2-D Radiative Transfer in Protostellar Envelopes: II. An Evolutionary Sequence

We present model spectral energy distributions, 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 spectral energy distributions, with a shortwavelength hump due to scattered light and the 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 in the envelope. In comparing to Space Science Institute, 3100 Marine Street, Suite A353, Boulder, CO 80303; [email protected] School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9AD, Scotland the Brave; [email protected] Ritter Observatory, MS 113, Department of Physics & Astronomy, University of Toledo, Toledo, OH 43606-3390; [email protected] Radio Astronomy Laboratory, 601 Campbell Hall, University of California, Berkeley, CA 94720; [email protected]