Bipolar outflows in OH/IR stars

We investigate the development of bipolar outflows during the early post-AG E evolution. A sample of 10 OH/IR stars with irregular OH spectra and unusu ally large expansion velocities is observed at high angular resolution. The sample includes bipolar nebulae (e.g., OH231.8+4.2), bright post-AGB stars (HD 101584) and reflection nebulae (e.g., Roberts 22). The IRAS colour-col our diagram separates the sample into different types of objects. One group may contain the immediate progenitors to the (few) extreme bipolar planeta ry nebulae. Two objects show colours and chemistry very similar to the plan etary nebulae with late IR-[WC] stars. One object is a confirmed close bina ry. A model is presented consisting of an outer AGE wind which is swept up by a faster post-AGB wind, with either the AGE or post-AGE wind being non-spher ically symmetric. The interface of the two winds is shown to exhibit a line ar relation between velocity and distance from the star, giving the impress ion of an accelerating outflow. The OH data confirm the predicted linear ve locity gradients, and also reveal torus-like, uniformly expanding component s. All sources are discussed in detail using optical/HST images where availabl e. ISO data for Roberts 22 reveal a chemical dichotomy, with both crystalli ne silicates and PAH features being present. IRAS 16342-3814 shows a dense torus with mass 0.1 M., and density of 10(8) cm(-3); HST data show four poi nt-like sources located symmetrically around the nebula, near the outer edg e of the dense torus. Lifetimes for the bipolar OH/IR stars are shown to be in excess of 10(4) yr , longer than normal post-AGE time-scales. This suggests that the toruses o r discs are near-stationary. We suggest that accretion from such a disc slo ws down the post-AGE evolution. Such a process could explain the link betwe en the long-lived bipolar nebular geometry and the retarded star.