C.-I. Lagerkvist, M. Dahlgren, P. Magnusson, in collaboration with I.P. Williams (Queen Mary and Westfield College, London) and A. Fitzsimmons (Queen's University, Belfast) have made a spectroscopic survey of the D-type asteroids.
The result is published in Astronomy and Astrophysics 282, 634-642 (1994).
A total of 19 asteroids previously classified as D-type have been observed in the wavelength range 3500 to 9600 A . 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) exhibit large-scale absorption (width about 2000 A ) characteristic of phyllosilicates and are confirmed as belonging to the C or P groups. A heliocentric colour gradient recently reported is presumed to be caused by the presence of silicates om asteroids lying within the main belt, but it is unknown whether this material is inherent 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 of D-type asteroids observed have a reflectance slope S' = 9.1 %/1000 A and albedo pv = 0.04, similar to that measured for comets P/Halley and P/Neujmin 1, but rather different to comets such as P/Tempel 2 and P/Arend-Rigaux.