Bipolar outflows in OH/IR stars

We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of 10 OH/IR stars with irregular OH spectra and unusually 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—colour diagram separates the sample into different types of objects. One group may contain the immediate progenitors to the (few) extreme bipolar planetary nebulae. Two objects show colours and chemistry very similar to the planetary nebulae with late IR-[WC] stars. One object is a confirmed close binary. A model is presented consisting of an outer AGB wind which is swept up by a faster post-AGB wind, with either the AGB or post-AGB wind being non-spherically symmetric. The interface of the two winds is shown to exhibit a linear relation between velocity and distance from the star, giving the impression of an accelerating outflow. The OH data confirm the predicted linear velocity gradients, and also reveal torus-like, uniformly expanding components. All sources are discussed in detail using optical/HST images where available. ISO data for Roberts 22 reveal a chemical dichotomy, with both crystalline silicates and PAH features being present. IRAS 16342-3814 shows a dense torus with mass 0.1 M⊙ and density of 108 cm−3; HST data show four point-like sources located symmetrically around the nebula, near the outer edge of the dense torus. Lifetimes for the bipolar OH/IR stars are shown to be in excess of 104 yr, longer than normal post-AGB time-scales. This suggests that the toruses or discs are near-stationary. We suggest that accretion from such a disc slows down the post-AGB evolution. Such a process could explain the link between the long-lived bipolar nebular geometry and the retarded star.