Olofsson, J. and Augereau, J.-C. and van Dishoeck, E. F. and Merín, B. and Grosso, N. and Ménard, F. and Blake, G. A. and Monin, J.-L. (2010) C2D Spitzer-IRS spectra of disks around T Tauri stars. V. Spectral decomposition. Astronomy and Astrophysics, 520 . A39. ISSN 0004-6361 http://resolver.caltech.edu/CaltechAUTHORS:20101109-094126013
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Context. Dust particles evolve in size and lattice structure in protoplanetary disks, due to coagulation, fragmentation and crystallization, and are radially and vertically mixed in disks due to turbulent diffusion and wind/radiation pressure forces. Aims. This paper aims at determining the mineralogical composition and size distribution of the dust grains in planet forming regions of disks around a statistical sample of 58 T Tauri stars observed with Spitzer/IRS as part of the Cores to Disks (c2d) Legacy Program. Methods. We present a spectral decomposition model, named “B2C”, that reproduces the IRS spectra over the full spectral range (5–35 μm). The model assumes two dust populations: a warm component responsible for the 10 μm emission arising from the disk inner regions (≲1 AU) and a colder component responsible for the 20–30 μm emission, arising from more distant regions (≲10 AU). The fitting strategy relies on a random exploration of parameter space coupled with a Bayesian inference method. Results. We show evidence for a significant size distribution flattening in the atmospheres of disks compared to the typical MRN distribution, providing an explanation for the usual flat, boxy 10 μm feature profile generally observed in T Tauri star spectra. We reexamine the crystallinity paradox, observationally identified by Olofsson et al. (2009 , A&A, 507, 327), and we find a simultaneous enrichment of the crystallinity in both the warm and cold regions, while grain sizes in both components are uncorrelated. We show that flat disks tend to have larger grains than flared disk. Finally our modeling results do not show evidence for any correlations between the crystallinity and either the star spectral type, or the X-ray luminosity (for a subset of the sample). Conclusions. The size distribution flattening may suggests that grain coagulation is a slightly more effective process than fragmentation (helped by turbulent diffusion) in disk atmospheres, and that this imbalance may last over most of the T Tauri phase. This result may also point toward small grain depletion via strong stellar winds or radiation pressure in the upper layers of disk. The non negligible cold crystallinity fractions suggests efficient radial mixing processes in order to distribute crystalline grains at large distances from the central object, along with possible nebular shocks in outer regions of disks that can thermally anneal amorphous grains.
|Additional Information:||© 2010 ESO. Received 18 December 2009; Accepted 1 July 2010. Published online 27 September 2010. The authors thank Christophe Pinte for his help on the bayesian study to derive uncertainties on the output parameters of the compositionnal fitting procedure. We thank the referee, Dan M. Watson, for his very constructive comments, that helped improving both the modeling procedure, especially on the question of large 6.0 μm crystalline grains, and the general quality of the paper. We also thank N. J. Evans II for his very useful comments that helped improving this study. We finally thank the Programme National de Physique Stellaire (PNPS) and ANR (contract ANR-07-BLAN-0221) for supporting part of this research. This research is also based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.|
|Subject Keywords:||stars: pre-main sequence; protoplanetary disks; circumstellar matter; infrared: stars; methods: statistical; techniques: spectroscopic|
|Official Citation:||C2D Spitzer-IRS spectra of disks around T Tauri stars - V. Spectral decomposition J. Olofsson, J.-C. Augereau, E. F. van Dishoeck, B. Merín, N. Grosso, F. Ménard, G. A. Blake and J.-L. Monin A&A 520 A39 (2010) DOI: 10.1051/0004-6361/200913909|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||09 Nov 2010 23:44|
|Last Modified:||26 Dec 2012 12:36|
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