In this thesis we 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 densities and temperatures in the gas envelopes
surrounding the youngest protostars are favourable for exciting a number of rotational
molecular transitions, observable in the submillimetre waveband. The line profiles of
these transitions contain information about both the physical state and dynamics o f the
envelope gas, and may potentially be used to test theoretical models of star formation.
We review the observational and theoretical background to the current picture of star
formation and protostellar evolution, and the previous molecular line studies of infall
in molecular clouds and protostellar envelopes.
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 A-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 our 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) show red-skewed line profiles, in direct
conflict with the infall expectation. We 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. We
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.
We 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 we find 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.
We 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. We 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 our best fit models are inconsistent with the Shu model, since for
both objects modelled, the infall velocities appear further advanced than the Shu model
would predict, given the density profile. We find better agreement with a form of collapse which assumes non-static initial conditions than with a static singular isothermal
sphere. We also find tentative evidence that the infall velocity is retarded from free-fall
towards the centre of the cloud.