Impact of the latest 22Ne+α reaction rates on nucleosynthesis in massive stars and galactic chemical evolution
Dr. Shuya Ota
National Nuclear Data Center
Brookhaven National Laboratory
In massive stars (initial mass of ≳ 9 M⊙), 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 [1,2], drastically reduced these uncertainties. In this work, we provide a set of 280 massive star nucleosynthesis models calculated using different 22Ne(α,n)25Mg and 22Ne(α,γ)26Mg rates published in the last few years, and perform a galactic chemical evolution (GCE) study to probe their impact on the weak s-process elemental abundances in the Galaxy [3]. The nucleosynthesis was calculated using the NuGrid’s MPPNP code [4] to produce the new sets of stellar yields, which were then used in the GCE simulations with the OMEGA+ code [5]. From GCE, we find that the different 22Ne+a rates cause 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-process elements become negligibly smaller than dispersions found in observations once the ωγ(α,n) is accurately determined within the uncertainty of 10–20% (typically reported experimental errors for the resonance) in future nuclear physics experiments.
[1] S. Ota et al., Phys. Lett. B 802, 135256 (2020), [2] S. Ota et al., Phys. Rev. C 104, 055806 (2021), [3] E. Kotar, S. Ota, et al., Astrophys. J. (submitted), [4] M. Pignatari et al., Astrophys. J. Suppl. 225, 24 (2016), [5] B. Cote et al., Astrophys. J. 835, 128 (2017).
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