Abstract
Context.CM-like asteroids (Ch and Cgh classes) are a major population within the broader C-complex, encompassing about 10% of the mass of the main asteroid belt. Their internal structure has been predicted to be homogeneous, based on their compositional similarity as inferred from spectroscopy and numerical modeling of their early thermal evolution.Aims.Here we aim to test this hypothesis by deriving the density of the CM-like asteroid (41) Daphne from detailed modeling of its shape and the orbit of its small satellite.Methods.We observed Daphne and its satellite within our imaging survey with the Very Large Telescope extreme adaptive-optics SPHERE/ZIMPOL camera and complemented this data set with earlier Keck/NIRC2 and VLT/NACO observations. We analyzed the dynamics of the satellite with ourGenoidmeta-heuristic algorithm. Combining our high-angular resolution images with optical lightcurves and stellar occultations, we determine the spin period, orientation, and 3D shape, using ourADAMshape modeling algorithm.Results.The satellite orbits Daphne on an equatorial, quasi-circular, prograde orbit, like the satellites of many other large main-belt asteroids. The shape model of Daphne reveals several large flat areas that could be large impact craters. The mass determined from this orbit combined with the volume computed from the shape model implies a density for Daphne of 1.77 ± 0.26 g cm−3(3σ). This densityis consistent with a primordial CM-like homogeneous internal structure with some level of macroporosity (≈ 17%).Conclusions.Based on our analysis of the density of Daphne and 75 other Ch/Cgh-type asteroids gathered from the literature, we conclude that the primordial internal structure of the CM parent bodies was homogeneous.
Highlights
The C-complex encompasses 50% of the mass of the asteroid belt
The high-quality correction delivered by the AO system of SPHERE (Fusco et al 2006, 2014) compared to previous generations of AO cameras, and the use of shorter wavelength, provides a twofold to threefold improvement in angular resolution
We present all the data compared with the predictions from the shape model in Appendix B
Summary
The C-complex encompasses 50% of the mass of the asteroid belt (or 14% if the four largest bodies, Ceres, Vesta, Pallas and Hygeia, are disregarded, DeMeo & Carry 2013, 2014). The high-quality correction delivered by the AO system of SPHERE (Fusco et al 2006, 2014) compared to previous generations of AO cameras, and the use of shorter wavelength (visible R band compared to the near-infrared J/H/K bands typically used with previous cameras), provides a twofold to threefold improvement in angular resolution This sharper resolution allows the detailed modeling of asteroid shapes and enhanced satellite detection capability, as recently illustrated on the main-belt asteroids (3) Juno, (6) Hebe, (16) Psyche, (89) Julia, (107) Camilla, and (130) Elektra (Viikinkoski et al 2015, 2018; Marsset et al 2016, 2017a,b; Yang et al 2016; Hanuš et al 2017a; Pajuelo et al 2018; Vernazza et al 2018). Considering that all these objects have experienced numerous collisions over the history of the solar system, some macroporosity, i.e. voids, can be expected
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