UNIVERSITY OF CAMBRIDGE INSTITUTE OF ASTRONOMY A DISSERTATION SUBMITTED TO THE UNIVERSITY OF CAMBRIDGE FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DISSECTING THE MILKY WAY WITH SPECTROSCOPIC STUDIES KEITH AUSTIN HAWKINS KINGS COLLEGE Submitted to the Board of Graduate Studies August 14, 2016 UNDER THE SUPERVISION OF PROF.GERRY GILMORE &DR.PAULA JOFRE´ “But in the heavens we discover by their light, and by their light alone, stars so distant from each other that no material thing can ever have passed from one to another; and yet this light, which is to us the sole evidence of the existence of these distant worlds, tells us also that each of them is built up of molecules of the same kinds as those which we find on earth.” – James Clerk Maxwell, Molecules, 1873 i ii DECLARATION OF ORIGINALITY I, Keith Austin Hawkins, declare that this Thesis entitled ‘Dissecting the Milky Way with Spec- troscopic Studies’, I confirm that this work was done wholly while in candidature for a research degree at this University, and that this thesis has not previously been submitted for a degree or any other qualification at this University or any other institution. This thesis is the result of my own research and contains nothing resulting from collaboration, except where explicitly noted. The length of this thesis does not exceed the stated limit of the Degree Committee of Physics and Chemistry of 60,000 words.The parts of this Thesis based on published work are as follows: Chapter 2 • “On the ages of the α-rich and α-poor populations in the Galactic halo’ Hawkins, K., Jofre,´ P., Gilmore, G., Masseron, T, 2014, MNRAS, 445, 2575 Chapter 3 • ‘Characterizing the high-velocity stars of RAVE: the discovery of a metal-rich halo star born in the Galactic disc’ Hawkins, K., Kordopatis, G., Gilmore, G., Masseron, T., Wyse, R. F. G., Ruchti, G., Bienaym, O., Bland-Hawthorn, J., Boeche, C., Freeman, K., Gibson, B. K., Grebel, E. K., Helmi, A., Kunder, A., Munari, U., Navarro, J. F., Parker, Q. A., Reid, W. A., Scholz, R. D., Seabroke, G., Siebert, A., Steinmetz, M., Watson, F., Zwitter, T., 2015a, MNRAS, 447, 2046 Chapter 4 • ‘Using Chemical Tagging to Redefine the Interface of the Galactic Halo and Disk’ Hawkins, K., Jofre,´ P., Gilmore, G., Masseron, T, 2015b, MNRAS, 453, 758 Chapter 5 • ‘Gaia FGK benchmark stars: new candidates at low-metallicities’ Hawkins, K., Jofre,´ P., Heiter, U, Soubiran, C., Blanco-Cuaresma, C., Casagrande, L., Gilmore, G., Lind, K., Magrini, L., Masseron, T., Pancino, E., Randich S., Worley, C. C., 2016a, A&A, 592, A70 Chapter 6 • ‘An Accurate and Self-Consistent Chemical Abundance Catalogue for the APOGEE/Kepler Sample’ Hawkins, K., Masseron, T., Jofre,´ P., Gilmore, G., Elsworth, Y., Hekker, S., 2016b, A&A, Accepted for publication Signed Date iii iv ACKNOWLEDGEMENTS There is an African proverb that says “it takes a village to raise a child.” I think this also holds for a doctoral student; “it takes a village to raise a Ph.D.” I have, from the time I was young, been surrounded by a village of amazing, supportive, and encouraging people that have made this Thesis possible. There is not enough space in this work to mention them all, but I will mention a few here. For those I do not mention fear not, because this Thesis is dedicated to all of my mentors, past and present. First, and foremost, I must thank my supervisors Prof. Gerry Gilmore and Dr. Paula Jofre.´ If it were not for you two, I would not have been able to do this. Your words of encouragement, and advice have taught me so much! This Thesis would not have been nearly as productive or fun if it were not for the ‘army’ of support I received from the various postdoc members of the group including: Thomas Masseron, Georges Kordopatis, Andy Casey, Clare Worley, Anna Hourihane, and Jason Sanders and various collaborators including Rosie Wyse, Greg Ruchti, Gus Williams, and the Gaia-ESO, and RAVE survey teams. All of you fit me into your busy schedules, whenever I needed, to coach me through various aspects of astrophysics and for that I am forever grateful. Particular thanks to Thomas Masseron for not only the many long conversations that improved my understanding of the technical details, but also for the basic stellar parameter code, BACCHUS, that was heavily used in this work. Like most things in life, I became passionate about astronomy because of a girl, Rachel Cuenot. Fifth grade me was enamored by her, in part, because she loved space at that time. She inspired me to become an astronomer before her family moved away. Thank you Rachel for the initial spark that emboldened me to peer into the telescope! I would also like to acknowledge my partner Ashleigh Chalk, Sahar Mansoor, Luca Matra,´ the Chalk-Newman family (Sally, Richard, Caz, and Joe), Hiren Joshi, Georgia Crick-Collins, Aimee´ Hall, Nicoletta Fala, Suzzie Wood, Shizuyo Ichikawa, and last but not least my fellow 2013(4,5) Marshall scholars, for the immense support, words of encouragement, nights out, and great experiences all around that have helped me settle in and find/make a home in Cambridge. To my officemates in H27 (Luca, Aimee,´ Nimisha, Sebastian,´ Tom, and Christina), we had some really great times! From H27-big-chat, to Luca’s tortoise noises. From backpacking the Alps with Luca and Co. to poker nights. You all have really made working at the IoA one of my most enjoyable experiences. I would not have been able to take up the PhD offer in Cambridge without the generous support of the Marshall Aid Commemoration Commission and King’s College Studentship. I particularly want to thank the Marshall Scholars program for many well-planned events that allowed me to learn a lot about the UK. Additionally, I would like to acknowledge Cary Frith, Beth Clodfelter, Profs. David Drabold, Markus Bottecher,¨ Joe Shields, John Johnson, Simon Schuler, Adam Kruas, Katy Garmany, Caitlin Casey, and Jarita Holbrook for encourag- ing me and supporting my efforts to do astronomy and make it more a more diverse community throughout the years before, during, and beyond my time at Ohio University. If it were not for you all, I would not have been able to obtain the Marshall Scholarship to complete this work. Finally, last but not least, this would not be a proper acknowledgment without thanking the role that my family has played in my life. I want to thank my mom, pops, my twin Kevin, Jasmyn, Tahnda, and Darrell for not only their support and encouragement over the last half- century but also dealing with me from start to finish! Keith A. Hawkins, Cambridge, 9th May, 2016 v vi DISSECTING THE MILKY WAY WITH SPECTROSCOPIC STUDIES KEITH AUSTIN HAWKINS SUMMARY In the last decade, the study of Galactic stellar populations has been completely transformed by the existence of large spectroscopic surveys including the Gaia-ESO survey (GES), the Sloan Digital Sky Survey (SDSS), the RAdial VElocity Experiment (RAVE), the APO Galactic Evolution Experiment (APOGEE), and others. These surveys have produced kinematic and chemical information for upwards of 105 stars. The field of Galactic astronomy consists of exploring this information to understand the Milky Way and other systems like it. As such, the collection of studies in this thesis are focused around examining several of these surveys to dissect the structure of the Milky Way with an emphasis on the Galactic halo. I begin with an introduction including the relevant prior knowledge of Galactic structure and, in particular, the ‘accreted’ and ‘in situ’ components of the Galactic halo (Chapter 1). In the following chapters, I dissect and explore the various components of the Milky Way in several phase-spaces including age, kinematics, and chemistry. In Chapter 2, I focus on the addressing the question of whether there is an age difference between the ‘accreted’ and ‘in situ’ components of the Galactic halo. I also discuss the de- velopment of a technique to measure chemical abundances from low-resolution stellar spectra which was used to separate the ‘accreted’ and ‘in situ’ components. In Chapter 3, I examine the chemical nature of high-velocity stars in the RAVE survey and address the role of disk heating in the formation of the Galactic halo. I also find evidence for a sample of metal-rich high velocity stars that are currently a part of the Galactic halo but likely born in the Galactic disk. In Chapter 4, I search for both the ‘accreted’ halo component and metal-rich high-velocity stars in the APOGEE survey, which samples a large volume of the Galaxy by targeting giant stars. I present evidence for the accreted halo, and a metal-poor thin disk, as well as propose a chemical-only approach to decompose the Galaxy. In working with the various surveys in the above chapters, particularly APOGEE, it became apparent that there are sometimes metallicity calibration issues which can plague the survey. I provide two possible solutions to this which I discuss in Chapters 5 and 6. Specifically, in Chapter 5, I propose a new set of candidate metal-poor benchmark stars which can be used to help calibrate large spectroscopic surveys. These new candidates are critical because they fill a parameter space where there is a clear lack of usable calibrators. In Chapter 6, I use a automated stellar parameter pipeline and a careful line selection to improve and include new chemical abundances within the APOGEE survey allowing for further study of the structure of the Galaxy. Finally, in Chapter 7, I discuss the impact of the work carried out in this thesis and present a glimpse of future prospects.