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D.E. Harker (NRC Associcate/NASA-Ames Research Center), D.H. Wooden (NASA/Ames Research Center), C.E. Woodward (University of Minnesota)
To understand the formation of protoplanetesimals in the early solar nebula one must ascertain how dust grains condense, collide, and grow into cm-sized rocks. Mg-rich crystalline silicates are thought to be pristine solar nebula condensates (Bell et al.~2000, PP IV, 897) or interstellar amorphous silicates annealed into crystals at ~1000~K temperatures (Hallenbeck et al.~1998, Icarus, 131, 198) in the inner hot zones of the accretion disk. Hence, our knowledge of solar nebula processes suggest Mg-rich crystals are signposts of pristine materials that were processed in the nebula, be they nebular condensates or annealed ISM grains. If radial migrations transport Mg-rich crystals out of the hot inner zones to larger radii, then we can expect the degree of crystallinity of grains throughout the disk to increase with time (Irvine et al.~1998, Faraday Diss., 109, 475; Nuth 1998; LPI, 20, 1801; Grady et al.~1999, BAAS, 194, #69.07).
We present 7.5 - 13.4~\micron\ spectrophotometry of three intermediate mass Herbig Ae/Be stars of similar spectral type obtained with the NASA/Ames HIFOGS. Between the three objects we will demonstrate that the 10~\micron\ silicate resonance features differ from amorphous silicate dominated to crystalline silicate dominated. ISO SWS observations that include the 20~\micron\ silicate resonance features are used to further examine the degree of crystallinity. The degree of crystallinity is not correlated with stellar age. However, a tantalizing trend is apparent between the increasing strength of the crystalline silicate features and the diminishment of near-IR to ~6~\micron\ flux in both SEDs (Malfait, et al.~1998, A&A, 331, 221) and the 6 - 45~\micron\ ISO SWS spectra; albeit the sample is small. We suggest that the drop in 2--6~\micron\ flux is an important discriminator of inner disk evolution, possibly indicating inner disk clearing by the formation of planetesimals. The degree of crystallinity deduced from the 10~\micron\ and 20~\micron\ silicate resonances appears to be correlated with evolution of the inner disk.
We acknowledge support from the NRC, NASA and NSF.