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Magnesium isotopes as a probe of the Milky Way chemical evolution

Thygesen, Anders Overaa

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The study of elemental abundance ratios from spectroscopy of stars has for a long time been used to investigate the structure and the chemical evolution history of the Milky Way. However, even with the ever-increasing number of stars with detailed abundances, many details about the Milky Way evolution are still not understood. While elemental abundance measurements already provide a lot of information, nucleosynthesis models predict not only bulk abundances of an element, but also its isotopic composition. When these can be measured, additional details about the nucleosynthesis can be obtained. The isotopic composition of elements in stars has only been measured for the lightest elements and even for these, observations of the highest quality are needed. In addition, detailed modeling of the line-formation in the stellar atmospheres is needed to correctly interpret the data.

The purpose of this thesis is to: - Investigate the chemical evolution history of the massive, high metallicity globular cluster 47 Tucanae, by performing an extensive study of a range of elements in cool giants. - Perform the first study of Mg isotopes in this cluster, to further constrain its chemical evolution history. In addition, this work represents the first study ever of the effects of using 3D stellar atmospheric models to derive the Mg isotopic mixture. - Perform the first study of Mg isotopes in stars in the inner disk of the Milky Way and the Milky Way bulge, including stars in the globular cluster NGC 6522. - Demonstrate that this type of study is feasible for stars in the bulge, and show how the Mg isotopic ratios can be used to constrain chemical evolution models for this part of the Galaxy.

The Mg isotopic ratios were successfully measured in all 21 observed stars and provided additional constraints on the chemical evolution history of 47 Tucanae. In addition, the first results for Mg isotopes with 3D stellar atmospheres gave improved fits to the MgH molecular features, compared to 1D. This also resulted in an increase of the measured fraction of 25Mg, improving the agreement with chemical evolution models.

For the inner disk and the bulge, we reached a level of accuracy on the Mg isotopic ratios that will allow us to distinguish between different chemical evolution models. For the one field star in the bulge, we see an indication of more efficient star formation compared to the disk, but a larger sample of stars is needed before firm claims can be made.

Item Type: Dissertation
Supervisor: Christlieb, Prof. Dr. Norbert
Date of thesis defense: 14 October 2015
Date Deposited: 28 Oct 2015 08:16
Date: 2015
Faculties / Institutes: Service facilities > Zentrum für Astronomie der Universität Heidelberg (ZAH) > ZAH: Landessternwarte
Service facilities > Graduiertenschulen > Graduiertenschule Fundamentale Physik (HGSFP)
Subjects: 500 Natural sciences and mathematics
520 Astronomy and allied sciences
530 Physics
Controlled Keywords: magnesium isotopes, globular cluster
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