Spectral Energy Distributions of Passive T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison withInfrared Space ObservatoryLWS Observations

We improve upon the radiative, hydrostatic equilibrium models of passivecircumstellar disks constructed by Chiang & Goldreich (1997). New featuresinclude (1) account for a range of particle sizes, (2) employment oflaboratory-based optical constants of representative grain materials, and (3)numerical solution of the equations of radiative and hydrostatic equilibriumwithin the original 2-layer (disk surface + disk interior) approximation. Weexplore how the spectral energy distribution (SED) of a face-on disk depends ongrain size distributions, disk geometries and surface densities, and stellarphotospheric temperatures. Observed SEDs of 3 Herbig Ae and 2 T Tauri stars,including spectra from the Long Wavelength Spectrometer (LWS) aboard theInfrared Space Observatory (ISO), are fitted with our models. Silicate emissionbands from optically thin, superheated disk surface layers appear in nearly allsystems. Water ice emission bands appear in LWS spectra of 2 of the cooleststars. Infrared excesses in several sources are consistent with verticalsettling of photospheric grains. While this work furnishes further evidencethat passive reprocessing of starlight by flared disks adequately explains theorigin of infrared-to-millimeter wavelength excesses of young stars, weemphasize how the SED alone does not provide sufficient information toconstrain particle sizes and disk masses uniquely.Comment: Accepted to ApJ, 35 pages inc. 14 figures, AAS preprin