Stace, HAW; (2015) Chemical-MHD Modelling Of Collapsing Pre-stellar Cores. Doctoral thesis , UCL (University College London).

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Abstract

AA multi-point magnetohydrodynamic (MHD) model of the early stages of collapsing prestellar cores is presented that takes into account the effects of ambipolar diffusion by deriving ionization levels from a network of chemical and grain-surface reactions. The contribution from both ions and charged grains is taken into account when determining the magnetic retardation effect on the flow of neutral particles. The resulting radial chemical profiles are used to determine molecular line profiles with the objective of identifying features in the case where ambipolar diffusion plays an important role in the dynamics of collapsing pre-stellar cores. It is found that a steep ionization drop at AV ~3 in the core is coincident with a rise in the dust-gas ratio: the drop in ionization acts as a barrier to ambipolar diffusion of charged grains; dust moves out from the centre of the core to supply the build-up of dust at the ambipolar diffusion barrier. The model predicts a broad velocity field with infall speeds increasing towards the centre; the associated spectral signature for optically thick lines is that of asymmetric line profiles with a dip velocity that is offset from the local standard of rest. The parameters that affect the model are analysed and their effect on collapse timescales, radial gas density distribution, synthetic line profiles and dust continuum emission determined. By varying parameters such as the cosmic ray ionization rate, initial atomic abundance fractions and grain chemistry, a picture of the sensitivity of collapsing pre-stellar cores to their environment is obtained.

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