Discovery of the interstellar cyanoacetylene radical cation HC3N+

Abstract

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 40 m 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.