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J.E. Kessler, G.A. Blake (California Institute of Technology)
In this study, silicate spectra of several high and low mass stars ranging from YSOs to debris disks were obtained using the Long Wavelength Spectrometer at the W. M. Keck Observatory. Comparison of the spectra obtained indicates interesting connections between the shape of the silicate feature and stellar mass and age. For example, although previous studies of the silicate emission feature in YSOs found ISM-like emission from TTauri stars and Herbig Ae stars alike (Cohen & Witteborn 1985, Hanner et al. 1995,1998, Meeus, private communication), for three MYr-old YSOs in our sample we find that the spectra of two, HAE stars (HD 163296 and WW Vul), are similar to those found for comets and the debris disk \beta Pictoris, while a WTTS (LkCa 15) spectrum shows a silicate feature which is much broader than that of the ISM but lacks the 11.2 \mum feature indicative of crystalline olivine. Such differences could reflect differing evolutionary stages between these YSOs or differences in the path of evolution of T-Tauri stars versus that of Herbig Ae/Be stars and their descendants.
In addition to these spectroscopic studies, several YSOs were imaged with the 10, 12 and 18 micron filters, and found to be unresolved even at the diffaction limit of the 10 m telescope. Calculations based on the Chiang & Goldreich model, run for a variety of dust emissivities, predict that between 10 and 33%~ of the disk emission at 10-20 \mum lies outside the LWS PSF (Chiang, priv. commun.), but from our images of these disks, this does not appear to be the case. This may be explained by unconstrained parameters such as the size of dust grains, the degree of dust settling and the temperature of the central star, which can effect the dust temperature and size of the emitting region in the 10-20 micron range (Chiang et al. 2000).
This research was funded by NASA GSRP fellowship NGT50231.
The author(s) of this abstract have provided an email address for comments about the abstract: kessler@gps.caltech.edu