A Solar‐like Cycle in Asymptotic Giant Branch Stars

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Abstract
I propose that the mechanism behind the formation of concentric semiperiodic shells found in several planetary nebulae (PNs) and proto-PNs and around one asymptotic giant branch (AGB) star is a solar-like magnetic activity cycle in the progenitor AGB stars. The time intervals between consecutive ejection events are ~200-1000 yr, which is assumed to be the cycle period (the full magnetic cycle can be twice as long, as is the 22 yr period in the Sun). The magnetic field has no dynamical effects; it regulates the mass-loss rate by the formation of magnetic cool spots. The enhanced magnetic activity at the cycle maximum results in more magnetic cool spots, which facilitate the formation of dust, hence increasing the mass-loss rate. The strong magnetic activity implies that the AGB star is spun up by a companion, via a tidal or common envelope interaction. The strong interaction with a stellar companion explains the observations that the concentric semiperiodic shells are found mainly in bipolar PNs.
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