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.