Modelling the sulphur chemistry evolution in Orion KL

Abstract: We study the sulphur chemistry evolution in the Orion KL along the gas and grain phases of the cloud. We investigate the processes that dominate the sulphur chemistry and to determine how physical and chemical parameters, such as the final star mass and the initial elemental abundances, influence the evolution of the hot core and of the surrounding outflows and shocked gas (the plateau). We independently modelled the chemistry evolution of both components 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 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. We also compare the predicted column densities with those inferred from observations of the species SO, SO2, CS, OCS, H2S and H2CS. The models that reproduce the observations of the largest number of sulphur-bearing species are those with an initial sulphur abundance of 0.1 times the sulphur solar abundance and a density of at least n_H=5x10^6 cm^-3 in the shocked gas region. 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 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 shocked gas, where SO and SO2 gas-phase formation reactions change from being dominated by O2 to being dominated by OH.