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Title: Protostellar infall : modelling submillimetre spectral line observations
Author: Buckley, Henry David
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1998
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Observations are presented of a sample of protostellar candidates (mainly Class 0 sources), in transitions of HCO+, H13CO+, CS, CO, and C18O. The HCO+ and CS transitions preferentially trace high density gas, whereas CO traces a much wider range of gas densities. A complex dynamical picture emerges, involving infall, rotation, and outflow. The CS and HCO+ line profiles observed towards several sources are in good qualitative agreement with radiative transfer predictions for infalling protosellar envelopes. In other objects the line profiles strongly disagree with the infall predictions. Outflows are detected in the high velocity CO(J = 3 → 2) emission observed towards most of the objects, although the morphology of the outflow gas in the vicinity of the protostar is often very complicated. Outflows are also apparent in the wings of the CS and HCO+ line profiles, however it is argued that in some objects outflow emission dominates the cores of these lines as well. The sources which show good qualitative evidence for infall all have strong centroid velocity gradient across their envelopes in CS and HCO+ maps. We consider whether outflow can explain the observed velocity gradients, and in most cases find that the outflow morphology, as traced by CO, is inconsistent with the direction of the centroid velocity gradient. This is used to argue that for these objects, the CS and HCO+ line core emission is dominated by the protosellar envelope rather than the outflow, and that the measured centroid velocity gradients are due to rotation of the envelope. Radiative transfer modelling is carried out on three of the best infall candidates in our sample, using the STEN-HOLM radiative transfer code. Reasonable fits are found for most of the observed line profiles using plausible model parameters, including a solid body rotation with the measured centroid velocity gradient. Possible reasons for the discrepancies between the observations and model predictions are suggested. For all three objects, a large turbulent velocity dispersion (compared with the sound speed) is required to explain the observed linewidths. The uniqueness of the model fits is examined, and we discuss possible alternative explanations for the observed line profiles. The formation of the characteristic asymmetric double-peaked line profile in infalling envelopes is discussed in detail, and some previous misconceptions are highlighted. The validity of the recently proposed 'two-slab' approximation, which allows infall line profiles to be modelled analytically, is assessed by testing it against the results of the exact radiative transfer code.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available