Sulphur chemistry in low mass star forming regions

Solar type stars are formed by the gravitational collapse of molecular clouds. This collapse goes with the formation a hot core region, inside the protostar, and shocked regions induced by protostellar outflows. In these hot regions, hot core and shocks, the sulphur depleted on the grain mantles evaporates in the gas phase in an unknown form and participate to a rich warm gas phase chemistry. The main goal of my thesis is the study of the possibility to use sulphur bearing species as chemical clocks of star formation. I first analysed millimeter observations of two low mass protostars and their environment with the aim to understand the behavior of S-molecules with respect to the gas density and temperature. I also made a theoretical study of SO and SO₂ emission in protostellar envelopes using a radiative transfer code associated with a dynamical collapsing model. I selected the best transitions of these two molecules to compute the SO and SO₂ abundance profiles through the protostar envelopes. Finally, I constructed a chemical interstellar model depending on time in order to check if S-bearing species can be used to date star forming regions. In fact, the abundance ratios of the main S-bearing species depend more on the physical conditions of the gas and on the form of sulphur evaporated from the grain than on the time. However, the situation is not totally hopeless. A careful comparison between observations and model predictions can give some useful hints on time estimates and on the mantle composition. As a result, my work suggests that sulphur is depleted on the grain mantles in the atomic form or in form quickly converted into S once evaporated. (author)