DUST PROCESSING AND GRAIN GROWTH IN PROTOPLANETARY DISKS IN THE TAURUS-AURIGA STAR-FORMING REGION

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 warm and cool grains 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 Mg-rich minerals and silica, while a few spectra with these features suggest the presence of other components. IRS spectra indicating the presence of large amounts of warm enstatite of similar to 400-500 K require crystalline silicates (enstatite or forsterite) at temperatures lower than the median temperature of the cool dust in the models, similar to 127 K; spectra showing a high abundance of other crystalline silicates (forsterite or silica) typically do not. A few spectra show 10 mu m complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that a low peak-to-continuum ratio of the 10 mu m complex always indicates grain growth. Most of the 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 could suggest that crystalline silicates are processed directly from amorphous silicates, whether through evaporation of the amorphous grains and condensation in chemical equilibrium or by annealing of the amorphous precursors. Alternatively, if one kind of crystalline silicate transforms into another kind, it suggests that the intermediate species transforms into the end-product species at a slower rate than the precursor transforms into the intermediate species; otherwise, there would be a negligible abundance of intermediate species. It is also found that the crystalline silicate abundance is correlated tightly with disk geometry, in the sense of higher crystallinity accompanying more-settled disks, which are commonly associated with growth and settling of grains. The abundance of large grains is also correlated with disks that are more highly settled, but with a wide range of large grain abundance for a given degree of settling. We interpret this range as that the settling of large grains is sensitive to individual disk properties. We also find that lower-mass stars have higher abundances of large grains in their inner regions.