Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using populations of optically thin dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra with narrow emission features associated with crystalline silicates require Mgrich minerals and silica, but a very small number suggest other components. Spectra indicating large amounts of enstatite at higher temperatures (400-500 K) also require crystalline silicates, either enstatite or forsterite, at temperatures lower (100-200 K) than those required for spectra showing high abundance of other crystalline silicates. A few spectra show 10 µm complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 µm complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. We also find that the more there is of one crystalline dust species, the more there is of the others. This suggests

Mid-infrared spectra of a few T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope show prominent narrow emission features indicating silica (crystalline silicon dioxide). Silica is not a major constituent of the interstellar medium; therefore, any silica present in the circumstellar protoplanetary disks of TTS must be largely the result of processing of primitive dust material in the disks surrouding these stars. We model the silica emission features in our spectra using the opacities of various polymorphs of silica and their amorphous versions computed from earth-based laboratory measurements. This modeling indicates that the two polymorphs of silica, tridymite and cristobalite, which form at successively higher temperatures and low pressures, are the dominant forms of silica in the TTS of our sample. These high temperature, low pressure polymorphs of silica present in protoplanetary disks are consistent with a grain composed mostly of tridymite named Ada found in the cometary dust samples collected from the STARDUST mission to Comet 81P/Wild 2. The silica in these protoplanetary disks may arise from incongruent melting of enstatite or from incongruent melting of amorphous pyroxene, the latter being analogous to the former. The high temperatures of ∼ 1200K-1300 K and rapid cooling required to crystallize tridymite or cristobalite set constraints on the mechanisms that could have formed the silica in these protoplanetary disks, suggestive of processing of these grains during the transient heating events hypothesized to create chondrules.

We present the Spitzer Infrared Spectrograph (IRS) spectrum of SR 20, a 5-10 AU binary T Tauri system in the r Ophiuchi star-forming region. The spectrum has features consistent with the presence of a disk; however, the continuum slope is steeper than the slope of an infinite geometrically thin, optically thick disk, indicating Ϫ4/3 l that the disk is outwardly truncated. Comparison with photometry from the literature shows a large increase in the mid-infrared flux from 1993 to 1996. We model the spectral energy distribution and IRS spectrum with a wallϩoptically thick irradiated disk, yielding an outer radius of AU, much smaller than predicted by0.39 ϩ0.03 models of binary orbits. Using a two-temperature minimization model to fit the dust composition of the IRS 2 x spectrum, we find that the disk has experienced significant grain growth: its spectrum is well fitted using opacities of grains larger than 1 mm. We conclude that the system experienced a significant gravitational perturbation in the 1990s.

In memory of Paola D'Alessio, who is dearly missed. ABSTRACTWe present spectra of 13 T Tauri stars in the Taurus-Auriga star-forming region showing emission in Spitzer Space Telescope Infrared Spectrograph (IRS) 5-7.5µm spectra from water vapor and absorption from other gases in these stars' protoplanetary disks. Seven stars' spectra show an emission feature at 6.6µm due to the ν 2 = 1-0 bending mode of water vapor, with the shape of the spectrum suggesting water vapor temperatures >500 K, though some of these spectra also show indications of an absorption band, likely from another molecule. This water vapor emission contrasts with the absorption from warm water vapor seen in the spectrum of the FU Orionis star V1057 Cyg. The other six of the thirteen stars have spectra showing a strong absorption band, peaking in strength at 5.6-5.7µm, which for some is consistent with gaseous formaldehyde (H 2 CO) and for others is