Discovery of C

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