Discovery of the elusive thioketenylium, HCCS<sup>+</sup>, in TMC-1
C. Cabezas, M. Agúndez, N. Marcelino, B. Tercero, Yasuki Endo, R. Fuentetaja, J. R. Pardo, P. de Vicente, J. Cernicharo
Abstract
We report the detection in TMC-1 of the cation HCCS + ( X̃ 3 Σ − ), which is the protonated form of the widespread radical CCS. This is the first time that a protonated radical has been detected in a cold dark cloud. Twenty-six hyperfine components from twelve rotational transitions have been observed with the Yebes 40 m and IRAM 30m radio telescopes. We confidently assign the characteristic rotational spectrum pattern to HCCS + based on the good agreement between the astronomical and theoretical spectroscopic parameters. The column density of HCCS + is (1.1 ± 0.1)×10 12 cm −2 , and the CCS/HCCS + abundance ratio is 50 ± 10, which is very similar to that of CS/HCS + (35 ± 8) and CCCS/HCCCS + (65 ± 20). From a state-of-the-art gas-phase chemical model, we conclude that HCCS + is mostly formed by reactions of proton transfer from abundant cations such as HCO + , H 3 O + , and H 3 + to the radical CCS.