Discovery of the interstellar cyanoacetylene radical cation HC₃N⁺

We report the first identification in space of HC₃N⁺, the simplest member of the family of cyanopolyyne cations. Three rotational transitions with half-integer quantum numbers from J=7/2 to 11/2 have been observed with the Yebes 40m radio telescope and assigned to HC₃N⁺, which has an inverted ²Π ground electronic state. The three rotational transitions exhibit several hyperfine components due to the magnetic and nuclear quadrupole coupling effects of the H and N nuclei. We confidently assign the characteristic rotational spectrum pattern to HC₃N⁺ based on the good agreement between the astronomical and theoretical spectroscopic parameters. We derived a column density of (6.0±0.6)×10¹⁰ cm⁻² and a rotational temperature of 4.5±1\,K. The abundance ratio between HC₃N and HC₃N⁺ is 3200±320. As found for the larger members of the family of cyanopolyyne cations (HC₅N⁺ and HC₇N⁺), HC₃N⁺ is mainly formed through the reactions of H₂ and the cation C₃N⁺ and by the reactions of H⁺ with HC₃N. In the same manner than other cyanopolyyne cations, HC₃N⁺ is mostly destroyed through a reaction with H₂ and a dissociative recombination with electrons.