Formation and migration histories of giant exoplanets in multi-stellar systems Thesis by Henry Hoang Khoi Ngo In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2017 Defended 19 May 2017 ii © 2017 Henry Hoang Khoi Ngo ORCID: 0000-0001-5172-4859 All rights reserved iii For Mom, Dad, Laura and our little chip iv “Decide in your heart of hearts what really excites and challenges you, and start moving your life in that direction. Every decision you make, from what you eat to what you do with your time tonight, turns you into who you are tomorrow, and the day after that. Look at who you want to be, and start sculpting yourself into that person. You may not get exactly where you thought you’d be, but you will be doing things that suit you in a profession you believe in. Don’t let life randomly kick you into the adult you don’t want to become.” —Chris Hadfield, Commander, Expedition 35, International Space Station v ACKNOWLEDGEMENTS I am grateful for everyone who has supported me as I worked towards this disser- tation. I want to thank my advisor, Heather Knutson, for her scientific advice and mentorship during the past five years. She encourages her students to pursue inter- esting research opportunities, to become excellent colleagues, and to have a healthy work-life balance. Heather will continue to be a role model for me in years to come. I also want to thank Dimitri Mawet, who agreed to join my thesis committee during his first weeks at Caltech, for providing an opportunity to work with the vortex coronagraph and for entrusting an exciting new planet finding project with me. I have benefited from the Planetary Science faculty’s open-door policy, which has made it easy to build a mentoring network. I am thankful for the advice from my committee members: Mike Brown, Geoff Blake and Konstantin Batygin. I appreciate the friendship and mentorship from the postdocs, especially Courtney Dressing, Gary Ruane, Björn Benneke, Brendan Bowler, and Sasha Hinkley. I am also thankful for friends, collaborators and advisors at the University of British Columbia and Queen’s University. Many people in the GPS Division and Planetary Science Department have improved my graduate student life. I am proud to be a member of the “Captains Planet”, with Lu Pan, Mike Wong, Danielle Piskorz, and Joe O’Rourke. Thank you for working with me on homework, studying with me for quals and providing moral support. I also appreciate scientific and life advice from grad students that went through before me: Alex Lockwood, Aaron Wolf, Josh Kammer, and Miki Nakajima. I have enjoyed fun scientific and not-so-scientific discussions with Peter Buhler, Chris Spalding, Nancy Thomas, and Peter Gao. Extra thanks to Peter Gao for your friendship over the last 10 years over two academic programs. Although I cannot name all the awesome students in our Division here, I am thankful for our dodgeball team (“The Late Heavy Bombardment”), our softball team (“Strike-slip”), and all the social events we do together. Special thanks to Mike Wong for collaborating with me on many of these projects. Finally, I am extremely grateful for the wonderful and caring staff members that have helped me along the way, both in the Division and the department, especially Dian Buchness, Liz Boyd, Julie Lee, Julia Zuckerman, Aleen Boladian, Margaret Carlos, Ulrika Terrones, Irma Black, and Mike Black. Moving to Caltech was a big change for Laura and me, but there were many wonderful vi people that helped us feel at home. Thank you to Laura Flower Kim and Daniel Yoder at the International Student Office; Natalie Gilmore, Felicia Hunt, and Kate McAnulty at the Graduate Office; and Kurt Murdoch and Kevin Lee at Human Resources. I am also grateful for the opportunity to improve graduate student life with the Graduate Student Council. Thank you to all of my colleagues on the Board of Directors over the past four years. I am very grateful for my parents’ love and support over the past 30 years. After the Vietnam War, they took great risks and made huge sacrifices. They left everything they knew behind to immigrate to Canada as refugees to create a better life for themselves and their future children. They taught me the importance of education and hard work, which has helped me become the person I am today. Finally, and most importantly, none of this would have been possible without Laura. Thank you for always being there for me. When I feel overwhelmed by a challenge, I know I can always count on your unconditional love and support. No words can capture how much love I have for you and how fortunate I feel to have you as my life partner in every sense of the word. Over the past 15 years, we have had many adventures and I am looking forward to many more in the years to come. vii ABSTRACT The first planets discovered outside of our solar system were very different from the solar system planets. These discoveries raised new challenges to planet formation models, which were designed to explain the origin of the solar system planets. One particularly intriguing population, the “hot Jupiters” were some of the first planets discovered. These gas giant planets have masses similar to Jupiter and Saturn, however, they were found on orbits 100 times closer to their star than Jupiter is to the sun. Proposed formation scenarios involve models that argue for formation at presently observed locations, but these are challenged by the lack of planet-building materials so close to the host star. Other models assume these planets form at more moderate locations, perhaps in a manner similar to Jupiter and Saturn, followed by inward migration via some other mechanism. These models are challenged by the lack of a known migration mechanism. This dissertation compiles three studies conducted over the past five years to in- vestigate the formation and migration histories of gas giant exoplanets. After the discovery of the first hot Jupiter, additional discoveries revealed some population characteristics that could provide evidence for certain formation or migration sce- narios. A large fraction of hot Jupiters were found to have eccentric orbits and/or misaligned orbits relative to the star’s spin axis. These properties suggest that a gravitational interaction with an additional massive object may have played a role in the dynamical history of these hot Jupiters. Studies of stellar multiplicity for nearby, sun-like stars have also revealed that multi-stellar systems are common. The studies presented in this dissertation investigate whether stellar companions to giant planet systems influence the planets. In the first and second study, we conduct a survey for stellar companions around stars that host hot Jupiters detected by the transiting method. The first study examines whether stars hosting misaligned planets are more likely to host a companion star. We found no such correlation, suggesting that stellar companions do not play a dominant role in causing planetary misalignment. In the second study, we look at the population of stellar companions as a whole to quantify the fraction of hot Jupiters that might have migrated due to stellar interactions. We find that less than 20% of hot Jupiters might have experienced this migration scenario. However, we do find that hot Jupiters are three times more likely to be in a wide multi-stellar system compared to nearby stars that do not host hot Jupiters, suggesting some other connection between the companion viii star and the giant planet. In the third study, we search for stellar companions around stars that host giant planets over a wide range of separations, from the close-in hot Jupiters to giant planets as far away as Jupiter is to our sun. These planets were found via the radial velocity method. We compare the giant planets’ orbital properties for single- and multi- stellar systems to determine whether planets in multi-stellar systems show some evidence for star-planet interactions. With the current dataset, we find no evidence to support the hypothesis that planets in multi-stellar systems have a different set of orbital properties. Finally, we present preliminary results of an ongoing survey to understand giant planet formation on the other extreme end. Instead of hot Jupiters on close-in orbits, this survey seeks to explain the origin of the very distant giant planetary mass objects found by direct imaging surveys. These objects are often found on separations that are ten to one hundred times farther away than Neptune is to our sun. Due to their distance and size, it’s not certain if these objects are some of the biggest planets in existence or if they are the smallest stars. This new survey will search for planets to serve as the link between known giant exoplanets and these unknown directly imaged objects. ix PUBLISHED CONTENT AND CONTRIBUTIONS Ngo, H., H. Knutson, S. Hinkley, et al. (2015). The Astrophysical Journal 800, 138. doi: 10.1088/0004-637X/800/2/138. H. Ngo conducted the observations, processed the data, analyzed the results and wrote the manuscript. H. Knutson advised this project. Reproduced with permission from the American Astronomical Society (copyright holder). Ngo, H., H. A. Knutson, S. Hinkley, et al. (2016). The Astrophysical Journal 827, 8. doi: 10.3847/0004-637X/827/1/8. H. Ngo conducted the observations, processed the data, analyzed the results and wrote the manuscript. H. Knutson advised this project. Reproduced with permission from the American Astronomical Society (copyright holder). Ngo, H., H.