We present Spitzer high spectral resolution IRS spectroscopy of three positions in the carbon-rich outflow of post-asymptotic giant branch star HD 44179, better known as the Red Rectangle. Surprisingly, the spectra show some strong unknown mid-infrared resonances, in the 13-20 μm range. The shape and position of these resonances vary with position in the nebula, and they are not correlated with the polycyclic aromatic hydrocarbon features. We conclude that these features are due to oxygen-rich minerals, located in a region that is believed to be predominantly carbon-rich. We provide possible explanations for the presence of oxygen-rich dust in the carbon-rich outflows. Simple Mg-Fe oxides (Mg1-xFex^O) are suggested as carriers of these unidentified features. © 2005. The American Astronomical Society. All rights reserved.
- Circumstellar matter
- Dust, extinction
- ISM: lines and bands stars: AGB and post-AGB
- Stars: Individual (HD 44179)