Long-term lithium abundance signatures following planetary engulfment

Abstract

Planetary engulfment events can occur while host stars are on the main sequence. The addition of rocky planetary material during engulfment will lead to refractory abundance enhancements in the host star photosphere, but the level of enrichment and its duration will depend on mixing processes that occur within the stellar interior, such as convection, diffusion, and thermohaline mixing. We examine engulfment signatures by modelling the evolution of photospheric lithium abundances. Because lithium can be burned before or after the engulfment event, it produces unique signatures that vary with time and host star type. Using MESA stellar models, we quantify the strength and duration of these signatures following the engulfment of a 1, 10, or 100 M_⊕ planetary companion with bulk Earth composition, for solar-metallicity host stars with masses ranging from 0.5 to 1.4 M_⊙. We find that lithium is quickly depleted via burning in low-mass host stars (≾ 0.7 M_⊙) on a time-scale of a few hundred Myrs, but significant lithium enrichment signatures can last for Gyrs in G-type stars (~ 0.9 M_⊙). For more massive stars (1.3−1.4 M_⊙), engulfment can enhance internal mixing and diffusion processes, potentially decreasing the surface lithium abundance. Our predicted signatures from exoplanet engulfment are consistent with observed lithium-rich solar-type stars and abundance enhancements in chemically inhomogeneous binary stars.