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Session 26 - Neutrinos & Stellar Abundances.
Oral session, Wednesday, January 07
Observations of globular cluster red giant branch (RGB) stars have shown star-to-star variations in the abundances of C, N, O, Na, Mg, and Al, contrary to predictions of standard stellar evolutionary theory. I have modeled the variations in the abundance profiles around the hydrogen-burning shell (H shell) of metal-poor red giant stars by combining four RGB stellar evolutionary sequences of different metallicities with a detailed nuclear reaction network. This approach has significant advantages over previous research: it allows for the variation in the temperature and density around the H shell; (2) it follows the effects of the changing H-shell structure as the sequence evolves; (3) it accounts for the effect of the metallicity on the abundance profiles; (4) it allows the reaction rates to be varied so that their uncertainties may be explored.
The results are in good qualitative agreement with the observations. All the models show a region above the H shell in which first C, then O, is depleted in the CN and ON nuclear burning cycles. Within the C-depleted region, the ^12C/^13C ratio is reduced to its equilibrium value. Just above the O-depleted region, Na is enhanced from proton captures on ^22Ne. In brighter models, Na becomes greatly enhanced within the O-depleted region as the NeNa cycle converts ^20Ne into ^23Na before attaining equilibrium inside the H shell. The more metal-poor models also show Al being increased around the H shell, first from ^25,26Mg, then from ^24Mg in the MgAl cycle. Despite the diminution ^24Mg suffers in synthesizing Al, the models show its abundance is increased due to the NeNa-cycle breakout reaction, ^23Na(p,\gamma)^24Mg. This latter result is at odds with observations that show ^24Mg is depleted in a sample of M 13 and NGC 6752 giants (Shetrone 1996, 1997).
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