C2D Spitzer-IRS spectra of disks around T Tauri stars IV. Crystalline silicates

Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 mu m where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object. Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at lambda > 20 mu m correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 mu m feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (<= 1 AU) emitting at mu similar to 10 mu m and a much colder region emitting at lambda > 20 mu m (<= 10 AU). We identify a crystallinity paradox, as the long-wavelength (lambda > 20 mu m) crystalline silicate features are detected 3.5 times more frequently (similar to 55% vs. similar to 15%) than the crystalline features arising from much warmer disk regions (lambda similar to 10 mu m). This suggests that the disk has an inhomogeneous dust composition within similar to 10 AU. The analysis of the shape and strength of both the amorphous 10 mu m feature and the crystalline feature around 23 mu m provides evidence for the prevalence of mu m-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of mu m-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.