The Disk Atmospheres of Three Herbig Ae/Be Stars

We present infrared (IR) spectrophotometry ($R \simeq 180$) of three HerbigAe/Be stars surrounded by possible protoplanetary disks: HD 150193, HD100546and HD 179218. We construct a mid-IR spectral energy distributions (SED) foreach object by using $7.6 - 13.2$ \micron HIFOGS spectra, 2.4 -- 45 \micron\spectrophotometry from the {\it ISO} SWS, the 12, 25, 60, and 100 \micron\photometric points from IRAS, and for HD 179218, photometric bolometric datapoints from the Mt. Lemmon Observing Facility. The SEDs are modeled by using anexpanded version of the \citet{chigol97} two-layer, radiative and hydrostaticequilibrium, passive disk. This expanded version includes the emission fromMg-pure crystalline olivine (forsterite) grains in the disk surface layer. HD150193 contains no crystals while HD 100546 and HD 179218 respectively showevidence of having crystalline silicates in the surface layers of their disks.We find that the inner region of HD100546 has a 37% highercrystalline-to-amorphous silicate ratio in its inner disk region ($\leq 5$ AU)compared to the outer disk region, while the inner disk region of HD 179218 hasa 84% higher crystalline-to-amorphous silicate ratio in its inner disk region($\leq 5$ AU) compared to the outer region. HD 150193 is best-fit by a smalldisk ($\sim 5$ AU in radius) while HD 100546 and HD 179218 are best fit bylarger disks ($\sim 150$ AU in radius).Comment: 20 pages, 5 figure