Physical characterization of double asteroid (617) Patroclus from 2007/2012 mutual events observations

We publish a set of 16 light curves of mutual events inside the synchronous system of the Jupiter Trojan (617) Patroclus. Patroclus is th only binary system of the six target asteroids of the forthcoming NASA Discovery-class mission Lucy. Determining the physical parameters of the system is therefore of primary importance in helping to plan the flyby mission.

Light curves were acquired during two follow-up campaigns of 6 months each between January-June 2007 and July-December 2012. Eight small eclipse events of amplitude of 0.2-0.3 mag were recorded in 2007. On the other hand, in 2012, the amplitudes of the phenomena were much deeper, between 0.6 and 0.8 mag, due to a nearly edge-on configuration of the system.

We refined the sidereal period to 102.78624 ± 0.00015h = 4.282760 ± 0.000005days. The J2000 ecliptic coordinates of the pole of the system were found to be λ = 235.3 ± 1.2° andβ = - 62.4 ± 0.2°. The volume ratio was determined equal to q = 0.69 ± 0.08. By using a model of inhomogeneous Roche ellipsoids in hydrostatic equilibrium, we derived a bulk density of 0.81 ± 0.16g/cm³ and a surface grain density of 2.69 ± 0.36g/cm³ in agreement with spectroscopic observations of this P-type asteroid.

As a validation, our solution was applied to revisit recent results obtained from observations of another type: AO astrometry and stellar occultation. Our model is thus perfectly able to account for these observations after fitting the mutual separation to the value of 695 ± 10km. Consequently, the area-equivalent diameter of the system as a whole is derived D_Α = 168.8 ± 2.6km.