THE EVOLUTION OF THE LITHIUM ABUNDANCES OF SOLAR-TYPE STARS .2. THE URSA MAJOR GROUP

We draw upon a recent study of the membership of the Ursa Major Group (UMaG) to examine lithium among 0.3 Gyr old solar-type stars. For most G and K dwarfs, Li confirms the conclusions about membership in UMaG reached on the basis of kinematics and chromospheric activity. G and K dwarfs in UMaG have less Li than comparable stars in the Pleiades. This indicates that G and K dwarfs undergo Li depletion while they are on the main sequence, in addition to any pre-main-sequence depletion they may have experienced. Moreover, the Li abundances of the Pleiades K dwarfs cannot be attributed to main-sequence depletion alone, demonstrating that pre-main-sequence depletion of Li also takes place. The Sun's Li abundance implies that the main-sequence mechanism becomes less effective with age, otherwise the Sun would have even less Li than it does. The hottest stars in UMaG (T(eff) greater than or similar to 6800 K) have Li abundances like those of hot stars in the Pleiades and Hyades and in T Tauris, and the two genuine UMaG members with temperatures near Boesgaard's Li chasm have Li abundances consistent with that chasm developing fully by 0.3 Gyr for stars with UMaG's metallicity ([Fe/H] = - 0.08 +/- 0.09). We see differences in the abundance of Li between UMaG members of the same spectral types, indicating that a real spread in the lithium abundance exists within this group.