c Copyright 2018 John C. Lurie Three Investigations of Low Mass Stars in the Milky Way Using New Technology Surveys John C. Lurie A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2018 Reading Committee: Mario Juri´c,Chair Kevin Covey Suzanne Hawley Program Authorized to Offer Degree: Astronomy University of Washington Abstract Three Investigations of Low Mass Stars in the Milky Way Using New Technology Surveys John C. Lurie Chair of the Supervisory Committee: Associate Professor Mario Juri´c Astronomy At least 80% of stars in the Milky Way have masses less than or equal to the Sun. These long lived stars are the most likely hosts of planets where complex life can develop. Although relatively stable on the timescale of billions of years, many low mass stars possess strong magnetic fields that are manifested in energetic surface activity, which may pose a hazard to both life and technology. Magnetic activity also influences the evolution of a low mass star through a feedback process that slows the rotation rate, which in turn tends to decrease the amount of activity. In this way, the rotation rate and activity level of a low mass star may provide an estimate of its age. Beyond their rotation-activity evolution as isolated objects, a small but important fraction of low mass stars have a close binary companion that influences the rotational and orbital properties of the system. Binary interaction can lead to phenomena such as supernovae, cataclysmic variables, and degenerate object mergers. From a larger perspective, low mass stars trace Galactic structure, and through their longevity serve as archives of the dynamical and chemical history of the Milky Way. Thus a full picture of low mass stars, and by extension the Milky Way, requires un- derstanding their rotation and activity; their interaction in close binaries; and their spatial and kinematic distribution throughout the Galaxy. Historically, these topics have been ap- proached from two separate but complementary modes of observation. Time series photomet- ric surveys measure the stellar variability caused by rotation, activity, and binary interaction, while wide field surveys measure the brightnesses and colors of millions of stars to map their distribution in the Galaxy. The first generation of digital detectors and computing technol- ogy limited intensive time series surveys to a small number of stars, and limited wide field surveys to little if any variability information. Today those limitations are falling away. This thesis is composed of three investigations of low mass stars using two recent surveys at the cutting edge of detector technology. The Kepler space telescope carried the largest camera ever launched into space, and continuously monitored the brightnesses of hundreds of thousands of stars with unprecedented precision and cadence. The Pan-STARRS survey was equipped with the largest camera ever constructed, and imaged 75% percent of the sky to greater depth than any previous optical survey. The first investigation in this thesis used Kepler observations of a binary system contain- ing two stars that are about one third the mass of the Sun. The convective motions in these stars extend to their centers, and so there is no interface with a radiative core to drive a solar-like dynamo that powers the magnetic activity of stars like the Sun. By virtue of being in a binary, the stars have the same age, providing a control for the interdependent effects of activity and rotation. The investigation found that the stars have nearly the same level of activity, despite one star rotating almost three times faster than the other. This suggests that in fully convective stars, there is a threshold rotation rate above which activity is no longer correlated with rotation. The second investigation also used Kepler observations, but in this case focused on low mass stars in close binaries, where tidal interactions are expected to circularize the orbit and synchronize the rotation rates to the orbital period. Prior to this investigation, there were few observational constraints on the tidal synchronization of stars with convective envelopes, and this investigation resulted in rotation period measurements for over 800 such stars. At orbital periods below approximately ten days, nearly all binaries are synchronized, while beyond ten days most binaries have eccentric orbits and rotation rates that are synchro- nized to the angular velocity at periastron. An unexpected result was that 15% of binaries with orbital periods below ten days are rotating about 13% slower than the synchronized rate. It was suggested that the equators of the stars are in fact synchronized, and that the subsynchronous signal originates from slower rotating high latitudes. The subsynchronous population presents a new test for theories of activity and differential rotation in tidally interacting binaries. The final investigation used Pan-STARRS observations to search for asymmetries in the disk of the Milky Way. In this case, low mass stars served as tracers of Galactic structure. Previous deep optical surveys avoided the Galactic plane, but Pan-STARRS enabled a com- prehensive search. In particular, asymmetries in the stellar density distribution may be the result of interactions with satellite galaxies, and the frequency and nature of the interac- tions provide an observational test case for theories of galaxy formation. The investigation revealed four asymmetries that extend over much of the visible disk. The observations are qualitatively consistent with mock observations of a Milky Way-like galaxy with a radial wave in its midplane. Although the origin of these asymmetries continues to be debated, the results support a new view of the Milky Way disk that is asymmetric at the 10-30% level in terms of the mean number of star counts. As a whole, the three investigations in this thesis bring together two different observa- tional approaches to take a holistic view of low mass stars in the Milky Way. In the near future, new surveys will provide even higher quality data for billions of stars. Therefore, it will be more important than ever to understand low mass stars both as complex astrophysical objects and as a major constituent of the Galaxy. TABLE OF CONTENTS Page List of Figures . iii List of Tables . xi Chapter 1: Introduction . 1 1.1 Low Mass Stars in the Milky Way . 1 1.2 A New Era in the Observation of Stars . 5 1.3 Stellar Rotation and Magnetic Activity . 9 1.4 Binary Interaction . 15 1.5 Galactic Structure and Formation . 22 1.6 Outline of this Thesis . 27 Chapter 2: GJ 1245: A Benchmark Wide Binary . 29 2.1 Introduction . 29 2.2 Observations and Analysis . 31 2.3 Starspot Evolution . 46 2.4 Flares . 50 2.5 Discussion . 60 Chapter 3: Tidal Synchronization of Kepler Eclipsing Binaries . 67 3.1 Introduction . 68 3.2 Data . 70 3.3 Classification and Rotation Period Analysis . 73 3.4 Tidal Synchronization . 83 3.5 Differential Rotation . 100 3.6 Additional Results . 107 3.7 Conclusion . 110 i Chapter 4: Asymmetries in the Milky Way Disk . 112 4.1 Introduction . 113 4.2 Pan-STARRS1 Data . 115 4.3 An Overview of the Asymmetric Milky Way . 120 4.4 The Asymmetry Mapping Technique . 125 4.5 Maps of the Asymmetric MW . 133 4.6 A Toy Model For Bending Modes . 138 4.7 Discussion . 142 4.8 Summary . 148 Chapter 5: Conclusion . 150 5.1 Summary . 150 5.2 Future Prospects . 152 5.3 Closing Thoughts . 157 5.4 Additional Acknowledgments . 159 5.5 Appendix to Chapter 3: Asynchronous EBs with Porb < 10 Days . 161 ii LIST OF FIGURES Figure Number Page 1.1 Top: Approximate main sequence lifetimes. Bottom: Number of stars (black line), and total stellar mass fractions (dashed red line) as a function of mass, based on the Kroupa (2001) IMF. 2 1.2 Left: Coronal loops emerging from the stellar surface seen in the ultraviolet by the Transition Region and Coronal Explorer. Right: A sunspot group seen in visible light by the Hinode spacecraft. NASA images in the public domain. 11 2.1 The 11 10 grid represents the spatial extent of the target pixel mask. The field of view× is shown by the 4400 arrow at the top, and the on-sky orientation of the mask is shown by the arrows labeled \E" and \N". Within each cell, the pixel-level light curve is plotted. The y-axis range in each pixel is 2000 e− s−1, and the timespan is 2 days, as denoted in the lower left corner. The color of each pixel corresponds to the strength of the starspot signals in each pixel, indicated by the color bars on the right. The locations of the PRF model sources for the A and B components are shown as a yellow circle and X, respectively. The expected positions of the stars based on their R.A. and decl. are plotted as a yellow open square and plus symbol, respectively. 35 2.2 The top left panel corresponds to a single observation of the target pixel mask, with the flux in each pixel indicated by the greyscale colorbar. The green border demarcates the PDC-SAP aperture. The PyKE PRF model is in the top right panel, which is summed within each pixel to produce the fit in the lower left panel. The lower right panel shows the residual between the observation and fit. Note that the residual color bar has both negative and positive values, and has a factor of 10 smaller range than the other panels. 38 iii 2.3 The ranges of angular separation in each Kepler quarter are plotted as vertical lines. Quarters are color-coded based on the observing season (spacecraft orientation) during which they were taken.