Sulphur chemistry in the envelopes of massive young stars

Abstract

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, H 2CS, 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 ∼10-8 for OCS, ∼10 -9 for H2S, H2CS, SO and SO2, and ∼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 ∼100-1000. This region of hot, abundant SO2 has been seen before in infrared absorption, and must be small, ≲0″.2 (180 AU radius). The derived abundance profiles are consistent with models of envelope chemistry which invoke ice evaporation at T ∼ 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 ∼10 do not correlate with previous systematic tracers of envelope heating. Without observations of H 2S and SO lines probing warm (≳100 K) gas, sulphur-bearing molecules cannot be used as evolutionary tracers during star formation.