Abstract
We present the analysis of high angular resolution observations of the triple Asteroid (87) Sylvia collected with three 8-10 m class telescopes (Keck, VLT, Gemini North) and the Hubble Space Telescope. The moons' mutual orbits were derived individually using a purely Keplerian model.
We computed the position of Romulus, the outer moon of the system, at the epoch of a recent stellar occultation which was successfully observed at less than 15 km from our predicted position, within the uncertainty of our model. The occultation data revealed that the Moon, with a surface-area equivalent diameter D-s = 23.1 +/- 0.7 km, is strongly elongated (axes ratio of 2.7 +/- 0.3), significantly more than single asteroids of similar size in the main-belt.
We concluded that its shape is probably affected by the tides from the primary. A new shape model of the primary was calculated combining adaptive-optics observations with this occultation and 40 archived light-curves recorded since 1978.
The difference between the J(2) = 0.024(-0.009)(+0.016) derived from the 3-D shape model assuming an homogeneous distribution of mass for the volume equivalent diameter D-v = 273 +/- 10 km primary and the null J(2) implied by the Keplerian orbits suggests a non-homogeneous mass distribution in the asteroid's interior.