Asteroids, Comets, Meteors (2012) 6416.pdf

PHYSICAL CHARACTERISTICS OF THE BINARY TROJAN ASTEROID (624) HEKTOR F. Marchis1,2, F. Vachier2, J. Durech3, J. Berthier2, M.H. Wong4, P. Kalas4, G. Duchene4,5,M. van Dam6, 1SETI Insti- tute ([email protected]) Carl Sagan Center, Mountain View CA USA, 2.IMCCE, Observatoire de Paris, France 3Astronomical Institute, Charles University in Prague, CzR 4Department of Asronomy, University of California, Berkeley USA, 5Observatoire de Grenoble, France 6. Flat Wavefronts, Christchurch, NZ

Introduction: In July 2006, [1] announced the primary [10], we will derive the mass, hence the bulk discovery of a 15-km diameter satellite around the L4 density of Hektor. Jupiter Trojan asteroid (624) Hektor from direct obser- Preliminary Results: Genoid, genetic-based algo- vations using the W.M. Keck telescope. Over the past rithm [10], finds several prelimary solutions with ac- 6 years, our group has collected additional observa- ceptable fitting parameter 25solar system should be made of a large por- based on our analysis per comparison with already tion of ices. well-characterized binary asteroids located in the Observations: From July 2006 to November 2011 main-belt. we collected twelve observations of (624) Hektor with the Keck II AO system and its near-IR camera in Kc band. In July 2006 we use the LGS AO data to reveal the presence of the satellite. In 2007-2011 Thanks to an improved Keck AO system, we recorded data in NGS AO mode to confirm the presence of the satellite shown in Figure 1. Because of its large contrast (ΔK=7.2 mag) and small separation (0.2-0.4 arcsec), the marginal detection of the satellite complicated the analysis of the data in comparison with binary main- belt asteroids (e.g. [4] & [5]). These observations confirm that Hektor’s primary, which is partially resolved (Fig. 1), is strongly elon- gated and possibly bilobated as suggested by [6]. Addi- tional photometric lightcurves were taken in R band Figure 1: Twelve Keck AO observations of (624) near the 2006, 2008 and 2009 oppositions using Nickel Hektor. The green circles indicate the position of the 1m and OHP-1.2m telescopes. 15km-satellite. The central areas show Hektor’s Finally, a stellar occultation was observed success- primary which is resolved 55 mas< Dann< 70 mas. fully from Japan by H. Tomioka & S. Uehara on Janu- ary 24 2008. The 10th magnitude star TYC057700887 References: was occulted by Hektor primary [7]. [1] Marchis et al. (2006) IAU 8732. [2] Marchis et Analysis: Combining our resolved AO observa- al. (2006) Nature 439, 565 [3] Mueller et al. (2010) tions with lightcurves from the literature and new ob- Icarus 205, 505-515, [4] Marchis et al. (2008) Icarus servations data plus the successful occultation data we 195, 1, 295-316 [5] Descamps et al. (2009) Hermione. derived an accurate shape model and pole solution of [6] Hartmann & Cruikshank (1978) Icarus 36, 3, 353- Hektor’s primary. Using the mid-IR observations from 366 [7] webpage Spitzer/IRS [8] and WISE [9], the equivalent diameter http://homepage2.nifty.com/mp6338/Hektor.html [8] of Hektor was estimated. Finally, using the astrometric Emery et al. (2006) 182, 2, 496-512 [9] Grav et al. positions of Hektor’s moon as inputs parameters for (2011), ApJ 742, 40 [10] Vachier, F. et al. (2012). Genoid fitting algorithm which can also take into ac- A&A, submitted. count the precessions due to the elongated shape of the Acknowledgements: FM was supported by the NASA grant NNX11AD62G.