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

TL;DR

This work addresses how uncertainties in the $^{22}$Ne$(\alpha,n)^{25}$Mg and $^{22}$Ne$(\alpha,\gamma)^{26}$Mg rates affect weak s-process nucleosynthesis in massive stars and the resulting Galactic chemical evolution. Using 280 massive-star models and OMEGA+-based GCE, the authors quantify how resonance strengths, particularly the $E_x=11.32$ MeV $\omega\gamma_{(\alpha,n)}$, drive variations in Cu, Zn, Ga, and Ge abundances, finding up to ~0.45 dex differences at solar metallicity. They perform resonance-sensitivity tests to show that constraining key resonances to within 10–20% substantially reduces GCE uncertainties to below observational dispersions, highlighting the pivotal role of targeted nuclear measurements at underground facilities. The results reinforce the importance of combined stellar-yield and GCE modeling for interpreting solar neighborhood abundances and guide future experimental efforts to refine neutron economy in massive-star s-process nucleosynthesis.

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.

Figures (20)

• Figure 1: Abundance profiles for relevant isotopes that help identify the different parts of the CCSN ejecta ($^1$H, $^4$He, $^{12}$C, $^{16}$O and $^{28}$Si), $^{63}$Cu ejected and after considering the radiogenic contributions ($^{63}$Cu*), are shown with respect to mass coordinate (mass fraction $X$ in y-axis) for the models of M$_{ZAMS}$=15M$_{\odot}$ at $Z$=0.02 (left panel) and $Z$=0.0001 (right panel). The relevant radiogenic parent species of $^{63}$Cu are also presented and plotted with dashed lines.
• Figure 2: The same as in Figure \ref{['fig:15_cu63']}, but for $^{65}$Cu.
• Figure 3: The same as in Figure \ref{['fig:15_cu63']}, but for $^{67}$Zn.
• Figure 4: The same as in Figure \ref{['fig:15_cu63']}, but for $^{68}$Zn.
• Figure 5: The same as in Figure \ref{['fig:15_cu63']}, but for $^{69}$Ga.