S-bearing molecules in Massive Dense Cores

Context. High-mass stars, though few in number, play a major role in the interstellar energy budget and the shaping of the Galactic environment. However, despite this importance, the formation of high-mass stars is not well understood, due to their large distances, short time scales, and heavy extinction. Aims. Chemical composition of the massive cores forming high-mass stars can put some constrains on the time scale of the massive star formation: sulphur chemistry is of specific interest due to its rapid evolution in warm gas and because the abundance of sulphur bearing species increases significantly with the temperature. Methods. Two mid-infrared quiet and two brighter massive cores are observed in various transitions (Eup up to 289 K) of CS, OCS, H2S, SO, SO2 and of their 34S isotopologues at mm wavelengths with the IRAM 30m and CSO telescopes. 1D modeling of the dust continuum is used to derive the density and temperature laws, which are then applied in the RATRAN code to model the observed line emission, and to derive the relative abundances of the molecules. Results. All lines, except the highest energy SO2 transition, are detected. Infall (up to 2.9 km s−1) may be detected towards the 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 variations in abundance ratios from source to source reveals that the SO and SO2 relative abundances increase with time, while CS and OCS decrease. Conclusions. Molecular ratios, such as [OCS/H2S], [CS/H2S], [SO/OCS], [SO2/OCS], [CS/SO] and [SO2/SO] may be good indicators of evolution depending on layers probed by the observed molecular transitions. Observations of molecular emission from warmer layers, hence involving higher upper energy levels are mandatory to include.