DPS 35th Meeting, 1-6 September 2003
Session 24. Asteroid Physical Studies II
Oral, Chairs: A. Cheng and L. Benner, Thursday, September 4, 2003, 1:30-3:00pm, DeAnza III

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[24.03] Searching and Studying Binary Asteroids with AO Systems

F. Marchis (UC-Berkeley), P. Descamps, J. Berthier, D. Hestroffer (IMCCE), I. de Pater (UC-Berkeley), A. Conrad, D. Le Mignant, F. Chaffee (W.M. Keck Observatory), D. Gavel (Moore Laboratory for AO at UCSC)

Our group has conducted adaptive optics observations of asteroids since 2001. Our main goal is the search and study of binary asteroids using several AO systems (Lick, Keck, VLT) and related technique such as Appulse (Berthier and Marchis, 2002) and Laser Guide Star observations (Marchis et al., AGU-EGS, 2003) to broaden the sample of asteroids observed from the main-belt out to the Kuiper Belt. We focussed our program last year on Trojan Asteroids. Six of them were observed using Appulses with Keck AO (~0.05-0.10", mv=15.4-18.5), 6 with the LGS at Lick (~0.25-0.35", mv<16) and 12 with the VLT/NACO system (~0.10-0.14"; mv<16.7). None of these observations reveals the presence of a companion. Based on this sample, and including 617 Patroclus binary asteroid discovered by Merline et al. (IAU, 7741, 2001), we deduce that the proportion of binary Trojan asteroids larger than 40 km is less than 4%. We will promote and discuss a technique of the analysis of negative discovery in large samples.

In January 2003, we conducted an observing campaign spanning 5 days of 121 Hermione with NACO, the new AO system offered at VLT. This C-type asteroid was discovered by Merline et al. (IAU, 7980, 2002). The companion, 6.1 mag fainter than the primary, is easily detected despite the faintness of the asteroid (mv~13). We use the method described in Marchis et al. (Icarus, 2003) to determine the orbit of the companion. Its orbital elements are a=794.7±2.1 km, and P=1.643±0.005 days. We derived a mass =1.47E19 kg, and a density of 3.1±0.8 g cm-3 (using IRAS diameter of 209±4.7 km). Considering typical densities of meteorite analogues (CI or CM carboneceous chondrite) would led to an extremely low macro-porosity of p<3%. This suggests that the volume of Hermione is ~30% larger, which is also supported by our resolved images of this body. This work supported by the National Science Foundation Science and Technology Center for Adaptive Optics, based partly on observations collected at the European Southern Observatory, Chile.

The author(s) of this abstract have provided an email address for comments about the abstract: fmarchis@astron.berkeley.edu

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.