Justtanont, Kuanhatai; (1992) Mass loss from evolved late type stars. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis can be divided into three parts which are related together. The first part deals with the modelling of the infrared energy distributions of oxygen-rich, late type stars, mainly OH/IR stars, in order to derive the dust mass loss rates. This is done by using the radiative transfer code, assuming the spherical symmetry, but takes into account the effects of thermal re-emission and multiple scattering. Also, the grain size distribution in circumstellar shells is assumed to follow the same power law governing the size distribution of interstellar grains. The optical properties of these grains, silicates, are investigated. For stars with large mass outflows, water-ice is expected to condense onto silicate grains. These star, seen with a deep 3.1μm absorption feature, due to water-ice, are successfully modelled using silicates derived here, with the published amorphous water-ice optical constants. The second part involves the study of gas kinetics of these stars, in particular the temperature distributions as results of collisional heating and radiative and adiabatic cooling. These, in turn, are used in the calculations of the CO line profiles, which also depend on mass loss rates. The dust and gas mass loss rates may represent different episodes at which mass is being lost from stars. The final part of this thesis involves the investigation of spectroscopic observations of post-AGB stars, a phase where mass loss has stopped and the dust shells are becoming detached from the stars, and/or having the evidence of bipolarity. Both the oxygen-rich and carbon-rich sources are studied as to establish the nature of their dust.

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