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 cryst alline silicate dust grains in protoplanetary disks, for a s tatistically 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 µm 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 emiss ion feature that can be essentially attributed to Mg-rich silicates. Fe-rich crystalline silicates are larg ely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at � > 20µm correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 µm feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (≤1 AU) emitting at �∼ 10µm and a much colder region emitting at � > 20µm (≤10 AU). We identify a crystallinity paradox, as the long-wavelength (� > 20µm) crystalline silicate features are 3.5 times more frequen tly detected (∼55% vs.∼15%) than the crystalline features arising from much warmer disk regions (�∼ 10µm). This suggests that the disk has an inhomogeneous dust composition within∼10 AU. The analysis of the shape and strength of both the amorphous 10µm feature and the crystalline feature around 23µm provides evidence for the prevalence of µm-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their pre sumed formation regions suggests effi cient outward radial transport mechanisms in the disks around TTauri disks. The presence of µm-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests effi cient (turbulent) vertical diffusion, likely accompanied by grain-grain fragmentation to balance the effi cient growth expected. In this scenario, the depletion of su bmicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.