Discovery of the interstellar cyanoacetylene radical cation HC3N+

We report the first identification in space of HC 3 N + , 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 40 m radio telescope and assigned to HC 3 N + , which has an inverted 2 Π 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 HC 3 N + based on the good agreement between the astronomical and theoretical spectroscopic parameters. We derived a column density of (6.0 ± 0.6)×10 10 cm −2 and a rotational temperature of 4.5 ± 1 K. The abundance ratio between HC 3 N and HC 3 N + is 3200 ± 320. As found for the larger members of the family of cyanopolyyne cations (HC 5 N + and HC 7 N + ), HC 3 N + is mainly formed through the reactions of H 2 and the cation C 3 N + and by the reactions of H + with HC 3 N. In the same manner than other cyanopolyyne cations, HC 3 N + is mostly destroyed through a reaction with H 2 and a dissociative recombination with electrons.