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

Abstract

The population of large asteroids is thought to be primordial and they are the most direct witnesses of the early history of our Solar System. Those satellites allow study of the mass, and hence density and internal structure. We study here the 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 over 15 years, we determine the orbit of its larger satellite to be circular, equatorial, and prograde, with RMS residuals of 7.8 mas. From 11 positions in three epochs only, in 2015 and 2016, we determine a preliminary orbit for the second satellite. We find the orbit to be somewhat eccentric and slightly inclined to the primary's equatorial plane, reminiscent of the inner satellites of other asteroid triple systems. Comparison of the near-infrared spectrum of the larger satellite reveals no significant difference with Camilla. Hence, these properties argue for a formation of the satellites by excavation from impact and re-accumulation of ejecta. We determine the spin and 3-D shape of Camilla. The model fits well each data set. We determine Camilla to be larger than reported from modeling of mid-infrared photometry, with a spherical-volume-equivalent diameter of 254 $\pm$ 36 km (3 $\sigma$ uncertainty), in agreement with recent results from shape modeling (Hanus2017+). Combining the mass of (1.12 $\pm$ 0.01) $\times$ 10$^{19}$ kg determined from the dynamics of the satellites and the volume from the 3-D shape model, we determine a density of 1,280 $\pm$ 130 SI. 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.