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 40m telescope. The observed C3N /C3N and C5N /C5N abundance ratios are similar to 140 and similar to 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 di fferent 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 CnN. 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.