Space and laboratory discovery of HC .

We report the detection in TMC-1 of the protonated form of CS. The discovery of the cation HCS was carried through the observation of four harmonically related lines in the Q band using the Yebes 40m radiotelescope, and is supported by accurate calculations and laboratory measurements of its rotational spectrum. We derive a column density (HCS) = (2.0 ± 0.5) × 10 cm, which translates to an abundance ratio CS/HCS of 65 ± 20. This ratio is comparable to the CS/HCS ratio (35 ± 8) and is a factor of about ten larger than the CO/HCO ratio previously found in the same source. However, the abundance ratio HCO/HCS is 1.0 ± 0.5, while CO/CS is just ~ 0.11. We also searched for protonated CS in TMC-1, based on calculations of its spectroscopic parameters, and derive a 3σ upper limit of (HCS)≤ 9×10 cm and a CS/HCS ≥ 60. The observational results are compared with a state-of-the-art gas-phase chemical model and conclude that HCS is mostly formed through several pathways: proton transfer to CS, reaction of S with c-CH, and reaction between neutral atomic sulfur and the ion CH .