C2D Spitzer-IRS spectra of disks around T Tauri stars

Abstract

<i>Aims. <i/>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).<i>Methods. <i/>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 <i>μ<i/>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.<i>Results. <i/>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 <i>λ<i/> > 20 <i>μ<i/>m correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 <i>μ<i/>m feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (<i>≤<i/>1 AU) emitting at <i>λ<i/> ~ 10 <i>μ<i/>m and a much colder region emitting at <i>λ<i/> > 20 <i>μ<i/>m (<i>≤<i/>10 AU). We identify a <i>crystallinity paradox<i/>, as the long-wavelength (<i>λ<i/> > 20 <i>μ<i/>m) crystalline silicate features are detected 3.5 times more frequently (~55% vs. ~15%) than the crystalline features arising from much warmer disk regions (<i>λ<i/> ~10 <i>μ<i/>m). This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The analysis of the shape and strength of both the amorphous 10 <i>μ<i/>m feature and the crystalline feature around 23 <i>μ<i/>m provides evidence for the prevalence of <i>μ<i/>m-sized (amorphous and crystalline) grains in upper layers of disks.<i>Conclusions. <i/>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 <i>μ<i/>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.