The Circumstellar Envelopes of Post-AGB Stars

Mid-IR images of a small sample of post-AGB stars have been obtained with the mid-IR camera, OSCIR, mounted on the 8-m Gemini North Telescope. Model calculations, using a 2-D dust RT code, have been performed in order to constrain the physical and chemical properties of the dust in the envelopes, for each object. Studies of individual objects in this transitional phase can be used to investigate the final period of AGB mass loss and improve our current understanding of the evolution of PN. Mid-IR images and modelling results for IRAS 22223+4327 are presented here. The observed mid-IR structure, which shows two emission peaks, is interpreted as the detection of two limb brightened edges of a dust torus. The position of the dust torus is in agreement with the position of the outflows observed in the archived HST optical images. This object deviates from axisymmetry, in the same way as several other post-AGB stars, where one of these peaks is brighter than the other. From the modelling of the SED of this object, estimates for the mass of the dust in the envelope and the mass-loss rate are derived.