On the Formation of Multiple Arcs around Asymptotic Giant Branch Stars

Two types of models for the formation of semi-periodic concentric multipleshells (M-shells) around asymptotic giant branch (AGB) stars and in planetarynebulae are compared against observations. Models that attribute the M-shellsto processes in an extended wind acceleration zone around AGB stars result inan optically thick acceleration zone, which reduces the acceleration efficiencyin outer parts of the extended acceleration zone. This makes such models anunlikely explanation for the formation of M-shells. Models which attribute theM-shell to semi-periodic variation in one or more stellar properties are mostcompatible with observations. The only stellar variation models on time scalesof 50-1500 years that have been suggested are based on an assumed solar-likemagnetic cycle. Although ad-hoc, the magnetic cycle assumption fits naturallyinto the increasingly popular view that magnetic activity plays a role inshaping the wind from upper AGB stars.Comment: 8 pages, Submitted to Ap