Astronomical identification of CN-, the smallest observed molecular anion

TL;DR

This paper reports the first astronomical detection of the cyanide anion CN- in space, supported by quantum calculations, revealing its formation mechanism and abundance in the C-star envelope IRC +10216.

Contribution

It provides the first detection of a diatomic negative ion in space and explores its formation process and abundance, expanding understanding of molecular anions in astrophysics.

Findings

CN- detected via rotational transitions in IRC +10216

CN- abundance is 0.25% relative to CN

C2H- remains undetected with very low abundance

Abstract

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN-, as well as quantum mechanical calculations of the excitation of this anion through collisions with para-H2. 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 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 large abundance derived for CN-, 0.25 % relative to CN, makes likely its detection in other astronomical sources. A parallel search for the small anion C2H- remains so far unconclusive, 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.