Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Professor John Landstreet and Professor Jan Cami

Abstract

The study of the chemical evolution of stars is of crucial importance since they play a major role in the enrichment of the chemistry of the universe. Throughout their lifetime, stars undergo several processes that can alter their chemistry. Gradually, the nucleosynthesis products from the interior of the star are radiatively and convectively levitated and mixed with the upper layers of the atmosphere. In the later stages of their evolution, low to intermediate mass stars (0.8-8.0~M☉) eject a significant fraction of these nucleosynthesis products, resulting in a circumstellar envelope of gas and dust around the central star with a very different and intriguing chemical composition. Through radiation pressure, this material will sail into the interstellar medium.


We offer the first extensive study concerning the atmospheric abundance determination using ultraviolet spectra, for two slow rotating non-magnetic early A-type (HD72660) and B-type (iota Herculis) stars. We carry out spectrum synthesis of the available UV spectra using the program ZEEMAN followed by a detailed abundance analysis. We find lines of several previously unstudied elements in the atmospheres of the two stars under study, implying the activity of
phenomena such as radiative levitation against gravity. In addition to finding new elements, we demonstrate that spectrum synthesis using ZEEMAN in the ultraviolet is indeed a reliable source for abundance determination.

This study is followed by a very careful and detailed infrared spectroscopy of a sample of evolved AGB stars in the final stages of their evolution. The sample is unique in a sense that properties such as distance, metallicity, initial mass of the parent star are comparable while the only variable is the age on the AGB phase. Throughout this study we shed light on the significance of interstellar extinction and its previously under-acknowledged influence on the infrared spectra. We introduce tools and methods that will allow us to separately model the spectrum of the photosphere and the molecular layers alone. We extract the dust spectra and present a qualitative analysis of the intriguing peculiarities.