Presentation #108.01 in the session Poster Presentations.
Spectroscopic studies of wide binary systems have revealed a substantial population where one star is enriched in refractory elements by up to ~0.1 dex relative to its companion. This has been tentatively attributed to the engulfment of a planet by the enriched star in each system. Chemical enrichment is observed in a large fraction (~30%) of systems, suggesting that planet engulfment is common. This is difficult to explain using standard models of dynamical evolution: simple hypotheses, such as planet-planet scattering in a ‘Kepler multi’ system or the Lidov-Kozai effect driven by an outer companion, fail to predict the observed prevalence of chemically enriched stars. We propose a scenario in which the engulfment originates from the tidal decay of a rocky ultra-short-period planet (USP). We construct a toy model of a population of planet-hosting Sun-like stars in which USPs form via gradual inward migration under external secular forcing, undergo rapid tidal decay at short orbital periods (<1 day), and enrich the stellar atmosphere when engulfed. Using that model, we show that one can simultaneously reproduce the observed fraction of stars with USPs (~1%) and the fraction with chemical enrichment at a typical age of a few Gyr. This implies constraints on both the average rate of USP formation per star and the average lifetime of an individual USP. We compare these constraints to predictions from existing models of USP migration and tidal evolution. In contrast with some previous studies, our results suggest that USPs can (and often do) undergo tidal decay during the main-sequence lifetimes of their host stars.