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 ² 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. 00. 6) 10^10 cm^-2 and a rotational temperature of 4. 51\, K. The abundance ratio between HC₃N and HC₃N^+ is 3200320. 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.