Sylvester, Roger James; (1995) Dust around main-sequence and supergiant stars. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is a study of the properties of the dust around two rather different types of star. The first part is concerned with the mid-infrared emission from a sample of 16 M-type supergiants. As well as silicate emission features, seven of the stars showed the UIR (unidentified infrared) emission bands, associated with carbonaceous material. According to standard theory, all the carbon in the outflows from these oxygen-rich stars should be bound up in CO molecules, preventing the formation of carbonaceous dust. The results were interpreted in terms of a non-equilibrium chemical model, which invoked chromospheric UV photons to dissociate CO, allowing carbonaceous material to form, and to excite the observed UIR-band emission. The larger part of the thesis considers Vega-excess stars - main sequence stars with excess infrared emission from circumstellar dust discs. Photometric and spectroscopic observations were carried out. A number of the stars displayed excess near-IR emission, indicating the presence of hot material. Mid-infrared spectroscopy enabled the grain composition to be identified: both silicates and carbonaceous species were detected. Millimetre and submillimetre photometry indicated that large grains are present around many of our sources, implying that significant grain coagulation has occurred. Most of the sources were modelled using a radiative transfer code, with disc geometry and multiple grain sizes. Two grain materials, astronomical silicate and amorphous carbon, were considered. Successful fits to the spectral energy distributions at mid-IR and longer wavelengths were found. The temperatures needed to produce near-IR excess emission were too high for grains in thermal equilibrium to survive. A model was therefore developed with very small grains undergoing thermal spiking due to single-photon absorption, which provided satisfactory fits for the hottest stars; the others had insufficient UV flux to excite the small grains.

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