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Magnesium Isotopes As a Probe of the Milky Way Chemical Evolution

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Magnesium Isotopes As a Probe of the Milky Way Chemical Evolution MAGNESIUM ISOTOPES AS A PROBE OF THE MILKY WAY CHEMICALEVOLUTION anders overaa thygesen supervisor: prof. dr. norbert christlieb referees: prof. dr. norbert christlieb prof. dr. eva grebel august 3, 2015 dissertation submitted to the combined faculties for the natural sciences and for mathematics of the ruperto-carola-university of heidelberg, germany for the degree of doctor of natural sciences put forward by: anders overaa thygesen born in randers, denmark oral examination: october 14, 2015 ABSTRACT 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 de- tails about the Milky Way evolution are still not understood. While ele- mental abundance measurements already provide a lot of information, nucleosynthesis models predict not only bulk abundances of an ele- ment, but also its isotopic composition. When these can be measured, additional details about the nucleosynthesis can be obtained. The iso- topic 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 exten- sive 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 rep- resents the first study ever of the effects of using 3D stellar atmo- spheric 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 con- strain 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 his- tory 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 frac- tion 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 v 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. vi ZUSAMMENFASSUNG Messungen von chemischen Elementhäufigkeitsverhältnissen aus Stern- spektren sind seit langer Zeit verwendet worden, um die Struktur und die chemische Entwicklungsgeschichte der Milchstraße zu untersuchen. Doch selbst mit der ständig steigenden Anzahl der Sterne mit detail- lierten Häufigkeiten sind viele Details über die Entwicklung der Milch- straße noch immer nicht verstanden. Während solche Häufigkeitsmes- sungen bereits eine Vielzahl von Informationen enthalten, prognostizie- ren Nukleosynthesemodelle nicht nur die pauschale Häufigkeit eines Elements, sondern auch dessen Isotopenzusammensetzung. Wenn diese gemessen werden kann, lassen sich zusätzliche Informationen über Kern- prozesse gewinnen. Bisher konnten Isotopenzusammensetzungen in Ster- nen nur für die leichtesten Elemente gemessen werden und selbst für diese sind Beobachtungen von höchster Qualität erforderlich. Zusätz- lich ist eine detaillierte Modellierung der Linienbildung in den Sternat- mosphären nötig, um die Daten richtig interpretieren zu können. Das Ziel dieser Arbeit ist: • die chemische Entwicklungsgeschichte des massereichen, metall- reichen Kugelsternhaufens 47 Tucanae durch eine umfassende Stu- die eines breiten Bereichs an Elementen in kühlen Riesensternen zu untersuchen; • erste Messungen von Mg-Isotopen in diesem Haufen durchzu- führen, um seine chemische Entwicklung weiter einzuschränken. Darüber hinaus stellt diese Arbeit die erste Studie über die Aus- wirkungen von dreidimensionalen Sternatmosphärenmodellen auf das abgeleitete Mg-Isotopengemisch dar. • Die erste Untersuchung von Mg-Isotopen in Sternen der inneren Scheibe der Milchstraße sowie ihrer zentralen Verdickung – dem “Bulge”; • eine Demonstration, daß diese Art von Studie für Sterne im Bulge machbar ist, und somit zu zeigen, wie die Mg-Isotopenverhältnisse Modelle der chemischen Entwicklung für diesen Teil der Galaxie einschränken können. Die Isotopenverhältnisse für Mg wurden erfolgreich in allen Sternen dieses Projekts gemessen und erlauben zusätzliche Einschränkungen über die chemische Entwicklung von 47 Tucanae. Weiterhin konnten vii die ersten Ergebnisse der MgIsotope aus dreidimensionalen Modellat- mosphären Molekülbanden von MgH weitaus besser repräsentieren als die gängigen eindimensionalen Modelle. Dies führte auch zu einem Anstieg des gemessenen Anteils von 25Mg, wodurch die Übereinstim- mung mit den chemischen Modellen verbessert wurde. Für die innere Scheibe und den Bulge erreichten wir ein Maß an Genauigkeit auf die Mg-Isotopenverhältnisse, das uns erlaubt, zwischen verschiedenen chemischen Entwicklungsmodellen zu unterscheiden. Für den Stern im Bulge sehen wir einen Hinweis auf effizientere Sternentstehung im Ver- gleich zur Scheibe, jedoch sind Messungen in einer größeren Anzahl von Sternen notwendig, bevor festere Aussagen getroffen werden kön- nen. viii "I’ll keep this wisdom in my flesh / I leave here believing more than I had / And there’s a reason I’ll be / A reason I’ll be back." — Eddie Vedder, No Ceiling, 2007 First and foremost I want to express my gratitude to my two supervi- sors, Prof. Norbert Christlieb and Dr. Luca Sbordone. You, and the rest of the group made me feel extremely welcome at the Landessternwarte from day one, and continued to contribute to make the LSW a pleas- ant and inspiring place for me to work throughout my studies. Thanks for giving me the freedom to pursue my project as independently as I wanted, and for your willingness to guide and teach me when I was in need of advice. A very special thanks to Luca for your seemingly infi- nite patience with me, for always taking time for discussions, and for your support throughout my time as a PhD student. It made a world of difference, both on a scientific and personal level. Your ability to always rekindle my enthusiasm for my research never ceases to amaze me. I would also like to extend my thanks to Hans Ludwig for your help with CO5BOLD, countless useful discussions, valuable input, and your willingness to explain things to me three, four, five times if needed. You always make me think more about my science. Thanks to my many collaborators who have been involved in my research, in particular Dr. David Yong, Dr. Amanda Karakas and Dr. Melissa Ness for many use- ful discussions and constructive feedback on my work (and more than one epic karaoke session). Thanks also to Dr. Andreas Koch for his help with the German translation of my abstract, and many interesting dis- cussions. It has been a pleasure to be a member of the Zentrum für Astronomie in Heidelberg, and I wish to acknowledge the generous funding from the SFB 881 "The Milky Way Galaxy", as well as a Distinguished Doc- toral Fellowship from the Heidelberg Graduate School for Fundamental Physics. This allowed me to accomplish my PhD, and made it possible for me to attend numerous meetings and conferences around the world, which have been invaluable experiences. It has also been a pleasure to be a member of the International Max Planck Research School and I want to thank Dr. Christian Fendt, whose engagement has contributed greatly to the well-being of all the students at IMPRS, as well as pro- vided useful support on several occasions. I also want to thank all the wonderful people in Heidelberg, and elsewhere, who made the last three years an amazing journey. In no particular order: Andrew, Tobias, Michael, Taisiya, Emer, Melanie, Ben, ix Richard, Paolo, Sonia, Ilya, Dave, László, Wie´nczysław, Joanna, Camille, Eric, Kathrin, Amelia, Paulina, Sundar, Liz, Roger, Ioana, Nikolay, all the swing dancing people and all the remaining people that I have un- intentionally left out. You have all helped make my time as a graduate student better and full of fond memories. A special thanks to Cata- rina and Anastasia for helping me keep my sanity through the last few months of my PhD, getting my mind off science, help keeping my cool and preventing my brain from exploding for the last month of my thesis writing. Til sidst, men ikke mindst, tak til jer derhjemme. Ude er godt, men hjemme bedst. Tak, mor og far, for at støtte mig på alle måder og for jeres utallige besøg, også når jeg indimellem har for travlt til at være fuldt til stede. Tak, Sigrid, for at være verdens sejeste søster og altid være der for mig. Det betyder alverden at jeg har jer at læne mig op af, selvom jeg er langt væk. Tak til Jakob, Rune, Lise, Bo, Jeppe, Cecilie, Gudmund og resten af flokken. Der er længe imellem vi ses, men jeg værdsætter det virkeligt at I bevarer kontakten, det gør en verden til forskel. x CONTENTS
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