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

We report optical and infrared imaging spectroscopy observations of the young binary object σ Orionis IRS1 A/B. The brighter component (σ Ori IRS1 A) of this binary system has M1 spectral type and a mass in the range of ≈ 0.3 – 0.8 M⊙. The fainter component (σ 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 H2 or [FeII]. The embedded stellar object σ 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. σ Ori IRS1 B is probably a young brown dwarf embedded in a proplyd that is being photoevaporated by the UV flux of the nearby multiple O and B star system σ Ori. Subject headings: stars: formation — stars: pre-main sequence — stars: lowmass, brown dwarfs