UNIVERSITY OF HAWAI'I LIBRARY The Enigmatic Surface of(3200) Phaethon: Comparison with cometary candidates A THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI'I IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN ASTRONOMY August 2005 By Luke R. Dundon Thesis Committee: K. Meech, Chairperson S. Bus D. Tholen To my parents, David and Colleen Dundon. III Acknowledgments lowe much gratitude to my advisor, Karen Meech, as well as the other members of my thesis committee, Dave Tholen and Bobby Bus. Karen, among many other things, trained me in the art of data reduction and good observing technique, as well as successful writing of telescope proposals. Bobby helped me perform productive near-IR spectral observation and subsequent data reduction. Dave provided keen analytical insight throughout the entire process of my project. With the tremendous guidance, expertise and advice of my committee, I was able to complete this project. They were always willing to aid me through my most difficult dilemmas. This work would not have been possible without their help. Thanks is also due to numerous people at the !fA who have helped me through my project in various ways. Dave Jewitt was always available to offer practical scientific advice, as well as numerous data reduction strategies. Van Fernandez allowed me to use a few of his numerous IDL programs for lightcurve analysis and spectral reduction. His advice was also quite insightful and helped focus my own thought processes. Jana Pittichova guided me through the initial stages of learning how to observe, which was crucial for my successful observations of (3200) Phaethon in the Fall of 2004. Henry Hsieh was there to offer practical advice about my project or about planetary science in general. Jim Heasley also aided me in my momentous first and only observing night on the Keck telescope, along with his collaborator, Lisa Prato from UCLA. Lisa provided me with her REDSPEC data reduction software, which made possible the first successfully obtained L­ band spectra on (3200) Phaethon. Successful observation and subsequent data reduction with NIRSPEC would also not have been possible without the invaluable aid of support iv astronomer Jim Lyke. At UKIRT, support astronomer Andy Adamson helped me both on the mountain and for numerous days and weeks after my observing run, always providing advice on successful spectral reduction strategies. His expertise aided me in detecting the near-IR spectral variation in (3200) Phaethon. At the IRTF Bobby Bus was there as support astronomer and committee member, always keeping an eye on my observing technique and ensuring that I obtained the best possible data for my target asteroid. With his practical advice I was able to take the appropriate steps to ensure that accurate data was obtained in my spectral observing runs. Outside expertise was also provided by Paul Abell of the Johnson Space Center, who provided advice on my L-band spectral analysis of Phaethon. Petr Pravec of the Ondrejov Observatory provided many thoughts and analytical advice on my photometric lightcurves of (3200) Phaethon and 1999 JD6. To both of them my thanks is due. Besides scientific expertise, moral support and practical advice were constantly provided by my current officemate, Scott Dahm, as well as my previous officemates Lisa Chien and Nick Moskovitz. My other classmates Hai Fu, Dave Harrington, Jeyan Kartaltepe, Mark Pitts, Steve Rodney and Bin Yang also provided tremendous support. Bin was always there to offer both scientific advice and moral support through my project, being herself another planetary graduate student. I must also extend thanks to the administrative staff at the HA, who helped in many countless ways, such as my travels to Chile, requests for interlibrary loans, and usage of audio-visual devices, to name a few. The computer support staff, including Pui Hin Roads, Naryan Raja, George Miyashiro, and Phillip Cahela, all provided tremendous help through my numerous computer-related dilemmas. Equally helpful were the support staff for IRAF (NOAO), who helped me through many data reduction strategies. Last but definitely not least, I am forever indebted to my family, Mom & Dad, Maureen, Mark and Molly, for their prayers and moral support through my graduate school adventure here at the Institute for Astronomy. v Abstract An investigation is presented on the surface properties of (3200) Phaethon via photometric and spectroscopic observations. (3200) Phaethon is a unique member of the near-Earth Asteroid population, as it is a remarkably blue (B-V=0.587) asteroid and the sole asteroidal parent body of a meteor stream, the Geminids. Numerous attempts have been made to explain its previous history, including ejection from a cometary nucleus, an asteroidal collision, or a close planetary encounter. We present results that show 0.3­ 0.9JLm and 0.8-2.5JLm hemispherical color variation across rotation. These color fluctuations correlate with the asteroid's lightcurve variation. An anti-correlation exists between the visible and near-IR color variation. The cause of the color variation may be a combination of particle size effects and compositional variation, possibly a result of space weathering. Additional asteroids and comets were observed to compare Phaethon's dynamical (orbit, rotation period) and physical (surface) properties. Ultimately, (3200) Phaethon may be evidence of a unique "transition object" that once contained a variable mixture of volatiles and rocky debris. VI Table of Contents Acknowledgments v Abstract ... vii List of Tables viii List of Figures IX Chapter 1: (3200) Phaethon . 1 1.1 Introduction. 1 1.2 Observations 12 1.2.1 Imaging 12 1.2.2 Imaging Results 14 1.2.3 Image Analysis 20 1.3 Spectroscopy ..... 29 1.3.1 Spectroscopic Analysis. 36 1.4 Discussion . 46 1.5 Conclusions/Summary 52 1.6 References....... 55 Chapter 2: Comparison Objects. 64 2.1 Introduction. 64 2.2 Imaging 71 2.3 Results. 76 2.3.1 1999 JD6 77 Vll 2.3.2 (142) Polana 80 2.3.3 1997 SEs .. 83 2.3.4 49P/ Arend-Rigaux 86 2.3.5 (1566) Icarus 88 2.3.6 (944) Hidalgo. 88 2.3.7 Imaging Analysis . 90 2.4 Spectroscopy ....... 95 2.4.1 Spectroscopy Analysis 101 2.5 Conclusions 104 2.6 References . 105 Chapter 3: Conclusions 111 3.1 (3200) Phaethon: an unusual object 111 3.2 References ............... 118 Vlll List of Tables 1.1 Properties of (3200) Phaethon . 5 1.2 Possible absorption features 10 1.3 Observing Instruments ... 12 1.4 Extinction Coefficients[mag/airmass] for (3200) Phaethon 13 1.5 Observing Conditions 14 1.6 (3200) Phaethon colors. 15 1.7 . Colors of (3200) Phaethon over multiple phase angles. 26 1.8 Observing Instruments . 30 1.9 Spectroscopic Conditions for (3200) Phaethon . 30 1.10 Average near-IR spectral slopes ......... 38 2.1 Comparison Objects .. 68 2.2 0 bserving Instruments . 73 2.3 Observing Conditions 74 2.4 Extinction Coefficients [mag/airmass] 75 2.5 Integration Times, SIN 75 2.6 1999 JD6 Colors .. 78 2.7 (142) Polana Colors 82 2.8 1997 SEs Colors .. 86 2.9 (1566) Icarus Colors 89 2.10 Comparison Object Colors. 93 IX 2.11 Observing Instruments. 96 2.12 Observing Conditions . 96 3.1 Asteroidal vs. Cometary Origin of (3200) Phaethon . 118 x List of Figures 1.1 Near-IR spectrum of (3200) Phaethon obtained by Dumas et at. (1998). The spectrum has been normalized to 2.2t-tm. A symbol in the upper left-hand corner displays the average error in the data. 9 1.2 Theta statistic plot for period determination of (3200) Phaethon. The deepest minimum occurs at 3.6048hr, indicative of the most likely period for (3200) Phaethon. ............................. .. 16 1.3 Phased R-filter lightcurve for (3200) Phaethon at a period of 3.6048hr. Overplotted are the V-R colors of (3200) Phaethon, to be discussed in the text. For this figure, peaks in the V-R curve correspond to bluer areas, and troughs correspond to redder areas. The lightcurve is phased to OO:OOUT, 19 November 2004. ................................ .. 17 1.4 Lightcurve constructed from data taken in January 1995, phased to OO:OOUT, November 19, 2004. The lightcurve amplitude is larger from 2004, along with rotational changes in lightcurve shape. Caution must be taken in comparing this lightcurve with the 2004 lightcurve, since the uncertainty in phasing (due to uncertainty in the known period) is +/-2.4hr, or 0.67 rotational phase.. 18 1.5 Interpolated and phased V-R measurements of (3200) Phaethon, at a period of 3.6048hr and zero phase OO:OOUT, 19 November 2004. There is an obvious sinusoidal trend of the V-R data for all three nights of observation. Overlaid are corrected field stars that were used for differential photometry in all November 2004 data. ........................ .. 19 Xl 1.6 R-filter lightcurve produced from 1997 data on (3200) Phaethon. Data were phased to 3.6048hr at the zero phase of OO:OOUT November 19, 2004. Uncertainty in phasing the 1997 data with the 2004 lightcurve is +/-1.7hr, or 0.47 rotational phase. ........................... .. 20 1. 7 B-V and V-R lightcurves from November 22, 1997. Calculations were phased to 3.6048hr and zero phase 00:00 UT, 19 November 2004. 21 1.8 R-I and V-I lightcurves from November 22, 1997. Calculations were phased to the same period and zero phase of Fig. 1. 7. ............... .. 22 1.9 BVRI broadband spectrum of (3200) Phaethon. The data is normalized to the I-filter (0.826j.tm) measurement. 23 1.10 Airmass test for V-R color measurements made for (3200) Phaethon (same color scheme as in Fig.