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.