Dig Sites of Stellar Archeology: Giant Stars in the Milky Way

Elemental abundances are the fingerprints of the stellar evolution; they provide critical information about the history of Galactic chemical evolution. Low and intermediate mass stars play the most important role in Galactic chemical enrichment and depending on many parameters such as initial mass, metallicity, and mass loss rate at red giant phase, age, helium abundance and rotation, they follow different paths on the HR-diagram.

Evolved giants (RHB and Red Clump stars), especially of lower masses than 2.5 M, are the best laboratories to investigate the extra-mixing mechanisms observed right after the luminosity function bump of red giant branch. Carbon and nitrogen (C/N) and 12C/13C ratios are the best indicators of deep mixing. The results show different ratios depending on the initial mass, metallicity and the stellar evolutionary stages. However, unambiguous relationships among these parameters have not been found yet. New observational and theoretical efforts are needed to unveil the relation between extra-mixing processes and the observed elemental abundances. Such extra-mixing is not explained by standard stellar evolution and therefore brings new challenges in theoretical studies.

Other evolutionary stages that witness drastic chemical changes in stellar atmospheres are AGB and post-AGB. Such stars are of critical importance to understand the last evolutionary stages of low and intermediate mass stars. Because of their fairly fast evolution, not many are known to date; these stars are still largely shrouded in mystery. A major influence of the third dredge-up phenomenon takes place in the post-AGB atmospheres which mainly alters the chemical abundance of 13C at the surface. This phenomenon also triggers the slow neutron-capture reactions (the s-process) in the stellar interiors. Some post-AGB stars are known to be enriched by the s-process elements, although some others do not show a single sign. The variety of the Galactic post-AGB population especially in chemical domain makes them even more difficult objects to investigate. The linkage between the post-AGBs enriched and non-enriched by certain elements remains unknown.

Giant stars are the ideal objects to dig down the ancient history of the Milky Way. The aim of this workshop is to bring up the current struggles and new challenges in both observational and theoretical studies on chemical evolution of the evolved stars, and discuss the new observational and theoretical approaches to improve our understanding on the Galactic chemical evolution.

Topics will include:

  • What happens after main sequence: the evolution of low and intermediate mass stars
  • Abundance alterations in red giants: extra-mixing processes
  • He-core burning stars in the field and stellar clusters: RHB vs. Red Clumps
  • AGB and Post-AGB stars: the third dredge-up and the complex atmospheres