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4.1.2 Classification of Member Stars

The infrared excess of a young star arises not from the reddening due to distant grains, but from emission relatively close to the stellar surface. That is, the phenomenon is circumstellar, rather than interstellar, in origin. As our present example illustrates, all stars within embedded clusters are subject to reddening by foreground dust, whether or not they have an infrared excess. Once the full spectral energy distribution of a source is established from observations, one may use the estimated AV through the cloud, together with the extinction curve, to calculate Ax, the extinction at all other wavelengths. In this manner, the "dereddened" energy distribution stemming from the star and its circumstellar matter can be reconstructed. Stars with heavy infrared excesses and those with none lie at the extremes of a rather well-defined morphological sequence of broadband spectra.

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