Letter to the Editor

QUIJOTE discovery of the cation radicals HC₅N⁺ and HC₇N^+⋆

¹ Dept. 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 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: 20 May 2024
Accepted: 31 May 2024

Abstract

We present the discovery with the QUIJOTE line survey of the cations HC₅N⁺ and HC₇N⁺ in the direction of TMC-1. Seven lines with half-integer quantum numbers from J = 25/2–23/2 to 37/2–35/2 have been assigned to HC₅N⁺ and eight lines from J = 55/2–53/2 to 71/2–69/2 to HC₇N⁺. Both species have inverted ²Π ground electronic states with very good estimates for their B₀ and A_SO constants based on optical observations. The lines with the lowest J of HC₅N⁺ exhibit multiple components due to the hyperfine structure introduced by the H and N nuclei. However, these different components collapse for the higher J. No hyperfine structure is found for any of the lines of HC₇N⁺. The derived effective rotational and distortion constants for HC₅N⁺ are B_eff = 1336.662 ± 0.001 MHz and D_eff = 27.4 ± 2.6 Hz, while for HC₇N⁺ they are B_eff = 567.85036 ± 0.00037 MHz and D_eff = 4.01 ± 0.19 Hz. From the observed intensities, we derived T_rot = 5.5 ± 0.5 K and N = (9.9 ± 1.0) × 10¹⁰ cm⁻² for HC₅N⁺, while we obtained T_rot = 8.5 ± 0.5 K and N = (2.3 ± 0.2) × 10¹⁰ cm⁻² for HC₇N⁺. The HC₅N/HC₅N⁺, C₅N/HC₅N⁺, C₅N⁻/HC₅N⁺, HC₇N/HC₇N⁺, HC₅N⁺/HC₇N⁺, and C₇N⁻/HC₇N⁺ abundance ratios are 670 ± 80, 4.8 ± 0.8, 1.2 ± 0.2, 1000 ± 150, 4.2 ± 0.5, and 2.2 ± 0.2, respectively. We have run chemical modelling calculations to investigate the formation and destruction of these new cations. We find that these species are mainly formed through the reactions of H₂ and the cations C₅N⁺ and C₇N⁺, and by the reactions of H⁺ with HC₅N and HC₇N, while they are mostly destroyed through a reaction with H₂ and a dissociative recombination with electrons. Based on the underestimation of the abundances of HC₅N⁺ and HC₇N⁺ by the chemical model by a factor ∼20, we suggest that the rate coefficients currently assumed for the reactions of these cations with H₂ could be too high by the same factor, something that will be worth investigating.