INTERPRETING INFRARED COLOR-COLOR DIAGRAMS - CIRCUMSTELLAR DISKS AROUND LOW-MASS AND INTERMEDIATE-MASS YOUNG STELLAR OBJECTS

We investigate the behavior of well-established evolutionary classes of young stellar objects in the JHK infrared color-color diagram. We examine observations of classical and weak-line T Tauri stars, Herbig AeBe stars, infrared protostars, and classical Be stars. We find that the different types of objects tend to occupy well-defined and different regions of the color-color diagram and that to some degree the evolutionary nature of a YSO can be determined from its position in JHK color-color space. In an attempt to provide a physical explanation for the observed JHK colors of YSOs, we explore the behavior of circumstellar disk models in the color-color diagram. We find that the JHK observations of classical T Tauri stars with infrared excesses can be well explained by standard disk models in agreement with earlier studies. However, standard disk models cannot account for the large infrared excesses and the distribution of AeBe stars in the color-color diagram unless (1) these model disks contain central holes and (2) the basic physical parameters of the disks always conspire to produce temperatures at the inner edges of the disks in the narrow range between 2000-3000 K. The close correspondence between the deduced temperatures at the inner edges of the disks and the dust sublimation temperature suggests that dust is the primary source of opacity at infrared wavelengths in circumstellar disks around AeBe stars.