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-AGB stars showing both PAH bandsand crystalline silicates. We present mid-infrared images, that resolve theobject for the first time. The morphology is similar to that of the `RedRectangle' (HD 44179), the prototype object with PAHs and crystallinesilicates. 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. ThePAH bands are enhanced at the outflow, while the continuum emission isconcentrated towards the center. Our findings support the suggestion that mixedchemistry and morphology are closely related. We discuss the ISO/SWS spectra ofIRAS 16279-4757. Several bands in the ISO/SWS spectrum show a match withanorthite: this would be the first detection of this mineral outside the solarsystem. Compared to HD 44179, the shapes of PAH bands are closer to those ofplanetary nebulae, possibly related to a population of small PAHs present HD44179, but absent around IRAS 16279-4757. Detailed examination of the spectrashows the individual character of these two objects. The comparison suggeststhat the torus found in IRAS 16279-4757 may have formed more recently than thatin HD 44179.Comment: accepted to Ap