Deep Circulation in Red Giant Stars: A Solution to the Carbon and Oxygen Isotope Puzzles?

Abstract

The long-standing puzzle of low ^(12)C/^(13)C in low-mass red giant branch (RGB) stars, and the more recent puzzle of low ^(18)O/^(16)O ratios in asymptotic giant branch (AGB) stars and in circumstellar Al_2O_3 grains preserved in meteorites, can be resolved by deep circulation currents below the bottom of the standard convective envelope. These currents transport matter from the nonburning bottom of the convective envelope down to regions where some CNO processing can take place ("cool bottom processing"). Modeling circulation with separate downward and upward streams, we found that, to resolve both discrepancies, the base of the extra mixing had to reach a temperature TP close to that of the H-burning shell, namely, Δ log T ≈ 0.17 from the base of the H-shell for both RGB and AGB stars. While the envelope composition depends sensitively on TP, it is insensitive to the speed or geometry of mixing. This indicates that our stream circulation model is generic, so that more sophisticated mixing models with the same TP would yield similar results. On the AGB, our models predict that stars with low ^(18)O/^(16)O can be either S or C stars but must have low ^(12)C/^(13)C (~4) and elevated ^(14)N. Cool bottom processing also destroys ^3He, so that galactic (D + ^3He) decreases with time; this removes the strongest lower limit on the baryon density Ω_b from big bang nucleosynthesis models.