Letter to the Editor

Discovery of C₅H⁺ and detection of C₃H⁺ in TMC-1 with the QUIJOTE line survey^⋆

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

Received: 23 December 2021
Accepted: 5 January 2022

Abstract

We report the discovery of the C₅H⁺ cation toward TMC-1 with the QUIJOTE line survey. Four lines from J = 7 − 6 up to J = 10 − 9 have been identified in perfect harmonic frequency relation that can be fit with B = 2411.94397 ± 0.00055 MHz and D = 138 ± 3 Hz. The standard deviation of the fit is 4.4 kHz. After discarding potential candidates, C₅H⁻ among them, we conclude that the carrier is C₅H⁺, for which accurate ab initio calculations provide B = 2410.3 MHz. We also report for the first time in a cold starless core the detection of the C₃H⁺ cation. The column densities we derive for C₅H⁺ and C₃H⁺ are (8.8 ± 0.5)×10¹⁰ cm⁻² and (2.4 ± 0.2)×10¹⁰ cm⁻², respectively. Hence, the C₅H⁺/C₃H⁺ abundance ratio is 3.7 ± 0.5. The fact that C₅H⁺ is more abundant than C₃H⁺ is well explained by dedicated chemical models and is due to the slow reactivity of C₅H⁺ with H₂, while C₃H⁺ reacts with H₂.