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Lurking in the Shadows: Wide-Separation Gas Giants As Tracers of Planet Formation

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Lurking in the Shadows: Wide-Separation Gas Giants As Tracers of Planet Formation Lurking in the Shadows: Wide-Separation Gas Giants as Tracers of Planet Formation Thesis by Marta Levesque Bryan In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2018 Defended May 1, 2018 ii © 2018 Marta Levesque Bryan ORCID: [0000-0002-6076-5967] All rights reserved iii ACKNOWLEDGEMENTS First and foremost I would like to thank Heather Knutson, who I had the great privilege of working with as my thesis advisor. Her encouragement, guidance, and perspective helped me navigate many a challenging problem, and my conversations with her were a consistent source of positivity and learning throughout my time at Caltech. I leave graduate school a better scientist and person for having her as a role model. Heather fostered a wonderfully positive and supportive environment for her students, giving us the space to explore and grow - I could not have asked for a better advisor or research experience. I would also like to thank Konstantin Batygin for enthusiastic and illuminating discussions that always left me more excited to explore the result at hand. Thank you as well to Dimitri Mawet for providing both expertise and contagious optimism for some of my latest direct imaging endeavors. Thank you to the rest of my thesis committee, namely Geoff Blake, Evan Kirby, and Chuck Steidel for their support, helpful conversations, and insightful questions. I am grateful to have had the opportunity to collaborate with Brendan Bowler. His talk at Caltech my second year of graduate school introduced me to an unexpected population of massive wide-separation planetary-mass companions, and lead to a long-running collaboration from which several of my thesis projects were born. I greatly appreciate his guidance on my first foray into direct imaging, as well as his continued support and perspective. I would like to thank other collaborators who made important contributions to the science presented in this thesis, including Henry Ngo, BJ Fulton, Bjorn Benneke, Sasha Hinkley, Adam Kraus, and Eve Lee. This thesis was also made possible by a number of extraordinary mentors who I encountered as an undergraduate. I would like to thank David Charbonneau, who I had the great fortune of meeting my first week of freshman year and who taught not one but five semesters of classes that I took. I am grateful for his invaluable support over the past decade, and for introducing me to exoplanets in the first place. Thank you also to David Latham for encouraging me and guiding me through my first two exoplanet research projects, and for helping me realize that I had found my research home base. I also want to thank Don Hall and John Kovac for providing mentorship and positive research experiences that encouraged me to continue on in astronomy. iv Thank you to Bekki Dawson and Meredith Hughes for being my official women in science graduate student mentors while I was an undergrad, and for continuing to provide measured advice and perspective throughout my time at Caltech. Thank you also to Courtney Dressing and Leslie Rogers for their advice and encouragement. I would also like to thank friends who have taken this journey with me. In particular, I would like to thank Michael Eastwood for his invaluable support through it all. Thank you also to Becky Jensen-Clem for being a wonderful officemate, friend, and potential twin. Finally, thank you to Anna Ho for being an amazing coffee shop work partner, yoga buddy, and all around friend. A special thank you to my mom for her love and support since the very beginning, and for providing me with the opportunities and encouragement to get where I am today. v ABSTRACT Over the past two decades, thousands of planets with an extraordinary diversity of properties have been discovered orbiting nearby stars. Many of these exoplanetary systems challenge our narrative for how planets form and evolve, motivating the search for observational clues to the underlying mechanisms that led to this diver- sity. In this quest, gas giant analogs to our own Jupiter and Saturn immediately stand out as the most visible relics of the planet formation process. They are products of their birth environment, with properties such as atmospheric and interior composi- tions, masses, and formation locations sculpted by protoplanetary disk and host star properties. They also actively shape their surroundings; early in their lifetimes, gas giants can alter the structure of the gas disk from which additional planetary bodies may coalesce and affect the transport of rocky and icy materials to the inner disk. After the gas has dissipated these same behemoths can push smaller planets around, causing them to migrate or even ejecting them from the system. Thus to explain the observed diversity of exoplanet systems, we must first understand how gas giant planets form and evolve. This thesis presents four studies that harness multiple observational techniques to explore this question of how gas giant planets outside our solar system form and evolve. In the first study, we searched for massive, long-period companions to 123 known exoplanetary systems using the radial velocity method. We found 20 systems with statistically significant RV trends, and used these data to produce the first statistical analysis of the frequency of outer gas giant companions in systems hosting inner gas giant planets. These companions appear to be common, with an occurrence rate of 52±5% for planets between 1 - 20 MJup and 5 - 20 AU. We also found that systems hosting hot Jupiters are more likely than warm and cold Jupiters to have an outer gas giant companion, consistent with the predictions of dynamical migration models. Finally, we found that planets between 0.1-5 AU in multi- body systems have higher average eccentricities than isolated planets, suggesting that dynamical interactions among gas giant planets play an important role in the evolution of most planetary systems. The second study explores the formation histories of wide-separation planetary- mass companions. One possible solution is to argue that these planets did not form in situ, but were instead scattered outward from their original formation locations via interactions with another body in the system. We carried out a near-infrared vi imaging survey using NIRC2 at Keck to search for close-in substellar companions to a sample of seven systems with confirmed planetary-mass companions on wide orbits. While we initially identified eight candidate companions, a second epoch of astrometry confirmed all eight to be background objects. From these new epochs of astrometry we also found that two of the previously confirmed companions showed evidence for orbital motion, which we used to constrain their orbital eccentricities to low to moderate values, contradicting predictions from scattering simulations. Taken together, these two pieces of evidence present a compelling argument against scattering as the explanation for the wide orbits of these planetary-mass companions. In the third study, we used near-IR high-resolution spectroscopy to measure ro- tational line broadening of three young (2-300 Myr) planetary-mass companions and combined these measurements with published rotation rates for two additional companions to provide the first look at the spin distribution of these objects. We compared this distribution to complementary rotation rate measurements for six brown dwarfs with masses < 20 MJup (three of which we measured), and found that these spin distributions are indistinguishable. This suggests either that these two populations formed via the same mechanism, or that processes regulating rotation rates are largely independent of formation history. We also found that the rotation rates for both populations are well below their break-up velocities and do not evolve significantly during the first few hundred million years after the end of accretion. This suggests that rotation rates are set early in a planetary-mass object’s lifetime, possibly by interactions with a circumplanetary disk. In the final study, we use radial velocity observations to search for massive, long- period gas giant companions in 65 systems hosting inner super-Earth (1 − 4 R⊕, 1 − 10 M⊕) planets in order to constrain formation and migration scenarios for this population. We find 10 systems with statistically significant trends indicating the presence of an outer companion. We quantify our sensitivity to the presence of long period companions in these system by fitting the sample with a power law distribution and find an estimated occurrence rate of 39±7% for companions between 0:5 − 20 MJup and 1 − 20 AU. A quantitative comparison to previous determinations of the frequency of Jupiter analogs indicates that the occurrence rate of Jupiter analogs in super-Earth systems appears to be higher than the occurrence rate of gas giant planets around field stars. We conclude that the presence of outer gas giant planets does not suppress the formation of inner super-Earths, and may instead facilitate their formation. vii PUBLISHED CONTENT AND CONTRIBUTIONS Bryan, M. L., B. Benneke, et al. (2018). “Constraints on the spin evolution of young planetary-mass companions”. In: Nature Astronomy 2, pp. 138–144. doi: 10.1038/s41550-017-0325-8. arXiv: 1712.00457 [astro-ph.EP]. M.L.B. conceived the project with input from coauthors, wrote observing pro- posals to get the data, led the observational program, reduced the data, analyzed the results, and wrote the paper. Bryan, M. L., B. P. Bowler, et al. (2016). “Searching for Scatterers: High-Contrast Imaging of Young Stars Hosting Wide-Separation Planetary-Mass Companions”. In: The Astrophysical Journal 827, 100, p. 100. doi: 10.3847/0004-637X/ 827/2/100. arXiv: 1606.06744 [astro-ph.EP]. M.L.B. contributed to the observations, reduced the data, analyzed the results, and wrote the paper.
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