Authors: Cernicharo, J.; Agundez, M.; Cabezas, C.; Fuentetaja, R.; Tercero, B.; Marcelino, N.; Endo, Y.; Pardo, J. R.; de Vicente, P.


Publication date: 2022/01/24

DOI: 10.1051/0004-6361/202142992

Abstract: We report the discovery of the C5H+ cation toward TMC-1 with the QUIJOTE line survey. Four lines from Jx2004;=7 – 6 up to J10 – 9 have been identified in perfect harmonic frequency relation that can be fit with Bx2004;=2411.94397 +/- 0.00055 MHz and Dx2004;=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 Bx2004;=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)x10(10) cm(-2) and (2.4 +/- 0.2)x10(10) 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 H-2, while C3H+ reacts with H-2.