A SPECTROSCOPIC SURVEY OF D-TYPE ASTEROIDS

We present CCD spectra of 19 asteroids previously classified as D-type covering the wavelength range 3500 less-than-or-equal-to lambda less- than-or-equal-to 9600 angstrom. A comparison with previously published photometric data shows excellent agreement. No small-scale absorption features are seen, but two asteroids (102 Miriam and 914 Palisana) ex hibit large-scale absorption (DELTAlambda approximately 2000angstrom) characteristic of phyllosilicates and are confirmed as belonging to th e C or P groups. A heliocentric colour gradient recently reported is p resumed to be caused by the presence of silicates on asteroids lying w ithin the main-belt, but it is unknown whether this material is inhere nt to the asteroid or has been deposited there by impacts. We confirm the existence of a reflectance slope - diameter correlation for D-type asteroids previously suspected from studies of Trojan asteroids, with smaller asteroids tending to be redder in colouration. The majority o f D-type asteroids observed have a reflectance slope S' = 9.1%/10(3) a ngstrom and albedo p(V) = 0.04, similar to that measured for comets P/ Halley and P/Neujmin 1, but rather different to comets such as P/Tempe l 2 and P/Arend-Rigaux.