Computer Science – Performance
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt........59b&link_type=abstract
Ph. D. thesis, Univ. of Edinburgh, (1997)
Computer Science
Performance
3
Scientific paper
I study the problem of how to identify, and measure the properties of infalling protostellar envelopes, through radiative transfer modelling of submillimetre spectral line observations. The physical concepts and assumptions used in radiative transfer modelling of rotational molecular lines are discussed. The observations of this thesis are modelled using an exact, non-LTE, spherically symmetric radiative transfer code (STENHOLM), which numerically solves the radiative transfer problem using the λ-iteration method. A detailed description of the code is given, including a number of new modifications. To test the performance of the code, comparisons are shown between the line profiles produced by STENHOLM and independent analytical and numerical calculations. 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. Of the ten objects included in the sample, five show qualitative signatures of infall (i.e. blue-skewed line profiles) in the high critical density tracers, CS and HCO+. Of the remaining objects, four show either no signature of infall or conflicting signatures in different tracers, and one (L483) shows red-skewed line profiles, in direct conflict with the infall expectation. I examine the evidence that the line profiles of the HCO+ and CS transitions observed towards each of the objects are confused by emission from outflows, by comparing wherever possible the morphology and centroid velocity gradients found in maps of these transitions with CO outflow maps. I find that the CS and HCO+ submillimetre transitions, which are usually thought of as good tracers of protostellar envelope gas by virtue of their large critical densities, are often significantly contaminated by outflow emission. For the three objects which show the strongest evidence for infall (NGC1333-IRAS2, IRAS 16293-2422 and Serpens SMM4) strong centroid velocity gradients are measured in the CS and HCO+ maps. I examine whether these velocity gradients are caused by outflow or rotation, and conclude that in the case of NGC1333-IRAS2, the outflow dominates the velocity gradient, whereas there is strong evidence that the IRAS 16293-2422 and Serpens SMM4 velocity gradients are due to rotation. Both these latter objects show evidence for elongation of their envelopes perpendicular to the rotation axis, suggesting they may be partially centrifugally supported. I examine the physical constraints which can be used to limit the number and range of parameters used in protostellar envelope models, and identify the turbulent velocity and tracer molecule abundance as the principle sources of uncertainty in the radiative transfer modelling. I explore the trends in the appearance of the predicted line profiles as certain key parameters in the models are varied. The formation of the characteristic asymmetric double-peaked line profile in infalling envelopes is discussed in detail, and some previous misconceptions concerning this problem are highlighted. Radiative transfer modelling is carried out on HCO+ and CS observations of NGC1333-IRAS2 and Serpens SMM4, using the STENHOLM radiative transfer code. Adequate fits are found for most of the observed line profiles using plausible infall model parameters, and possible reasons for the discrepancies are suggested. The density and velocity profiles in the best fit models are inconsistent with the Standard Model, since for both objects modelled, the infall velocities appear further advanced than the Standard Model would predict, given the density profile. I find better agreement with a form of collapse which assumes non-static initial conditions than with a static singular isothermal sphere. I also find tentative evidence that the infall velocity is retarded from free-fall towards the centre of the cloud.
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