Does an NeNa Cycle Exist in Explosive Hydrogen Burning?

Abstract

According to common assumptions, matter in the mass range A ≥ 20 is processed through the so-called NeNa cycle during hydrogen-burning nucleosynthesis. The existence of such a reaction cycle implies that the (p, α) reaction on 23Na is more likely to occur than the competing (p, γ) reaction. However, recently evaluated thermonuclear rates for both reactions carry relatively large uncertainties and allow for both possibilities, i.e., a and an open NeNa cycle. We measured the 23Na(p, γ)24Mg reaction at the Laboratory for Experimental Nuclear Astrophysics. The present experimental results, obtained with our sensitive γ-ray detection apparatus, reduce the 23Na + p reaction rate uncertainties significantly. We demonstrate that a closed NeNa cycle does not exist at stellar temperatures of T = 0.2-0.4 GK. The new results have important implications for the nucleosynthesis in classical novae, including the amount of 26Al ejected by the thermonuclear explosion, the elemental abundances of Mg and Al observed in nova shells, and observations of Mg and Al isotopic ratios in primitive meteorites.