Particle emissivity in circumstellar disks

Submillimeter continuum observations of 29 pre-main-sequence objects in Taurus and Orion are used to study the wavelength dependence of particle emission. These objects are mostly T Tauri stars whose long-wavelength emission is thought to originate in circumstellar disks. The flux densities imply power-law frequency distributions with spectral indices between two and three in almost all cases. If the emission is optically thin, the particle emissivities have power-law indices between -1 and 1; otherwise, these values are lower limits. It is argued that in most cases the emission is optically thin at wavelengths near 1 mm, so the measured indices should be close to the true values. The indices derived for the circumstellar particles are substantially smaller than those thought to obtain in the diffuse interstellar medium and dense molecular clouds; they imply that the millimeter-wave opacities are larger near these stars than in the interstellar medium. Particle shape and composition changes can produce the observed behavior. The effect is to decrease the required amount of disk material needed to produce the observed millimeter-wave continuum emission.