A STUDY OF HILDA ASTEROIDS .2. COMPOSITIONAL IMPLICATIONS FROM OPTICAL SPECTROSCOPY

We obtained 49 reflectance spectra of 23 Hilda asteroids in the wavele ngth ranges 3850-7650 and 3850-10150 Angstrom during two observing run s in 1992. The primitive D-types dominate and comprise 34% of the numb ered Hilda asteroids, while 28% and 2% are P and C-types, respectively . This is in contrast to the results of earlier investigations by Tede sco & Gradie (1982) and Gradie et al. (1989) who found fewer D-type as teroids in the Hilda group. We confirm the result by Dahlgren & Lagerk vist (1995) that there is a spectral slope - asteroid size relation am ong 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 foun d, implying that no aqueous alteration of the surfaces has occurred. T he absence of C-types within the Hilda group suggests that the C popul ation is confined to smaller heliocentric distances than thought befor e, with very few C asteroids further out than the Cybele group. The ph ase 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.