Physical, spectral, and dynamical properties of asteroid (107) Camilla and its satellites

The population of large 100+ km asteroids is thought to be primordial. As such, they are the most direct witnesses of the early history of our Solar System available. Those among them with satellites allow study of the mass, and hence density and internal structure. We study here the dynamical, physical, and spectral properties of the triple asteroid (107) Camilla from lightcurves, stellar occultations, optical spectroscopy, and high-contrast and high-angular-resolution images and spectro-images. Using 80 positions measured over 15 years, we determine the orbit of its larger satellite, S/2001 (107) 1, to be circular, equatorial, and prograde, with root-mean-square residuals of 7.8 mas, corresponding to a sub-pixel accuracy. From 11 positions spread over three epochs only, in 2015 and 2016, we determine a preliminary orbit for the second satellite S/2016 (107) 1. We find the orbit to be somewhat eccentric and slightly inclined to the primary's equatorial plane, reminiscent of the properties of inner satellites of other asteroid triple systems. Comparison of the near-infrared spectrum of the larger satellite reveals no significant difference with Camilla. Hence, both dynamical and surface properties argue for a formation of the satellites by excavation from impact and re-accumulation of ejecta in orbit. We determine the spin and 3-D shape of Camilla. The model fits well each data set: lightcurves, adaptive-optics images, and stellar occultations. We determine Camilla to be larger than reported from modeling of mid-infrared photometry, with a spherical-volume-equivalent diameter of 254 ± 36 km (3σuncertainty), in agreement with recent results from shape modeling (Hanus et al., 2017, A&A 601). Combining the mass of (1.12 ± 0.01) × 10¹⁹ kg (3σuncertainty) determined from the dynamics of the satellites and the volume from the 3-D shape model, we determine a density of 1,280 ± 130 kg · m−3 (3 σ uncertainty). From this density, and considering Camilla's spectral similarities with (24) Themis and (65) Cybele (for which water ice coating on surface grains was reported), we infer a silicate-to-ice mass ratio of 1–6, with a 10–30% macroporosity.