Lithium in the low-mass stars of the Coma Berenices open cluster

Abstract

ABSTRA C T I report lithium abundances and radial velocities for 15 late F, G and K stars in the Coma Berenices open cluster. All the stars are proper motion members, and the radial velocities enable us to confirm membership for 11 single stars and identify three SB1 systems and one short-period SB2 system that are very probable cluster members. The pattern of Li abundances is similar to that in the Hyades, with some evidence for higher Li abundances in Coma Ber for Teff < 5700 K. I argue that these data do not support standard models for Li depletion involving only convective mixing, because the Coma Ber stars have consistently determined lower metallicities and shallower convection zones than the Hyades and should have suffered a great deal less Li depletion than observed. Instead, it seems that the Li abundances in these and other young clusters are primarily ranked according to their age rather than their metallicity. I propose a scenario where the extreme metallicity dependence of pre-main-sequence Li depletion is tempered by a mechanism that severely inhibits convective mixing, where continued main-sequence depletion occurs via non-standard mixing mechanisms, and where any dependence of Li depletion on metallicity takes at least several hundred Myr to emerge.