vm - Astronomy & Astrophysics A&A 467, 737747...

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A&A 467, 737–747 (2007) DOI: 10.1051 / 0004-6361:20066626 c ± ESO 2007 Astronomy & Astrophysics 3.2mm lightcurve observations of (4) Vesta and (9) Metis with the Australia Telescope Compact Array T. G. Müller 1 and P. J. Barnes 2 1 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany e-mail: tmueller@mpe.mpg.de 2 School of Physics A28, University of Sydney, Sydney NSW 2006, Australia e-mail: peterb@physics.usyd.edu.au ; Received 24 October 2006 / Accepted 8 March 2007 ABSTRACT Context. (4) Vesta and (9) Metis are large main-belt asteroids with high albedos. There are strong indications for heterogeneous sur- faces for both targets from imaging techniques in the visible and near-IR range, very likely connected to impact structures. Despite that, the thermal spectral energy distributions from mid-IR to the mm-range have, until now, been consistent with a homogeneous regolith-covered surface and the thermal light-curves are dominated by the shape and spin vector properties. Aims. With millimetre-observations at 93.0 and 95.5 GHz we tried to characterise the emission properties of the surface material. The coverage of the full rotation period allowed a detailed study of the heterogeneity of the surface. Methods. We combined our carefully-calibrated mm-observations with sophisticated thermophysical modelling techniques. In this way it was possible to derive emissivity properties and to disentangle the e f ects caused by shape, albedo or various thermal properties. Results. The rotationally averaged Fuxes are explained very well by our thermophysical model techniques when using an emissivity in the mm-range of about 0.6 for (4) Vesta and about 0.7 for (9) Metis. The mm-lightcurves follow for a large fraction of the rotation period the shape-introduced variations. The rotational phases with clear deviations are connected to structures which are visible in the HST images of (4) Vesta and the Keck AO-images of (9) Metis. The observed lightcurve amplitudes are peak-to-peak 30% for (4) Vesta and 25% for (9) Metis, while the shape-related amplitudes are only 5 and 4%, respectively. Conclusions. The emissivities at mm-wavelengths are lower than in the far-IR, con±rming that particles with sizes of about 100 µ m inFuence the mm-behaviour. Previously identi±ed bright spots at visible / near-IR wavelength are connected to sharp emissivity drops. The dark Olbers region on (4) Vesta causes an excess in mm-emission on top of the shape introduced light-curve. The thermophys- ical model predictions match the overall Fux levels very well, but cannot reproduce certain lightcurve features due to the lack of information on the grain size distribution. The 3-mm observations are very powerful for the study of surface heterogeneities. Key words. minor planets, asteroids – radio continuum: solar system – infrared: solar system – techniques: photometric – radiation mechanisms: thermal 1. Introduction (4) Vesta and (9) Metis are large main-belt asteroids with well- characterised shape, spin-vector, size and albedo properties (e.g., Thomas et al. 1997; Torppa et al. 2003; Storrs et al. 1999;
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vm - Astronomy & Astrophysics A&A 467, 737747...

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