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Paper: Properties of AGB Star Progenitors
Volume: 199, Asymmetrical Planetary Nebulae II: From Origins to Microstructures
Page: 51
Authors: Wood, P. R.
Abstract: In this paper, two separate topics are discussed. Firstly, shells around AGB stars are considered. The highly spherical shells seen in microwave lines around some AGB stars are readily explained in terms of helium shell flash behaviour. On the other hand, deep images of AGB and post-AGB stars show that in some of them, the almost spherically symmetric AGB wind is modulated on a timescale of a few hundred years, much shorter than the helium shell flash timescale. Some single star explanations for this modulated are investigated: no single star mechanism appears satisfactory, although merging of mass shells produced near the AGB star by pulsation and dust formation needs further investigation. Secondly, binarity in AGB stars is discussed. In particular, it is noted that observations of variable AGB stars in the LMC indicate that ~25&#37 of them show long secondary periods consistent with that expected for the orbital period of a close companion. If it turns out that the long secondary periods are indeed the result of a close orbiting companion, then it is almost certain that these companions will lead to the axially symmetric mass ejection observed in many planetary nebulae.
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