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Title: The initial conditions of star formation
Author: Jessop, Nicholas Edgar
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1999
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This thesis is a study of the physical conditions typical in regions of the Inter Stellar Medium which are likely to go on to form stars. In recent years considerable progress has been made in the understanding of low mass star formation following various studies of opaque regions in near by parts of the Galaxy. The regions' high densities and high opacities make them favourable environments for star formation to occur. Using the IRAS Sky Survey Atlas a number of high latitude clouds are selected, and optical depths maps of these clouds are constructed. The most opaque regions in these clouds are identified and catalogued as a set of cloud cores. The column density and mass of each core are calculated. The majority of the cores are found to be gravitationally bound, and a small fraction are found to have signs of protostellar content; they contain IRAS point sources within them with spectral characteristics typical of Protostars or embedded Young Stellar Objects. An analysis of the typical properties of this set of cores and the typical properties found in previous studies of more opaque and more dense regions in the Inter Stellar Medium reveals that the timescale in which a cloud core forms a protostar decreases as both the opacity and density of the core increases. The results compare well with physical models of star formation in which the prestellar evolution is regulation by ambipolar diffusion, and the strong influence of various sources of ionizing on the star formation timescale is discussed. The very earliest protostars typically have a massive infalling envelope (visible in submillimetre continuum and molecular line observations), vigorous outflows, and radio emission due to shocks. The latter two processes are thought to be powered by the dynamically infalling envelope. Cores without any sign of protostellar content often contain dense regions similar in mass, but less dense, than the protostars' envelopes. Dense and massive enough to go on to form stars, these objects are precursors to the protostars; prestellar cores. A high resolution submillimetre study of a subset of these cores carried out with the JCMT in Hawaii is presented. Various isotopes of CO were detected towards the cores. One core in particular, L1689B, proved bright enough to map over a considerable region in both C18O(J=2→1) and C18O(J=3→2). All these observations are presented. In order to fully interpret the observations of L1689B, and to compare its properties with theoretical models of the early stages of star formation, a parameterised representation of L1689B is modelled with a radiative transfer code in order to produce predicted maps which can be compared with the observations. It is found that simple models of L1689B, assuming an isothermal gas temperature and constant abundance account for L1689B's appearance. Either a temperature drop or a drop in CO abundance towards its centre are needed to explain the observations. Neither however are uniquely implied by the C18O observations alone. A reanalysis of a submillimetre continuum maps of L1689B is made, and using these results the degeneracy is partly lifted. L1689B appears to have both a drop in temperature and a fall in abundance, both of which could be causally linked one to the other, or more likely, both of which are dependent on an external factor like the lack of ultra violet penetration towards the centre. Either way it is shown that the central region of L1689B is unsupported by pressure gradients, and susceptible to rapid free fall unless the existence of magnetic fields is assumed. This core therefore seems to be being supported by magnetic fields and evolving by ambipolar diffusion.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available