New Insights into Circumstellar Disk Evolution in the Era of ALMA Thesis by Scott Alexander Barenfeld In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Astrophysics CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2019 Defended October 5, 2018 ii © 2019 Scott Alexander Barenfeld ORCID: 0000-0001-5222-6851 All rights reserved iii ACKNOWLEDGEMENTS The list of people who have helped guide me to where I am today is too long to fit into a single section of my thesis. While there are so many people I would like to thank, I must start with my two advisors, John Carpenter and Anneila Sargent. Working with each of them has been an absolute privilege. John’s high standards and attention to detail have constantly pushed me to be the best scientist that I can be. While his vision was instrumental in developing the project that would become my thesis, he always encouraged me to think independently about my research and find answers on my own. Though I did not begin working closely with Anneila until the second half of my time at Caltech, her influence has proven invaluable as well. Through the process of repeatedly returning paper drafts to me looking, as she put it, “like the Dead Sea Scrolls,” she has made me a far better writer. I am extremely thankful for the encouragement she has given me in recent months to push ahead and finish my thesis more quickly than I thought was possible. I am also grateful to my thesis committee members, Dimitri Mawet, Chuck Steidel, and Phil Hopkins, and in particular to my committee chair, Lynne Hillenbrand, who served as my research advisor during my first two years at Caltech. Of course, it takes more than just advisors to get someone through a Ph.D. I would therefore like to also thank Andrea Isella, Luca Ricci, and Tiffany Meshkat, all of whom I worked closely with during my time here, and of course, all of the administrators and staff in Cahill, who keep our department running smoothly. Looking beyond Caltech, I would not be where I am today if not for my undergraduate research advisors at the University of Rochester and Arecibo Observatory, Eric Mamajek, Eric Bubar, Chris Salter, and Tapasi Ghosh. And of course, Janet Fogg, who has been a parent-away-from-home to every student who has come through Rochester’s Physics and Astronomy department. My time at Rochester was hugely transformative, thanks in large part to the lifelong friends I made there, including Marc Nuzzo, Gabrielle Mariano, and Rachel Blomberg. In addition, I will never forget the time I spent screaming my lungs out as part of the “actually pretty decent” University of Rochester Pep Band. I want to thank all of my fellow graduate students with whom I have shared Cahill for making our department such an enjoyable place to be. In particular, I would like to thank Marin Anderson, who had the unfortunately luck of having a desk next to mine in our first year as I was learning how to juggle. It was fortunate for iv me, however, as in addition to evidently having very quick reflexes, she has been a great friend ever since. Also, thank you to Swarnima Manohar, the only person I’ve ever met who enjoys Marvel movies as much as I do, and to Lee Rosenthal, a close second. Thank you to Gina Duggan for working so hard to make the department, and Caltech as a whole, a better place, and to Mark Duggan for all of the awesome game nights. Thank you to Ryan Monroe for all the fun times in the department and on the slopes of Mammoth. Broken thumb or not, I still beat you down the hill! Thank you to Jake Jencson, who has more love for the state of Ohio than I’ve ever seen in another human being, to Denise Schmitz, for some amazing sessions of Dungeons and Dragons, and to Rachel Theios, for all the adorable pictures of Bailey and all the times spent chatting in each others’ offices when one of us needed a break. And last, but not least, thank you to Donal O’Sullivan, my officemate and best man, for giving a reason to look forward to coming into work every day. As every graduate student knows, working towards a Ph.D. has its ups and downs. As such, I would like to thank Lee Coleman and the rest of the Caltech Counseling Center for getting me and so many others through our toughest moments. At times like those, it was always so helpful to have things to focus on outside of Caltech, which is why I am so grateful to Seed Consulting Group for the opportunity to volunteer with them in my spare time. Working with Seed was the most rewarding thing I have done during my time as a graduate student, and I would like to thank all of the consultants I teamed with for making Seed possible. Seed would not exist without the vision and passion of Ajay Bawa, so I am incredibly grateful to him for starting something that has been truly special to be a part of. Of course, none of this would have happened without my parents. Whether through watching documentaries on the Discovery Channel, visiting the Bronx Zoo, or going on incredible adventures to see nature in person, my parents instilled in me a fascination with the natural world from a young age. As I’ve grown, they have always been incredibly supportive of whatever I decided to pursue, and were always there for me to turn to whenever I struggled. I am so grateful for the close relationship we still share. Beyond my parents, my extended family has played a huge part in my life, especially my grandparents. I still remember, while visiting my Grandfather in Florida during high school, watching back-to-back documentaries on String Theory and Stephen Hawking’s latest ideas about black holes. I didn’t understand a single word in either one, but I knew right then that physics and astronomy were what I wanted to pursue. v Finally, I could not be more grateful to my incredible wife, Faye. You have been the most supportive person anyone has ever had in their lives during our nearly ten years together. I could go on and on about just the last few weeks, as you’ve single-handedly gotten us ready to move to New York so that I could concentrate on my thesis. I cannot thank you enough for always being there for me no matter how difficult things have gotten. I cannot believe that when I started at Caltech, we had only just finished our first year of living together in campus housing, and now we are married and getting ready to make our second move across the country together. And after all this time, there is still no one I would rather spend 5 straight days stuck in a car with! vi ABSTRACT In recent decades, it has become clear that solar systems such as our own form within the circumstellar disks of gas and dust that surround young stars. Thus, to understand how these solar systems come to be, it is necessary to study the conditions within these disks. Until recently, such studies have required a focus on intrinsically brighter and younger disks that are easier to observe. However, a full picture of planet formation requires the characterization of older disks to determine how these systems change over time. The unprecedented capabilities of the Atacama Large Millimeter/submillimeter Array (ALMA) now present the opportunity to study populations of these disks in detail for the first time. In this thesis, I present a study of over 100 such disks in the 5-11 Myr old Upper Scorpius OB Association (Upper Sco) using ALMA, as well as Keck Observatory, with the aim of comparing the properties of these systems to younger disks in order to shed new light on disk evolution. Following background discussion on disks and their evolution, ALMA measure- ments of the continuum and CO line fluxes of these disks at 0.88 mm are reported in Chapter 3. The continuum fluxes are used to show that the majority of these systems contain less than 1 M⊕ of dust. It is then shown that dust masses around these stars are on average a factor of ∼ 4:5 lower than their younger counterparts in the Taurus star-forming region, placing important constraints on the mass evolution of these systems. Finally, constraints on depletion of gas in these disks are discussed using the CO measurements. The spatial distributions of the gas and dust within these Upper Sco disks are modeled in Chapter 4. The radial extents of gas and dust are measured and compared, with several systems showing evidence of the gas being more extended. The sizes of the dust disks are compared to younger systems, showing that these disks shrink by a factor of approximately three as they age. These results suggest that dust evolves from the outside-in within disks, perhaps through radial drift. Despite this evolution, dust disks in Upper Sco fall on the same correlation between size and millimeter luminosity as their younger counterparts. This implies a link between the radial structures of disks of different ages, perhaps indicating that these systems are composed of optically-thick dust substructure. Of course, an understanding of planetary system formation would be incomplete vii without accounting for the presence of stellar companions, which are common around young stars and are expected to shorten disk lifetimes by truncating their sizes.