Letter to the Editor

Discovery of the elusive thioketenylium, HCCS⁺, in TMC-1^⋆

¹ Grupo 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 Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
³ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
⁴ Department of Applied Chemistry, Science Building II, National Yang Ming Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 300098, Taiwan

Received: 2 December 2021
Accepted: 21 December 2021

Abstract

We report the detection in TMC-1 of the cation HCCS⁺ (X̃ ³Σ⁻), which is the protonated form of the widespread radical CCS. This is the first time that a protonated radical has been detected in a cold dark cloud. Twenty-six hyperfine components from twelve rotational transitions have been observed with the Yebes 40 m and IRAM 30m radio telescopes. We confidently assign the characteristic rotational spectrum pattern to HCCS⁺ based on the good agreement between the astronomical and theoretical spectroscopic parameters. The column density of HCCS⁺ is (1.1 ± 0.1)×10¹² cm⁻², and the CCS/HCCS⁺ abundance ratio is 50 ± 10, which is very similar to that of CS/HCS⁺ (35 ± 8) and CCCS/HCCCS⁺ (65 ± 20). From a state-of-the-art gas-phase chemical model, we conclude that HCCS⁺ is mostly formed by reactions of proton transfer from abundant cations such as HCO⁺, H₃O⁺, and H₃⁺ to the radical CCS.