hide
Free keywords:
minor planets
asteroids: general
asteroids: individual: Daphne
methods: observational
techniques: high angular resolution
Astrophysics - Earth and Planetary Astrophysics
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 our Genoid meta-heuristic algorithm. Combining our high-angular resolution images with optical lightcurves and stellar occultations, we determine the spin period, orientation, and 3D shape, using our ADAM shape 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 (3 σ). 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. Based on observations made with (1) ESO Telescopes at the La Silla Paranal Observatory under programs <A href="http://archive.eso.org/wdb/wdb/eso/s ched_rep_arc/query?progid=281.C-5011(A)">http://281.C-5011</A> (PI Dumas), <A href="http://archive.eso.org/wdb/wdb/eso/sched_rep_arc/query? progid=099.D-0098(A)">http://099.D-0098</A> (SPHERE GTO), and <A href="h ttp://archive.eso.org/wdb/wdb/eso/abstract/query?progid=199.C-0074(A)">h ttp://199.C-0074(A)</A> (PI Vernazza); and (2) the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.The reduced and deconvolved AO images and the 3D shape model are publicly available at <A href="http://observations.lam.fr/astero/">h ttp://observations.lam.fr/astero/</A> and at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A> (ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz- bin/qcat?J/A+A/623/A132">http://cdsarc.u-strasbg.fr/viz- bin/qcat?J/A+A/623/A132</A>
