Abundances in Globular Cluster Red Giant Stars

Details Abundances in Globular Cluster Red Giant Stars Abundances in Globular Cluster Red Giant Stars

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...191.2603c&link_type=abstract

American Astronomical Society, 191st AAS Meeting, #26.03; Bulletin of the American Astronomical Society, Vol. 29, p.1253

Astronomy and Astrophysics

Astronomy

1

Scientific paper

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: (1) 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 (12) C/(13) C ratio is reduced to its equilibrium value. Just above the O-depleted region, Na is enhanced from proton captures on (22) Ne. In brighter models, Na becomes greatly enhanced within the O-depleted region as the NeNa cycle converts (20) Ne into (23) Na before attaining equilibrium inside the H shell. The more metal-poor models also show Al being increased around the H shell, first from (25,26) Mg, then from (24) Mg in the MgAl cycle. Despite the diminution (24) Mg suffers in synthesizing Al, the models show its abundance is increased due to the NeNa-cycle breakout reaction, (23) Na(p,γ)(24) Mg. This latter result is at odds with observations that show (24) Mg is depleted in a sample of M 13 and NGC 6752 giants (Shetrone 1996, 1997).

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