Space and laboratory discovery of HC₃ S

We report the detection in TMC-1 of the protonated form of C₃S. The discovery of the cation HC₃S⁺ was carried through the observation of four harmonically related lines in the Q band using the Yebes 40m radiotelescope, and is supported by accurate ab initio calculations and laboratory measurements of its rotational spectrum. We derive a column density N(HC₃S⁺) = (2.0 ± 0.5) × 10¹¹ cm^-2, which translates to an abundance ratio C₃S/HC₃S⁺ of 65 ± 20. This ratio is comparable to the CS/HCS⁺ ratio (35 ± 8) and is a factor of about ten larger than the C₃O/HC₃O⁺ ratio previously found in the same source. However, the abundance ratio HC₃O⁺/HC₃S⁺ is 1.0 ± 0.5, while C₃O/C₃S is just ~ 0.11. We also searched for protonated C₂S in TMC-1, based on ab initio calculations of its spectroscopic parameters, and derive a 3σ upper limit of N(HC₂S⁺)≤ 9×10¹¹ cm^-2 and a C₂S/HC₂S⁺ ≥ 60. The observational results are compared with a state-of-the-art gas-phase chemical model and conclude that HC₃S⁺ is mostly formed through several pathways: proton transfer to C₃S, reaction of S⁺ with c-C₃H₂, and reaction between neutral atomic sulfur and the ion C₃H⁺ ₃.