Testing stellar models with observations from AGB and post-AGB stars

Abstract

Low- to intermediate-mass stars (0.8 - 7 M{u2299}) are interesting in a broad astronomical context because they produce large amounts of electromagnetic radiation, they eject large amounts of mass, and the ejected mass contains many products of internal nucleosynthesis. However, theoretical studies of the evolution of red-giants are inhibited by uncertainties in important processes such as mass loss, convective mixing, and grain formation in stellar winds. From an observational point of view it is difficult to determine stellar parameters such as the initial mass and metallicity. This makes it hard to constrain theoretical models with observations. The objective of my Ph.D. thesis is to provide a deeper understanding of the physical mechanisms and the nucleosynthesis that occurs in the late stages of stellar evolution for low-to intermediate-mass stars by deriving and using the best observational constraints available. The first two parts of this study were carried out using three intermediate-age clusters in the Magellanic Clouds. Accurate initial masses and mass loss rates were estimated using the pulsation properties of cluster-AGB stars. Subsequently, updated stellar evolution models constrained with observations of these cluster-AGB stars were constructed. Surfaces abundances were calculated as a function of initial mass and metallicity and compared to observationally derived abundances. The final parts of this thesis are based on an observational study of post-AGB stars. Post-AGB stars bear signatures of the structural and chemical composition changes that occur during the AGB phase of evolution and therefore can be used to constrain AGB models. To be able to utilise the wealth of information that can be gained from these objects, a catalog of post-AGB stars in the Magellanic Clouds has been created firstly by selecting candidates based on the existence of a mid-infrared excess and secondly by obtaining low resolution optical spectra. The optical spectra and broadband photometry from the optical to mid-infrared were used to derive luminosities, effective temperatures and masses for the post-AGB stars. These catalogs of spectroscopically verified post-AGB stars are expected to be a valuable resource for the study of the late stages of stellar evolution as a function of initial mass and metallicity because of the known distances to the Magellanic Clouds. In summary, this thesis, using observations of AGB and post-AGB stars, has paved the path towards constraining and improving stellar evolution and nucleosynthesis models, and it provides a catalog of post-AGB candidates in the Magellanic Clouds that can be used in future studies to further constrain models of AGB nucleosynthesis.