Physical and dynamical properties of the main belt triple Asteroid (87) Sylvia

•We improve the shape and size of (87) Sylvia by combining different data set.•We derive moons’ orbital parameters using a Keplerian model.•Sylvia would be composed of a dense core embedded into an irregularly shaped material.•First direct estimate of the shape and size of its satellite Romulus. We present the analysis of high angular resolution observations of the triple Asteroid (87) Sylvia collected with three 8–10m 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 15km from our predicted position, within the uncertainty of our model. The occultation data revealed that the Moon, with a surface-area equivalent diameter DS=23.1±0.7km, 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 J2=0.024-0.009+0.016 derived from the 3-D shape model assuming an homogeneous distribution of mass for the volume equivalent diameter DV=273±10km primary and the null J2 implied by the Keplerian orbits suggests a non-homogeneous mass distribution in the asteroid’s interior.