Discovery of the elusive thioketenylium, HCCS+, in TMC-1
  We report the detection in TMC-1 of the cation HCCS+ (3Sigma-), 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
40m 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)e12 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.
