Circumstellar gas and dust emission and mass loss from evolved stars
This dissertation presents a study of the circumstellar gas and dust emission and their relation to the mass-loss in evolved stars. Considering the physical process of heating and cooling of the dust, a model to calculate the circumstellar dust emission is developed. The dust distribution is expressed in terms of the physical parameters of the envelope and the optical properties are assumed to be of a power-law type. Calculations for dust envelopes extending over a broad range of optical opacities are presented. There is special emphasis on calculations for IRAS fluxes at 12, 25, and 60 $\mu$m and the near infrared flux at 2.2 $\mu$m. Their dependence on the parameters of the model, namely, the effective temperature and luminosity of the central star, the power-law index, and the dust absorption coefficient of the grains is presented. The results are combined in color-color diagrams and it is found that they have little dependence on the luminosity and effective temperature of the central star, thus making them solely dependent on the dust properties. The calculated colors and spectra for envelopes of carbon-rich and oxygen-rich stars agree with those observed in these types of stars. Using an available model for the circumstellar gas emission, an analysis of reported CO J = 1-0 and 2-1 observations of nearby carbon-rich stars is done. The observational data supports the validity of the gas emission model. The gas model is related to the dust model through the physical parameters they have in common. This fact allows one to connect the results from the two models in order to constrain the space of their physical parameters.