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.