0 v 1 2 8 N ov 1 99 9 Silicate Emission in T Tauri Stars : Evidence for Disk Atmospheres ? 1 Antonella Natta

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 μm. All nine stars show evidence of silicate emission at 10 μm, 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 < ∼1 μm is the dominant source of opacity. These observations support the notion that extended disk atmospheres contribute substantially to the mid–IR flux of young stars. Subject headings: circumstellar matter: disks — dust: silicate – stars: pre–main sequence – infrared: spectra 1 Based on observations obtained with ISO. ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA. Hubble Fellow also at the Max–Planck–Institut für Astronomie, Königstuhl 17, Heidelberg, Germany, D–69117