QUIJOTE discovery of the cation radicals HC5N+ and HC7N+

TL;DR

This paper reports the first detection of the cation radicals HC5N+ and HC7N+ in space, providing their spectral properties, abundance ratios, and insights into their formation and destruction mechanisms in the interstellar medium.

Contribution

The discovery of HC5N+ and HC7N+ in TMC-1, including their spectral constants, abundance ratios, and chemical formation pathways, which were previously unknown.

Findings

HC5N+ and HC7N+ detected in TMC-1 with specific rotational lines.

Derived physical parameters: Trot and column densities for both cations.

Chemical models suggest formation mainly via reactions with H2 and H+, but current reaction rates may be overestimated.

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 2Pi ground electronic states with very good estimates for their B0 and AS0 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 Beff = 1336.662+/- 0.001 MHz and Deff = 27.4+/-2.6 Hz, while for HC7N+ they are Beff = 567.85036+/-0.00037 MHz and Deff = 4.01+/-0.19 Hz. From the observed intensities, we derived Trot = 5.5+/-0.5K and N = (9.9+/-1.0)x 1010 cm-2 for HC5N+, while we obtained Trot = 8.5+/-0.5K and N = (2.3+/-0.2)x1010 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 around 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.