Polycyclic aromatic hydrocarbons and crystalline silicates in the bipolar post-asymptotic giant branch star IRAS 16279-4757

IRAS 16279-4757 belongs to a group of post-asymptotic giant branch (post-AGB) stars showing both polycyclic aromatic hydrocarbon (PAH) bands and crystalline silicates. We present mid-infrared images that resolve the object for the first time. The morphology is similar to that of the "Red Rectangle" (HD 44179), the prototype object with PAHs and crystalline silicates. A two-component model and images suggest a dense oxygen-rich torus, an inner, low-density, carbon-rich region, and a carbon-rich bipolar outflow. The PAH bands are enhanced at the outflow, while the continuum emission is concentrated toward the center. Our findings support the suggestion that mixed chemistry and morphology are closely related. We discuss the Infrared Space Observatory Short-Wavelength Spectrometer (ISO SWS) spectra of IRAS 16279-4757. Several bands in the ISO SWS spectrum show a match with anorthite: this would be the first detection of this mineral outside the solar system. Compared to HD 44179, the shapes of PAH bands are closer to those of planetary nebulae, possibly related to a population of small PAHs present in HD 44179 but absent around IRAS 16279-4757. Detailed examination of the spectra shows the individual character of these two objects. The comparison suggests that the torus found in IRAS 16279-4757 may have formed more recently than that in HD 44179.