Hydrogen burning of sodium-23 in globular cluster red giant stars

Details Hydrogen burning of sodium-23 in globular cluster red giant stars Hydrogen burning of sodium-23 in globular cluster red giant stars

Oct 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003phdt........14r&link_type=abstract

Thesis (PhD). THE UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL, Source DAI-B 64/04, p. 1782, Oct 2003, 110 pages.

Physics

1

Scientific paper

The standard model of stellar evolution predicts constant surface abundances for globular cluster stars on the red giant branch. Contrary to these expectations, stars within a given globular cluster can show large variations in the abundance of C, N, and O as well as certain light metals, such as Na and Al. Also observed are global anticorrelations, such as that between O and Na. The question that rises is whether these variations are due to primordial differences in material that formed the cluster or from extra physics beyond the standard theory of stellar evolution, such as mixing. Answering this question requires reliable thermonuclear reaction rates. The 23Na(p,α)20Ne and 23Na(p,γ) 24Mg reactions are part of the Ne-Na cycle and are important for nucleosynthesis in globular cluster red giant stars. Both reactions are uncertain at astrophysically interesting temperatures, leading to uncertainties in the predicted abundances. For nuclear reactions which proceed through narrow resonances, the reaction rates are directly proportional to the resonance strengths. The absolute strength of the ELabR = 338 keV resonance in 23Na(p,α)20Ne was measured and used as a standard strength in determining the reaction rate. Prior to the present work, no standard strengths for this reaction were available. For the 23Na(p,γ)24Mg reaction, the uncertainty in the reaction rate comes from a resonance at EcmR = 144 keV. Traditionally this resonance has been difficult to measure because of low count rates combined with a large background. In the present work a technique for minimizing background via coincidences was developed. This technique was used to search for and obtain a much improved upper limit for the EcmR = 144 keV resonance. The upper limit in the present work is over an order of magnitude lower than the current literature value. Some astrophysical implications resulting from the new 23Na + p reaction rates are explored via a nuclear reaction network and stellar models.

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