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

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 $27\mathrm{\%}$ (9/33). Secondary masses reveal one planetary companion ($M\sin i\approx 7{\mathrm{M}}_{\mathrm{J}\mathrm{u}\mathrm{p}}$), three brown dwarfs ($M\sin i=30-33{\mathrm{M}}_{\mathrm{J}\mathrm{u}\mathrm{p}}$), and five stellar-mass companions ($M\sin i=0.2-0.8{\mathrm{M}}_{\odot }$). Our findings suggest that Li-rich red giants with lower lithium abundance ($\mathrm{A}(\mathrm{L}\mathrm{i})\approx 1.5\mathrm{d}\mathrm{e}\mathrm{x}$) tend to be in binaries as compared to those with higher lithium abundance, and Li-rich red giants with $\log g=2-3\mathrm{d}\mathrm{e}\mathrm{x}$ 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.