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.
