Interstellar nitrile anions: Detection of C₃N^- and C₅N^- in TMC-1

We report on the first detection of C₃N^- and C₅N^- towards the cold dark core TMC-1 in the Taurus region, using the Yebes 40 m telescope. The observed C₃N/C₃N^- and C₅N/C₅N^- 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 C₃N/C₃N^- and C₅N/C₅N^- 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 HC₃N and HC₅N as possible routes to form C _n N^-. The detection of C₅N^- 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 C₅N^- and the neutral radical C₅N.