CCD Reflectance Spectra of Selected Asteroids. II. Low-Albedo Asteroid Spectra and Data Extraction Techniques

Narrowband reflectance spectra were obtained of 27 asteroids with a CCD spectrograph from 1988 to 1992 as part of a search for weak absorption features in the visible and near-infrared reflectance spectra of low-albedo asteroids (C, P, and D classes and subclasses) that could be indicative of aqueous alteration products. Spectra were acquired primarily of asteroids located in the outer part of the main asteroid belt (mean semimajor axis 2.5 ≤ a ≤ 3.2 AU), a region where asteroids whose spectra have features diagnostic of oxidized iron in phyllosilicates have been previously identified (Sawyer, Ph.D. dissertation, University of Texas, 1991; F. Vilas, E. C. Hatch, S. M. Larson, S. P. Sawyer, and M. J. Gaffey, Icarus 102, 225-231, 1993). Three outer-belt (a ≥ 3.2 AU) asteroid spectra were also obtained. The spectrum of 1 Ceres shows a weak absorption feature near 0.60 μm consistent with earlier narrowband reflectance spectra (Vilas and McFadden, Icarus 100, 85-94, 1992). The spectrum of 606 Brangane suggests that it may be one of the S-class asteroids whose spectra show no mafic silicate absorption feature at 1.0 μm. The technique for extracting absorption features that have depths of 1-5% of the background continuum from a spectrum is also demonstrated. Examples of the repeatability of one feature among different asteroids, in spectra of the same asteroid reduced using different solar analogue standard stars, and in spectra of the same asteroid taken on different observing nights. are shown.