A study of the ten- and twenty-micron silicate features in celestrial infrared sources

Using two low spectral resolution infrared spectrometers and the 2.3meter telescope of the Wyoming Infrared Observatory, the author has gathered photometry and imaging data on a number of compact HII regions and protostellar sources at wavelengths from 8 to 24 μm. Models using three components of dust, using a standard emissivity law for silicate grains, in which two of the dust components are warm enough to emit significantly at these wavelengths and the third component in cold but is seen through its extinction effect, successfully fit the observed spectra of many sources. These models are refinements to two-component models often used to fit the observed 8-13 μm spectra of these sources. The warmer dust component is typically hotter, and the cool component is typically cooler, than dust temperatures derived from two-component fits. The exact configuration of these three dust components varies among the different sources studied. In the Trapezium IR nebula, two emitting components at temperatures of ∼300 K and 100 K suffice; no overlying extinction is needed. In W51 IRS 1 a layered structure is indicated, with the warmer emitting component lying behind cool dust, which in turn lies behind the cold component. For Mon R2 the best model suggests that the warmer dust lies within a shell of cool dust. Pixel-by-pixel fits to sources for which the best data were obtained result in maps of dust optical depths and temperatures for each component. For the Trapezium, these maps indicate a growing dominance of the cool component with distance from the 10μm intensity peak. The fitted maps of W51 indicate that the southern part of the nebula suffers less extinction than the northern part. Fits to images of W3 A are more difficult to interpret, suggesting that the fitted model may not be realistic for this source