Dust Processing in Disks around T Tauri Stars

The 8-14 micron emission spectra of 12 T Tauri stars in the Taurus/Aurigadark clouds and in the TW Hydrae association obtained with the InfraredSpectrograph (IRS; The IRS is a collaborative venture between CornellUniversity and Ball Aerospace Corporation funded by NASA through the JetPropulsion Laboratory and the Ames Research Center.) on board Spitzer areanalyzed. Assuming the 10 micron features originate from silicate grains in theoptically thin surface layers of T Tauri disks, the 8-14 micron dust emissivityfor each object is derived from its Spitzer spectrum. The emissivities are fitwith the opacities of laboratory analogs of cosmic dust. The fits include smallnonspherical grains of amorphous silicates (pyroxene and olivine), crystallinesilicates (forsterite and pyroxene), and quartz, together with large fluffyamorphous silicate grains. A wide range in the fraction of crystalline silicategrains as well as large silicate grains among these stars are found. The dustin the transitional-disk objects CoKu Tau/4, GM Aur, and DM Tau has thesimplest form of silicates, with almost no hint of crystalline components andmodest amounts of large grains. This indicates that the dust grains in theseobjects have been modified little from their origin in the interstellar medium.Other stars show various amounts of crystalline silicates, similar to the widedispersion of the degree of crystallinity reported for Herbig Ae/Be stars ofmass <2.5 solar masses. Late spectral type, low-mass stars can have significantfractions of crystalline silicate grains. Higher quartz mass fractions oftenaccompany low amorphous olivine-to-amorphous pyroxene ratios. It is also foundthat lower contrast of the 10 micron feature accompanies greater crystallinity.Comment: AASTEX, 39 pages text, 14 figures, 4 tables, scheduled to be published July 2006 in the Astrophysical Journa