Neptune's ring arcs: VLT/NACO near-infrared observations and a model to explain their stability

Abstract

Context. Neptune's incomplete ring arcs have been stable since their discovery in 1984 although these structures should be destroyed in a few months through differential Keplerian motion. Regular imaging data are needed to address the question of the arc stability. Aims. We present the first NACO observations of Neptune's ring arcs taken at 2.2 μm (Ks band) with the Very Large Telescope in August 2007, and propose a model for the arc stability based on co-orbital motion. Methods. The images were aligned using the ephemerides of the satellites Proteus and Triton and were suitably co-added to enhance ring or satellite signals. Resonance theory and N-body simulations were used to model the arcs' confinement. Results. We derive accurate mean motion values for the arcs and Galatea and confirm the mismatch between the arcs' position and the location of the 42:43 corotation inclination resonance. We propose a new confinement mechanism where small co-orbital satellites in equilibrium trap ring arc material. We constrain the masses and locations of these hypothetical co-orbital bodies. © 2014 ESO.