Sulfur-Bearing Molecules In Massive Star-Forming Regions: Observations
  Of OCS, CS, H2S and SO
  We studied sulfur chemistry of massive star-forming regions through
single-dish submillimeter spectroscopy. OCS, O13CS, 13CS, H2S and SO
transitions were observed toward a sample of massive star-forming regions with
embedded UCH II or CH II regions. These sources could be divided into H II-hot
core and H II-only sources based on their CH3CN emission. Our results show that
the OCS line of thirteen sources is optically thick, with optical depth ranging
from 5 to 16. Column densities of these molecules were computed under LTE
conditions. CS column densities were also derived using its optically thin
isotopologue 13CS. H2S is likely to be the most abundant gas-phase sulfuretted
molecules in hot assive cores. Both the column density and abundance of
sulfur-bearing molecules decrease significantly from H II-hot core to H II-only
sources. Ages derived from hot core models appear to be consistent with
star-formation theories, suggesting that abundance ratios of [CS]/[SO],
[SO]/[OCS] and [OCS]/[CS] could be used as chemical clocks in massive
star-forming regions.
