Letter to the Editor

Discovery of the interstellar cyanoacetylene radical cation HC₃N^{+ ⋆}

¹ Dept. de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: carlos.cabezas@csic.es; jose.cernicharo@csic.es
² Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 300098, Taiwan
³ Observatorio Astronómico Nacional (OAN, IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁴ Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain

Received: 12 June 2024
Accepted: 28 June 2024

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.