Mid-infrared spectroscopy of disks around classical T Tauri stars

We present the first Spitzer Space Telescope Infrared Spectrograph observations of the disks around classical T Tauri stars: spectra in the 5.2-30 mum range of six stars. The spectra are dominated by emission features from amorphous silicate dust, and a continuous component from 5 to 8 mum that in most cases comprises an excess above the photosphere throughout our spectral range. There is considerable variation in the silicate feature/continuum ratio, which implies variations of inclination, disk flaring, and stellar mass accretion rate. In most of our stars, structure in the silicate feature suggests the presence of a crystalline component. In one, CoKu Tau/4, no excess above the photosphere appears at wavelengths shortward of the silicate features, similar to 10 Myr old TW Hya, Hen 3-600, and HR 4796A. This indicates the optically thick inner disk is largely absent. The silicate emission features with peaks at 9.7 and 18 mum indicate small dust grains are present. The extremely low 10-20 mum color temperature of the dust excess, 135 K, indicates these grains are located more than 10 AU from the star. These features are suggestive of gravitational influence by planets or close stellar companions and grain growth in the region within 10 AU of the star, somewhat surprising for a star this young (1 Myr).