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We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type, and were uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU, but with massive gas-rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term `cold disks' to distinguish these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Though rare, cold disks are likely in transition from an optically thick to an optically thin state, and so offer excellent laboratories for the study of planet formation., 13 pages, 3 figures, accepted to ApJL

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., 27 pages, 13 figures, accepted for publication by ApJ, formatted with emulateapj using revtex4 v4.0