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Issue A&A
Volume 412, Number 1, December II 2003
Page(s) 133 - 145
Section Formation, structure and evolution of stars
DOI 10.1051/0004-6361:20031409



A&A 412, 133-145 (2003)
DOI: 10.1051/0004-6361:20031409

Sulphur chemistry in the envelopes of massive young stars

F. F. S. van der Tak1, A. M. S. Boonman2, R. Braakman2 and E. F. van Dishoeck2

1  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2  Sterrewacht, Postbus 9513, 2300 RA Leiden, The Netherlands

(Received 9 December 2002 / Accepted 20 August 2003)

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 SO 2 emission arises in high-velocity gas, either infalling or outflowing. For the low-velocity gas, excitation temperatures are 25 K for H 2S, 50 K for SO, H 2CS, NS and HCS +, and 100 K for OCS and SO 2, indicating that most observed emission traces the outer parts ( T<100 K) of the molecular envelopes, except high-excitation OCS and SO 2 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 H 2S, H 2CS, SO and SO 2, and ~10 -10 for HCS + and NS. In the inner envelopes ( T>100 K) of six sources, the SO 2 abundance is enhanced by a factor of ~100-1000. This region of hot, abundant SO 2 has been seen before in infrared absorption, and must be small, $\la$0 $\farcs$2 (180 AU radius). The derived abundance profiles are consistent with models of envelope chemistry which invoke ice evaporation at $T\sim 100$  K. Shock chemistry is unlikely to contribute. A major sulphur carrier in the ices is probably OCS, not H 2S 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 ( $\ga$100 K) gas, sulphur-bearing molecules cannot be used as evolutionary tracers during star formation.


Key words: ISM: molecules -- molecular processes -- stars: circumstellar matter -- stars: formation

Offprint request: F. F. S. van der Tak, vdtak@mpifr-bonn.mpg.de

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