A study of Hilda asteroids II. Compositional implications from optical spectroscopy

We obtained 49 reflectance spectra of 23 Hilda asteroids in the wavelength ranges 3850-7650 and 3850-10150 Å during two observing runs in 1992. The primitive D-types dominate and comprise 34 % of the numbered Hilda asteroids, while 28 % and 2 % are P and C-types, respectively. This is contrast to the results of earlier investigations by Tedesco & Gradie (1982) and Gradie et al. (1989) who found fewer D-types asteroids in the Hilda group. We confirm the result by Dahlgren & Lagerkvist (1995) that there is a spectral slope-asteroid size relation among the Hilda asteroids, implying a size dependent surface composition. No evidence of absorption features are seen in the spectra of the P and D-types, in particular no features due to phyllosilicates are found, implying that no aqueous alteration of the surfaces has occurred. The absence of C-types within the Hilda group suggests that the C population is confined to smaller heliocenter distances than thought before, with very few C asteroids further out than the Cybele group. The phase reddening effect is absent for the P and D-types observed. Though it is not a Hilda member one spectrum of 279 Thule was obtained and is also presented.