Hot CNO-Ne Cycle Hydrogen Burning. Thermonuclear Evolution at Constant Temperature and Density

Abstract

The relative concentrations of nuclei achieved in hydrogen burning on carbon, nitrogen, and oxygen at high temperatures are calculated as a function of time. Calculations have been performed for static burning conditions, for temperatures in the range 0.1 < T9 1 and densities 1 p < 10 g cm - 3. The Ne-Na cycle also operates under these high-temperature conditions. A network comprising 28 nuclei from 12C to 25Mg, related by 60 (p, y), (p, a), (a, y), (a, n), and positron decay reactions, has been used in this study. At higher temperatures the (p, y) and (p, a) thermonuclear reaction rates are typically much faster than the positron decay rates, quite in contrast to the situation existing under the usual CNO bi-cycle burning conditions in stellar cores. This distinguishing feature leads to nucleosynthesis which differs significantly from that calculated at lower temperatures. Specifically, large enhancements of nuclei of masses A = 15, 17, 18, 19, 21, and 22 are found to occur at various stages of the burning at constant temperature (the stable nuclei on these isobars being 15N, 170, O, 19F, 21Ne, and 22Ne). Under some conditions, 13C/12C ratios exceeding unity are achieved, in contrast to the ratio 13C/12C = 0.29 characteristic of equilibrium in the CNO bi-cycle at lower temperatures, T < 70 million degrees. The physical conditions determined to be most promising for nucleosynthesis provide insight as to possible sites for the formation of many of these lighter isotopes in astrophysical environments. Subject headings: interiors, stellar - massive stars - nuclear reactions - abundances