Observational Constraints on the Origin of the Aluminum Enhancements and Magnesium Depletions in Bright M13 Giants

Abstract

The most recent observations of the abundances of oxygen, sodium, magnesium, and aluminum provide strong support for the idea that "deep mixing" occurs within Ml3 giants. In deep-mixing scenerios, 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(BAAS, 27, 1995 AJ, 112, 2639 AJ, 112, , 1996 estimates of magnesium isotope ratios provide especially powerful constraints on the reshuffling temperature for the brightest aluminum-rich Ml3 giants. We find that proton-capture nucleosynthesis can give an excellent account of the abundances of aluminum and the isotopes of magnesium in the brightest aluminum-rich giants in Ml3-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 ~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 Ml3 giants rather than steadily (at lower 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-0 abundance anticorrelation observed in globular-cluster giants and the Na-N correlation observed in field halo giants.

Na-O anticorrelation and HB

“…In currently evolving low-mass stars the CNO processing shell only reaches its maximum temperature of about 70 MK very near the tip of the RGB (e.g. Langer et al 1996), while we observe stars still far from this magnitude range and yet able to show the Si-Al correlation in their photospheric abundances.…”

Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields

“…In the same vein, granulation effects are anticipated to have a limited impact on the strength of the Na i lines used (Collet et al 2007). The MgAl cycle operates above such high temperatures (∼7 × 10 7 K; Langer et al 1997) that Al is not expected to be produced at the expense of Mg prior to the AGB phase. The variation of the Al abundance within our sample is comparable Variations of some key abundance ratios across the logT eff -log g plane.…”

On OH line formation and oxygen abundances in metal-poor stars

“…Furthermore, concerns regarding the primordial nature of the oxygen in field giants have been voiced as mixing of nuclear-processed material may pollute the surface, as evident in many globular cluster giants (e.g. Langer et al 1997). The O i triplet on the other hand is more easily discerned in metal-poor dwarfs but also more susceptible to departures from LTE (cf.…”