  In order to investigate possible connections between dust processing and disk
properties, 8--13 um spectra of 34 young stars, with a range of circumstellar
environments and spectral types A to M, were obtained using the Long Wavelength
Spectrometer at the W. M. Keck Observatory. The broad 9.7 um amorphous silicate
feature which dominates this wavelength regime evolves from absorption in
young, embedded sources, to emission in optically revealed stars, and to
complete absence in older debris disk systems for both low- and
intermediate-mass stars. The peak wavelength and FWHM are centered about 9.7
and ~2.3 um, corresponding to amorphous olivine, with a larger spread in FWHM
for embedded sources and in peak wavelength for disks. In a few of our objects
that have been previously identified as class I low-mass YSOs, the observed
silicate feature is complex, with absorption near 9.5 um and emission peaking
around 10 um. Although most of the emission spectra show broad classical
features attributed to amorphous silicates, variations in the shape/strength
may be linked to dust processing, including grain growth and/or silicate
crystallization. We study quantitatively the evidence for evolutionary trends
in the 8--13 um spectra through a variety of spectral shape diagnostics. Based
on the lack of correlation between these diagnostics and broad-band infrared
luminosity characteristics for silicate emission sources, we conclude that
although spectral signatures of dust processing are present, they can not be
connected clearly to disk evolutionary stage (for optically thick disks) or
optical depth (for optically thin disks). The diagnostics of silicate
absorption features (other than the central wavelength of the feature),
however, are tightly correlated with optical depth.

Astrophysics