Proton capture chains in globular cluster stars .2. Oxygen, sodium, magnesium, and aluminum abundances in M13 giants brighter than the horizontal branch

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 (similar to 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. (C) 1996 American Astronomical Society.