Discovery of the interstellar cyanoacetylene radical cation HC${}_3$N${}^{+}$

Abstract

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 40m radio telescope and assigned to HC${}_3$N${}^{+}$, which has an inverted ${}^2\hat{I}$ 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$\pm$0.6)$\times$10${}^{10}$ cm${}^{-2}$ and a rotational temperature of 4.5$\pm$1\,K. The abundance ratio between HC${}_3$N and HC${}_3$N${}^{+}$ is 3200$\pm$320. As found for the larger members of the family of cyanopolyyne cations (HC${}_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.

Accepted as a Letter to the Editor in Astronomy & Astrophysics on June 28th 2024