Opacity-induced over-expansion of asymptotic giant branch stars

I argue that the behavior of the photospheric opacity in oxygen-rich (similar to solar abundance) upper asymptotic giant branch stars may cause these stars to substantially expand for a few thousand years. I term this process over-expansion. This may occur when the photospheric (effective) temperature drops to Tp ∼ 2900 K, and because the opacity sharply increases as temperature decreases down to Tp ∼ 2000 K. The much higher opacity implies a much lower photospheric density, necessary to stabilize the envelope structure. As mass loss proceeds, the star eventually contracts to become a post-asymptotic giant branch star. Some possible outcomes of the over-expanded phase are discussed. (1) The overexpanded phase may be connected to the formation of semi-periodic concentric arcs (rings; shells). (2) The over-expanded phase may be related to the positive correlation between the mass loss rate and the transition to axisymmetric mass loss geometry. (3) An over-expanded asymptotic giant branch star, which doubles its radius, is somewhat more likely to swallow a low mass companion.