Looking for Companionship: Radial Velocity Follow-Up of Lithium-Rich Giants with ESPRESSO

TL;DR

This paper addresses the origin of lithium enrichment in red giants by testing the binary fraction among Li-rich giants using ESPRESSO radial-velocity observations. The authors apply the The Joker sampler to constrain orbital parameters and derive companion masses, identifying one planet, three brown dwarfs, and five stellar-mass companions among 33 Li-rich giants, for an overall companion rate of $27\%$ (9/33). They find trends suggesting binary prevalence at lower lithium abundances and within the $\log g$ range $2$–$3$, and discuss two formation scenarios: (i) lithium production via mass transfer from an intermediate-mass AGB companion, and (ii) tidal interactions from sub-stellar companions that induce Li production; Gaia DR4 astrometry is needed to confirm or distinguish these mechanisms. The study highlights the potential role of companions in Li enrichment and lays groundwork for future multi-epoch and Gaia-enabled analyses.

Abstract

Lithium-rich red giants have been a long-standing mystery in stellar astrophysics. A leading theory to explain these chemically peculiar and rare objects is interactions with a close companion. To investigate their companion fraction, we collected high-resolution spectra of 33 Li-rich red giants using ESPRESSO, and used The Joker constrain their orbital parameters. We find an overall companion rate of $27\%$ (9/33). Secondary masses reveal one planetary companion ($ M\sin i \approx 7 \; \rm M_{Jup}$), three brown dwarfs ($ M\sin i=30-33 \; \rm M_{Jup}$), and five stellar-mass companions ($M\sin i= 0.2-0.8 \;\rm M_\odot$). Our findings suggest that Li-rich red giants with lower lithium abundance ($\rm A(Li) \approx 1.5 \; dex$) tend to be in binaries as compared to those with higher lithium abundance, and Li-rich red giants with $\log g = 2-3 \rm \; dex$ have a higher companion rate than those outside of this range. We offer two potential formation mechanisms of our Li-rich sample: (i) the progenitor mass of stellar mass companions suggest that these objects were potentially lithium-producing, intermediate-mass AGB stars; (ii) the sub-stellar companions were initially in multi-planet systems, but dynamical instability caused the tidal dissipation of close-in planets thereby enhancing the red giant in lithium. Extended baselines and dedicated follow-up with Gaia DR4 astrometry are required to confirm the orbital parameters of our systems and distinguish between mechanisms.

Figures (2)

• Figure 1: Kiels diagram showing the full sample of Li-rich red giants from sayeed_2024 in red and ESPRESSO target sample from this work in gold overlaid on the full GALAH sample (with good quality flags) for reference. Our ESPRESSO target sample covers the overall distribution of the reference Li-rich sample along the red giant branch.
• Figure 2: $A\textrm{(Li)}$ of our ESPRESSO targets as a function of broadening velocity coloured by their surface gravity, $\log g$. The complete sample of Li-rich sample in GALAH from S24 is shown as a density distribution for reference. Our ESPRESSO sample is diverse, spanning evolutionary state, lithium abundance, and broadening velocity.