THE ASTRONOMICAL JOURNAL      VOLUME 105, NUMBER 6, PAGE 2299         JUNE 1993

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

                              DAVID R. SODERBLOM

     Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
                                Maryland 21218
                     Electronic mail: soderblom@stsci.edu

                           CATHERINE A. PILACHOWSKI

         Kitt Peak Observatory, P.O. Box 26732, Tucson, Arizona 85726
                        Electronic mail: catyp@noao.edu

                               STEPHEN B. FEDELE

     Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
                                Maryland 21218
                       Electronic mail: fedele@stsci.edu

                                BURTON F. JONES

Lick Observatory, Board of Studies of Astronomy and Astrophysics, University of
                   California, Santa Cruz, California 95064
                    Electronic mail: jones@helios.ucsc.edu


                                   ABSTRACT

    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 the UMaG (T(eff) >~ 6800K) 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.