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(Abridged) We demonstrate that the amount of extra mixing required to fit theobserved low C/N and 12C/13C ratios in first giant branch (FGB) stars is alsosufficient to explain the C and N abundances of Galactic AGB stars. We simulatethe effect of extra mixing on the FGB by setting the composition of theenvelope to that observed in low-mass FGB stars, and then evolve the models tothe tip of the AGB. The inclusion of FGB extra mixing compositional changes hasa strong effect on the C and N abundance in our AGB models, leading tocompositions consistent with those measured in Galactic C-rich stars. Thecomposition of the models is also consistent with C abundances measured inmainstream silicon carbide grains. While our models cover the range of Cabundances measured in C stars in NGC 1846, we cannot simultaneously match thecomposition of the O and C-rich stars. Our models only match the O isotopiccomposition of K and some M, MS giants, and are not able to match the Ocomposition of C-rich AGB stars. By increasing the 16O intershell abundance(based on observational evidence) it is possible to reproduce the observedtrend of increasing 16O/18O and 16O/17O ratios with evolutionary phase. Weconclude 1) if extra mixing occurs during the AGB it likely only occursefficiently in low metallicity objects, or when the stars are heavily obscuredmaking spectroscopic observations difficult, and 2) that the intershellcompositions of AGB stars needs further investigation.

Is Extra Mixing Really Needed in Asymptotic Giant Branch Stars?