2006ApJ...649..900S

We present a grid of near-infrared (IR) synthetic images of pre-main-sequence stars at different stages of evolution, which we simulate by varying envelope mass, disk radius and mass, and outflow cavity shape. Our aim is to determine how variations in physical properties of young stellar objects (e.g., mass infall rate, disk size) affect their observed colors and morphology, and use this information to highlight observable differences between different evolutionary states. We show that the near-IR colors are a function of envelope mass infall rate and inclination; hence both parameters must be constrained if colors are to be used to infer a source's true evolutionary state. Sources with more opaque envelopes have redder diffuse colors, because the scattered light suffers reddening as it propagates through the envelope. Somewhat counterintuitively, colors are reddest at intermediate inclinations (i∼45°-70°) and then become bluer edge-on, where light is ∼100% scattered. Thus a source with relatively blue colors could be an evolved source or a younger source oriented edge-on. Importantly, we find that at inclinations where scattered light dominates, it is erroneous to derive extinction A_Vfrom observed colors; fully half of all objects will underestimate A_Vby at least an order of magnitude. We use our models to interpret six protostellar sources in the Taurus-Auriga molecular cloud observed with HST NICMOS. Of the six young stellar objects modeled in this paper, five require an infalling envelope to match the colors and should thus be classified as young embedded sources. The remaining source, Haro 6-5B, is a disk source, having already dispersed its envelope.