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

Abstract

(4) Vesta and (9) Metis are large main-belt asteroids with high albedos. With millimetre-observations at 93.0 and 95.5 GHz we characterised the emission properties of the surface material. The coverage of the full rotation period allowed a detailed study of the heterogeneity of the surface. The rotationally averaged fluxes 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. The emissivities at mm-wavelengths are lower than in the far-IR, confirming that particles with sizes of about 100 mikron influence the mm-behaviour. The 3-mm observations are very powerful for the study of asteroid surface heterogeneities.