S-bearing molecules in massive dense cores

摘要

Context. Although few in number, high-mass stars play a major role in the interstellar energy budget and the shaping of the Galacticenvironment; however, the formation of high-mass stars is not well understood, because of their large distances, short time scales, andheavy extinction.Aims. The chemical composition of the massive cores forming high-mass stars can put some constraints on the time scale of themassive star formation: sulfur chemistry is of specific interest thanks to its rapid evolution in warm gas and because the abundance ofsulfur-bearing species increases significantly with the temperature.Methods. Two mid-infrared quiet and two brighter massive cores were observed in various transitions (E_(up) to 289 K) of CS, OCS,H_2S,SO,and SO_2and of their ~(34)Sisotopologues at mm wavelengths with the IRAM 30m andCSOtelescopes. The 1D modeling ofthe dust continuum is used to derive the density and temperature laws, which were then applied in the RATRAN code to modeling theobserved line emission and to deriving the relative abundances of the molecules.Results. All lines are detected, except the highest energy SO2 transition. Infall (up to 2.9 kms~(-1) ) may be detected towardsthe core W43MM1. The inferred mass rate is 5.8-9.4 10~(-2)M_⊙/yr. We propose an evolutionary sequence of our sources(W43MM1→IRAS18264-1152→IRAS05358+3543→IRAS18162-2048), based on the SED analysis. The analysis of the varia-tions in abundance ratios from source to source reveals that the SO and SO2 relative abundances increase with time, while CS andOCS decrease.Conclusions. Molecular ratios, such as OCS/H_2S, CS/H_2S,SO/OCS,SO_2/OCS, SO, and SO may be good indica-tors of evolution, depending on layers probed by the observed molecular transitions. Observations of molecular emission from warmerlayers, so that involving higher upper energy levels must be included.