The structure and the origin of the molecular gas along chemically rich outflows
Bipolar outflows are ubiquitously present in Young Stellar Objects. They are the mechanism through which the accreting central object loses the excessive angular momentum and they play a major role in stopping the gravitational collapse and probably in fixing the final mass of the forming star. A few outflows show a rich molecular emission characterized by small (d < 0.1 pc), low velocity (v < 10km s_1) clumps. The aim of this thesis is to investigate the origin and the nature of the low velocity molecular clumps by the use of a chemical model that simulates the clump formation and its subsequent interaction with the outflow, coupled with a radiative transfer model that reproduces the line profile for a direct comparison with the observations. Different scenarios for the formation of the clumps have been investigated within a large range of physical conditions. The models have been compared with single dish observations of the clumps observed along the CB3 and LI 157 outflows. The best models are those where the clump is formed, at least partially, before the advent of the outflow with the advent of the outflow the clump undergoes a short period of non-dissociative shock and the consequent release of the icy mantle, together with the high temperature chemistry, leads to the observed chemical enrichment. The structure of the molecular gas has also been investigated with high spatial resolution observations carried out with the Plateau de Bure Interferometer. The interferometric data confirm the results of the previous analysis: what is seen as a single molecular clump with size < 0.1 pc by the single dish telescope when observed at high resolution is in fact a complex structure with sub-clumps of the order of 0.02 pc different both in the physical conditions and their chemical composition.