Quantitative Grain Size Estimation on Airless Bodies from the Negative Polarization Branch. II. Dawn Mission Targets (4) Vesta and (1) Ceres

Context. Sunlight scattered from the surface of an airless body is generally partially polarized, and the corresponding polarization state includes information about the scattering surface, such as albedo, surface grain sizes, composition, and taxonomic types. Aims. We conducted polarimetry of two large airless bodies, the Dawn mission targets (1) Ceres and (4) Vesta, in the near-infrared region. We further investigated the change in the polarimetric phase curves over the wavelengths expected from previous works. Methods. We used the Nishiharima Infrared Camera (NIC) installed at the Nishi-Harima Astronomical Observatory (NHAO) to observe these objects at multiple geometric configurations in the J, H, and $\mathrm{K_s}$ bands ($ \lambda \sim 1.2\mathrm{-}2.3 \mathrm{\mu m} $). Results. Polarimetric parameters were determined and compared with previously reported experimental results. In particular, Vesta exhibits a characteristic change in the negative polarization branch as the wavelength increases to the $\mathrm{K_s}$ band, which we interpret as an indication of the dominant existence of $D \sim 10\mathrm{-}20 \mathrm{\mu m}$ particles. Our approach is supported by empirical reasoning and coincides well with an independent, theory-driven approach based on thermal modeling. Conclusions. This work demonstrates how near-infrared polarimetry can be utilized to quantitatively determine the particle size of airless objects. This finding will have important implications for asteroid taxonomy and regolith evolution.