Bipolar Outflows from OH/IR Stars

Abstract

We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of ten 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 symetric. 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 confirms the predicted linear velocity gradients, and also reveals toruslike, 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 PAHs features being present. IRAS 16342−3814 shows a dense torus with mass 0.1 M ⊙ and density of 10 8 cm −3 ; HST data shows 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 10 4 yr, longer than normal post-AGB timescales. This suggests that the toruses or disks are nearstationary. We suggest that accretion from such a disk slows down the post-AGB evolution. Such a process could explain the link between the long-lived bipolar nebular geometry and the retarded star.

The binary central star of the bipolar pre-planetary nebula IRAS 08005−2356 (V510 Pup)

“…They determined the size of the nebula to be ⇠ 2.68 " ⇥1.42 " . Outflows have also been resolved in OH data (Zijlstra 2001) and in CO and SiO by Sahai & Patel (2015), who reported CO outflow velocities reaching up to 200 km s 1 .…”