THE ASTRONOMICAL JOURNAL    VOLUME 112, NUMBER 2, PAGE 545          AUGUST 1996

     PROTON CAPTURE CHAINS IN GLOBULAR CLUSTER STARS. I. EVIDENCE FOR DEEP
         MIXING BASED ON SODIUM AND MAGNESIUM ABUNDANCES IN M13 GIANTS

                           CATHERINE A. PILACHOWSKI

   National Optical Astronomy Observatories, Kitt Peak National Observatory,
                  P.O. Box 26732, Tucson, Arizona 85726-6732
                        Electronic mail: catyp@noao.edu

                              CHRISTOPHER SNEDEN

    McDonald Observatory and Department of Astronomy, University of Texas,
      Austin, Texas 78712, and National Optical Astronomy Observatories,
  Kitt Peak National Observatory, P.O. Box 26732, Tucson, Arizona, 85726-6732
                   Electronic mail: chris@verdi.as.texas.edu

                                ROBERT P. KRAFT

     UCO/Lick Observatory, Board of Studies in Astronomy and Astrophysics,
            University of California, Santa Cruz, California 95064
                      Electronic mail: kraft@ucolick.org

                                 G. E. LANGER

   Department of Physics, Colorado College, Colorado Springs, Colorado 80903
               Electronic mail: elanger@academic.cc.colorado.edu


                                   ABSTRACT

    Sodium abundances have been derived for 130 giant stars in the globular
    cluster M13 from spectra of the Na I doublet at lambda 5682 and 5685 A
    obtained using the KPNO 4-m Hydra fiber positioner and bench
    spectrograph.  Magnesium abundances have also been obtained for the
    brightest 98 stars in the sample from the nearby Mg I line at lambda
    5711 A.  The stars observed in M13 range from the faintest at Mv ~ +1.0
    and log g ~ 2.6 up to the tip of the giant branch, and include 18 stars
    on the asymptotic giant branch.  Among the lower luminosity giants, the
    sodium abundances have a large star-to-star range, approximately from
    -0.3 <= [Na/Fe] <= +0.5.  However, the sodium abundances of the most
    luminous giants (Mv < -1.7) are usually high; typically, [Na/Fe] >=
    +0.3, with a much smaller star-to-star scatter.  The asymptotic giant
    branch stars have smaller sodium abundances on average than do the red
    giant branch tip stars.  The spread in [Na/Fe] ratios is larger in M13
    than it is among halo field giants of comparable metallicity; M13
    contains many more stars with high [Na/Fe] ratios than can be found in
    the field, even at relatively low luminosities on the giant branch.
    Magnesium is uniformly overabundant ([Mg/Fe] ~ +0.3) in all stars with
    a low sodium abundance, but the [Mg/Fe] ratio ranges from approximately
    -0.3 to +0.3 in stars with a high sodium abundance.  These sodium and
    magnesium abundance variations in M13 are discussed in the context of
    proton capture and deep mixing hypotheses.  In addition to the CN and
    ON hydrogen burning chains previously discussed in the literature, the
    NeNa and MgAl burning chains have also contributed to the abundance
    mixture observed in M13 giants.  At least some of the products of
    proton capture chains have been produced in situ in the giants, and
    brought to the surface, most probably via deep mixing.  Evidence in
    support of the occurrence of proton capture nucleosynthesis and deep
    mixing among M13 stars includes (1) the absence of sodium-poor stars at
    the red giant tip, (2) the fact that asymptotic branch stars have lower
    sodium abundances on average than do stars near the red giant tip, and
    (3) the existence of a positive correlation between sodium and nitrogen
    abundances as well as a partial anti-correlation of sodium and
    magnesium abundances.