Observational constraints on the origin of the aluminum enhancements and magnesium depletions in bright M13 giants

The most recent observations of the abundances of oxygen, sodium, magnesium, and aluminum provide strong support for the idea that ''deep mixing'' occurs within M13 giants. In deep-mixing sceneries, material from the envelopes of globular-cluster giants is assumed to be mixed well into the hydrogen-burning shells of some (but not all) stars. There the abundances of a number of light elements are reshuffled by proton capture before the material is returned to the envelope above. The recent observations also put strong constraints on the conditions under which the reshuffling occurs. Shetrone's (BAAS, 27, 1432 1995; AJ, 112, 2639, 1996) estimates of magnesium isotope ratios provide especially powerful constraints on the reshuffling temperature for the brightest aluminum-rich M13 giants. We find that proton-capture nucleosynthesis can five an excellent account of the abundances of aluminum and the isotopes of magnesium in the brightest aluminum-rich Wants in M13-and of the abundances of oxygen and sodium as well. In order to match the observations, the abundances in about ninety percent of the material that makes up the envelopes of these stars would have to have been reshuffled by proton capture at a temperature near 70 million kelvins (MK). The maximum temperatures in the hydrogen shells of canonical models for bright M13 giants is only similar to 55 MK. We briefly discuss ways in which the observations and the models might be reconciled. Our ''best bet'' is that hydrogen burning occurs intermittently (at higher temperatures) in some M13 giants rather than steadily (at fewer temperatures) as it does in canonical models. We also find that recent rates for the Ne-Na cycle reactions (El Eid and Champagne, ApJ, 451, 298, 1995) suggest solutions to two earlier puzzles: the shape of the Na-O abundance anticorrelation observed in globular-cluster giants and the Na-N correlation observed in field halo giants.