High lithium-7 (7Li) abundances in giants are indicative of nonstandard physical processes affecting the star. Mechanisms that could produce this signature include contamination from an external source, such as planets, or internal production and subsequent mixing to the stellar surface. However, distinguishing between different families of solutions has proven challenging, and there is no current consensus model that explains all the data. The lithium-6 (6Li) abundance may be a potentially important discriminant, as the relative 6Li and 7Li abundances are expected to be different if the enrichment were to come from internal production or from engulfment. In this work, we model the 6Li and 7Li abundances of different giants after the engulfment of a substellar mass companion. Given that 6Li is more strongly affected by Galactic chemical evolution than 7Li, 6Li is not a good discriminant at low metallicities, where it is expected to be low in both star and planet. For modeled metallicities ([Fe/H] > −0.5), we use a “best-case” initial 6Li/7Li ratio equal to the solar value. 6Li increases significantly after the engulfment of a companion. However, at metallicities close to solar and higher, the 6Li signal does not last long in the stellar surface. As such, detection of surface 6Li in metal-rich red giants would most likely indicate the action of a mechanism for 6Li enrichment other than planet engulfment. At the same time, 6Li should not be used to reject the hypothesis of engulfment in a 7Li-enriched giant or to support a particular 7Li-enhancement mechanism.
