Metal-poor stars as metal-rich impostors via planet engulfment

Abstract

Planet engulfment is a common phenomenon that leads to the accretion of planetary mass and the deposition of orbital angular momentum into the stellar envelope. As the final stage of star--planet interactions, this event is of great significance for understanding the post-engulfment evolution of stars, searching for observational evidence of such events, and investigating the complex interplay between stars and planets. This study focuses on the long-term variations in stellar metallicity and surface rotational velocity following planet engulfment, as well as the impact of rotationally induced mixing on the degree of stellar metal enrichment. We utilized the MESA code to explore how different stellar and planetary properties influence the metallicity and rotation of the host star after an engulfment event. Our models show that engulfment events during the later main sequence (MS) stage result in significantly higher enrichment than those at the zero-age main sequence While rotational mixing generally reduces the enrichment magnitude, metal-poor stars can maintain super-solar abundances for several gigayears, whereas high-metallicity stars are nearly insensitive to rotation. While planetary engulfment induces an instantaneous spin-up, this angular momentum injection is transient, and the star quickly re-converges to its nominal rotational evolution track. Therefore, the rotational acceleration during the MS cannot serve as a long-term criterion. The intensity and duration of stellar metal enrichment are heavily influenced by the thickness of the stellar convective envelope and their MS lifetime. Stars with longer MS lifetimes tend to possess thicker convective envelopes. Our research indicates that searching for evidence of planetary engulfment involves a critical trade-off between the long-lived but diluted signals in 0.7 M_⊙ stars and the intense yet short-lived enrichment in 1.2 M_⊙ stars. In conclusion, 1.0,M_⊙ stars that have relatively long MS lifetimes and significant metal enrichment levels represent superior targets for observation.