Two-Dimensional Radiative Transfer in Protostellar Envelopes. III. Effects of Stellar Temperature

We model how the mid-IR colors of young stellar objects (YSOs) vary with stellar temperature. The spectral energy distribution (SED) of each object has contributions from the thermal emission of circumstellar dust, from direct stellar photospheric emission, and from scattered stellar emission. We first isolate the effects of stellar contributions (direct + scattered) to the SED using homologous "Class I" models: the distribution of circumstellar matter is chosen to scale with stellar temperature T-* such that the shape of the thermal contribution to the SED remains constant. The relative contribution of stellar direct and scattered light varies with T-*, changing the 1-10 mum (MIR) colors. Stellar light contributes more to the mid-IR emission of YSOs with lower temperature stars (T-* similar to 4000 K) because the emission peak wavelength of the star is closer to that of the thermal radiation. In YSOs with hotter central stars, since the peak of the stellar and thermal spectra are more separated in wavelength, the 1-10 mum spectrum is closer to a pure thermal spectrum and the objects are redder. Next we consider realistic Class 0, I, and II source models and find that the other dominant effect of varying stellar temperature on YSO SEDs is that of the inner disk wall: in high-T-* models, the dust destruction radius is much farther out, with a consequently larger inner disk wall that contributes relatively more to the 2-10 mum flux. This effect partially offsets that of the stellar contribution, leading to varying behaviors of the 2-10 mum flux: In Class 0 sources, the trend is for higher T-* models to have redder colors. In Class I sources, the trend applies with some exceptions. In Class II sources, 2-10 mum colors become redder, going from T-* 4000 to 8000 K because of decreasing stellar contribution at T-* 8000 K, and then become blue again from 8000 to 31,500 K because of an increasing inner disk wall contribution. Near edge-on inclinations, the color behavior is completely different. Our modeled mid-IR protostellar colors have implications for interpretations of Spitzer IRAC observations of star formation regions. It is commonly assumed that the slope of the SED at 1-10 mum is directly related to evolutionary state. We show that inclination effects, aperture size, scattered light, and stellar temperature cause a broad spread in the colors of a source at a single evolutionary state. Color-magnitude diagrams can help sort out these effects by separating sources with different T-* on the basis of their different brightness (for sources at the same distance).