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A. S. Rivkin (MIT), E. S. Howell (Arecibo Obs.)
E-class asteroids in the taxonomy of Tholen (1984) have flat spectra through the 0.3--1.0 \mum region, and high albedos (usually above 0.3). they dominate the Hungaria region of the asteroid belt, though a few E asteroids (notably the largest ones, 44 Nysa and 64 Angelina) can be found scattered much further out. These asteroids have been associated with the aubrite (enstatite achondrite) meteorites, which are thought to be of igneous origin (Gaffey et al. 1989, Kelley and Gaffey 1995). Rivkin et al. (1995) found an absorption feature at 3 \mum on 44 Nysa and a possible feature on 64 Angelina. Rivkin (1997) presented observations of 6 E-class asteroids (none of them Hungaria region asteroids), and found the 3-\mum absorption feature on 4 of them. Because the 3-\mum absorption is diagnostic for water- and/or hydroxyl-bearing minerals, this implies a different history for these E asteroids than what is indicated by the aubrites.
Bus (1999) during a spectroscopic study found that some E-class asteroids have an absorption feature near 0.5 \mum. Troilite has been suggested as the agent responsible for this visible feature (Cloutis and Burbine 1999), though some serious objections to troilite remain (Fornasier and Lazzarin 2001). Cloutis and Burbine suggested that troilite rather than hydrated minerals could be responsible for the 3-\mum feature as well, based on spectra of iron-bearing zinc sulfides and new troilite spectra they obtained. We consider this hypothesis as well as other alternative interpretations of the 3-\mum absorption band, using the set of available laboratory and astronomical data as well as geological arguments and newly-obtained IRTF data.
We acknowledge support from the NASA Planetary Astronomy Program. GBA.