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Investigating Exoplanet Habitability and the Stellar Magnetism of Cool Stars Across Half the Southern Sky Via Superflares, Starspots, and Stellar Rotation

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Investigating Exoplanet Habitability and the Stellar Magnetism of Cool Stars Across Half the Southern Sky Via Superflares, Starspots, and Stellar Rotation INVESTIGATING EXOPLANET HABITABILITY AND THE STELLAR MAGNETISM OF COOL STARS ACROSS HALF THE SOUTHERN SKY VIA SUPERFLARES, STARSPOTS, AND STELLAR ROTATION Ward S. Howard A dissertation submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics and Astronomy. Chapel Hill 2021 Approved by: Nicholas M. Law Andrew W. Mann Fabian Heitsch Reyco Henning Daniel M. Scolnic ©2021 Ward S. Howard ALL RIGHTS RESERVED ii ABSTRACT Ward S. Howard: Investigating exoplanet habitability and the stellar magnetism of cool stars across half the Southern sky via superflares, starspots, and stellar rotation (Under the direction of Nicholas M. Law) Stellar flares are stochastic events that occur when a star's magnetic field re-connects, releasing intense radiation across the electromagnetic spectrum. Rocky planets in the habitable zones of M-dwarfs are often subjected to superflares, events of at least 1033 erg and 10-1000× the energy of the largest solar flares. Frequent superflares can erode the ozone layer of an Earth-like atmosphere and allow lethal amounts of UV flux to reach the surface. Conversely, too few flares may result in insufficient UV radiation to power pre-biotic chemistry due to the inherent faintness of M-dwarfs in the UV. Cool stars are often found to exhibit superflares. Cool stars are the most common type of star, and are known to frequently host rocky planets. As a result, they may host most of the universe's Earth-size planets orbiting in the habitable zones of main sequence stars. My EvryFlare Survey uses observations from the Evryscope array of small telescopes and the Transiting Exoplanet Survey Satellite (TESS) to answer two questions about superflares and their impacts on the habitability of terrestrial planets orbiting cool stars: (1) How frequently are superflares emitted from the nearby cool stars, both in the present and in the first 200 Myr after formation? (2) What impact does superflare UV emission have on planetary atmospheres and surface habitability of planets orbiting cool stars? The EvryFlare Survey has resulted in the detection of 575 superflares from 284 stars. Results include a superflare from Proxima Cen, the nearest host star to a rocky planet in the habitable zone. I used these events to measure a decrease in superflare rates with increasing age, rotation, and starspot coverage. I will discuss the effects of superflares on ozone loss to planetary atmospheres, including one superflare with sufficient energy to photo-dissociate iii all ozone in an Earth-like atmosphere in a single event. I present the largest-ever survey of simultaneous observations of dozens of M-dwarf superflares with Evryscope and TESS to measure the flare blackbody and estimate UV-C continuum emission. I find superflare temperatures increase with flare energy. The largest and hottest flare briefly reached an estimated 42,000 K. During superflares, I estimate rocky HZ planets orbiting <200 Myr stars typically receive a top-of-atmosphere UV-C flux of ∼120 W m−2 and up to 103 W m−2, 100-1000× the time-averaged XUV flux from Proxima Cen. Finally, I will describe a data analysis project with Robo-AO, exploring the performance of laser guide star adaptive optics systems in the absence of tip-tilt correction. iv To my wife, Katelyn Tassan Howard. v ACKNOWLEDGMENTS The research contained in this dissertation would not have been possible without the support of a number of people. I am incredibly grateful for the formative roles that each of them has played during my time here at UNC. Pursuing a PhD is a long and grueling journey that brings to mind stories such as The Lord of the Rings. As Frodo wouldn't have got far without Sam, I wouldn't have made it to this point without each of the following people. First, special thanks are due to my dissertation advisor, Nicholas Law. Nick showed me how \good science" is done, and how to be a good collaborator. Nick's years of insightful comments such as \Starting a sentence with `whereas' is something only the Declaration of Independence can do," have helped me write more clearly and concisely. Nick, thank you for introducing me to the field of stellar flares and habitability. I enjoyed learning about the topic alongside you. Thank you for showing me so many times how to handle success and failure with a level head. On a related note, thank you for teaching me how to write grants. Since I popped my head into the CTIO dome and asked you \Is that Alestorm?" I have found countless hours of research motivation from your little-known Scottish power metal bands. Finally, your confidence in my ability to succeed has often given me the nudge I needed to do the next hard thing. Next, I would like to recognize each of my other committee members for their thoughtful questions, directions, and time. To Profs. Andrew Mann, Fabian Heitsch, Daniel Scolnic, and Reyco Henning: thank you for partnering with me in this dissertation journey. Andrew, thank you for letting me stop by your office to discuss active M-dwarfs on so many occasions when you had a million spreadsheets to finish. I would also like to acknowledge my academic mentors who helped me reach graduate school in the first place: Profs. Brad Barlow, Steven Gibson, Geoffrey Poore, Bill Nettles, Fonsie Guilaran, David Ward, Michael Salazar, and vi Lorin Matthews. Brad, thank you for mentoring me and helping me to pursue my dream of doing astronomy all the way from high school to PhD applications, the quals, and into post doc applications. Throughout the PhD, I have learned so much from the other members of my lab and from my cohort. Hank, thank you for always sparing a listening ear or imparting wisdom from your coding wizardry. I also owe you a debt of gratitude for your help with Evryscope light curves. Amy, thank you for good discussions of flare stars and planets while humoring my bad puns. Carl, thank you for taking me under your wing and teaching me the ropes when I was a new grad student. Octavi, Jeff, Phil, Ramses, Nathan, Alan, and Lawrence, I am grateful to have worked alongside each of you. To my friends from my graduate cohort (broadly defined), thank you for keeping me sane while working Jackson problems and studying for quals. Particular thanks are due to Joseph Karlik, Anna Reine, Chris Haufe, Paul Smith, Andrew Loheac, and Emilia Zywot for good conversations and outrageously funny tabletop game nights. In my broader Chapel Hill community, I want to thank my pastors and mentors Eric Gravelle, Blair Waggett, AJ Farthing, Ricky Harris, Don Tyndall, and Hank Tarlton for their guidance. I would also like to thank Drs. Glynis Cowell and Tacia Kohl for their wisdom. I am eternally grateful to Fred and Nancy Brooks for hosting the Intervarsity retreat at Caswell where I met my wife. To my friends Mark Stouffer, David Little, Jeffrey Robbins, Mark Reeves, and Serge Severenchuk: I am grateful for each of you. Beyond Chapel Hill, thank you to my friends Connor Ferrell, Grant Riley, Seth Brake, Kenan Keller, Hans and Meredith Noyes, Kelly and Liam Goldsmith, and Tai and Micah Donor. I would like to thank my family for their continual encouragement. Mom and Dad, your example encouraged me to do my best in my studies without finding my identity in them. You've supported me since day one. Lauren, I am grateful for your years of sisterly encouragement and support. Beth Tassan, Mimi, Papaw, and Grandpa, thank you for your calls, kind cards, and good food. You lifted my spirits on so many days. Gina, your help vii moving and lunch visits are greatly appreciated. Phil and Christie, thank you for reminding me that my studies aren't everything. To my wife, ever since we went on our long walk at Caswell, I've known I wanted to marry you. You've been my constant encouragement on all the good and hard days of the PhD and have inspired me to achieve my best work. Working on our doctorates together has been the best of adventures! I am beyond grateful for your consistent and selfless support. Lastly, I want to thank my Savior for drawing me to faith and into the family of God. Jesus, you are my best hope, greatest treasure, and best inspiration to do good science. viii TABLE OF CONTENTS LIST OF TABLES . xv LIST OF FIGURES . xvi LIST OF ABBREVIATIONS . xix 1 INTRODUCTION . 1 1.1 The occurrence of superflares from the Sun and nearby cool stars . 1 1.2 Blackbody temperature of solar and stellar flares . 4 1.3 The complex relationships between flares, starspots, and spin-down . 6 1.3.1 Starspots and the magnetic environments that trigger flares . 6 1.3.2 Stellar flares, starspots, and rotation as a probe of spin-down . 7 1.4 The curious case of flares that do not occur randomly . 8 1.5 The effect of superflares on potentially-habitable exoplanets. 10 1.5.1 The habitability impacts of the increased UV radiation of hot superflares 12 1.5.2 Proxima b: a case study for the habitability of temperate rocky planets orbiting flare stars . 13 1.6 Superflare discovery . 14 1.7 Photometric surveys of rotating cool stars . 18 1.8 Long-term Evryscope observations of all bright flare stars in the South . 18 1.9 Overview of Contents . 21 1.9.1 Breakdown of Work by Chapter . 21 1.9.2 Significant Contributions to Other Research .
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