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Multiplicity of Solar-Type Stars

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Multiplicity of Solar-Type Stars A Survey of Stellar Families: Multiplicity of Solar-Type Stars A Dissertation Presented in Partial Fulfillment of Requirements for the Degree of Doctor of Philosophy in the College of Arts and Sciences Georgia State University 2009 by Deepak Raghavan Committee: Dr. Harold A. McAlister, Chair Dr. Douglas R. Gies, Member Dr. Todd J. Henry, Member Dr. David W. Latham, Member Dr. Brian D. Mason, Member Dr. A. G. Unil Perera, Member Dr. Russel J. White, Member Date Dr. H. Richard Miller, Chair Department of Physics & Astronomy A Survey of Stellar Families: Multiplicity of Solar-Type Stars by Deepak Raghavan Under the Direction of Harold A. McAlister Abstract I present the results of a comprehensive assessment of companions to solar-type stars. The sample of 454 stars, including the Sun, was selected from the Hipparcos catalog with π > 40 mas, σπ/π < 0.05, 0.5 ≤ B − V ≤ 1.0, and positioned on an HR diagram within a band extending 1.5 magnitudes below and 2 magnitudes above an iterative best-fit main sequence. The resulting sample is an exhaustive set of stars with V -band flux between 0.1 and 10 times that of the Sun, providing a physical basis for the term “solar-type”. New observational as- pects of this work include a survey for separated fringe packet companions with long-baseline interferometry at the Center for High Angular Resolution Astronomy (CHARA) Array, blink- ing multi-epoch archival images to identify wide common proper motion companions, and speckle interferometry. Apart from evaluating companion information from the various cat- alogs covering many different techniques, I also obtained and included unpublished results from extensive radial velocity monitoring programs. The many sources leveraged enable a thorough search and evaluation for stellar and brown dwarf companions. The results presented here include eight new companion discoveries, four of which are wide common proper motion pairs discovered by blinking archival images, and four more are from the spectroscopic data. The overall observed fractions of single, double, triple, and higher order systems are 57% ± 3%, 33% ± 2%, 8% ± 1%, and 3% ± 1%, respectively. If all candidate companions identified are found to be real, the percentages would be 54% ± 2%, 34% ± 2%, 9% ± 2%, and 3% ± 1%, respectively. The incompleteness analysis indicates that only a few undiscovered companions remain for this well-studied sample. Including these missed companions and estimating the percentage of candidates that are real leads to multiplicity fractions of 55% ±3%, 34% ±2%, 9% ±2%, and 2% ±1% for single, binary, triple, and higher order systems. These results include all stellar and brown dwarf companions, and show that a majority of the solar-type stars are single, in contrast to earlier expectations. This large sample enables a check of the dependence of multiplicity on various physical parameters. Bluer, more massive stars are more likely to have companions than redder, less massive ones, consistent with the trend seen over the entire spectral range, but showing a steeper drop at solar-type stars than previously believed. Consistent with prior studies, younger, more active stars are seen as more likely to have companions than older, less active ones, suggesting that companions may be stripped over time by dynamical interactions. A preliminary, but important indication seen here is that brown dwarfs, like planets, seem to prefer stars with higher metallicity, tentatively suggesting that brown dwarfs may form like planets, at least when they are companions to stars. The period distribution derived is unimodal and roughly Gaussian with a peak and me- dian value of 300 years. The period-eccentricity relation shows the expected circularization for periods below 12 days, followed by a roughly flat distribution. The mass-ratio distribution shows a clear discontinuity near a value of one, indicating a preference for twins, which are not confined to short orbital periods. The period-mass-ratio relationship shows that multiple formation mechanisms are likely responsible for the formation of stellar systems – fission for like pairs and fragmentation or random associations for longer-period systems of all mass ratios. This distribution also shows a paucity of low-mass companions, confirming that the brown dwarf desert extends out to wide separations. The ratio of planet hosts among single, binary, and multiple systems are statistically indistinguishable, suggesting that planets are as likely to form around single stars as they are around components of binary or multiple systems. Hence, the common occurrence of binary systems is likely to increase the real-estate available for planets, and perhaps life. INDEX WORDS: Stellar multiplicity, Binary stars, Solar-type stars, Solar neighbor- hood, Exoplanet systems, Brown dwarfs, Survey, Long baseline inter- ferometry, Radial velocity A Survey of Stellar Families: Multiplicity of Solar-Type Stars by Deepak Raghavan A Dissertation Presented in Partial Fulfillment of Requirements for the Degree of Doctor of Philosophy in the College of Arts and Sciences Georgia State University 2009 Copyright by Deepak Raghavan 2009 A Survey of Stellar Families: Multiplicity of Solar-Type Stars by Deepak Raghavan Major Professor: Harold A. McAlister Committee: Douglas R. Gies Todd J. Henry David W. Latham Brian D. Mason A. G. Unil Perera Russel J. White Electronic Version Approved: Office of Graduate Studies College of Arts & Sciences Georgia State University May 2009 iv – 0 – To my parents Thank you for inculcating the spirit of learning in me v – 0 – Acknowledgments When I decided to pursue a doctorate in astronomy more than six years ago, it was not with complete certainty, but with a sense of curiosity, wonder, and yes, some apprehension. I had decided to chart a unique course for myself, and to travel down a path with few footsteps, especially given where I was coming from. I would not have been able to complete this journey without the steadfast support of my family and friends, and I would like to extend my most sincere gratitude to them. I could not have done this without the support of my dear wife, Priya, who has not only always stood by me, but also been such a tremendous source of support and wisdom, letting me probe and explore, while always being there when I needed a listening ear or a supporting hand. My children, Anjan and Aditi, while sometimes curious, and sometimes worried as to what daddy was doing back in school, never wavered in their implicit support. I thank my parents and siblings for inculcating the spirit of learning in me, which has become a significant driver of my life. My mother led by example, completing her own Ph.D. in Hinduism in 2007. I am very proud of her accomplishment and inspired by it. I appreciate my friends for their indulgence when I would excitedly talk about astronomy and for allowing me to miss parties or leave them early to go to AROC for observing. My thanks also go out to my committee members and the astronomy faculty members for the invaluable support and guidance they have provided me throughout the process. Special vi thanks to Todd Henry for directing my master’s work on exoplanet systems, for the idea of this thesis work, and for his consistently insightful feedback. I have truly enjoyed working with my advisor, Hal McAlister, and would like to recognize his consistent encouragement, support, and direction, not to mention his pioneering efforts in making the CHARA Array a reality and providing us graduate students with copious observing time on such a world- class instrument. Special thanks to Dave Latham for allowing my work to benefit from his meticulous radial velocity monitoring over 30 years and for being so supportive, gathering new data as required and reducing the data on hundreds of stars to enhance the results of this effort. Thanks to Brian Mason for always knowing the answer to any question about double stars and for taking me along on the speckle observing runs; and to Doug Gies, whose consistent advice and ideas enabled me to give a better shape to this effort. This work would not have been possible without the help extended by astronomers around the world, who responded to my persistent requests with support and encouragement. Geoff Marcy graciously allowed me access to his treasure trove of the highest precision radial velocities, enabling me to derive more robust statistics. Artie Hatzes, Bill Cochran, Hugh Jones, and Jason Wright also helped me improve the radial velocity coverage. Bill Hartkopf, Andrei Tokovinin, Dimitri Pourbaix, and Richard Gray were very helpful with their insights and analysis on specific stellar systems. My fellow graduate students enriched this journey with their humor, wit, and open acceptance of a who some might have considered “an outsider”, for which I am indebted to them. Special thanks to Rajesh Deo and Justin Cantrell for taking care of our computers vii while working on their own research, and to Alvin Das for creating a template that made it easier to navigate the LaTeX dissertation jungle. Thanks also to the staff at the CHARA Array for keeping such a sophisticated instrument working so well that it seemed like I was witnessing science fiction come alive. I am grateful to have grown up in India, whose society gave me a strong foundation, built with a desire to learn and accomplish. I am also proud to call the the Unites States of America home, and am thankful to my fellow citizens for accepting and embracing me beyond my wildest expectations. There is no other country in the world where I could have traveled the paths I did, charting a uniquely individual course for my life, without once having to justify or explain the twists and turns I decided to take.
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