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Title: Time dependent multifluid magnetohydrodynamic models of C-type shocks in weakly ionized, dusty media
Author: Ashmore, Ian
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2011
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During the star formation process, low mass protostars go through a period of mass loss which involves expelling as much as half of the material from the accretion disk along their polar axes. The ejected material affects the environment surrounding the protostar as it interacts with the quiescent material of the core. Some of the core material is entrained and accelerated above the local sound speed, ensuring that shocks are present at the interfaces with the undisturbed cloud. Because of the high densities, appreciable magnetic field and low fractional ionization in such regions, ambipolar diffusion allows the ram pressure of the flow to be dissipated over an extended distance instead of one or two collisions. This precludes collisional ionization and reduces molecular dissociation in the shock front and therefore guarantees efficient cooling. The entrained cloud material is known as a molecular outflow and the extended associated shocks are denoted C-type. In addition, the conditions in the shocks produce streaming velocities between the charged and neutral species. This allows the dust grains present to become negatively charged and facilitates collisions and chemical reactions both in the gas phase and on grain surfaces. Although previous work exists into the structure and processes in C-type shocks, computational limitations have restricted most studies to either shocks at steady state or ones which are perpendicular to the magnetic field. The conditions in molecular clouds make steady shocks unlikely and the outflows make an arbitrary angle with the magnetic field making it unlikely that an individual outflow is even nearly perpendicular to the upstream field. A new MHD scheme is available which allows numerical models the structures of non-steady oblique shocks to be made. This thesis details multifluid MHD models of the C-type shocks in molecular outflows using this scheme. Steady perpendicular and oblique C-type shock structures are obtained for suitable molecular outflow conditions and compared to previous models to confirm the schemes accuracy. The first non-steady simulations are undertaken to investigate the interaction of steady, oblique C-type shocks with perturbations in the upstream density which are chosen to simulate the clumpy nature of molecular cloud cores. Finally terms are developed to describe the systematic collisions between charged grains and neutrals which are thought to return icy mantle and refractory grain core material to the gas phase in a process known as sputtenng. A region of parameter space suitable for the molecular outflows from low mass protostars is explored. The results are compared to previous observations and theoretical investigations. Chemical segregations are noted along the outflow axis, most notably between HCO+, which is common in the upstream region and the edge of the precursor and water and SiO, which become common in the shock and remain so in the downstream region. It is found that the formation and sputtering of water directly causes the HCO+ abundance to fall. This finding is in harmony with recent observations. Further investigation also reveals a that the material sputtered from the grains has a dependence on the angle between the shock normal and the upstream magnetic field. Such a dependence follows from the expressions employed for the sputtering rates, but has not been previously noted or quantified. It is found that the angular dependance is important over a wide region of the parameter space and the insights gained are likely to affect the interpretation of observations of the gas phase products of sputtering in the future.
Supervisor: Caselli, Paula ; Caselli, Tom Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available