The Formation and Evolution of Planetary Systems: Grain Growth and Chemical Processing of Dust in T Tauri Systems

This paper is one in a series presenting results obtained within the Formation and Evolution of Planetary Systems
\n(FEPS) Legacy Science Program on the Spitzer Space Telescope. Here we present a study of dust processing and
\ngrowth in seven protoplanetary disks. Our spectra indicate that the circumstellar silicate dust grains have grown to
\nsizes at least 10 times larger than observed in the interstellar medium and show evidence for a non-negligible (~5% in
\nmass fractions) contribution from crystalline species. These results are similar to those of other studies of protoplanetary
\ndisks. In addition, we find a correlation between the strength of the amorphous silicate feature and the shape
\nof the spectral energy distribution. This latter result is consistent with the growth and subsequent gravitational settling
\nof dust grains toward the disk midplane. Furthermore, we find a change in the relative abundance of the different
\ncrystalline species: more enstatite than forsterite is observed in the inner warm dust population at ~1 AU, while
\nforsterite dominates in the colder outer regions at ~5-15 AU. This change in the relative abundances argues for a
\nlocalized crystallization process rather than a radial mixing scenario in which crystalline silicates are being transported
\noutwards from a single formation region in the hot inner parts of the disk. Finally, we report the detection of
\nemission from polycyclic aromatic hydrocarbon (PAH) molecules in five out of seven sources.We find a tentative
\nPAH band at 8.2 µm that was previously undetected in the spectra of disks around low-mass pre-main-sequence
\nstars.