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The Lithium Plateau in Super Metal-Rich Stars By Donald B. Lee-Brown Submitted to the graduate degree program in the Department of Physics and Astronomy and the Graduate Faculty of the University of Kansas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Bruce Twarog, Ph.D., Chairperson Barbara Anthony-Twarog, Ph.D. Committee members David Mechem, Ph.D. Gregory Rudnick, Ph.D. Stephen Sanders, Ph.D. Date defended: May 9, 2018 The Thesis Committee for Donald B. Lee-Brown certifies that this is the approved version of the following thesis : The Lithium Plateau in Super Metal-Rich Stars Bruce Twarog, Ph.D., Chairperson Date approved: May 17, 2018 ii Abstract I present a study of the lithium abundances for a sample of super metal-rich (SMR, [Fe/H] > + 0.20) main sequence dwarfs in the solar neighborhood. The SMR stars were selected to have surface temperatures in the region of the lithium plateau, a nar- row region in temperature space where stars are predicted by models to preserve their initial surface Li abundances while on the main sequence. Despite these predictions, observations of lower-metallicity stars indicate that significant depletion occurs dur- ing the first few Gyr of stellar evolution. SMR stars, which represent the extreme end of Galactic chemical evolution, present an opportunity to constrain proposed mecha- nisms to explain this depletion. 100 SMR candidates were selected from existing sur- veys and followed up with spectroscopic observations using the Hydra spectrograph on the WIYN 3.5m telescope at Kitt Peak National Observatory, as well as photomet- ric observations using the 0.9m WIYN and 40” telescopes at Kitt Peak and Mount Laguna Observatory, respectively. Using the results from ANNA, a new tool that uses a neural network to parameterize stellar spectra, as well as a more traditional equiva- lent width based analysis, 44 single stars with [Fe/H] > + 0.20 were identified and Li was measured for each star. Consistent with previous studies, the SMR stars can be divided into a sample of stars with measurable Li and a sample with upper limits only. Examining the low-Li stars’ evolutionary states reveals that they are consistent with being evolved Li dip stars and therefore depleted their surface Li while on the main sequence before evolving to the cooler temperatures of the Li plateau. Considering only the high-Li sample, the stars are all consistent with having ages in the range 3 – 4.5 Gyr, indicating that they have already significantly depleted their Li. We find iii no young (age < 1 Gyr) SMR stars in the sample, which may explain an observed turnover in the positive correlation between [Fe/H] and initial Li abundance at super- solar metallicities – the only SMR stars in the solar neighborhood are already too old to measure initial Li as they have depleted down to the 2-3 Gyr plateau value. The Li plateau for the SMR sample was measured to be A(Li) = 2.55 dex, which agrees with observations of the similarly-aged super metal-rich cluster NGC 6253 as well as more metal-poor clusters, confirming that the Li plateau abundance in stars older than 2-3 Gyr is apparently insensitive to stellar metallicity. Examining the kinematics and available elemental abundances of the SMR stars, they are shown to be indistinguish- able from lower-metallicity thin disk stars aside from their high [Fe/H], consistent with an origin in the inner thin disk. iv Acknowledgements Graduate school is a bit like visiting a haunted house. You go in with what can charita- bly be called ignorance, quickly find yourself terrified by surprises leaping out at you from the dark, and by the halfway mark all you can think about is how badly you want to get out. This is followed by resignation to the fact that you will probably die before you see the light of day again. Eventually, you miraculously find yourself on the other side but realize that while you were gone the rest of the world kept on going about its business and your friends are now married and bought houses. Despite all this, upon further reflection you decide that it really wasn’t that bad and tell everyone who will listen how awesome the whole experience was. As one might guess, I hate haunted houses, but if I were transported back in time I’d do my Ph.D. all over again and there are a lot of people who deserve a tremendous amount of credit for that sentiment. First off, I could not have asked for better mentorship from my advisors, Bruce Twarog and Barbara Anthony-Twarog. Their guidance, support, flexibility, kindness, and especially patience carried me through graduate school and I think the best indicator of the impact they’ve had is that I tend think of them as family. Really, really smart family who are an inexhaustible resource for all things astronomy. And just for good measure so the record shows it, I’m going to reiterate the boundless patience they had with their errant graduate student, who was always working on some “short side project” or another and putting off his thesis work. I was fortunate to have the opportunity to spend a few years working on a different project with Gregory Rudnick, who generously spent a considerable amount of his own time and resources helping me learn about extragalactic evolution and in general v just being a really good person to talk to about my career goals. I don’t think I realized how generous that was on his part (and that of my advisors - see discussion of patience above) until well after the fact, so thank you. Many thanks go to Constantine Deliyannis at Indiana University, who donated signif- icant time to this project and was extremely flexible about trading observing time in order for me to get observations of my desired targets. The staff at WIYN and KPNO were also extremely generous and helpful - in particular thanks go to Di Harmer, who was an invaluable resource when working with Hydra and gave a lot of help with best observing practices, as well as Heidi Schweiker, who generously arranged for me to use part of a testing and engineering night after a string of bad weather. My committee members certainly deserve thanks for agreeing to be locked in a room with me for hours at a time until I managed to fumble my way through everything I needed to. There are a lot of other people who contributed to this project unofficially in some way or another over the years, and here are some who deserve special credit: Teresa Symons and Grayson Petter both contributed in material ways to this project. Teresa’s clever photPARTY code proved invaluable in greatly accelerating the process of analyzing my photometry, and she was always ready to patiently explain how to use it. Grayson served as the primary “beta tester” of ANNA one summer, and his bug- hunting abilities helped ANNA reach a stable form much faster than it would have otherwise. Whenever I meet with prospective students, I make sure to highlight the pleasant and open atmosphere in our department and especially the astronomy wing. Sinan Deger, Nesar Ramachandra, John Martens, Mindy Townsend, Justin Mann, Jenn Cooper, and all the other graduate students in the department work hard to make this true. So does our department office staff and administration - I’ve gotten so many good restaurant vi recommendations from Joel Sauerwein in the office that I really think he should make an app for them. Michael Reeks, who is barely beating me to the Ph.D. at UVA, has been my friend-of- all-trades: best man, confidant, foodie friend, responsibilibuddy, math resource, hiking partner. His friendship is all the more surprising, given that I got both of us suspended in high school for sneaking out late at night. Jeff Wilkerson at my alma mater, Luther College, was the first person to get me inter- ested in astronomy - before I worked with him, the extent of my astronomy experience was sitting in my backyard as a kid thinking that it was all really boring while my dad tried to get me to look through a telescope. My parents have always been supportive of the winding path I’ve taken in life to get where I am, and graduate school was no different. They’ve good-naturedly asked to read my papers (sorry about that, Mom and Dad) and have always welcomed me back home on short notice. Also they didn’t kick me out of the house after they found out I’d been hiding uneaten sandwiches in the heat vents for years throughout elementary school, so that’s something. Finally, my wife Hannah deserves many more thanks than can be written here. Not only did she support me throughout this adventure after moving to Kansas with me, she did so while winning pharmacy school. She also puts up with my steadily growing pile of homebrewing equipment sitting in the corner of our apartment, and my general Grinch-like demeanor most evenings. Thank you, and I love you. vii Contents 1 Introduction1 1.1 Background: Stellar Evolution............................1 1.1.1 Standard Stellar Evolution Theory......................4 1.2 Lithium: A Tracer of our Ignorance.........................4 1.2.1 The Cosmological Lithium Problem.....................5 1.2.2 Galactic Lithium Enrichment.........................7 1.2.3 Lithium Depletion on the Main Sequence.................. 10 1.2.3.1 The Li Dip............................. 10 1.2.3.2 The Li Plateau........................... 13 1.2.3.3 Low Mass Stars.........................