ABSTRACT Title of dissertation: A STATISTICAL CHARACTERIZATION OF THE ATMOSPHERES OF SUB-SATURN PLANET CANDIDATES IN THE KEPLER ARCHIVE Holly Ann Sheets, Doctor of Philosophy, 2016 Dissertation directed by: Professor L. Drake Deming Department of Astronomy Exoplanet atmospheric characterization is still in its early stages. Large sur- veys like the Kepler Mission provide thousands of planet candidates, but follow-up observations to characterize the individual candidates are often difficult to obtain. In this thesis, I develop a method to detect small atmospheric signals in Kepler’s planet candidate light curves by averaging light curves for multiple candidates with similar orbital and physical characteristics. I also consider two applications of this method: at secondary eclipse, to determine the average albedo of groups of planet candidates, and near transit, to determine whether on average the planets have cloud-free, low- mean-molecular-weight atmospheres, or cloudy/hazy/high-mean-molecular-weight atmospheres. This approach allows the measurement of properties that are un- measurable for candidates individually, because of their low signal-to-noise, and it prevents biasing of the results by false positives (candidates that are not actually planets) and outliers by not depending on only a few measurable candidates. I first develop the method and apply it to the secondary eclipses of 32 close-in Kepler planet candidates between 1 and 6 R⊕ with short cadence data available, in order to determine their average albedo. I then adapt the method to the long cadence data, accounting for the effects of the longer integration time. The increase in the number of candidates available in long cadence allows for finer radius groupings of 1 to 2 R⊕, 2to4 R⊕, and 4 to 6 R⊕. The short cadence average includes 6,238 individual eclipses, while the long cadence averages contain 80,492 eclipses from 56 candidates in the 1 to 2 R⊕ bin, 22,677 eclipses from 38 candidates in the 2 to 4 R⊕ bin, and 4,572 eclipses from 16 candidates in the 4 to 6 R⊕ bin. In both studies, I find that these planet candidates are generally dark, though there are bright outliers like Kepler-10b, and I discuss the implications of these results for understanding the atmospheres of these planets. Finally, I apply the method to Kepler planet candidates in short cadence near transit, looking for a brief brightening due to light that is refracted through the atmospheres of the planets and directed toward the observer just before and just after transit. Refracted light is strongest in planetary atmospheres that are cloud-free and have a low mean molecular weight. Preliminary results suggest this strong refraction effect is not present in the selected group of 10 candidates with radii between 0.8 and 3 R⊕, but I begin to develop a more detailed model and sketch out future plans to improve the model and to continue testing for the presence of refracted light with greater sensitivity. A STATISTICAL CHARACTERIZATION OF THE ATMOSPHERES OF SUB-SATURN PLANET CANDIDATES IN THE KEPLER ARCHIVE by Holly Ann Sheets Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2016 Advisory Committee: Professor L. Drake Deming, Chair/Advisor Professor Suvi Gezari Professor Douglas P. Hamilton Dr. Shawn Domagal-Goldman Professor Rachel Pinker, Graduate Dean’s Representative c Copyright by Holly Ann Sheets 2016 Preface I am grateful for support in this work from NASA’s Virtual Planetary Labo- ratory (Victoria Meadows of the University of Washington, P. I.) and from NASA’s Astrophysics Data Analysis Program (ADAP) under grant NNX15AE53G, entitled “A Statistical Characterization of the Atmospheres of Kepler’s Small Planets” (L. Drake Deming of the University of Maryland, P.I.). This thesis contains work published in the Astrophysical Journal as well as work currently in preparation for submission to the same journal. Chapter 2 was published under the title “Statistical Eclipses of Close-in Kepler Sub-Saturns” (Sheets & Deming, 2014). Chapter 3 will soon be submitted. Results from Chap- ter 2 were presented at the Kepler Science Conference II in November 2013, the 223rd Meeting of the American Astronomical Society in January 2014, and the 46th Annual Meeting of the Division for Planetary Sciences in November 2014. Prelimi- nary results from Chapter 3 were presented at the 225th Meeting of the American Astronomical Society in January 2015. Results from Chapter 3 were presented at the Statistics and Exoplanets Focus Meeting at the XXIX General Assembly of the International Astronomical Union in August 2015, the 47th Annual Meeting of the Division for Planetary Sciences in November 2015, and the 227th Meeting of the American Astronomical Society in January 2016. This thesis includes data collected by the Kepler mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. All of the data from Kepler presented in this thesis were obtained from the Mikulski Archive ii 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 NNX13AC07G and by other grants and contracts. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. iii Acknowledgments Thank you to the students, faculty, and staff in the Department of Astronomy whom I have gotten to know during my journey here and who have made that journey easier. Thanks especially to my advisor, Professor Drake Deming, for his guidance, encouragement, and patience when the words wouldn’t write themselves. Thanks also to Professor Stuart Vogel, for his efforts as department chair to promote diver- sity and inclusion in the department, and for all that he has done on behalf of the graduate students in the department. Thanks to my committee members for their time and valuable input on my work. Thanks to Professor Doug Hamilton for being a sounding board when I needed it. Thanks to Professor Derek Richardson, Dr. Melissa Hayes-Gehrke, Alice Olm- stead, and the rest of the A120/121 overhaul team for including me in the process. I’ve learned so much about teaching and education research from this experience, which will hopefully come in handy in the near future. Thanks also to Observatory Director Elizabeth Warner for indulging my love of looking through telescopes. Thanks to my family and friends for their support throughout the years. Lastly, thanks to my dog, Mira, who is my greatest friend and confidant. iv Table of Contents List of Tables vii List of Figures viii List of Abbreviations x 1 Introduction 1 1.1 Outline of Thesis ............................. 22 2 Statistical Eclipses of Short Cadence Close-in Kepler Sub-Saturns 24 2.1 Introduction ................................ 24 2.2 Kepler Observations ........................... 26 2.3 Averaging Candidate Light Curves ................... 30 2.4 Reflected and Thermal Light Modeling ................. 32 2.5 Results and Discussion .......................... 33 2.5.1 Close-in Candidates ........................ 33 2.5.2 Control Group .......................... 37 2.5.3 Individual Candidates ...................... 48 2.5.4 Advantages and Limitations of the Technique ......... 54 2.6 Implications for Atmospheres and Surfaces of Sub-Saturns ...... 57 2.6.1 Presence of Atmospheres in My Sample ............. 57 2.6.2 Expected Albedos ......................... 59 2.7 Chapter 2 Summary ........................... 61 3 Statistical Eclipses of Long Cadence Close-in Kepler Sub-Saturns 63 3.1 Introduction ................................ 63 3.2 Kepler Data and Candidate Selection .................. 64 3.3 Averaging the Long Cadence Data ................... 72 3.3.1 Scaling and Stacking the Individual Eclipses .......... 72 3.3.2 Notes on Individual Candidates ................. 78 3.3.2.1 KOI 1662.01 ...................... 78 3.3.2.2 KOIs 2396.01 and 2882.02 ............... 80 3.3.2.3 KOI 2795.01 ...................... 80 v 3.3.2.4 KOI 4294.01 ...................... 81 3.3.2.5 KOI 4351.01 ...................... 83 3.3.2.6 KOI 4924.01 ...................... 84 3.4 Modeling Reflected Light and Thermal Emission ............ 84 3.5 False Positive and Other Detrimental Scenarios ............ 85 3.5.1 Dilution by Unknown Objects in Aperture ........... 85 3.5.2 Eccentric Eclipsing Binaries ................... 87 3.5.3 Half-Period Eclipsing Binaries .................. 88 3.5.4 Eccentric Planets ......................... 88 3.6 Results and Discussion .......................... 91 3.6.1 Super-Earths ........................... 91 3.6.1.1 Excluding Kepler-10b ................. 91 3.6.1.2 Including Kepler-10b .................. 92 3.6.2 Mini-Neptunes .......................... 95 3.6.3 Super-Neptunes .......................... 95 3.6.4 Comparison to Short Cadence ..................100 3.6.5 Comparison to Demory (2014) ..................109 3.6.6 Biases in the Albedo Due to Averaging .............110 3.6.7 Implications of the Low Albedos .................111 3.7 Chapter 3 Summary ...........................113 4 Refraction and Future Work 115 4.1 Background ................................115 4.2 Averaging the Data ............................121