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Published March 20, 2005 | Published
Journal Article Open

8-13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature


Silicate features arising from material around pre-main-sequence stars are useful probes of the star and planet formation process. In order to investigate possible connections between dust processing and disk properties, 8-13 μm spectra of 34 young stars, exhibiting a range of circumstellar environments and including spectral types A-M, were obtained using the Long Wavelength Spectrometer at the W. M. Keck Observatory. The broad 9.7 μm amorphous silicate (Si–O stretching) feature that dominates this wavelength regime evolves from absorption in young, embedded sources, to emission in optically revealed stars, and to complete absence in older "debris" disk systems for both low- and intermediate-mass stars. This is similar to the evolutionary pattern seen in Infrared Space Observatory (ISO) observations of high/intermediate-mass young stellar objects (YSOs). The peak wavelength and FWHM are centered about 9.7 and ~2.3 μm, respectively, corresponding to amorphous olivine, with a larger spread in FWHM for embedded sources and in peak wavelength for disks. In a few of our objects that have been previously identified as class I low-mass YSOs, the observed silicate feature is more complex, with absorption near 9.5 μm and emission peaking around 10 μm. Although most of the emission spectra show broad classical features attributed to amorphous silicates, small variations in the shape/strength may be linked to dust processing, including grain growth and/or silicate crystallization. For some of the Herbig Ae stars in the sample, the broad emission feature has an additional bump near 11.3 μm, similar to the emission from crystalline forsterite seen in comets and the debris disk β Pictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Ae star, HD 179218, clearly show strong, narrow emission near 11.3 μm. We study quantitatively the evidence for evolutionary trends in the 8-13 μm spectra through a variety of spectral shape diagnostics. Based on the lack of correlation between these diagnostics and broadband infrared luminosity characteristics for silicate emission sources, we conclude that although spectral signatures of dust processing are present, they cannot be connected clearly to disk evolutionary stage (for optically thick disks) or optical depth (for optically thin disks). The diagnostics of silicate absorption features (other than the central wavelength of the feature), however, are tightly correlated with optical depth and thus do not probe silicate grain properties.

Additional Information

© 2005 American Astronomical Society. Received 2004 March 23; accepted 2004 November 30. Support for JEK-S was provided by NASA through the Graduate Student Researchers Program, NGT5-50231, and through the Spitzer Space Telescope Postdoctoral Fellowship Program, under award 1256316. G. A. B. acknowledges additional support from the NASA Origins program. M. R. M. acknowledges support from NASA contract 1224768 to the University of Arizona administered through the Jet Propulsion Laboratory. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors would also like to thank Randy Campbell for his assistance with Keck LWS operation and data reduction. The authors additionally acknowledge Jonathan Foster for his assistance with the 10 and 20 m photometric reduction and Josh Eisner for access to his SED database of class I sources. The authors would also like to thank the reviewer for many useful comments.

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