Does an NeNa Cycle Exist in Explosive Hydrogen Burning?

Abstract

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

Nuclear astrophysics in the laboratory and in the universe

“…The resulting resonance strength is ωγ ≤ 5.17 × 10 −9 eV, a reduction of the previous upper limit 44 by a factor of 29. As a consequence, the NeNa cycle is essentially closed for temperatures below about T 9 = 0.1 and thus is decoupled from the MgAl region.…”

“…10. Monte Carlo-based reaction rates of 40 Ca(α,γ ) 44 Ti. For a better comparison, all rates are normalized to the recommended Monte Carlo rate.…”

“…10. The calculated experimental Monte Carlo-based 40 Ca(α,γ ) 44 Ti reaction rates are shown as a contour plot (black-red-yellow; see legend on the right), where the colors signify the coverage probability between any given rate boundaries. For a better comparison, all rates are normalized to the recommended Monte Carlo rate.…”

“…It is obvious that the new Monte Carlo rates are significantly different from previous results. The 40 Ca(α,γ ) 44 Ti reaction is crucial for the production of the γ -ray emitter 44 Ti in core-collapse supernovae and, therefore, we are planning to explore these new rates with new stellar model calculations in the near future.…”

Experimental study ofβ-delayed proton decay ofAl23for nucleosynthesis in novae

“…This leads eventually into the MgAl cycle and to a reduced abundance of 22 Na in the end products. The rates of these depleting reactions have been of considerable interest, and recent studies include 21 Na(p,γ) 22 Mg [6,7], 22 Mg(p,γ) 23 Al [8][9][10], 22 Na(p,γ) 23 Mg [11][12][13][14][15][16][17][18], 23 Na(p,γ) 24 Mg [19] and 23 Mg(p,γ) 24 Al [20]. It is believed that novae could become the first type of explosive process for which all the nuclear input to the nucleosynthesis calculations is based on experimental data [21].…”