SPECTRAL ENERGY DISTRIBUTIONS OF PASSIVE T TAURI AND HERBIG Ae DISKS: GRAIN MINERALOGY, PARAMETER DEPENDENCES, AND COMPARISON WITH INFRARED SPACE OBSERVAT ORY LWS OBSERVATIONS

We improve upon the radiative, hydrostatic equilibrium models of passive circumstellar disks con-structed by Chiang & Goldreich. New features include (1) an account for a range of particle sizes, (2) employment of laboratory-based optical constants of representative grain materials, and (3) numerical solution of the equations of radiative and hydrostatic equilibrium within the original two-layer (disk surface plus disk interior) approximation. We systematically explore how the spectral energy distribution (SED) of a face-on disk depends on grain size distributions, disk geometries and surface densities, and stellar photospheric temperatures. Observed SEDs of three Herbig Ae and two T Tauri stars, including spectra from the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO), are Ðtted with our models. Silicate emission bands from optically thin, superheated disk surface layers appear in nearly all systems. Water ice emission bands appear in LWS spectra of two of the coolest stars. Infrared excesses in several sources are consistent with signiÐcant vertical settling of photospheric grains. While this work furnishes further evidence that passive reprocessing of starlight by Ñared disks adequately explains the origin of infrared-to-millimeter wavelength excesses of young stars, we emphasize by explicit calculations how the SED alone does not provide sufficient information to constrain particle sizes and disk masses uniquely. Subject headings: accretion, accretion disks È circumstellar matter È radiative transfer