Impact of the latest 22Ne+α reaction rates on nucleosynthesis in massive stars and galactic chemical evolution

Abstract

In massive stars (initial mass of > 9 solar masses), the weak s (slow neutron capture) process produces elements between Fe and Zr, enriching the Galaxy with these elements through core-collapse supernova explosions. The weak s-process nucleosynthesis is driven by neutrons produced in the 22Ne(α, n)25Mg reaction during convective He-core and C-shell burning. The yields of heavy elements thus depend on the 22Ne(α, n)25Mg and the competitive 22Ne(α, γ)26Mg reaction rates which are dominated by several narrow-resonance reactions. While the accuracy of these rates has been under debate for decades, recent experimental efforts, including ours, drastically reduced these uncertainties. In this work, we use a set of 280 massive star nucleosynthesis models calculated using different 22Ne(α, n)25Mg and 22Ne(α, γ)26Mg rates, and a galactic chemical evolution (GCE) study to probe their impact on the weak s-process elemental abundances in the Galaxy. The GCE was computed with the OMEGA+ code, using the new sets of stellar yields with different 22Ne+α rates. From GCE, we find that these rates are causing up to 0.45 dex of variations in the [Cu/Fe], [Ga/Fe], and [Ge/Fe] ratios predicted at solar metallicity. The greatest impact on the stellar nucleosynthesis and GCE results derives from uncertainties in the (α,n) strength (ωγ(α,n)) of the Ex=11.32 MeV resonance. We show that the variations observed in the GCE calculations for the weak s-proess elements become negligibly smaller than dispersions found in observations once the ωγ(α,n) is accurately determined within the uncertaintiy of 10 to 20% (typically reported experimental errors for the resonance) in future nuclear physics experiments.

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