Georgia State University ScholarWorks @ Georgia State University Physics and Astronomy Dissertations Department of Physics and Astronomy 12-14-2017 Nearby M Dwarf Stars and the Wide Main Sequence Tiffany D. Clements Georgia State University Follow this and additional works at: https://scholarworks.gsu.edu/phy_astr_diss Recommended Citation Clements, Tiffany D., "Nearby M Dwarf Stars and the Wide Main Sequence." Dissertation, Georgia State University, 2017. https://scholarworks.gsu.edu/phy_astr_diss/100 This Dissertation is brought to you for free and open access by the Department of Physics and Astronomy at ScholarWorks @ Georgia State University. It has been accepted for inclusion in Physics and Astronomy Dissertations by an authorized administrator of ScholarWorks @ Georgia State University. For more information, please contact [email protected]. NEARBY M DWARF STARS AND THE WIDE MAIN SEQUENCE by Tiffany Danielle Clements Under the Direction of Todd Henry, PhD ABSTRACT This dissertation addresses the underlying causes for the observed widening of the main sequence for low mass stars and presents the results of three different studies using two different astrophysical observing methods to assess the properties of a sample of nearby, M dwarf stars and how these properties affect a star's position on an H-R diagram. The first study is the assessment of the activity of the internal magnetic fields of 76 southern nearby, M dwarf stars through measurements of the relative changes in their V magnitudes over time periods of years. This long-term variability is then analyzed with respect to the vertical positions of these stars on the main sequence to determine the effects of stellar activity on these positions. The second study uses a similar technique as the first study (relative photometry) only on short-term timescales of only hours. This study serves to assess the effects of magnetic activity and variability at the surfaces of 120 low mass stars on their main sequence positions. The final study presented here analyzes the high-resolution spectra of 80 nearby, M dwarf stars to assess their compositions and activity indicators and how these properties affect the placement of these stars above, below, or on the central distribution of the main sequence. This study also includes the use of published metallicity values for many of the stars in our sample to better assess the importance of this property on main sequence position. We find that variability plays an important role in the elevation of stars, particularly young stars, above the main sequence, likely due to an extension of their chromospheres caused by heightened levels of stellar activity. For stars below the main sequence we find a strong connection between their positions and abundances, as low metallicity results in a temperature displacement that ultimately places these stars below the main sequence. These results suggest that while each property is important, heightened activity is a stronger in- fluence for stars above the main sequence while low metallicity is the primary property that places these stars below the main sequence. INDEX WORDS: astronomy, solar neighborhood, stars: low-mass, stars: subdwarfs, stars: abundances, stars: activity, stars: flare, techniques: photomet- ric, techniques: spectroscopic NEARBY M DWARF STARS AND THE WIDE MAIN SEQUENCE by Tiffany Danielle Clements A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the College of Arts and Sciences Georgia State University 2017 Copyright by Tiffany Clements 2017 NEARBY M DWARF STARS AND THE WIDE MAIN SEQUENCE by Tiffany Danielle Clements Committee Chair: Todd Henry Committee: Douglas Gies Russel White Brian Thoms Suzanne Hawley Electronic Version Approved: Office of Graduate Studies College of Arts and Sciences Georgia State University December 2017 iv CHAPTER 0 DEDICATION This dissertation is dedicated to my wonderful husband and to my daughter. My husband, Keith Clements, has been my biggest supporter and has kept me going for the last six years. Between reassuring me that I can do this and picking up the extra work left while I was traveling or working well into the evenings he has kept me grounded and pushed me to do anything I set my mind to. He has kept me both sane and fed through the last six years and I am so grateful for all that he has done. My daughter, Artemis, while she may not be here yet, her constant kicks and stretches over the last several months have kept me motivated and sprinting for the finish line. I can't wait to finally meet her and guide her through the world until she finds her own place in it, as I have. v CHAPTER 0 ACKNOWLEDGMENTS Many thanks go to my adviser and mentor for the past six years, Dr. Todd Henry. Your to-do lists, guidance, and many paper edits have kept me on the path to this point and allowed me to evolve from \Protodoctor" to Dr. Clements. The RECONS Team has always been a wonderful group that never questions working together and using observing time for each other to make sure we all succeed together. Wei- Chun Jao has always made himself available to answer my questions and taken the time to show me how to do something. Sergio Dieterich, Adric Riedel, and Jennifer Winters have all helped me to edit a paper, made their data readily available, used observing time to help with my projects, or answered any questions I've emailed them throughout my time at Georgia State. Michele Silverstein is the closest RECONS member to me and we've gone through a lot of this together. We've learned how to navigate our way through the group, attended conferences, and shared our observing time and expertise. I'm grateful to have had such a wonderful colleague and friend with whom to go through all of this. My family and friends have always been supportive of my goals and taken the time to be there for both my complaints and successes. Thank you to my parents for raising me to know that if I want something badly enough and work hard enough, then I can do it, and for never doubting that I would make it this far. I love you both. Thank you to my sisters, Diana and Sara, and my brother, Josh, for being the best (and most complicated) family I could have asked for. You've all been so understanding about my missing so many family events and vi gatherings because of the difficulty of traveling so far during grad school. Missing out on time with you has been one of the hardest parts of all of this, but I've cherished every moment I've had to visit or talk with you. Finally, to my friends, those of you not in Astronomy have found it so fascinating and \cool" to learn about what I do and your enthusiasm has always kept me motivated, those of you in Astronomy have understood how frustrating and complex this field can really be and have lamented with me, supported me, and always kept me grounded with how brilliant each of you are. Thank you all for your support, advice, and (Charlie) housing and feeding me for the past three years that I've lived out-of-state. Finally, thanks to Alvin Das, the original creator of the LaTeX package for the preparation of a GSU dissertation, and the most recent editor of the package, Jeremy Jones. This dissertation would have probably never been written if it weren't for you as you've saved many Astronomy graduate students months of trying to find a way to format these daunting documents just right. vii CHAPTER 0 TABLE OF CONTENTS ACKNOWLEDGMENTS . v LIST OF TABLES . x LIST OF FIGURES . xi 1 INTRODUCTION . 1 1.1 M Dwarf Stars ................................. 1 1.2 The Wide Main Sequence .......................... 2 1.3 Activity in Low Mass Stars ......................... 8 1.4 Off-Main Sequence Stars in This Study: Young Stars and Subdwarfs 9 1.5 The Bigger Picture .............................. 11 1.6 Overview of This Thesis ........................... 12 2 SELECTION OF TARGETS . 16 2.1 Origin of the Data ............................... 16 2.2 Elimination of Close Binaries ........................ 18 2.3 Defining the Center of the Dispersion ................... 20 2.4 Defining Our Boxes: What They Represent . 21 3 LONG-TERM VARIABILITY STUDY . 25 3.1 Overview of the Long-Term Variability Study . 25 3.2 Stars in this Study ............................... 27 3.3 Observations and Reductions ........................ 28 3.4 Analysis and Results ............................. 30 3.4.1 Individual Stars ............................ 33 3.4.2 ROSAT Study ............................. 37 viii 3.5 Conclusions ................................... 38 4 SHORT-TERM VARIABILITY STUDY . 52 4.1 Overview of the Short-Term Variability Study . 52 4.2 Stars in Study ................................. 54 4.3 Observations and Reductions ........................ 57 4.4 Analysis and Results ............................. 60 4.4.1 Control Stars Results ......................... 63 4.4.2 Individual Variability Detections . 64 4.5 Conclusions ................................... 67 5 SPECTROSCOPY STUDY . 85 5.1 Overview of the Spectroscopy Study ................... 85 5.2 Stars In Study ................................. 85 5.3 Observations and Reductions ........................ 87 5.4 Analysis and Results ............................. 89 5.4.1 Trends in Spectral Features ..................... 90 5.4.2 Metallicities ............................... 95 5.4.3 Radial Velocity Measurements ................... 98 5.4.4 Notable Stars .............................. 100 5.5 Conclusions ................................... 103 6 CONCLUSIONS . 133 6.1 Comparison to Other Works . 133 6.2 Overall Results ................................. 137 6.3 Potential Improvements . 139 6.4 Future Work .................................. 141 REFERENCES . 143 ix APPENDIX COMPLETE SAMPLE DATA . 150 x CHAPTER 0 LIST OF TABLES Table 2.1 Telescopes and Instruments Used. 23 Table 3.1 Properties of the Low Mass Stars in the Long-Term Variability Study. 40 Table 3.2 ROSAT Detection Data. 42 Table 4.1 Properties of the Low Mass Stars in the Short-Term Variability Study.