Spitzer observations of V4332 sagittarii: Detection of alumina dust

We present broadband 24, 70, and 160 μm photometry and 5-35 and 55-90 μm spectra of the eruptive variable V4332 Sgr from Spitzer observations. The distinguishing feature of the 5-35 μm spectrum is an unusually broad absorption feature near 10 μm at the position generally associated with silicate-rich dust. Through radiative transfer modeling, we show that this broad feature cannot arise from silicates alone but requires the inclusion of alumina (Al ₂O₃) as a dust condensate. The case for including Al ₂O₃ is further strengthened by the presence of the AlO radical, a potentially important molecule in forming Al₂O ₃. The present detection indicates that porous alumina manifests itself through a broadening of the 9.7 μm silicate feature and additionally displays, on the shoulder of the silicate feature, a component at ∼11.5 μm. We discuss how further observations of V4332 Sgr may have the potential of verifying some general predictions of the dust condensation process.