The Opposition Effect of the Asteroid 4 Vesta

We present the results of photometric observations carried out with four small telescopes of the asteroid 4 Vesta in the \(B\), \(R_{\rm C}\), and \(z'\) bands at a minimum phase angle of 0.1 \(\timeform{D}\). The magnitudes, reduced to unit distance and zero phase angle, were \(M_{B}(1, 1, 0) = 3.83 \pm 0.01, M_{R_{\rm C}}(1, 1, 0) = 2.67 \pm 0.01\), and \(M_{z'}(1, 1, 0) = 3.03 \pm 0.01\) mag. The absolute magnitude obtained from the IAU \(H\)--\(G\) function is \(\sim\)0.1 mag darker than the magnitude at a phase angle of 0\(\timeform{D}\) 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 \(R_{\rm C}\) 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 \(\rho\) = 0.4--0.7, yielding the bluk density of 0.9--2.0 \(\times\) \(10^3\) kg \(\mathrm{m^{-3}}\). It is evident that the opposition effect for Vesta makes a contribution to not only the shadow-hiding effect, but also the coherent backscattering effect that appears from ca. \(1\timeform{D}\). 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 (\(B_{C0}\)).