2022A&A...657L...4C

We report the detection in TMC-1 of the cation HCCS⁺ (XX^~3Σ^–), 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)x10¹² 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⁺, H₃O⁺, and H₃⁺ to the radical CCS.