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

In order to investigate possible connections between dust processing and diskproperties, 8--13 um spectra of 34 young stars, with a range of circumstellarenvironments and spectral types A to M, were obtained using the Long WavelengthSpectrometer at the W. M. Keck Observatory. The broad 9.7 um amorphous silicatefeature which dominates this wavelength regime evolves from absorption inyoung, embedded sources, to emission in optically revealed stars, and tocomplete absence in older debris disk systems for both low- andintermediate-mass stars. The peak wavelength and FWHM are centered about 9.7and ~2.3 um, corresponding to amorphous olivine, with a larger spread in FWHMfor embedded sources and in peak wavelength for disks. In a few of our objectsthat have been previously identified as class I low-mass YSOs, the observedsilicate feature is complex, with absorption near 9.5 um and emission peakingaround 10 um. Although most of the emission spectra show broad classicalfeatures attributed to amorphous silicates, variations in the shape/strengthmay be linked to dust processing, including grain growth and/or silicatecrystallization. We study quantitatively the evidence for evolutionary trendsin the 8--13 um spectra through a variety of spectral shape diagnostics. Basedon the lack of correlation between these diagnostics and broad-band infraredluminosity characteristics for silicate emission sources, we conclude thatalthough spectral signatures of dust processing are present, they can not beconnected clearly to disk evolutionary stage (for optically thick disks) oroptical depth (for optically thin disks). The diagnostics of silicateabsorption features (other than the central wavelength of the feature),however, are tightly correlated with optical depth.Comment: 27 pages, 13 figures, accepted for publication by ApJ, formatted with emulateapj using revtex4 v4.