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Published August 10, 2008 | Published
Journal Article Open

The Formation and Evolution of Planetary Systems: Grain Growth and Chemical Processing of Dust in T Tauri Systems


This paper is one in a series presenting results obtained within the Formation and Evolution of Planetary Systems (FEPS) Legacy Science Program on the Spitzer Space Telescope. Here we present a study of dust processing and growth in seven protoplanetary disks. Our spectra indicate that the circumstellar silicate dust grains have grown to sizes at least 10 times larger than observed in the interstellar medium and show evidence for a non-negligible (~5% in mass fractions) contribution from crystalline species. These results are similar to those of other studies of protoplanetary disks. In addition, we find a correlation between the strength of the amorphous silicate feature and the shape of the spectral energy distribution. This latter result is consistent with the growth and subsequent gravitational settling of dust grains toward the disk midplane. Furthermore, we find a change in the relative abundance of the different crystalline species: more enstatite than forsterite is observed in the inner warm dust population at ~1 AU, while forsterite dominates in the colder outer regions at ~5-15 AU. This change in the relative abundances argues for a localized crystallization process rather than a radial mixing scenario in which crystalline silicates are being transported outwards from a single formation region in the hot inner parts of the disk. Finally, we report the detection of emission from polycyclic aromatic hydrocarbon (PAH) molecules in five out of seven sources.We find a tentative PAH band at 8.2 µm that was previously undetected in the spectra of disks around low-mass pre-main-sequence stars.

Additional Information

© 2008. The American Astronomical Society. Received 2007 May 17; accepted 2008 February 21. Print publication: Issue 1 (2008 August 10). FEPS gratefully acknowledges support from NASA through JPL grants 1224768, 12224634, and 1224566.We would like to thank Dan Watson and Pat Morris for helpful discussions regarding data reduction, Deborah Padget and Tim Brooke for assistance with the observing plan, and Betty Stobie for assistance with software development, as well as the rest of the FEPS team, the IRS instrument team, and colleagues at the Spitzer Science Center for their support. M. R. M. is also supported through NASA's Astrobiology Institute. J. B. and Th. H. acknowledge support from the EU Human Potential Network contract HPRN-CT-2002000308.

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October 18, 2023