Abstract Title of Dissertation: Characterizing young debris disks through far-infrared and optical observations Jessica K. Donaldson, Doctor of Philosophy, 2014 Dissertation directed by: Professor Douglas P. Hamilton Department of Astronomy University of Maryland and Dr. Aki Roberge Exoplanets & Stellar Astrophysics Laboratory NASA Goddard Space Flight Center Circumstellar disks are the environments where extrasolar planets are born. De- bris disks in particular are the last stage of circumstellar disk evolution, the youngest of which may harbor still-forming terrestrial planets. This dissertation focuses on examining the properties of dust grains in the youngest debris disks as a proxy to study the unseen parent planetesimal population that produces the dust in destruc- tive collisions. The parent planetesimals are important to understanding the late stages of terrestrial planets because they can deliver volatile material, such as water, to young terrestrial planets. We used the Herschel Space Observatory to study young debris disks (ages ∼ 10 − 30 Myr) in the far-infrared where the thermal emission from the dust grains is brightest. We constructed spectral energy distributions (SEDs) of 24 debris disks and fit them with our debris disk models to constrain dust parameters such as temperature, dust location, and grain size. We also looked for correlations between the stellar and disk parameters and we found a trend between the disk temperature 0:85 and stellar temperature, which we fit as a power-law of Tdisk / T∗ . One bright, well studied disk in our sample, HD32297, has a well populated SED, allowing us to fit it with a more detailed model to determine dust grain composition. The HD32297 disk has also been imaged in scattered light, so we used the image to constrain the dust location before fitting the SED. We found the dust grains are composed of a highly porous and icy material, similar to cometary grains. This suggests there are icy comets in this system that could deliver water to any terrestrial planets in the disk. We followed up this system by observing it with the Hubble Space Telescope to get simultaneous spatial and spectral data of the disk. These data let us look for compositional changes with disk radius. We found the disk has a very red color at optical wavelengths in the innermost radius we probed (∼ 110 AU). This could indicate the presence of organic material, or it could be a property of the scattering phase function of large grains. Further analysis of this data is ongoing. Characterizing young debris disks through far-infrared and optical observations by Jessica K. Donaldson Dissertation submitted to the Faculty of the Graduate School of the University of Maryland at College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2014 Advisory Committee: Professor Douglas P. Hamilton, chair Dr. Aki Roberge Professor L. Drake Deming Professor Lee G. Mundy Professor Douglas C. Hamilton © Jessica K. Donaldson 2014 Preface Portions of this dissertation have been published elsewhere. Chapter 3 was previ- ously published in its entirety in The Astrophysical Journal (Donaldson et al. 2012). Results in Chapter 3 were also presented at the 217th meeting of the American Astro- nomical Society in January 2011. Chapter 4 will be submitted in its entirety to The Astrophysical Journal in the near future. Results form Chapter 4 have already been presented at The Universe Explored by Herschel conference at ESA/ESTEC in Oc- tober 2013. Chapter 5 was previously published in its entirety in The Astrophysical Journal (Donaldson et al. 2013) and its results presented at the 4th National Capital Area Disk Meeting at The Space Telescope Science Institute in July 2012. Prelim- inary results from Chapter 6 were presented at the 221st meeting of the American Astronomical Society in January 2013. ii Acknowledgements A great many people have helped me over the years to get to this point in my career, and I would like to thank them all for their support. Most importantly, I would like to thank my advisor, Aki Roberge, who was happy to take me on as a first year student and give me a rich new dataset to work with. Aki has provided me with countless hours of support over the last few years. She has taught me a great deal and has been an invaluable resource in furthering my education in astronomy research. She has also been a great mentor and guide through the nuances of a career in academia. I am especially grateful for the help in networking and introducing me to so many astronomers from all over the world. Many other colleagues and collaborators have also contributed to my success. I would like to thank the entire Herschel GASPS team for mak- ing me immediately feel welcome, and for all the help and encouragement I've received over the years. Specifically I would like to thank Bill Dent, Carlos Eiroa, Sasha Krivov, Francois Menard, Jean-Charles Augereau, iii Geoff Mathews, Christine Chen, Gwendolyn Meeus, Goran Sandell, and Pablo Riviere-Marichalar for contributing to Chapters 3 and 4 with their comments. Sasha Krivov especially deserves many thanks for his prompt and in depth comments on my paper drafts and for the long and lively discussions we've had both in person and via email. I also want to thank Jeremy Lebreton and Jean-Charles Augereau for providing me with a copy of the GRaTer code to use in Chapter 5 and answer my many question on its use. I would also like to thank Alycia Weinberger for giving me access to the data used in Chapter 6. Alycia has always been very approachable and happy to answer all of my questions. I would also like to thank Alycia for inviting me to shadow her during a remote observing run. Several former and current NASA postdocs have mentored me, being good listeners and providing sage wisdom, not just at Goddard, but also as familiar faces at conferences. These include Hannah Jang-Condell, John Debes, and Chris Stark. Grad student Erika Nesvold also deserves credit for listening and understanding. I would like to thank Prof. Doug P. Hamilton for always appearing delighted to answer many and varied questions both from me and from Aki as she learns to navigate the University's various regulations. Many friends and family have also helped me succeed through many years of support. I would like to thank my parents for supporting and encouraging me, especially my father who infected me with an interest in science at a young age. Mia Bovill, who shared a small windowless office with me during my first two years of grad school, helped me through my early fears and I would have had a much harder time without her iv reassurances. I owe a huge debt to my lifelong friends who have always been there for me. Rachel Reich, Sarah Stanley, Noramone Potter, Leah Clark, and Danielle Dunn have provide years of emotional support and I don't know what I would have done without them. v Contents List of Tables ix List of Figures x 1 Introduction 1 1.1 Planet Formation . .2 1.1.1 Star Formation . .2 1.1.2 From Dust to Protoplanets . .4 1.1.3 Giant Planet Formation . .6 1.1.4 Terrestrial Planet Formation . .8 1.2 Observational Characteristics of Circumstellar Disk Classes . .9 1.3 Debris Disks . 12 1.3.1 Origin of dust in debris disks . 12 1.3.2 Detection Methods . 13 1.3.3 Modeling Debris Disk SEDs . 18 1.4 GASPS { A Herschel Key Programme . 20 2 Absorption and Scattering of Radiation by Dust Grains in Debris Disks 25 2.1 Optical Constants . 25 2.2 Thermal Equilibrium . 29 2.3 Radiation Pressure . 30 2.4 SED modeling code for optically thin disks . 33 2.5 Scattered light . 34 2.5.1 Phase function . 34 2.5.2 Projection of a 3D inclined disk . 36 3 Herschel PACS Observations and Modeling of Debris Disks in the Tucana-Horologium Association 38 3.1 Introduction . 38 3.2 Observations and Data Reduction . 40 3.2.1 Photometry . 43 vi 3.2.2 Spectroscopy . 45 3.3 Blackbody and Modified Blackbody Fits . 48 3.4 Dust Disk Model . 53 3.4.1 Model Parameters . 54 3.4.2 Results . 56 3.5 Resolving the HD105 Debris Disk . 63 3.5.1 Radial Profile . 63 3.5.2 Determining the Outer Radius . 66 3.6 Discussion . 67 3.6.1 Minimum Grain Size . 68 3.6.2 Inner Holes . 69 3.6.3 An Unusual Debris Disk? . 69 3.7 Summary/Conclusion . 71 4 Young Debris Disks in the Herschel GASPS Survey: Relations Between Dust and Stellar Properties 79 4.1 Introduction . 79 4.2 Sample, Observations, and Data Reduction . 81 4.3 Analysis . 93 4.4 Spectral energy distributions . 99 4.4.1 Modified blackbody fits . 100 4.5 Multiplicity in systems hosting debris disks . 101 4.6 Correlations . 109 4.7 Detection Limits . 111 4.8 Interpretation of the temperature trend . 113 4.9 Discussion . 118 4.10 Summary . 120 5 Modeling the HD32297 Debris Disk with far-IR Herschel Data 121 5.1 Introduction . 121 5.2 Observations and Data Reduction . 123 5.3 Analysis . 125 5.3.1 Herschel PACS photometry . 125 5.3.2 Herschel SPIRE photometry . 125 5.3.3 Herschel PACS spectroscopy . 126 5.3.4 Column density of C II in HD32297 . 127 5.4 SED Modeling . 130 5.4.1 SED data . 130 5.4.2 Stellar Properties . 133 5.4.3 Surface Density Profile . 135 5.4.4 Dust Disk Modeling Strategy . 136 vii 5.4.5 Outer disk modeling with GRATER ..............