UNIVERSITY OF HAWAI'I LIBRARY A Physical Survey of Centaurs A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI'I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN ASTRONOMY MAY 2003 By James M. Bauer Dissertation Committee: Karen J. Meech, Chairperson Tom Dinell Alan Stockton David J. Tholen Alan Tokunaga © Copyright 2003 by James M. Bauer All Rights Reserved iii To my wife, Chija "Oh, night that was my guide, Oh, night more lovely than the rising Sun... " IV Acknowledgements I wish to thank my dissertation committee, especially the chair, my graduate adviser and exceptionally generous and tolerant mentor, Karen J. Meech, but also Alan 'Ibkunaga, Dave Tholen, Alan Stockton and Tom Dinel!. Much needed advice and direction was contributed by all the members ofmy committee, and I will ever feel an abundance of respect, friendship, and gratitude toward each member. Yan Fernandez generously provided much additional guidance which was indispensable, to say the least. Likewise, Tobias Owen very graciously contributed many hours and imparted much wisdom to this work. I'd like to thank Ted Roush, for his instruction on the use and administration of his Hapke scattering code, and his advice in the interpretation of spectra, and above all his patience with a slow, but determined student. Though not officially committee members, Drs. Fernandez, Roush, and Owen were as integral a part of the writing of this dissertation as the committee members. John Kormendy and Mike Delisio, who were originally part of the committee, but later left for positions out of state also deserve and have my gratitude. I'm thankful for the logistical support, always smooth and reliable, of Diane Tokumura and the very professional financial administrating of Gale Yamada. For their direct help at the telescopes, I wish to thank the UH 2.2m staff, especially John Dvorak, Paul Degrood, and Andrew Pickles, the IRTF staff, including John Rayner, Bill Vacca, Bobby Bus, Dave Griep, and Bill Golish and the UKIRT staff, including Thor Wold. Mike Cushing deserves special acknowledgment for his efforts on the SpeXtool, vital in the data reduction. My good friends Scott Dahm, Gabriela Canalizo, Haakon Dahle, Kathy Roth, Liz Barrett, Chris Mullis, Barry Rothberg, and JD Armstrong for their help in observing, technical advice, and v overall good counsel. Henry Hsieh contributed his P /Tempel1 work from his AST699 paper as a reference for a comet nucleus phase curve. Ralf Kaiser provided me with an overall understanding of the issues involved in outer solar system chemistry. I thank Dave Jewitt, especially for encouraging my work with C/2001 T4 in particular, a very interesting object indeed. I similarly benefited from many informative discussions with John Davies regarding the photometry of small outer solar system bodies. Many others helped, including Tony Farnham, Jana Pittichova, Peter Capak, Scott Sheppard, Cathy Ishida, and Mike Connelley with the observing. Kevin Jim was a resource for help with both the technical aspects of NIR imaging and his work on compiling the thesis macros as well. Tom Geballe set me on the initial path of NIR spectral observations in the outer solar system; no one could wish for a better instructor. I wish to thank Trevor Jim for the task of converting and correcting the UH Thesis macros so that they would work properly with modern forms of Latex. Thanks to Jason Surace, Joe Jensen, Chris Dudley, Jeff Goldader, and the many others who created the initial versions of the Thesis LaTeX macros. Data from JPL's Horizon's ephemeris service (http://ssd.jpl.nasa.gov/horizons.html), and M. Showalter's Uranus Viewer plotting routine (http://ringside.arc.nasa.gov/www/tools/ tools.html) were used for the planning and data analysis. Image processing in much of this work has been performed using the IRAF program. IRAF is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. The IDL astronomy reduction libraries of M. Buie were also used in part of the data reduction, though they were somewhat modified by the author. The image of Miranda's surface in Chapter 2 was acquired from JPL's solar system web site, and was produced by A. T. Oner. Modified IRAF scripts of B. Cavanah (1997) were used in the flux calibration of the spectral data. I also acknowledge support for this work from the planetary astronomy program NAG5-4495. Perhaps the most important to thank are saved for the end, as it often seems not in their vocations or original expectations to provide the help that they did, or it may not vi be to the writer's immediate benefit to acknowledge. In the end, though, I'm sure it is to our advantage... First, my family, especially my parents, but also the support of my siblings. My long suffering wife of now one year, to whom this work is dedicated, and she's certainly seen me at my worst now, I hope... .I thank also my friends from our Friday gatherings, including Dennis Klein, Derek Nordstrom, Lea Dacanay, the Dillons, the Davis's, Paul Sosso, the Laus, the Lees, Kieren Morris, Lisa Gunaugh, Rita Eastmond, Gwen Reyes, Chelsae Polatti, Todd Hickman, Marie McCabe and Dr. Mark Alexander. Finally, and most of all, though it may sound trite, I thank the Creator of creation, without whom none of this would have been possible. I'm certain to have forgotten many, and I can only humbly ask their forgiveness. Because I do no hope to turn again because I do not hope because I do not hope to turn Desiring this man's gift and that man's scope -T.S. Eliot vii Abstract There are forty four known small planetary bodies with orbits that are contained within the heliocentric distances of Jupiter and Neptune. It is thought that the origin of these bodies is the Kuiper Belt, the predicted reservoir of the current short period comet population. Yet, only two bodies, Chiron and C/NEAT (2001 T4), have been shown to possess a visible coma. We've undertaken an observational survey of these bodies to obtain detailed characterization of the physical properties of the Centaurs to search for evidence of activity, and to use the physical characteristics to make inferences about primordial conditions in the outer solar nebula and evolutionary processes among different dynamical regimes in the outer nebula. We present the results of optical observations of 24 Centaurs, which yield a 3-0" correlation of color with semimajor axis, with redder Centaurs being farther from the Sun. The survey also revealed the rotation light curve period for 2 Centaurs, and the phase-darkening slope parameters, G, for 5 Centaurs which range from -0.18 to 0.13, agreeing with the steepest of main belt asteroid phase curve responses. We show spectral evidence of a variegated surface for 1999 UG5 and find the second reddest Centaur object is the active Centaur C/NEAT (2001 T4). We also present spectral evidence of crystalline water ice and ammonia species on our comparison object, the Uranian satellite Miranda. viii Table of Contents Acknowledgements V Abstract ... Vlll List of Tables xi List of Figures xiii Chapter 1: Introduction: Dynamical, Physical, and Evolutionary Relationships of Outer Solar System Bodies .. 1 1.1 The Nature of Centaurs 2 1.1.1 Statistics & Theories . 3 1.1.2 What is a Centaur? 8 1.2 Observation Methods .... 10 1.2.1 Centaur Coma Search & Color Photometry 11 1.2.2 Lightcurves and Colors 13 1.2.3 Phase Curves ...... 14 1.2.4 NIR Surface Spectroscopy 17 1.3 Conclusion ............. 18 Chapter 2: The Near Infrared Spectrum of Miranda: Evidence of Crystalline Water Ice.. 22 2.1 Introduction.......... 23 2.2 Observations and Data Reduction 26 2.2.1 Spectra . 26 ix 2.2.2 Photometry 31 2.3 Analysis . 35 2.3.1 Identification of Spectral Features 35 2.3.2 Spectral Modeling 36 2.3.3 Spatial Mixtures 39 2.4 Discussion . 45 2.4.1 Surface Variations 45 2.4.2 Comparison with Other Outer Solar System Surfaces. 46 2.5 Conclusions . 49 Chapter 3: Visible Photometric Survey of 24 Centaurs 51 3.1 Introduction . 52 3.2 Observations & Reduction. 53 3.2.1 Colors .... 63 3.2.2 Phase Curves 66 3.2.3 Surface Brightness Profiles 69 3.3 Analysis & Discussion 73 3.4 Conclusions . 78 Chapter 4: Observations of the Centaur 1999 UG5: Evidence of a Unique Outer Solar System Surface.. 88 4.1 Introduction. 88 4.2 Observations 89 4.2.1 Photometry . 89 4.2.2 Spectroscopy 92 4.3 Analysis ...... 95 4.3.1 Photometry 95 4.3.2 Spectroscopy 100 4.4 Discussion . ..... 106 Chapter 5: An Optical Survey of the Active Centaur C/NEAT (2001 T4) 112 x 5.1 Introduction . 112 5.2 Observations 114 5.3 Reduction & Analysis 115 5.4 Discussion . 123 5.5 Conclusions 128 Chapter 6: Discussion 130 6.1 Rotation Light Curves 131 6.2 Spectroscopy & Colors 133 6.3 Phase Curves 135 6.4 Conclusions . 137 6.4.1 Observations 137 6.4.2 Interpretation . 139 6.4.3 Future Work 142 xi List of Tables 1.1 Centaur Spectral Model Fits From Literature, 1999 19 2.1 Observing Geometry & Instrumental Configuration 27 2.2 Full Disk Albedos 34 2.3 Ice Optical Constants 41 2.4 Spatial Mixture Model Fits 43 3.1 Observing Conditions ... 55 3.2 UH Centaur Color Photometry 56 3.2 UH Centaur Color Photometry 57 3.2 UH Centaur Color Photometry 58 3.2 UH Centaur Color Photometry 59 3.2 UH Centaur Color Photometry 60 3.2 UH Centaur Color Photometry 61 3.2 UH Centaur Color Photometry 62 3.3 Colors and Orbital Elements of Centaurs 65 3.4 Phase Curve R-Band Photometry Summary 70 3.5 Photometry and Phase Curve Lumme-Bowell Model Fits 71 3.6 1998 SG35 UH R-Band Photometry 72 3.7 Colors Linear Fits ........