Modelling the sulphur chemistry evolution in Orion KL⋆
Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón-Ajalvir, km. 4,
Torrejón de Ardoz, Madrid,
2 Department of Physics & Astronomy, University College London, Gower St. London WC1E 6BT, UK
Accepted: 21 May 2014
Context. We present a study of the sulphur chemistry evolution in the region Orion KL along the gas and grain phases of the cloud.
Aims. Our aim is to investigate the processes that dominate the sulphur chemistry in Orion KL and to determine how physical and chemical parameters, such as the final mass of the star and the initial elemental abundances, influence the evolution of the hot core and of the surrounding outflows and shocked gas (the plateau).
Methods. We independently modelled the chemistry evolution of the hot core and the plateau using the time-dependent gas-grain model UCL_CHEM and considering two different phase calculations. Phase I starts with the collapsing cloud and the depletion of atoms and molecules onto grain surfaces. Phase II starts when a central protostar is formed and the evaporation from grains takes place. We show how the stellar mass, the gas density, the gas depletion efficiency, the initial sulphur abundance, the shocked gas temperature, and the different chemical paths on the grains leading to different reservoirs of sulphur on the mantles affect sulphur-bearing molecules at different evolutionary stages for both components. We also compare the predicted column densities with those inferred from observations of the species SO, SO2, CS, OCS, H2S, and H2CS.
Results. The models that reproduce the observations of the largest number of sulphur-bearing species in both components are those with an initial sulphur abundance of 0.1 times the sulphur solar abundance (0.1 S⊙) and a density of at least nH = 5 × 106 cm-3 in the shocked gas region.
Conclusions. We conclude that most of the sulphur atoms were ionised during Phase I, consistent with an inhomogeneous and clumpy region where the UV interstellar radiation penetrates and leading to sulphur ionisation. We also conclude that the main sulphur reservoir on the ice mantles was H2S. In addition, we deduce that a chemical transition currently takes place in the plateau shocked gas, where SO and SO2 gas-phase formation reactions change from being dominated by O2 to being dominated by OH.
Key words: astrochemistry / ISM: abundances / ISM: clouds / ISM: molecules
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© ESO, 2014