Bipolar outflows in OH/IR stars

Abstract

We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of ten OH/IR stars is observed at high angular resolution, including bipolar nebulae (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 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 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. The OH data confirms the predicted linear velocity gradients, and reveals 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 PAHs features being present. IRAS 16342-3814 shows a dense torus; 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 disks are near-stationary. 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.Comment: 31 pages, LaTeX using mn.sty. MNRAS, accepted for publicatio

H2O Maser Observations of Candidate Post‐AGB Stars and Discovery of Three High‐Velocity Water Sources

“…Hubble Space Telescope (HST ) images show two asymmetric bipolar reflection lobes on either side of a dark lane. The OH masers are aligned with the inner edges of the lobes, with outflow velocities up to 70 km s À1 (Sahai et al 1999;Zijlstra et al 2001). Near-infrared images (Sahai et al 2005) reveal corkscrewlike structures in the lobes that are consistent with a collimated precessing jet of diameter 100 AU and period 50 yr.…”

Polarization properties of OH masers in AGB and post-AGB stars

“…These objects sometimes experience OH bursts of highly polarized emission at 1612 MHz usually not associated with bursts at 1665 and 1667 MHz (Szymczak & Gérard 2005). In many post-AGB objects there is a disruption of detached AGB 1612 MHz shells that are no longer spherically symmetric and the appearance of a well-defined bipolar morphology at 1667 MHz is observed (Zijlstra et al 2001). In these partially-filled 1612-MHz structures, observed at high angular resolution, Zeeman components are much less spatial blended than in dense (10 −4 M , Habing 1996), more fully filled shells that are observed around OH/IR objects (e.g., Bowers & Johnston 1990).…”

OH-selected AGB and Post-AGB Objects. II. Blue versus Red Evolution Off the Asymptotic Giant Branch

“…Those values are close to what van der Veen found for the two spectral classes (Table 4) and correspond closely to the scale heights given by Corradi (2000) for bipolar (130 pc) and elliptical (320 pc) PNs. In addition, the location of the '' bipolar outflow '' region defined by Zijlstra et al (2001) suggests that the LI sources may be related to bipolar sources. The higher fraction of irregular spectra among LI sources and the preferred location of sources with irregular spectra in this '' bipolar outflow '' region, as found in Paper I, indicate that on their way '' back '' to the red PN region of the IRAS two-color diagram the LI sources may go through a highly irregular, high-outflow mass-loss phase.…”