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 = lambda less than or = 9600A. 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 (Delta lambda approximately 2000A) characteristic of phyllosilicates and are confirmed as belonging to the C or P groups. A heliocentric color gradient recently reported is presumed to be caused by the presence of silicates on 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 exisitence 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 coloration. The majority of D-type asteroids observed have a reflectance slope S' = 9.1% 10³ A and albedo pv = 0.04, similar to that measured to comets P/Halley and P/Neujmin 1, but rather different to comets such as P/Tempel 2 and P/Arend-Rigaux.