Proton capture chains in globular clusters. I.

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.