Bipolar outflows from OH/IR stars
Zijlstra A.A., Chapman J.W., te Lintel Hekkert P., Likkel L.,
Comeron F., Norris R.P., Molster F.
and Cohen R.J., 2001, MNRAS, 322, 280
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 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 with mass 0.1 Msun and density of
108 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
104 yr, longer than normal post-AGB timescales. This suggests that
the toruses or 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.
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Last update: Aug 9, 2001