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Published July 15, 2018 | public
Journal Article Metadata-only

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

Pajuelo, M.
Dumas, C.

Abstract

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) × 10^(19) kg (3σ uncertainty) 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.

Additional Information

© 2018 Published by Elsevier Inc. Received 25 July 2017, Revised 14 February 2018, Accepted 7 March 2018, Available online 8 March 2018. Based on observations obtained at: (1) the Hubble Space Telescope, operated by NASA and ESA; (2) the Gemini Observatory and acquired through the Gemini Observatory Archive, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil); (3) the European Southern Observatory, Paranal, Chile – 071.C-0669 (PI Merline), 073.C-0062 and 074.C-0052(PI Marchis), 087.C-0014(PI Marchis), 088.C-0528 (PI Rojo), 095.C-0217 and 297.C-5034 (PI Marsset) – and (4) 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. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 071.C-0669 (PI Merline), 073.C-0062 & 074.C-0052 (PI Marchis), 088.C-0528 (PI Rojo), 095.C-0217 & 297.C-5034 (PI Marsset). Some of the data presented herein were obtained at 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. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. Some of these observations were acquired under grants from the National Science Foundation and NASA to Merline (PI). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). We wish to acknowledge the support of NASA Contract NAS5-26555 and STScI grant GO-05583.01 to Alex Storrs (PI). Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration. We thank the AGORA association which administrates the 60 cm telescope at Les Makes observatory, under a financial agreement with Paris Observatory. Thanks to A. Peyrot, J.-P. Teng for local support, and A. Klotz for helping with the robotizing. Thanks to all the amateurs worldwide who regularly observe asteroid lightcurves and stellar occultations. Many co-authors of this study are amateurs who observed Camilla, and provided crucial data. We thank J. Ďurech for providing his implementation of Dobrovolskis (1996) method. The authors acknowledge the use of the Virtual Observatory tools Miriade (Berthier et al., 2008), MP3C (Delbo et al., 2017), TOPCAT, and STILTS (Taylor, 2005). This research used the facilities of the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency (Gwyn et al., 2012).

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