Studies of SOHO Comets Matthew M. Knight Advisor: Michael F. A'Hearn On the cover: The image on the cover was taken on 1998 June 1 at 17:27 UT using the SOHO/LASCO C2 coronagraph with the orange filter. The upper comet is C/1998 K10 (SOHO-54) and the lower comet is C/1998 K11 (SOHO-55). These were two of the brightest comets ever seen by SOHO, and reached perihelion within ∼4 hours of each other. The horizontal line extending from the nuclear condensation of C/1998 K11 is bleeding due to saturation. The Sun is behind the occulting disc at the center of the image. The image is approximately 3.3◦ tall and 2.5◦ wide. Abstract Title of Dissertation: Studies of SOHO Comets Matthew Manning Knight, Doctor of Philosophy, 2008 Dissertation directed by: Professor Michael F. A’Hearn Department of Astronomy We present here a study of the Kreutz, Marsden, and Kracht comets observed by SOHO including photometric reductions and analysis, numerical modeling, and physical modeling. We build off the work of Biesecker et al. (2002) and analyze the results of our photometric study of more than 900 lightcurves of Kreutz comets observed by SOHO. We find that they do not have a bimodal distance of peak brightness as previously reported, but instead peak from 10.5–14 R (prior to per- ihelion), suggesting there is a continuum of compositions rather than two distinct subpopulations. The lightcurves have two rates of brightening, typically ∝r−7.3±2.0 when first observed by SOHO then rapidly transitioning to ∼r−3.8±0.7 between 20–30 R . It is unclear at what distance the steeper slope begins, but it likely does not extend much beyond the SOHO field of view. We derive nuclear sizes up to ∼50 meters in radius for the SOHO observed comets, with a cumulative size distribution of N(>R)∝R−2.2 for comets larger than 5 meters in radius. This size distribution cannot explain the six largest members of the family seen from the ground, sug- gesting that either the family is not collisionally evolved or that the distribution is not uniform around the orbit. The total mass of the distribution up to the largest expected size (∼500 meters) is ∼4×1014 g, much less than the estimated mass of the largest ground observed members. After correcting for the changing discovery circumstances, the flux of comets reaching perihelion has increased since 1996, and the increase is seen in comets of all sizes. We consider the Marsden and Kracht comets together due to their apparent dynamical linkage (e.g. Ohtsuka et al. (2003); Sekanina and Chodas (2005)). Sea- sonal effects of the viewing geometry make it impossible to build a characteristic lightcurve of either group. Many are seen to survive perihelion and most reach a peak brightness within ∼6 hours of perihelion with no preference for peaks before or after perihelion. Most are barely above the threshold for detection, and the largest is probably smaller than 30 meters in radius. Our dynamical simulations suggest that the orbital distribution of the Kracht group can be produced by low velocity fragmentation events and close approaches to Jupiter over the last 50–250 years. We construct fragmentation trees for the Marsden and Kracht groups and predict that 7–8 comets in each group may be visible on their next perihelion passage. This research was supported by NASA Planetary Atmospheres grants NAG513295 and NNG06GF29G. Studies of SOHO Comets by Matthew Manning Knight 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 2008 Advisory Committee: Professor Michael F. A’Hearn, chair Doctor Douglas A. Biesecker Professor Douglas P. Hamilton Professor Derek C. Richardson Professor Paul D. Feldman Professor James Farquhar c Matthew Manning Knight 2008 Preface The work presented here has not been published, but has been presented as posters and talks at the 2004–2007 AAS Division of Planetary Science meetings, the 2005 Asteroids, Comets, and Meteors meeting, and the January 2008 American Astro- nomical Society meeting. It has made use of the University of Maryland’s time at Kitt Peak National Observatory and has benefited from collaboration with scien- tists from Laboratoire d’Astronomie Spatiale in Marseille, France, NASA’s Goddard Space Flight Center, and the Naval Research Laboratory. Due to the quantity of data produced, the work included herein will be published in multiple places. More than 20,000 reduced and calibrated images of Kreutz comets observed by SOHO have been submitted to the Planetary Data System for archiving and will be made public after passing peer review. Later updates will include the Kreutz comets observed since 2005 and the non-Kreutz comets. The photometry discussed here will be published in International Comet Quarterly and the Planetary Data System. The analysis of the Kreutz comets will be submitted to Icarus shortly, and the analysis of the Marsden and Kracht comets will be submitted to Icarus either independently or jointly with the Marseille group. ii To my parents, John and Nancy Knight, whose support and encouragement have allowed me to follow my dreams and made me who I am today. iii Acknowledgements Thanks to my advisor, Michael A’Hearn for helping me reach my po- tential as a scientist. He has encouraged me to explore my scientific in- terests, indulged my desires to travel, and allowed me to make mistakes along the way. He has matched deep insight with a quick wit to keep me terrified of disappointing him for the better part of a decade. I can’t tell you how many times I’ve been proud of myself for coming up with some great idea only to flip through my notebook and find a suggestion from Mike 10 months ago that told me to do just that. Thanks also to Doug Hamilton and Doug Biesecker for serving as co-advisors and helping me with the occasional topics Mike doesn’t know everything about. I am indebted to the many planetary scientists who have taken me un- der their wings and helped me to feel like I am among friends at confer- ences and telescopes. These include Beatrice M¨uller,Nalin Samarasinha, Yan Fernandez, Laura Woodney, Tony Farnham, Lucy McFadden, Casey Lisse, Derek Richardson, Donna Pierce, Jianyang Li, Dennis Wellnitz, iv Olivier Groussin, and Amy Lovell. Astronomy is immensely more re- warding when you work with friends. Thanks to the graduate students, postdocs, professors, and staff who have made all these years of graduate school so much more fun than I thought they’d be. I couldn’t possibly name everyone, but the following people deserve special notice: Nicholas Chapman, Kevin Walsh, Rahul Shetty, Rob Piontek, Vanessa Lauburg, Stephanie Zonak, Franziska Koeck- ert, Lisa Wei, John Vernaleo, Claudia Knez, David Garofalo, Ke Zhang, David Rupke, Cole Miller, Kayhan G¨ultekin,Glen Petitpas, Chul Gwon, Ji Hoon Kim, Woong-Tae Kim, and Nikolas Volgenau. Special thanks to all the Dirty Snowballs past, present, and future for indulging my sports star fantasies. We may never have won an IM title, but I hope you all had as much fun as I did. Thanks to Aung, Kyaw, and everyone at Mandalay. I am already longing for chicken 11 and I haven’t even moved away yet! Thanks to all the teachers who inspired and challenged me along the way, most notably Miss Robertson, Mr. Kreisa, Mr. Lynch, Mr. Wallace, Professor Deaver, and Professor Elzinga. Thanks to my family for their love, encouragment, hot meals, and free laundry. I am particularly grateful to my uncle Don Manning for all those free tickets and for his longstanding offer to proofread my papers. I also wish to remember my late grandfather, Bus Knight, from whom I apparently inherited the only bit of science genes in the family. I have found that lying on the floor in the sun next to a purring cat makes all of life’s troubles less important. Thanks to all those cats who v have given me a new perspective on life. Finally, thanks to my wife, Stacy Teng. She has put up with my being “No Fun Matthew” for months on end and has managed to make me smile whenever I’ve needed it. vi Contents List of Tables x List of Figures xi 1 Introduction 1 1.1 A Brief History of Sungrazing Comets . 1 1.2 The Importance of Studying Sungrazing Comets . 6 1.2.1 Sungrazers: Probing the Extremes of the Solar System . 6 1.2.2 Sungrazers: An Abundant Collection of Fragmented Comets . 8 2 Photometric Reductions 12 2.1 Overview of SOHO/LASCO . 12 2.1.1 LASCO . 14 2.1.2 LASCO Observing Sequences . 15 2.1.3 SOHO Mission Interruptions . 16 2.2 Calibrating Images . 17 2.3 Aperture Photometry . 20 2.3.1 Aperture Size . 21 2.3.2 Conversion to Visual Magnitudes . 23 2.3.3 Normalization of the Magnitude . 23 2.3.4 Estimating the Error . 28 3 Kreutz Comets 31 3.1 Overview . 31 3.2 Orbital Elements . 33 3.3 Temporal Clustering . 39 3.4 Lightcurves . 43 3.4.1 Peak Distance and the “Universal Curves” . 43 3.4.2 Brightening and Fading . 49 3.4.3 Orange – Clear Magnitude Difference . 52 3.4.4 Size Distribution . 54 3.4.5 The Effect of Normalizing the Photometry . 56 vii 3.5 Population . 62 3.5.1 Yearly Detection Rate . 62 3.5.2 Seasonal Variability . 65 3.5.3 True Arrival Rate .