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arXiv · 2306.03323

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

Maryum Sayeed·Melissa K. Ness·Benjamin T. Montet·Matteo Cantiello·Andrew R. Casey·Sven Buder·Megan Bedell·Katelyn Breivik·Brian D. Metzger·Sarah L. Martell·Leah McGee-Gold
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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 analysing 1155 Li-rich giants (A(Li) $\geq$ 1.5) in GALAH DR3. To expose peculiar traits of Li-rich stars, we construct a reference sample of Li-normal (doppelg\"anger) stars with matched evolutionary state and fiducial supernova abundances. Comparing Li-rich and doppelg\"anger spectra reveals systematic differences in the H-$\alpha$ 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 $v_\textrm{broad} \gtrsim 20$ km s$^{-1}$) compared to doppelg\"angers. On average, Li-rich stars have higher abundances than their doppelg\"angers, for a subset of elements, and Li-rich stars at the base of RGB have higher mean $s-$process abundances ($\geq 0.05$ dex for Ba, Y, Zr), relative to their doppelg\"angers. External mass-transfer from intermediate-mass AGB companions could explain this signature. Additional companion analysis excludes binaries with mass ratios $\gtrsim$ 0.5 at $\gtrsim$ 7 AU. We also discover that highly Ba-enriched stars are missing from the Li-rich population, possibly due to low-mass AGB companions which preclude Li-enrichment. 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.

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Maryum Sayeed, Melissa K. Ness, Benjamin T. Montet, Matteo Cantiello, Andrew R. Casey, Sven Buder, Megan Bedell, Katelyn Breivik, Brian D. Metzger, Sarah L. Martell, Leah McGee-Gold. 2023-06-06. Many Roads Lead to Lithium: Formation Pathways For Lithium-Rich Red Giants. https://doi.org/10.3847/1538-4357%2Fad1936

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