Lithium abundance dispersion in the Pleiades and M 34

Abstract

Context. K-type stars of similar effective temperatures in clusters younger than about 250 Myr are characterised by a wide dispersion in lithium abundance whose origin is not understood. Aims. The present study addresses this question in light of rotation period measurements recently conducted in the Pleiades and M 34 open clusters. Methods. The study investigates the dependence of lithium abundance with effective temperature distinguishing stars that belong to different rotation sequences in diagrams where measured rotation periods are plotted against B − V colour indices. Results. At any given effective temperature below 5500 K, stars on the fast rotator sequence in the Pleiades and M 34 have significantly higher lithium abundances that stars with same masses and ages located on the slow rotator sequence. Conclusions. This correlation suggests that shear-induced turbulence at the base of the convection zone accelerates the depletion of lithium in rapidly rotating late G- and K-type stars during a brief episode of strong rotational braking by stellar wind in their early evolution on the main sequence. The epoch at which this strong rotational braking occurs does not depend on stellar mass only but also on the rotation history of each individual star. I argue that the large dispersion in surface lithium abundance of K stars in the Pleiades and M 34 could be a consequence of this phenomenon.