2018A&A...611L...1A

We present the first identification in interstellar space of the thioformyl radical (HCS) and its metastable isomer HSC. These species were detected toward the molecular cloud L483 through observations carried out with the IRAM 30m telescope in the λ3mm band. We derive beam-averaged column densities of 7x10¹²cm^–2 for HCS and 1.8x10¹¹cm^–2 for HSC, which translate into fractional abundances relative to H₂ of 2x10^–10 and 6x10^–12, respectively. Although the amount of sulfur locked by these radicals is low, their detection allows placing interesting constraints on the chemistry of sulfur in dark clouds. Interestingly, the H₂CS/HCS abundance ratio is found to be quite low, ∼1, in contrast with the oxygen analog case, in which the H₂CO/HCO abundance ratio is around 10 in dark clouds. Moreover, the radical HCS is found to be more abundant than its oxygen analog, HCO. The metastable species HOC, the oxygen analog of HSC, has not yet been observed in space. These observational constraints are compared with the outcome of a recent model of the chemistry of sulfur in dark clouds. The model underestimates the fractional abundance of HCS by at least one order of magnitude, overestimates the H₂CS/HCS abundance ratio, and does not provide an abundance prediction for the metastable isomer HSC. These observations should prompt a revision of the chemistry of sulfur in interstellar clouds.