Sulphur chemistry in the envelopes of massive young stars

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 25 K for H2S, 50 K for SO, H2CS, NS and HCS+, and 100 K for OCS and SO2, indicating that most observed emission traces the outer parts (T < 100 K) 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 &SIM; 10(-8) for OCS, &SIM; 10(-9) for H2S, H2CS, SO and SO2, and &SIM; 10(-10) for HCS+ and NS. In the inner envelopes (T > 100 K) of six sources, the SO2 abundance is enhanced by a factor of similar to 100 - 1000. This region of hot, abundant SO2 has been seen before in infrared absorption, and must be small, less than or similar to0.2 (180 AU radius). The derived abundance profiles are consistent with models of envelope chemistry which invoke ice evaporation at T similar to 100 K. 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 similar to 10 do not correlate with previous systematic tracers of envelope heating. Without observations of H2S and SO lines probing warm (greater than or similar to 100 K) gas, sulphur-bearing molecules cannot be used as evolutionary tracers during star formation.