Detection of interstellar HCS and its metastable isomer HSC: new pieces in the puzzle of sulfur chemistry

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 thanks to observations carried out with the IRAM 30m telescope in the 3 mm band. We derive beam-averaged column densities of 7e12 cm-2 for HCS and 1.8e11 cm-2 for HSC, which translate to fractional abundances relative to H2 of 2e-10 and 6e-12, respectively. Although the amount of sulfur locked by these radicals is low, their detection allows to put interesting constraints on the chemistry of sulfur in dark clouds. Interestingly, the H2CS/HCS abundance ratio is found to be quite low, around 1, in contrast with the oxygen analogue 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 analogue, HCO. The metastable species HOC, the oxygen analogue of HSC, has not been yet observed in space. These observational constraints are confronted 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.Comment: Accepted for publication in A&A Letter

Detection of interstellar HCS and its metastable isomer HSC: New pieces in the puzzle of sulfur chemistry

4 pags., 2 figs., 1 tab.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 30 m telescope in the λ3 mm band. We derive beam-averaged column densities of 7 × 10 cm for HCS and 1.8 × 10 cm for HSC, which translate into fractional abundances relative to H of 2 × 10 and 6 × 10, 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 HCS/HCS abundance ratio is found to be quite low, ~1, in contrast with the oxygen analog case, in which the HCO/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 HCS/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.We acknowledge funding support from the European Research Council (ERC Grant 610256: NANOCOSMOS) and from Spanish MINECO through grant AYA2016-75066- C2-1-P. M.A. also acknowledges funding support from the Ramón y Cajal programme of Spanish MINECO (RyC-2014-16277).Peer Reviewe