Is Extra Mixing Really Needed in Asymptotic Giant Branch Stars?

TL;DR

This study shows that extra mixing in first giant branch stars can explain the observed chemical abundances in AGB stars, but its efficiency and effects vary with metallicity and require further investigation into intershell compositions.

Contribution

It demonstrates that the same extra mixing processes in FGB stars can account for AGB star abundances, highlighting the need to explore intershell compositions further.

Findings

Extra mixing explains low C/N and 12C/13C ratios in FGB and AGB stars.

Models match C abundances in C-rich stars and silicon carbide grains.

Increased 16O intershell abundance reproduces isotopic ratios in evolved stars.

Abstract

(Abridged) We demonstrate that the amount of extra mixing required to fit the observed low C/N and 12C/13C ratios in first giant branch (FGB) stars is also sufficient to explain the C and N abundances of Galactic AGB stars. We simulate the effect of extra mixing on the FGB by setting the composition of the envelope to that observed in low-mass FGB stars, and then evolve the models to the tip of the AGB. The inclusion of FGB extra mixing compositional changes has a strong effect on the C and N abundance in our AGB models, leading to compositions consistent with those measured in Galactic C-rich stars. The composition of the models is also consistent with C abundances measured in mainstream silicon carbide grains. While our models cover the range of C abundances measured in C stars in NGC 1846, we cannot simultaneously match the composition of the O and C-rich stars. Our models only match the O isotopic composition of K and some M, MS giants, and are not able to match the O composition of C-rich AGB stars. By increasing the 16O intershell abundance (based on observational evidence) it is possible to reproduce the observed trend of increasing 16O/18O and 16O/17O ratios with evolutionary phase. We conclude 1) if extra mixing occurs during the AGB it likely only occurs efficiently in low metallicity objects, or when the stars are heavily obscured making spectroscopic observations difficult, and 2) that the intershell compositions of AGB stars needs further investigation.