The cosmic evolution of lithium-7 (Li) remains one of the most fascinating challenges in astrophysics due to its complex origins and its behaviour in different astrophysical environments. Li is unique among light elements, with an abundance influenced by primordial nucleosynthesis, cosmic rays, and an as-yet unidentified stellar source. This doctoral thesis aims to untangle the complex processes that govern the production and depletion of Li over cosmic time, providing new insights into stellar and Galactic evolution.
I conducted several studies addressing different aspects of the cosmic evolution of Li. The first study focused on the re-evaluation of large samples of field stars to determine the upper envelope of Li abundance in high-metallicity stars in the Milky Way. This work highlighted the importance of selection effects in previous analyses and provided new constraints for Galactic evolution models, revealing that high-metallicity stars can retain higher Li abundances than previously thought, particularly in hot dwarf stars.
The second study examined young dwarf and giant stars with extreme [α/Fe] ratios, discovering a new population of stars, which we named "y-exαfe" (young stars with extreme [α/Fe] ratios). These stars displayed Li abundances similar to those of young, [α/Fe]-normal stars, suggesting that they may have formed during recent star formation events, potentially triggered by interactions between the Milky Way and the Sagittarius dwarf galaxy.
In the third study, I developed and utilized advanced stellar models that incorporate sophisticated magnetohydrodynamic processes to explain the observed Li plateau in Population II stars. This research provided a coherent explanation for Li depletion patterns observed in different stellar populations, revealing the influence of stellar internal structure and angular momentum transport mechanisms on Li abundance.
The fourth study explored the role of novae in enriching the Galactic disc with Li. By integrating recent theoretical yields and delay-time distributions into a multi-zone Galactic chemical evolution model, I quantified the contribution of novae to the overall Li abundance in the Milky Way. The results indicated that novae could account for a significant portion of the meteoritic Li observed today, alongside contributions from primordial nucleosynthesis and cosmic rays.
Overall, this thesis provides a comprehensive overview of the cosmic evolution of Li, offering new theoretical and observational perspectives that advance our understanding of stellar and Galactic processes. These results contribute to the broader field of astrophysics by shedding light on the complex interplay between stellar evolution, nucleosynthesis, and Galactic chemical evolution.
