2022A&A...657L..16C

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)x10¹⁰ cm^–2 and (2.4 ±0.2)x10¹⁰ 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.