THE ASTRONOMICAL JOURNAL    VOLUME 113, NUMBER 1, PAGE 279     JANUARY 1997

         PROTON CAPTURE CHAINS IN GLOBULAR CLUSTER STARS. II. OXYGEN,
           SODIUM, MAGNESIUM, AND ALUMINUM ABUNDANCES IN M13 GIANTS
                      BRIGHTER THAN THE HORIZONTAL BRANCH

                                ROBERT P. KRAFT

     UCO/Lick Observatory, Board of Studies in Astronomy and Astrophysics,
            University of California, Santa Cruz, California 95064

                              CHRISTOPHER SNEDEN

    McDonald Observatory and Department of Astronomy, University of Texas,
                              Austin, Texas 78712

                    GRAEME H. SMITH AND MATTHEW D. SHETRONE

     UCO/Lick Observatory, Board of Studies in Astronomy and Astrophysics,
            University of California, Santa Cruz, California 95064

                                 G. E. LANGER

   Department of Physics, Colorado College, Colorado Springs, Colorado 80903

                           CATHERINE A. PILACHOWSKI

     National Optical Astronomy Observatories, Tucson, Arizona 85726-6732

    Observations of 11 giants in the globular cluster M13 fainter than
    M_V=-1.7 have been obtained with the High Resolution Echelle
    Spectrograph on the Keck I Telescope. When combined with similar data
    for brighter giants obtained with the 3.0-m Shane Telescope's Hamilton
    Echelle spectrograph, these data allow examination of the detailed
    abundances of oxygen, sodium, magnesium, and aluminium over a wide
    range of luminosity on the red giant branch. We find that oxygen
    depletions, correlated with sodium enhancements, can be found in giants
    with luminosities down to nearly the level of the horizontal branch at
    M_V=0.3. Aluminum abundances in M13 giants also span a wide range (~1.4
    dex) and are anticorrelated with the observed magnesium
    abundances. However, field halo giants of comparable metallicity and
    evolutionary state exhibit (normal) high abundances of O and Mg and low
    abundances of Na and Al. Stars in M13 with high sodium and aluminum and
    low oxygen and magnesium abundances are found at all giant branch
    luminosities, as are stars with more normal light element abundances
    (as defined by field halo stars of comparable metallicity). The
    detailed abundance patterns of the light elements oxygen, sodium,
    magnesium, and aluminum can be explained most easily as a result of
    deep mixing and proton capture nucleosynthesis. Evidence in support of
    this explanation is (a) the constancy of the abundances of Al+Mg and
    C+N+O, (b) the shift in the distribution of sodium abundances toward
    higher values and of oxygen abundances toward lower values in cluster
    giants with surface gravities below log g = 1.0. The action of deep
    mixing, however, is seen not only at the top of the giant branch, but
    already in some giants even as faint as the level of the horizontal
    branch. (Copyright) 1996 American Astronomical Society.