Holdship, JR; (2017) Shock chemistry in star forming environments. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Molecules are abundant in many astrophysical environments. The observation of these molecules and the modelling of the chemistry that leads to their formation is a powerful tool for improving our understanding of the regions in which they are found. In this thesis, a chemical model is developed and applied to astrophysical shocks to understand a number of processes in star forming regions. Shocks often result in discontinuous changes in the temperature and density of the gases they affect. The turbulent fragmentation theory of star formation suggests that such shocks are responsible for the over-densities that lead to the formation and collapse of prestellar cores from molecular clouds. Focusing on low mass objects, a chemical model of prestellar cores formed through shocks is developed and compared to models of similarly dense gas formed through gravitational freefall. Observational predictions of shock induced differences in the chemistry of these cores are reported and compared to the literature. Not only are molecules abundant in the gas phase but they are also observed in solid state, frozen onto the surfaces of interstellar dust grains. This freeze out process is efficient in molecular clouds but the composition of these ices is poorly constrained for all but the most abundant molecules. Shocks represent a powerful tool to study these ices as they not only change the gas properties but can also sputter and shatter dust grains releasing frozen molecules into the gas phase. Observations of sulfur-bearing molecules in a protostellar outflow, L1157-B1, are presented and analysed, giving an overview of the sulfur content of the recently shocked gas. The abundances of important sulfur molecules are then compared to chemical models of C-type shocks in an attempt to constrain the main solid state form of sulfur before the shock and the shock properties of the observed outflow.

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