The Disk Atmospheres of Three Herbig Ae/Be Stars

Abstract

We present infrared (IR) spectrophotometry ($R \simeq 180$) 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 $7.6 - 13.2$ \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 data points from the Mt. Lemmon Observing Facility. The SEDs are modeled by using an expanded version of the \citet{chigol97} 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. 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-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 has a 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 small disk ($\sim 5$ AU in radius) while HD 100546 and HD 179218 are best fit by larger disks ($\sim 150$ AU in radius).