3.2 mm lightcurve observations of (4) Vesta and (9) Metis with the Australia Telescope Compact Array

Context. (4) Vesta and (9) Metis are large main-belt asteroids with high albedos. There are strong indications for heterogeneous surfaces 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 mater ial. The coverage of the full rotation period allowed a detailed study of the he terogeneity 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 disentangl e the effects caused by shape, albedo or various thermal properties. Results. The rotationally averaged fluxes are explained very well by o ur 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 shapeintroduced variations. The rotational phases with clear deviations are connected to structures which are visible in th e 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, confirming that particles with sizes of about 100 � m influence the mm-behaviour. Previously identified bright spots at vis ible/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 thermophysical model predictions match the overall flux levels very well, but cannot reproduce certa in 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.