PAHs and crystalline silicates in the post-AGB star IRAS 16279-4757

TL;DR

This study presents mid-infrared imaging and spectral analysis of the post-AGB star IRAS 16279-4757, revealing complex morphology, mixed chemistry, and potential mineral detections, advancing understanding of circumstellar environments in evolved stars.

Contribution

First resolved mid-infrared images of IRAS 16279-4757 showing detailed morphology and chemical composition, including potential detection of anorthite outside the solar system.

Findings

Morphology similar to the Red Rectangle with a dense oxygen-rich torus

Enhanced PAH emission at bipolar outflows

Possible first detection of anorthite mineral in space

Abstract

IRAS 16279-4757 belongs to a group of post-AGB stars showing both 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 towards the center. Our findings support the suggestion that mixed chemistry and morphology are closely related. We discuss the 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 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.