2006ApJ...636..362I

We use three-dimensional radiative transfer models to show the effects of clumpy circumstellar material on the observed infrared colors of high-mass stars embedded in molecular clouds. We highlight differences between three-dimensional clumpy and one-dimensional smooth models that can affect the interpretation of data. We discuss several important properties of the emergent spectral energy distribution (SED). More near-infrared light (scattered and direct from the central source) can escape than in smooth one-dimensional models. The near- and mid-infrared SED of the same object can vary significantly with viewing angle, depending on the clump geometry along the sight line. Even the wavelength-integrated flux can vary with angle by more than a factor of 2. Objects with the same average circumstellar dust distribution can have very different near- and mid-IR SEDs, depending on the clump geometry and the proximity of the most massive clump to the central source. Although clumpiness can cause similar objects to have very different SEDs, there are some observable trends. Near- and mid-infrared colors are sensitive to the weighted average distance of clumps from the central source and to the magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared emission remains a robust measure of the total dust mass. We present simulated SEDs, colors, and images for 2MASS and Spitzer filters. We compare them to observations of some ultracompact H II regions and find that three-dimensional clumpy models fit better than smooth models. In particular, clumpy models with fractal dimensions in the range 2.3-2.8, smooth-to-clumpy ratios of ≲50%, and density distributions with shallow average radial density profiles fit the SEDs best (<ρ>∝r^α, -1.0<α<0.0).