Interpreting infrared color-color diagrams - Circumstellar disks around low- and intermediate-mass young stellar objects

Classical and weak-line T Tauri stars, Hernig AeBe stars, infrared protostars, and classical Be stars are found to occupy well-defined and different regions in the JHK infrared color-color diagram, with some overlap. The location of each of these young stellar objects on the diagram is determined to a large extent by its evolutionary state. The pattern of colors displayed by classical T Tauri stars with near-infrared excesses can be reproduced very well with standard circumstellar disk models. But standard disk models cannot account for the large infrared excesses and the distribution of AeBe stars unless (1) models contain central holes and (2) the physical parameters of the disks and central stars always conspire to produce temperatures in the narrow range between 2000-3000 K at the inner edges of the disks. The fact that this temperature range is near the sublimation temperature of interstellar dust suggests that dust is the primary source of opacity at infrared wavelengths in circumstellar disks around AeBe stars.