Olofsson, J. and Augereau, J.-C. and van Dishoeck, E. F. and Merín, B. and Lahuis, F. and Kessler-Silacci, J. and Dullemond, C. P. and Oliveira, I. and Blake, G. A. and Boogert, A. C. A. and Brown, J. M. and Evans, N. J., II and Geers, V. and Knez, C. and Monin, J.-L. and Pontoppidan, K. (2009) C2D Spitzer-IRS spectra of disks around T Tauri stars: IV. Crystalline silicates. Astronomy and Astrophysics, 507 (1). pp. 327-345. ISSN 0004-6361 http://resolver.caltech.edu/CaltechAUTHORS:20091211-101144639
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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 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 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 λ > 20 μm 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 ~ 10 μm and a much colder region emitting at λ > 20 μm (≤10 AU). We identify a crystallinity paradox, as the long-wavelength (λ > 20 m) crystalline silicate features are detected 3.5 times more frequently (~55% vs. ~15%) than the crystalline features arising from much warmer disk regions (λ ~ 10 μm). 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 μm feature and the crystalline feature around 23 μm 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 presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of μm-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.
|Additional Information:||© 2009 ESO. Received 13 March 2009; accepted 26 August 2009. The authors warmly thank Bram Acke for his help on the IDL routine to extract the crystalline features characteristics, Jérome Aléon and Matthieu Gounelle for sharing with us their precious knowledge of small solid Solar System bodies, and Joel Green for the rich discussions we had. We also thank the anonymous referee for the very useful comments that helped improving this study. It is also a pleasure to acknowledge fruitful discussions with the Grenoble FOST team members, as well as the participants to the ANR (Agence Nationale de la Recherche of France) project ANR-07-BLAN-0221. This research is partly supported by the Programme National de Physique Stellaire (PNPS).|
|Subject Keywords:||stars: pre-main sequence; planetary systems: protoplanetary disks; circumstellar matter; infrared: stars; methods: statistical; techniques: spectroscopic|
|Official Citation:||C2D Spitzer-IRS spectra of disks around T Tauri stars - IV. Crystalline silicates J. Olofsson, J.-C. Augereau, E. F. van Dishoeck, B. Merín, F. Lahuis, J. Kessler-Silacci, C. P. Dullemond, I. Oliveira, G. A. Blake, A. C. A. Boogert, J. M. Brown, N. J. Evans, V. Geers, C. Knez, J.-L. Monin and K. Pontoppidan A&A 507 (1) 327-345 (2009) DOI: 10.1051/0004-6361/200912062|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||11 Dec 2009 19:41|
|Last Modified:||26 Dec 2012 11:38|
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