Letter to the Editor

Space and laboratory discovery of HC₃S^+⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: jose.cernicharo@csic.es
² Department of Applied Chemistry, Science Building II, National Chiao Tung University, 1001 Ta-Hsueh Rd., Hsinchu 30010, Taiwan
³ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁴ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain

Received: 27 November 2020
Accepted: 9 January 2021

Abstract

We report the detection in TMC-1 of the protonated form of C₃S. The discovery of the cation HC₃S⁺ was carried through the observation of four harmonically related lines in the Q band using the Yebes 40 m radiotelescope, and is supported by accurate ab initio calculations and laboratory measurements of its rotational spectrum. We derive a column density N(HC₃S⁺) = (2.0 ± 0.5)×10¹¹ cm⁻², which translates to an abundance ratio C₃S/HC₃S⁺ of 65 ± 20. This ratio is comparable to the CS/HCS⁺ ratio (35 ± 8) and is a factor of about ten larger than the C₃O/HC₃O⁺ ratio previously found in the same source. However, the abundance ratio HC₃O⁺/HC₃S⁺ is 1.0 ± 0.5, while C₃O/C₃S is just ∼0.11. We also searched for protonated C₂S in TMC-1, based on ab initio calculations of its spectroscopic parameters, and derive a 3σ upper limit of N(HC₂S⁺) ≤ 9 × 10¹¹ cm⁻² and a C₂S/HC₂S⁺ ≥ 60. The observational results are compared with a state-of-the-art gas-phase chemical model and conclude that HC₃S⁺ is mostly formed through several pathways: proton transfer to C₃S, reaction of S⁺ with c-C₃H₂, and reaction between neutral atomic sulfur and the ion C₃H₃⁺.