Many Roads Lead to Lithium: Formation Pathways For Lithium-rich Red Giants

Abstract

Stellar models predict that lithium (Li) inside a star is destroyed during the first dredge-up phase, yet 1.2% of red giant stars are Li-rich. We aim to uncover possible origins of this population, by analyzing 1099 Li-rich giants ( A (Li) ≥ 1.5) in GALAH DR3. To expose peculiar traits of Li-rich stars, we construct a reference sample of Li-normal (doppelgänger) stars with matched evolutionary state and fiducial iron-peak and alpha-process abundances ([Fe/H] and [Mg/Fe]). Comparing Li-rich and doppelgänger spectra reveals systematic differences in the H α and Ca-triplet line profiles associated with the velocity broadening measurement. We also find twice as many Li-rich stars appear to be fast rotators (2% with v broad ≳ 20 km s −1 ) compared to doppelgängers. On average, Li-rich stars have higher abundances than their doppelgängers, for a subset of elements, and Li-rich stars at the base of RGB have higher mean s -process abundances (≥0.05 dex for Ba, Y, Zr), relative to their doppelgängers. External mass-transfer from intermediate -mass AGB companions could explain this signature. Additional companion analysis excludes binaries with mass ratios ≳0.5 at ≳7 au. Finally, we confirm a prevalence of Li-rich stars on the red clump that increases with lithium, which supports an evolutionary state mechanism for Li-enhancement. Multiple culprits, including binary spin-up and mass-transfer, are therefore likely mechanisms of Li-enrichment.