Lithium in stars more massive than ∼6 M⊙ is expected to be totally depleted by the time when the stars reach the He core burning phase, as a consequence of rotationally induced mixing and other mechanisms. Some massive AGB stars present very high lithium abundances, a phenomenon explained by the activation of the Cameron-Fowler mechanism during the early thermal-pulse phase. An unexpected result of our ongoing survey of moderately young open clusters, aimed at covering stellar evolution for stars in the 6 to 10 M⊙ range, is the high fraction of red (super)giants with strong Li lines. We find many more objects than expected under the assumption that the fraction of Li-rich giants is similar to that seen in the field (dominated by solar-type stars). Moreover, the stars with high lithium abundances are always the coolest and brightest in the respective clusters. Although the interpretation of these detections is not straightforward, they hint at a very early activation of the lithium production mechanism in stars in this mass range.
Memorie della Societa Astronomica Italiana
- Pub Date:
- stars: evolution;
- stars: supergiants;
- stars: fundamental parameters;
- open clusters and associations;
- Hertzsprung-Russell and colour-magnitude diagrams