup>-<sup/>, 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<sup>-<sup/>, 0.25% relative to CN, indicates that its detection in other astronomical sources is likely. A parallel search for the small anion C<sub>2<sub/>H<sup>-<sup/> remains inconclusive, despite the previous tentative identification of the <i>J<i/> = 1–0 rotational transition. The abundance of C<sub>2<sub/>H<sup>-<sup/> in IRC +10216 is found to be vanishingly small, <i><<i/>0.0014% relative to C<sub>2<sub/>H.