Two-dimensional Radiative Transfer in Protostellar Envelopes. II. An Evolutionary Sequence

We present model spectral energy distributions (SEDs), colors, polarization, and images for an evolutionary sequence of a low-mass protostar from the early collapse stage ( Class 0) to the remnant disk stage (Class III). We find a substantial overlap in colors and SEDs between protostars embedded in envelopes (Class 0 - I) and T Tauri disks (Class II), especially at mid-IR wavelengths. Edge-on Class I-II sources show double-peaked SEDs, with a short-wavelength hump due to scattered light and a long-wavelength hump due to thermal emission. These are the bluest sources in mid-IR color-color diagrams. Since Class 0 and I sources are diffuse, the size of the aperture over which fluxes are integrated has a substantial effect on the computed colors, with larger aperture results showing significantly bluer colors. Viewed through large apertures, the Class 0 colors fall in the same regions of mid-IR color-color diagrams as Class I sources and are even bluer than Class II - III sources in some colors. It is important to take this into account when comparing color-color diagrams of star formation regions at different distances or different sets of observations of the same region. However, the near-IR polarization of the Class 0 sources is much higher than the Class I-II sources, providing a means to separate these evolutionary states. We varied the grain properties in the circumstellar envelope, allowing for larger grains in the disk midplane and smaller grains in the envelope. In comparing with models with the same grain properties throughout, we find that the SED of the Class 0 source is sensitive to the grain properties of the envelope only - that is, grain growth in the disk in Class 0 sources cannot be detected from the SED. Grain growth in disks of Class I sources can be detected at wavelengths greater than 100 mum. Our image calculations predict that the diffuse emission from edge-on Class I and II sources should be detectable in the mid-IR with the Space Infrared Telescope Facility (SIRTF) in nearby star-forming regions ( out to several hundred parsecs).