Interstellar nitrile anions: Detection of C<sub>3</sub>N<sup>−</sup> and C<sub>5</sub>N<sup>−</sup> in TMC-1
J. Cernicharo, N. Marcelino, J. R. Pardo, M. Agúndez, B. Tercero, P. de Vicente, C. Cabezas, C. Bermúdez
Abstract
We report on the first detection of C 3 N − and C 5 N − towards the cold dark core TMC-1 in the Taurus region, using the Yebes 40 m telescope. The observed C 3 N/C 3 N − and C 5 N/C 5 N − abundance ratios are ∼140 and ∼2, respectively; that is similar to those found in the circumstellar envelope of the carbon-rich star IRC +10216. Although the formation mechanisms for the neutrals are different in interstellar (ion-neutral reactions) and circumstellar clouds (photodissociation and radical-neutral reactions), the similarity of the C 3 N/C 3 N − and C 5 N/C 5 N − abundance ratios strongly suggests a common chemical path for the formation of these anions in interstellar and circumstellar clouds. We discuss the role of radiative electronic attachment, reactions between N atoms and carbon chain anions C n − , and that of H − reactions with HC 3 N and HC 5 N as possible routes to form C n N − . The detection of C 5 N − in TMC-1 gives strong support for assigning to this anion the lines found in IRC +10216, as it excludes the possibility of a metal-bearing species, or a vibrationally excited state. New sets of rotational parameters have been derived from the observed frequencies in TMC-1 and IRC +10216 for C 5 N − and the neutral radical C 5 N.