  Aims: Our objectives were first to evaluate the possibility for using the NUV
absorption as diagnostics of hydrated minerals based on the recent datasets of
primitive asteroids and hydrated carbonaceous chondrites, and second to
investigate the reflectance spectrophotometry of the primitive asteroids in the
NUV as functions of heliocentric distance and size. Methods: The NUV and
visible reflectance spectrophotometry of more than 9,000 primitive asteroids
was investigated using two spectrophotometric surveys, the Eight Color Asteroid
Survey (ECAS) and the Sloan Digital Sky Survey (SDSS), which cover wavelengths
down to 0.32 um and 0.36 um, respectively. We classified asteroids from the
main asteroid belt, the Cybele and Hilda zones, and Jupiter Trojans based on
Tholen's taxonomy and described the statistical distribution of primitive
asteroid types. We also examined the relationship of the NUV, 0.7 um, and 2.7
um absorptions among primitive asteroids and hydrous carbonaceous chondrites CI
and CM. Results: We found strong correlations between the NUV and the OH-band
(2.7 um) absorptions for primitive asteroids and hydrated meteorites,
suggesting the NUV absorption can be indicative of hydrated silicates.
Moreover, there is a great difference in the NUV absorption between the large
asteroids (diameter d > 50 km) and small asteroids (d < 10 km) in the taxonomic
distribution. The taxonomic distribution of asteroids differs between the inner
main belt and middle-outer main belt. Notably, the C types are dominating large
members through the main belt and the F types are dominating small asteroids of
the inner main belt. The asteroids beyond the main belt consist mostly of P and
D types, although P types are common everywhere in the main belt. The peculiar
distribution of F types might indicate a different formation reservoir or
displacement process of F types in the early Solar System.
