On Lithium-Rich Red Giants. II. Engulfment on the Giant Branch of Trumpler 20

Abstract: The Gaia-ESO survey recently reported on a large sample of lithium (Li) abundance determinations for evolved stars in the rich open cluster Trumpler 20. They argue for a scenario where virtually all stars experience post main sequence mixing and Li is preserved in only two objects. We present an alternate explanation, where Li is normal in the vast majority of cluster stars and anomalously high in these two cases. We demonstrate that the Li upper limits in the red giants can be explained with a combination of main sequence depletion and standard dredge-up, and that they are close to the detected levels in other systems of similar age. In our framework, the two detected giants are anomalously Li-rich, and we propose that both could have been produced by the engulfment of a substellar mass companion of 16(+6-10) M_J. This would imply that ~5% of 1.8 solar mass stars in this system, and by extension elsewhere, should have substellar mass companions of high mass that could be engulfed at some point in their lifetimes. We discuss future tests that could confirm or refute this scenario.