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Lind, Karin (2010): Chemical analysis of globular star clusters: theory and observation. Dissertation, LMU München: Fakultät für Physik



A few minutes after the Big Bang that created our Universe, nucleosynthesis reactions forged some lighter isotopes, including 7Li. Ever since then, such reactions have taken place in the hot stellar interiors, providing the stars with the energy to shine and forming all chemical elements necessary for life as we know it. In this thesis I describe how we have inferred the chemical composition of stars in a globular cluster, hosting one of the oldest stellar populations in our Galaxy, in order to address two fundamental astrophysical problems. The first problem is related to lithium, and the prediction of its primordial abundance from present cosmological theories of how the Universe was born. The Li abundances that we measure in the envelopes of globular cluster stars are lower than this estimate and we investigate the possibility that Li has been drained from the stellar surfaces. This thesis presents observational evidence that low-mass stars experience a small increase in their surface Li abundances during the course of their late phases of evolution, supporting this hypothesis. The second question is related to the formation and evolution of globular clusters. It appears that these dense stellar environments early underwent a unique form of self- enrichment, by retaining the gas outflow from slow stellar winds. The material was then incorporated into a second stellar generation with identical chemical signatures to the first, except for a handful of lighter elements, including sodium. I here discuss several aspects of this stellar pollution process. Only with a correct physical description of the radiative and convective energy transport through the stellar atmosphere, and the equilibrium state of the atoms, ions and molecules that form the tenuous gas, can we make solid predictions of the emergent spectrum and derive accurate abundances. In particular, I discuss how the simplifying assumption of local thermal equilibrium give rise to systematic errors in the analysis of Li and Na spectral lines, commonly mis-estimating the elemental abundances in stars by 10–50%, and in certain cases considerably more. Moreover, we have for the first time investigated the combined influence from departures from local thermal equilibrium and hydro-static equilibrium in the determination of the solar Na abundance.