PAHs and crystalline silicates in
the post-AGB star IRAS 16279-4757
Matsuura M., Zijlstra A. A., Molster F.J., Hony S.,
Waters L.B.F.M., Kemper F., Bowey J.E., Chihara H., Koike C.,
Keller L.P., 2004, ApJ, accepted for publication
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.
paper (pdf, 330kb)