The Disk Atmospheres of Three Herbig Ae/Be Stars

We present infrared (IR) spectrophotometry (R ≃ 180) of three Herbig Ae/Be stars surrounded by possible protoplanetary disks: HD 150193, HD100546 and HD 179218. We construct a mid-IR spectral energy distributions (SED) for each object by using 7.6− 13.2 μm HIFOGS spectra, 2.4 – 45 μm spectrophotometry from the ISO SWS, the 12, 25, 60, and 100 μm photometric points from IRAS, and for HD 179218, photometric bolometric data points from the Mt. Lemmon Observing Facility. The SEDs are modeled by using an expanded version of the Chiang & Goldreich (1997) two-layer, radiative and hydrostatic equilibrium, passive disk. This expanded version includes the emission from Mg-pure crystalline olivine (forsterite) grains in the disk surface layer. Each of the three objects studied vary in the amount of crystals evident from their spectrophotometry. HD 150193 contains no crystals while HD 100546 and HD 179218 respectively show evidence of having crystalline silicates in the surface layers of their disks. We find that the inner region of HD100546 has a 37% higher crystalline-toamorphous silicate ratio in its inner disk region (≤ 5 AU) compared to the outer disk region, while the inner disk region of HD 179218 has a 84% higher crystallineto-amorphous silicate ratio in its inner disk region (≤ 5 AU) compared to the Visiting Astronomer, Cerro Tololo Interamerican Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.