2025A&A...699L...2E

We report the first detection of HS₂ 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_–10, 1_ and 3_0, 3_–20, 2_). Assuming a rotational temperature of 7 K, we derived an HS₂ column density of 5.7 × 10¹¹ cm^–2, using a local thermodynamic equilibrium model that reproduces the observed spectra. The abundance of HS₂ relative to H₂ 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 HS₂ using a chemical model, which reproduces the observed abundance of HS₂ and indicates that dissociative recombination reactions from the ions H₂S$ ^{+}_{\mathrm{2}} $ and H₃S$ ^{+}_{\mathrm{2}} $ play a major role in forming HS₂.