Silicate emission in T Tauri stars: Evidence for disk atmospheres?

We present low-resolution mid-infrared spectra of nine classical T Tauri stars associated with the Chamaeleon I dark cloud. The data were obtained with the PHOT-S instrument on board the Infrared Space Observatory (ISO) in the two wavelength ranges 2.5-4.9 and 5.9-11.7 mu m. All nine stars show evidence of silicate emission at 10 mu m, which is the only prominent feature in the spectra. We discuss a model for the origin of these features in a hot, optically thin surface layer of the circumstellar disks surrounding the central young stars (i.e., a disk atmosphere). We report excellent agreement of our observations with predictions based upon this simple model for most stars in our sample, assuming that a mixture of amorphous silicates of radius less than or similar to 1 mu m is the dominant source of opacity. These observations support the notion that extended disk atmospheres contribute substantially to the mid-infrared flux of young stars.