News from Low Mass Star Nucleosynthesis and Mixing

TL;DR

This paper reviews evidence of non-convective mixing in low-mass stars during RGB and AGB phases, highlighting its role in nucleosynthesis, the increasing abundance of neutron-rich nuclei, and the significance of very low mass stars for Galactic chemical evolution.

Contribution

It provides a synthesis of observational data and discusses the necessity of deep mixing mechanisms in low-mass stars, challenging traditional diffusive models and emphasizing their role in Galactic nucleosynthesis.

Findings

Deep mixing occurs in RGB and AGB stars.

Neutron-rich nuclei abundances are increasing in the Galaxy.

Very low mass stars are increasingly important for nucleosynthesis.

Abstract

Light and intermediate nuclei as well as s-process elements have been detected in presolar grains and in evolved red giants. The abundances of some of these nuclei cannot be accounted for by canonical stellar models and require non-convective mixing below the envelope, occurring during the phases of the Red Giant Branch (RGB) and of the Asymptotic Giant Branch (AGB). Similar mechanisms appear to be necessary to account for the formation of the neutron source driving s processing. We present a short review of these phenomena and we comment on the picture that emerges from the set of available data on the evolution and nucleosynthesis in low mass stars. Our conclusions include: i) the need for deep mixing in both RGB and AGB stars; ii) the suggestion that these phenomena occur at a non-negligible velocity, possibly incompatible with diffusive processes; iii) the verification that the abundances of neutron-rich nuclei are presently increasing in the Galaxy, contrary to previous expectations and hence that the s process has new surprises to offer us; iv) the recognition of the growing importance of very low mass stars for Galactic nucleosynthesis.