Near-infrared observations of primitive asteroids and a possible extinct comet

Photometry was obtained simultaneously in the near-infrared and in the visible, of several D-type asteroids, thought to be composed of the most primordial material in the asteroid belt. These objects have been previously classified as D-type based on their low albedo and broadband colors in the visible region of the spectrum. One of the objects observed is 944 Hidalgo, which is possibly an extinct comet, based on its orbital elements. The photometry further constrains the surface mineralogy and is a test of whether differences exist between these objects in different regions of the asteroid belt. Refining the knowledge of the surface composition of these objects will allow a better measure of the amount of thermal processing which has taken place in these so-called primitive asteroids. Spectral slope is assumed to be sensitive to surface composition. There is some variability in spectral slope among those objects classified as D-type, with indications of increasing slope into the near-infrared with heliocentric distance. The spectral features are quite subtle as expected for these low albedo objects, but there is possible evidence of subtle features in some of the D-type asteroids and possibly some active comets. Further analysis of these spectra and the implications for the outer region of the asteroid belt is presented.