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

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 Al2O3 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 temperatureTPclose to that of the H-burning shell, namely, DlogT20.17 from the base of the H-shell for both RGB and AGB stars. While the envelope composition depends sensitively onTP, it is insensitive to the speed or geometryofmixing.Thisindicatesthatourstreamcirculationmodelisgeneric,sothatmoresophisticatedmixing models with the sameTPwould 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(14) and elevated 14 N. Cool bottom processing also destroys 3 He, so that galactic (D 1