2003A&A...412..133V

The sulphur chemistry in nine regions in the earliest stages of high-mass star formation is studied through single-dish submillimeter spectroscopy. The line profiles indicate that 10-50% of the SO and SO₂ emission arises in high-velocity gas, either infalling or outflowing. For the low-velocity gas, excitation temperatures are 25K for H₂S, 50K for SO, H₂CS, NS and HCS⁺, and 100K for OCS and SO₂, indicating that most observed emission traces the outer parts (T<100K) of the molecular envelopes, except high-excitation OCS and SO₂ lines. Abundances in the outer envelopes, calculated with a Monte Carlo program, using the physical structures of the sources derived from previous submillimeter continuum and CS line data, are ∼10^–8 for OCS, ∼10^–9 for H₂S, H₂CS, SO and SO₂, and ∼10^–10 for HCS⁺ and NS. In the inner envelopes (T>100K) of six sources, the SO₂ abundance is enhanced by a factor of ∼100-1000. This region of hot, abundant SO₂ has been seen before in infrared absorption, and must be small, ≲0.2" (180AU radius). The derived abundance profiles are consistent with models of envelope chemistry which invoke ice evaporation at T∼100K. Shock chemistry is unlikely to contribute. A major sulphur carrier in the ices is probably OCS, not H₂S as most models assume. The source-to-source abundance variations of most molecules by factors of ∼10 do not correlate with previous systematic tracers of envelope heating. Without observations of H₂S and SO lines probing warm (>100K) gas, sulphur-bearing molecules cannot be used as evolutionary tracers during star formation.