Photospheric Opacity and Over-expanded Envelopes of Asymptotic Giant Branch Stars

I suggest 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 overexpansion. This may occur when the photospheric (effective) temperature drops to Tp ∼ 3000 K, and because the opacity sharply increases as temperature further decreases down to Tp ∼ 2000 K. The much higher opacity implies a much lower photospheric density, which stabilizes 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 over-expanded 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; and (3) An over-expanded asymptotic giant branch star, which doubles its radius, is somewhat more likely to swallow a low mass companion. Subject headings: stars: AGB and post-AGB − circumstellar matter − stars: mass-loss − planetary nebulae: general