AAS 201st Meeting, January, 2003
Session 20. Star Formation I
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[20.11] Constraining the Relative Mass Fractions of Amorphous and Crystalline Silicates in the Disk Atmospheres of Three Herbig Ae Stars Using Radiative Transfer Models

D.E. Harker (University of California, San Diego/CASS), D.H. Wooden (NASA Ames Research Center), C.E. Woodward (University of Minnesota)

Intermediate age stars surrounded by disks (Class II sources) are ideal objects for investigating the processes which lead to planetary formation. The disks around Class II objects can be directly observed as opposed to Class~0 and Class~I sources which are embedded in a surrounding molecular cloud. Specifically, Herbig Ae stars (1 -- 3 M\odot) provide an ideal environment for observing the ongoing processes in disks around Class~II sources since they are bright and many are well studied. A goal of our research is to quantify the crystalline silicate component of the submicron grains in protoplanetary disks to constrain the degree of processing (heating or annealing) and radial mixing of dust in the disk. Dust processing is precursory to grain aggregation and protoplanet formation.

In this work, we present NASA Ames HIFOGS 7 -- 14~\micron\ spectrophotometric observations of three Herbig Ae stars: HD~150193, HD~100546, and HD~179218. We will present an analysis of the silicate mineralogy in each of the three systems emphasizing the solid-state spectral features in the 10~\micron\ region. The spectrum of HD~150193 shows that amorphous silicates make up most of the dust emission while HD~179218 and HD~100546 show emission from crystalline silicates. We will also construct a spectral energy distribution by adding ISO SWS spectra and IRAS photometry to the HIFOGS data sets.

We model the SEDs of these systems using the two-layer passive disk model developed by Chiang & Goldreich (1997, ApJ, 490, 368) and updated by Chaing et al.\ (2001, ApJ, 547, 1077). To expand the mineralogy of the model, we add crystalline olivine grains to the disk surface layer. Presented will be the results of our modeling which includes a quantification of the relative mass fractions of submicron crystalline and amorphous silicates required to reproduce the assembled SEDs.

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