Probing Binary Architectures of Lithium-rich Giants in GALAH with COSMIC and Stellar Models

Surface lithium is depleted when a star goes through the first dredge-up phase, yet 1% of red giants are found to be Li rich. The formation mechanism for these remains uncertain. We combine observational constraints from Galactic Archaeology with HERMES ( GALAH ) Li-rich giants, with the binary population synthesis software Compact Object Synthesis and Monte Carlo Investigation Code ( COSMIC) to investigate the system properties of these objects assuming binary mass transfer. By evolving nine million binary systems, we find that binary histories most consistent with the observational constraints are mass transfer from an intermediate-mass asymptotic giant branch (AGB) donor to a main-sequence (MS) star now observed as a Li-rich red giant. In GALAH , 9% of MS stars have A (Li) = 2.5–3.2 dex making it plausible to create red giants with A (Li) = 1.5–2.2 dex via MS mass transfer, but they cannot explain the more enriched giants with A (Li) ≳ 2.2 dex. Nucleosynthetic yields from stellar models show that AGB stars with initial masses of 4.25–5 M ⊙ and 8 M ⊙ contain the most lithium in their ejecta. Intermediate-mass AGB stars comprise 29% of the COSMIC results, with present-day separations s = 3.3 ± 0.5 au and mass ratios q = 0.5–1.6. We achieve 95% agreement in mean enhancements in Ba and Y between the GALAH observations and stellar models of 6 and 8 M ⊙ AGB stars, assuming a 1% mass transfer efficiency. We find a low mass transfer efficiency best reproduces the GALAH observations suggesting that the preferred mass transfer mechanism for Li enrichment is via wind Roche-lobe overflow. While we constrain the most plausible binary parameters assuming AGB mass transfer creates Li-rich giants, discrepancies in the nucleosynthesis comparisons and the small fraction of Li-enhanced MS stars suggests additional enrichment mechanisms are likely.