Discovery of C5H+ and detection of C3H+ in TMC-1 with the QUIJOTE line surveyx22c6;

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.

Automated summary

We report the discovery of the C5H+ and C3H+ cations toward TMC-1 with the QUIJOTE line survey. Through perfect harmonic frequency relation and accurate ab initio calculations, we have identified the carrier of C5H+ and determined the column densities of both C5H+ and C3H+ as well as their abundance ratio.