Stellar Evolution at the Crossroads: Resolving the Nature of RV Tauri Variable Systems Using Unprecedented Observations From the Kepler and XMM–Newton Space Telescopes By Laura Daniela Vega Dissertation Submitted to the Faculty of the Graduate School of Vanderbilt University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Astrophysics March 31, 2021 Nashville, Tennessee Approved: Keivan G. Stassun, Ph.D. Andreas A. Berlind, Ph.D. Patricia T. Boyd, Ph.D. J. Kelly Holley-Bockelmann, Ph.D. Rodolfo Montez Jr., Ph.D. Eric M. Schlegel, Ph.D. DEDICATION I dedicate this dissertation to all those who have inspired, encouraged, mentored, cared, and supported me on this journey to become an astrophysicist. ii ACKNOWLEDGMENTS This PhD could not have been possible on my own. I would like to take this moment to express my sincere thanks to all my family and friends who have cheered me on from the start. And to all the gentle people who have crossed my path, who I have either learned from or who have simply been kind to me at any point in this journey, thank you. Gabriel, thank you for your encouraging words and the countless moments you boosted my self-esteem when I did not believe in myself... Thank you for believing in me. Un agradecimiento especial a mama, daddy, Adriana, y Joel por su amor incondicional. Daddy (my Luna) and Mama (my Vega): gracias por llevarnos a Mexico´ a visitar familia cada verano desde que eramos´ pequenos.˜ Admirando el cielo repleto de estrellas durante nuestros viajes de noche en carretera, en el bello desierto montaoso Mexicano, fue una de mis inspiraciones para querer estudiar el universo. ¡Lo logre,´ mama! Jesse, thank you for your love, support, and caring. And for all the little but meaningful things you do for us, they have kept me going, especially when finishing this dissertation, they do not go unnoticed. And for our little kitty cat, Logan, who makes me smile just by being a cat. To my advisor Keivan and my research mentor Rudy: my sincerest thanks for giving me the opportunity of working with you at Vanderbilt, and at the Center for Astrophysics. I am very grateful for your constant support, motivation, guidance, time, and patience, as well as the sincere words of advice you have given me during some difficult moments in my life. I would also like to thank the rest of the members of my dissertation committee, Eric Schlegel, Padi Boyd, Kelly Holley-Bockelmann, and Andreas Berlind: thank you for your time, expertise, advice, and kindness. I would like to acknowledge Vanderbilt’s Psychological & Counseling Center and Center for Student Wellbeing, especially Samantha York: my academic coach, meditation guide, and Writer’s Accountability Group mentor. I would not have made it this far without the support and resources they provided. I would like to thank the Fisk-Vanderbilt Master’s-to-PhD Bridge Program family for all of the support they have given me. I am grateful to my NASA OSTEM family and to the SREB Institute for Teaching and Mentoring. I would like to acknowledge the wonderful work of the NASA’s Goddard Space Flight Center’s iii Astrophysics Science Division Communications Team in the production of a NASA news feature for our research. I am very grateful this dissertation work was financially supported by the NASA OSTEM MUREP Harriett G. Jenkins Predoctoral Fellowship, Grant Nos. NNX15AU33H and 80NSSC19K1292; by the partial federal support from the Latino Initiatives Pool, administered by the Smithsonian Latino Center; and by the partial support from NSF PAARE grant AST-1358862. This research has made use of XMM–Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). This research has made use of The Submillimeter Array, a joint project between the Smithso- nian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. This research has made use of the DASCH project at Harvard, which is grateful for partial support from NSF grants AST-0407380, AST-0909073, and AST-1313370. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, and of the VizieR catalog access tool, CDS, Strasbourg, France (DOI: 10.26093/cds/vizier). The original description of the VizieR service was published in 2000 (Ochsenbein et al., 2000). This work includes data collected by the Kepler mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. The data was obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non- HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. This work makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/Cali- fornia Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. iv TABLE OF CONTENTS Page DEDICATION . ii ACKNOWLEDGMENTS . iii LIST OF TABLES . viii LIST OF FIGURES . ix CHAPTERS . 1 Introduction . 1 1.1 RV Tauri Variables . 1 1.2 Dissertation Outline . 5 2 Evidence for Binarity and Possible Disk Obscuration in Kepler Observations of the Pul- sating RV Tau Variable DF Cygni . 8 2.1 Original Abstract . 9 2.2 Introduction . 10 2.3 Data: Kepler Time Series Observations . 12 2.4 Analysis and Results . 13 2.4.1 General Features of the DF Cyg Light Curve . 14 2.4.2 Arrival Time Variations in the Deep and Shallow Minima . 16 2.4.3 Redetermination of the Long-term Periodicity . 17 2.4.4 SED: Stellar Radius and Dust Disk . 20 2.5 Discussion: Disk Occultation Scenario for the Long-period Behavior . 22 2.5.1 Evidence for a Binary Companion with an 800 Day Period . 22 v 2.5.2 Disk Occultation Scenario . 23 2.6 Summary and Conclusions . 28 3 Multiwavelength Observations of the RV Tauri Variable System U Monocerotis: Long- Term Variability Phenomena That Can Be Explained by Binary Interactions with a Cir- cumbinary Disk . 30 3.1 Original Abstract . 31 3.2 Introduction . 32 3.3 The U Monocerotis System . 35 3.3.1 U Mon as an RV Tauri Variable of RVb Type . 37 3.3.2 U Mon as a Binary Star System . 37 3.3.3 The U Mon Circumbinary Disk . 38 3.3.4 Magnetic Activity in U Mon . 39 3.4 Data . 39 3.4.1 Radial Velocity Observations . 39 3.4.2 Light-curve Observations . 40 3.4.2.1 AAVSO . 40 3.4.2.2 DASCH . 41 3.4.2.3 Combining AAVSO and DASCH Light-curve Data . 43 3.4.3 SMA Observations . 43 3.4.4 XMM-Newton Observations . 44 3.4.5 Spectral Energy Distribution Data . 46 3.5 Results . 47 3.5.1 Secular Variations of the U Mon Light Curve over the Past Century . 47 3.5.2 Orbital Properties of the U Mon Binary Star System . 50 3.5.3 Spectral Energy Distribution . 50 3.5.4 Properties of the Circumbinary Disk’s Inner Edge . 52 3.5.5 Submillimeter Emission from U Mon . 53 vi 3.5.6 X-Ray and UV Emission from U Mon . 54 3.6 Discussion . 56 3.6.1 A 60 yr Trend in the Light Curve . 56 3.6.2 Circumbinary Disk Interaction . 57 3.6.3 Nature and Origin of X-Ray Emission from U Mon . 59 3.7 Conclusions . 62 4 Conclusions and Future Work . 65 4.1 Conclusion . 65 4.2 Future Work . 66 4.2.1 Multiwavelength Observations of RV Tauri Systems . 66 4.2.2 Circumbinary Disks . 68 4.2.3 Accretion Disks . 68 BIBLIOGRAPHY . 71 vii LIST OF TABLES Table Page 2.1 The long-period transitional behavior of DF Cyg observed by Kepler. 16 2.2 The deep minima arrival time of DF Cyg in the Kepler light curve. 19 3.1 Observed and derived physical properties for the U Mon system used in our analysis. 36 3.2 New flux measurements for U Mon. 47 3.3 Results of model fits to visibilities of U Mon measured with the SMA. 53 3.4 X-ray spectral fit results for U Mon. 55 viii LIST OF FIGURES Figure Page 1.1 A segment of DF Cygni’s Kepler light curve, an RV Tauri-type variable star, show- ing the characteristic pulsation pattern of alternating deep and shallow minima in brightness over time. 2 1.2 The relationship between brightness and pulsation (i.e., the Leavitt Law) for Type II Cepheid variables in the Large Magellanic Cloud. The cyan and red symbols are “peculiar” W Vir stars and highly-reddened RV Tauri stars, respectively, and were not used in the linear regression model show in this plot. Figure take from Manick et al. (2017). 3 1.3 RV Tauri stars reside in a part of the HR diagram where stellar evolutionary tracks of high mass and low mass stars meet, they were thought to evolve from either massive or low-mass stars. Recent studies favor a low-mass stellar origin. Figure adapted from Cox (1974). 4 1.4 Spectral energy distributions of six bright RV Tauri stars showing an excess of in- frared light starting at ∼2 mm.