Multiplicity in the Jupiter Trojan Population: Low density of 617 Patroclus with Keck LGS AO and Perspective

International audienceThe system 617 Patroclus, the only binary Trojan known, was discovered in 2001 with the Hokupa'a Gemini-8m AO system. Because of their faintness (magnitude in visible mv>15.5), Trojan asteroids cannot be directly observed by most of the AO systems. In 2004, a Laser Guide Star (LGS) AO system was offered on the Keck-10m. We initiated an observing campaign, recording direct images of the 617 Patroclus double system in broadband filters with the NIRC2 near-infrared camera. The orbital parameters of the system were estimated using two methods and by including additional Gemini archive data (2001-2002), we obtained a consistent and accurate solution. The two components, separated by 680±20 km, revolve around their center of mass in 4.289±0.004 days in a roughly circular orbit (e ˜0.02-0.02). Using the thermal measurements by Fernandez et al. (2003), we derive (with a beaming parameter eta =0.94) a radius of R1= 60.9 km and R2= 56.3 km (with an error of 1.6 km and an Av= 0.04). Even considering the uncertainty in the volume of the components, the density of 617 Patroclus (rho = 0.8±0.15 g/cm3) is extremely low, if compared with the bulk-density of known binary C-type main-belt asteroids (rho 1.2 g/cm3, see Marchis et al., ACM, 2005). Assuming that the system is made of the same material as Ganymede or Callisto (uncompressed density of 1.6 g/cm3), the bulk density yields to a macro-porosity of ˜50%. A more realistic smaller porosity (p ˜15 different composition, such as more water ice in the interior of 617 Patroclus, suggesting a formation in the distant outer regions of the solar system (Morbidelli et al., Nature, 2005). The origin of this tightly bound system will be also discussed (Marchis et al., Nature, in press, 2005). We will present a quality comparison of the LGS Keck system with various techniques and different AO systems, discussing the interest of this technique to broaden the search for multiple asteroids in all minor planet populations