Novel pathways to CN− within interstellar clouds and circumstellar envelopes: implications for IS and CS chemistry

Abstract

Prospects for detection of the diatomic negative ion CN− within dense interstellar clouds and/or circumstellar envelopes are reassessed in the light of new pathways to small negative ions in such environments. Formation of CN− by charge transfer from PAH− to CN, and by dissociative attachment of free electrons to MgCN and MgNC, is discussed. These pathways are expected to lead to much higher CN− abundances than can be formed by those reactions which have been included in previous models of interstellar negative-ion chemistry. Investigation of electron attachment to Mg(CN) is assisted by ab initio calculations, at the G2 level of theory, upon the structure and total energy of singlet and triplet states of MgCN− and MgNC−. The model of CN− chemistry which we present here shows that interstellar abundances of PAH− are probably too low to produce detectable CN−, but the efficient occurrence of dissociative attachment to MgNC and MgCN within the circumstellar envelope IRC+ 10216 may produce CN− at an abundance of ∼2 × 10−10n(H2), which should permit the detection of CN− via its rotational spectrum.