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We have identified four circumstellar disks with a deficit of dust emissionfrom their inner 15-50 AU. All four stars have F-G spectral type, and wereuncovered as part of the Spitzer Space Telescope ``Cores to Disks'' LegacyProgram Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequencestars. Modeling of the spectral energy distributions indicates a reduction indust 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 betweenthe inner and outer disk has led us to use the term `cold disks' to distinguishthese unusual systems. However, hot dust [0.02-0.2 Mmoon] is still presentclose to the central star (R ~0.8 AU). We introduce the 30/13 micron, fluxdensity ratio as a new diagnostic for identifying cold disks. The mechanismsfor dust clearing over such large gaps are discussed. Though rare, cold disksare likely in transition from an optically thick to an optically thin state,and so offer excellent laboratories for the study of planet formation.

Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps