The sulfur depletion problem: upper limits on the H2S2, HS2, and S2
  gas-phase abundances toward the low-mass warm core IRAS16293-2422
  A fraction of the missing sulfur in dense clouds and circumstellar regions
could be in the form of three species not yet de- tected in the interstellar
medium: H2S2, HS2, and S2 according to experimental simulations performed under
astrophysically relevant conditions. These S-S bonded molecules can be formed
by the energetic processing of H2S-bearing ice mantles on dust grains, and
subsequently desorb to the gas phase. The detection of these species could
partially solve the sulfur depletion problem, and would help to improve our
knowledge of the poorly known chemistry of sulfur in the interstellar medium.
To this purpose we performed dedicated ground-based observations toward the
low-mass warm core IRAS16293-2422. Observations in the submillimeter regime
were obtained with the APEX 12 m telescope during 15 hours of observation,
targeting a wide selection of the predicted rotational transitions of the three
molecules. The 1{\sigma} noise rms values were extracted in the spectral
regions where the targeted species should have been detected. These values were
a factor of 2-7 lower than those reached by previous observations toward the
same source, and allowed us to estimate a 1{\sigma} upper limit to their
molecular abundances of 8.1E-9 , 1.1E-8 , and 2.9E-7 relative to H2, for H2S2,
HS2, and S2, respectively. The upper limit abundances of the three molecules
containing the S2 unit are up to two orders of magnitude lower than the H2S
abundance in the source, and one order of magnitude lower than the expected
abundances from the experimental simulations using ice analogs. Subsequent
gas-phase chemistry after desorption could lower the abundances of the three
species to undetectable levels in our observations.
