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

• Published in: The Astrophysical journal 2024-03, Vol.964 (1), p.42
• Main Authors: Sayeed, Maryum, Ness, Melissa K., Montet, Benjamin T., Cantiello, Matteo, Casey, Andrew R., Buder, Sven, Bedell, Megan, Breivik, Katelyn, Metzger, Brian D., Martell, Sarah L., McGee-Gold, Leah
• Format: Article
• Language: English
• Subjects: Abundance; Astronomical models; Asymptotic giant branch stars; Binary stars; Chemical abundances; Companion stars; Giant stars; H alpha line; Iron; Lithium; Lithium stars; Mass ratios; Mass transfer; Red giant stars; Stars; Stellar evolution; Stellar models; Zirconium
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad1936

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.