c2d Spitzer IRS spectra of disks around T Tauri stars.: I.: Silicate emission and grain growth

Infrared similar to 5-35 mu m spectra for 40 solar mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar mass stars limited primarily to the 10 mu m region. The observed 10 and 20 mu m silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with micron-sized grains indicating fast grain growth (from 0.1 to 1.0 mu m in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 mu m, indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 mu m ratios and require even larger grains emitting at 20 mu m than at 10 mu m. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 mu m feature strength versus shape trend is not correlated with age or H alpha equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength versus shape trend is related to spectral type, however, with M stars showing significantly flatter 10 mu m features (larger grain sizes) than A/B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type-dependent factors (e.g., X-rays, UV radiation, and stellar/disk winds).