Griffin, Ian Paul; (1991) The circumstellar environments of late type stars. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis a comprehensive model for the thermal emission from dust associated with the circumstellar environments of late type stars has been developed. The model fully takes into account the effects of non-isotropic scattering, absorption and thermal re-emission, and can calculate the radiative equilibrium temperature distributions consistent with the radiation field for a multiple grain size, multiple grain component dust distribution. The model is applied to a number of well studied late type stars, with both optically thin and optically thick dust shells whose chemistry reflects the either oxygen or carbon- rich nature of the underlying stellar photosphere. Published laboratory measurements of optical constants are used to investigate which of a variety of proposed dust condensates best fit the observations; in oxygen rich environments this is found to be amorphous olivine whilst in carbon-rich envelopes it is provided by amorphous carbon, sometimes in conjunction with silicon carbide grains. Results from a multiple wavelength observing programme of late type stars with anomalous dust shells are presented, and are compared with published models. The radiative transfer model developed for red giants with normal dust shells is used to show that a binary model for carbon stars observed to have oxygen rich dust shells has severe problems. A new model is developed for these anomalous stars by comparing their observed dust shells with those of S stars.

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