The opposition effect of the asteroid 4 Vesta

Abstract

We present the results of photometric observations carried out with four small telescopes of the asteroid 4 Vesta in the B, RC, and z′ bands at a minimum phase angle of 0°.1. The magnitudes, reduced to unit distance and zero phase angle, were MB(1, 1, 0) = 3.83 ± 0.01, MRC(1, 1, 0) = 2.67 ± 0.01, and Mz′(1, 1, 0) = 3.03 ± 0.01 mag. The absolute magnitude obtained from the IAU H - G function is ∼ 0.1 mag darker than the magnitude at a phase angle of 0° determined from the Shevchenko function and Hapke models with the coherent backscattering effect term. Our photometric measurements allowed us to derive geometric albedos of 0.35 in the B band, 0.41 in the RC band, and 0.31 in the z′ bands by using the Hapke model with the coherent backscattering effect term. Using the Hapke model, the porosity of the optically active regolith on Vesta was estimated to be ρ = 0.4-0.7, yielding a bulk density of 0.9-2.0 × 103 kgm-3. It is evident that the opposition effect for Vestamakes a contribution not only to the shadow-hiding effect, but also the coherent backscattering effect that appears from ∼ 1°. The amplitude of the coherent backscatter opposition effect for Vesta increases with a brightening of reflectance. By comparison with other solar system bodies, we suggest that multiple-scattering on an optically active scale may contribute to the amplitude of the coherent backscatter opposition effect (BC0).