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Sigma Orionis IRS1 A and B: A Binary Containing a Proplyd

We report optical and infrared imaging spectroscopy observations of the young binary object Sigma Orionis IRS1 A/B. The brighter component (Sigma Ori IRS1 A) of this binary system has M1 spectral type and a mass in the range of 0.3 to 0.8 solar masses. The fainter component (Sigma Ori IRS1 B) has a unique morphology and spectrum. The unresolved stellar object is surrounded by an extended envelope that is slightly offset from the position of this star. The envelope's spectrum shows strong emission lines of H and HeI but no shock-excited emission from molecular hydrogen or from [FeII]. The embedded stellar object Sigma Ori IRS1 B has an absorption spectrum characteristic of a late M photosphere, but with an additional approximately equal amount of dust continuum flux veiling the absorption lines. Sigma Ori IRS1 B is probably a young brown dwarf embedded in a proplyd that is being photo-evaporated by the UV flux of the nearby multiple O and B star system Sigma Ori.