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

TL;DR

This study investigates the binary fraction of lithium-rich red giants using high-resolution spectra, revealing diverse companions and suggesting potential formation mechanisms linked to stellar evolution and planetary interactions.

Contribution

First detailed radial velocity follow-up of lithium-rich giants with ESPRESSO, providing new insights into their companion types and formation scenarios.

Findings

27% of Li-rich giants have companions, including planets, brown dwarfs, and stars.

Li-rich giants with lower lithium abundance are more often in binaries.

Higher binary rate observed in giants with specific surface gravity ranges.

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.