Assessing the processes behind planet engulfment and its imprints

Context. Newly formed stars are surrounded by a protoplanetary disc composed of gas and dust, part of which ends up forming planets. During the system’s evolution, some of the planetary material may end up falling into the host star and being engulfed by it, leading to potential variation in the stellar composition. Aims. The present study explores how planet engulfment may impact the chemical composition of the stellar surface and discusses what the rate of events with an observable imprint would be for Sun-like stars. Methods. We used data on the formation and evolution of 1000 planetary systems from the New Generation Planetary Population Synthesis (NGPPS) calculations by the Generation III Bern model to analyse the conditions under which planet engulfment may occur. Additionally, we used stellar models computed with Cesam2k20 (Code d’Evolution Stellaire Adaptatif et Modulaire) to account for how the stellar internal structure and its processes may affect the dilution of the signal caused by planet engulfment. Results. Our results show that there are three different phases associated with different mechanisms under which engulfment events may happen. Moreover, systems that undergo planet engulfment are more likely to come from protoplanetary discs that are more massive and more metal-rich than non-engulfing systems. Engulfment events leading to an observable signal happen after the dissipation of the protoplanetary disc when the convective envelope of the stars becomes thinner. With the stellar convective layer shrinking as the star evolves in the main sequence, they display a higher variation of chemical composition. This variation also correlates with the amount of engulfed material. By accounting for the physical processes happening in the stellar interior and in the optimistic case of being able to detect variations above 0.02 dex in the stellar composition, we find an engulfment rate no higher than 20% for Sun-like stars that may reveal detectable traces of planet engulfment. Conclusions. Engulfment events that lead to observable variation in the stellar composition are rare due to the specific conditions required to result in such signatures.