The Disk Atmospheres of Three Herbig Ae/Be Stars

Abstract

We present infrared (IR) spectrophotometry (R 180) of three Herbig Ae/Be stars surrounded by possible protoplanetary disks: HD 150193, HD 100546, and HD 179218. We construct a mid-IR spectral energy distributions (SEDs) for each object by using 7.6-13.2 μm HIFOGS spectra, 2.4-45 μm spectrophotometry from the Infrared Space Observatory Short-Wavelength Spectrometer, the 12, 25, 60, and 100 μm photometric points from IRAS, and for HD 179218, photometric bolometric data points from the Mount Lemmon Observing Facility. The SEDs are modeled by using an expanded version of the Chiang & Goldreich 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 HD 100546 has a 37% higher crystalline-to-amorphous silicate ratio in its inner disk region (≤5 AU) than in 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 (≤5 AU) than in the outer region. All three objects are best fitted using a grain-size distribution power law that falls as a-3.5. HD 150193 is best fitted by a small disk (~5 AU in radius), while HD 100546 and HD 179218 are best fitted by larger disks (~150 AU in radius). Furthermore, HD 100546's disk flares larger than those of HD 150193 (25% more at 5 AU) and HD 179218 (80% more at 5 AU). We discuss the implications of our results and compare them with other modeling efforts.