  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.
