Giant Planets Transiting Giant Stars a DISSERTATION SUBMITTED TO
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Giant Planets Transiting Giant Stars A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI`I AT MANOA¯ IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ASTRONOMY August 2019 By Samuel Kai Grunblatt Dissertation Committee: Daniel Huber, Chairperson Eric J. Gaidos Andrew W. Howard Christoph J. Baranec Rolf-Peter Kudritzki Jonathan P. Williams c Copyright 2019 by Samuel K. Grunblatt All Rights Reserved ii To I. Piehler, H. Grunblatt, and H. and E. Blake iii Acknowledgements As my six years as a graduate student in Hawaii draws to a close, I'm grateful to a wide range of individuals who have made this thesis possible. Without a broad network of support and enthusiasm for my research, obtaining this degree would not have been possible. A famous scientist once said, "If I have seen further it is by standing on the shoulders of Giants." Though my achievements certainly pale in comparison, the sentiment rings true to my work. Thank you to all of you who helped me get to this point{it wouldn't have happened without you. First, I'd like to acknowledge the support of the US taxpayers. Without the funding that all of us provide, the NASA and NSF funds used to pay my salary would not be available. I'd also like to thank the citizens of the state of Hawaii. Your (retroactive) choice to allow scientific research at the summit of Maunakea was essential to the success of this research. Given the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawai`ian community, I am very grateful to have been given the opportunity to use this unique place to best further exoplanet and astronomical studies for the common good. Next, I'd like to thank my advisors. I'm incredibly honored to be the first graduating Ph.D. student of Dr. Daniel Huber. I will be forever grateful for running into you at the IAU meeting in 2015, where you offered a thesis project to me in a 15 minute conversation in the lobby. You took a potentially life-changing risk on me, and then never hesitated to answer my questions or make yourself available to me when I needed it most. More importantly, you've shown me how to be an outstanding scientist yet a well-balanced person at the same time. But I've been lucky enough to have many other great mentors throughout my scientific career. I'd also like to thank Eric Gaidos, for similarly taking a leap of faith to become the chair of my thesis committee before ever having worked with me. Your intellectual curiosity inspires me every time I come into contact with it. Andrew Howard generously gave me the first taste of publishable-quality iv research, and continues to inspire my interest in exoplanets and attention to detail. Stuart Jefferies taught me the beauty and importance of being able to explain difficult concepts in approachable terms. Rolf Kudritzki made the value of this even clearer in my most memorable class as a graduate student. Before I arrived in Hawaii, there were many others who helped me onto the proper path. Joe Patterson{thanks for being more than happy to help out an eager undergrad, and keeping his astronomy enthusiasm alive and well throughout those four years. Your lack of hesitation to get me involved in observing has played a huge role in my life since then. Thanks to Chryssa Kouvelioutou and Jochen Greiner, for taking the time to give me my first tastes of doing research at an institute, and giving me the confidence in my research I needed to continue. Thanks to Dr. Fred Chromey for taking the time to teach a high schooler how to do astronomy research and use IRAF even when he had nothing to gain from it. Thanks to Mr. Kirk Reinhardt for believing in the potential of Kingston High School students and always giving us access to the tools we needed to succeed. And lastly, thanks to Dr. Norbert Magnussen for inspiring and encouraging me to become an astrophysicist almost twenty years ago. I'd also like to thank all of my fellow students for their support along the way. Thanks to all of the UH IfA incoming grad classes of 2012, 2013, and 2014 for collaborating with me on homeworks and commiserating when things got tougher than anything I had seen in undergrad. A special thanks to my former housemates, Conor and Max, for driving me insane and later keeping me sane as we all faced this new challenge together. And of course to my apartment mate of the past four years, Travis{your patience is a thing of legend, and your friendship is something I'll treasure for the rest of my life. Thanks for all the great memories and conversations, scientific and otherwise. And to my own student, Stephanie: mentoring you over the past two years has taught me so much, and your success and drive inspires me every day. I'm so proud of what you've accomplished, and I can't wait to see where you're headed next. And finally, I'd like to thank my loving family. Whether it was the ridiculously large body of knowledge of Dad's or the unrivaled work ethic of Mom that inspired me to make this choice, you have both supported me 100% without ever giving it a second thought. You gave me the self-confidence, ambition, and downright desire to make this happen. Thanks to my sister Eva, for being one of my closest friends and the best sister I could ever hope for. I'd also like to thank my grandmothers, Inge Piehler and Hilda Grunblatt, and my \adopted" grandparents Helen and Ed Blake for always v believing in me and making me into the person I am now. I know that you'd be here supporting me every chance you got if you were still here today, and for that I can never thank you enough. vi Abstract Every Sun-like star will eventually evolve into a red giant, a transition which can profoundly affect the evolution of a surrounding planetary system. The timescale of dynamical planet evolution and orbital decay has important implications for planetary habitability, as well as post-main sequence star and planet interaction, evolution and internal structure. In this thesis, I investigate the population of giant planets transiting low luminosity red giant branch stars observed by the NASA K2 mission. I report the discovery of two new planets orbiting evolved stars, and confirm the existence of a third, doubling the number of evolved (R∗ > 3.5 R ,Teff < Teff; ) stars with known transiting planets. By developing new tools to mitigate stellar variability in evolved star light curves, I robustly measure the planetary radii of these systems. I find that all of these planets are inflated, the first evidence that planets may be inflated directly by an increase in incident stellar radiation, and thus comprise a previously unknown class of re-inflated planets. I also obtain radial velocity measurements of planets orbiting evolved stars to constrain their orbital properties and the efficiency of re-inflation. I find that close-in giant planets orbiting evolved stars display a preference for moderately eccentric orbits, a previously predicted outcome of late-stage planetary system evolution. Finally, I perform a comprehensive planet occurrence study using all oscillating low luminosity red giant branch stars observed in the first 16 campaigns of K2. I measure stellar masses and radii to 6% precision or better using asteroseismology, and find a comparable fraction of close-in giant planets around evolved stars as main sequence stars. A higher fraction of inflated close-in gas giants is also found around evolved stars. These discoveries imply that planet engulfment happens more slowly than previously predicted, and that the effects of stellar evolution on the occurrence of close-in planets larger than Jupiter is not significant until stars have begun ascending substantially up the red giant branch (& 6 R ). Further surveys of these stars by the NASA TESS mission will reveal the dependence of late-stage planetary evolution on star and planet properties. vii Table of Contents Acknowledgements . iv Abstract . vii List of Tables . xii List of Figures . xiii Chapter 1: Introduction . 1 1.1 Exoplanet Demographics . 2 1.1.1 Exoplanet surveys: a recent history . 2 1.1.2 The K2 Mission . 5 1.1.3 Planets around evolved stars . 6 1.2 Asteroseismology . 10 1.3 Gaussian Process Analysis . 12 Chapter 2: K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star . 16 2.1 Introduction . 16 2.2 Observations . 18 2.2.1 K2 Photometry . 18 2.2.2 Imaging with Keck/NIRC2 AO . 18 2.2.3 Spectroscopy with UH88/SNIFS, IRTF/SpeX, and Keck/HIRES . 18 2.2.4 Radial Velocity Measurements . 20 2.3 Host Star Characteristics . 21 2.3.1 Spectroscopic Analysis . 21 2.3.2 Asteroseismology . 22 2.4 Lightcurve Analysis and Planetary Parameters . 26 2.4.1 Gaussian process transit model . 26 viii 2.4.2 Radial Velocity Analysis: Planetary Confirmation and False Positive Assessment 30 2.5 Discussion . 33 2.5.1 Is EPIC 211351816.01 Inflated? . 33 2.5.2 Planet Inflation Scenarios . 34 2.5.3 Planetary Engulfment . 39 2.6 Conclusions . 40 Chapter 3: Seeing double with K2: Testing Re-inflation with Two Remarkably Similar Planets Around Red Giant Branch Stars . 41 3.1 Introduction . 41 3.2 Observations . 43 3.2.1 K2 Photometry .