First detection of HS 2 in a cold dark cloud

We report the first detection of HS 2 towards the cold dark cloud TMC-1. This is the first observation of a chemical species containing more than one sulphur atom in this type of sources. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the Q band (31–50 GHz). The detection is confirmed by the observation of the fine and hyperfine components of two rotational transitions (2 0, 2 –1 0, 1 and 3 0, 3 –2 0, 2 ). Assuming a rotational temperature of 7 K, we derived an HS 2 column density of 5.7 × 10 11 cm −2 , using a local thermodynamic equilibrium model that reproduces the observed spectra. The abundance of HS 2 relative to H 2 is 5.7 × 10 −11 , which means that it is about seven times more abundant than its oxygenated counterpart HSO. We also explored the main formation and destruction mechanisms of HS 2 using a chemical model, which reproduces the observed abundance of HS 2 and indicates that dissociative recombination reactions from the ions H 2 S 2 + and H 3 S 2 + play a major role in forming HS 2 .