Reevaluation of the Ne22(α,γ)Mg26 and Ne22(α,n)Mg25 reaction rates

Abstract

Background: The competing $^{22}\mathrm{Ne}(\ensuremath{\alpha},\ensuremath{\gamma})^{26}\mathrm{Mg}$ and $^{22}\mathrm{Ne}(\ensuremath{\alpha},n)^{25}\mathrm{Mg}$ reactions control the production of neutrons for the weak $s$ process in massive and asymptotic giant branch (AGB) stars. In both systems, the ratio between the corresponding reaction rates strongly impacts the total neutron budget and strongly influences the final nucleosynthesis. A number of experimental studies have been performed over recent years which necessitate the reevaluation of the $^{22}\mathrm{Ne}(\ensuremath{\alpha},\ensuremath{\gamma})^{26}\mathrm{Mg}$ and $^{22}\mathrm{Ne}(\ensuremath{\alpha},n)^{25}\mathrm{Mg}$ reaction rates. Evaluations of the reaction rates following the collection of new nuclear data presently show differences of up to a factor of 500, resulting in considerable uncertainty in the resulting nucleosynthesis.Purpose: To reevaluate the $^{22}\mathrm{Ne}(\ensuremath{\alpha},\ensuremath{\gamma})^{26}\mathrm{Mg}$ and $^{22}\mathrm{Ne}(\ensuremath{\alpha},n)^{25}\mathrm{Mg}$ reaction rates using updated nuclear data from a number of sources including updating spin and parity assignments.Methods: With updated spin and parity assignments, the levels which can contribute to the reaction rates are identified. The reaction rates are computed using a Monte Carlo method which has been used for previous evaluations of the reaction rates in order to focus solely on the changes due to modified nuclear data.Results: The evaluated $^{22}\mathrm{Ne}(\ensuremath{\alpha},\ensuremath{\gamma})^{26}\mathrm{Mg}$ reaction rate remains substantially similar to that of Longland et al. but, including recent results from Texas A, the $^{22}\mathrm{Ne}(\ensuremath{\alpha},n)^{25}\mathrm{Mg}$ reaction rate is lower at a range of astrophysically important temperatures. Stellar models computed with newton and mesa predict decreased production of the weak branch $s$ process due to the decreased efficiency of $^{22}\mathrm{Ne}$ as a neutron source. Using the new reaction rates in the mesa model results in $^{96}\mathrm{Zr}/^{94}\mathrm{Zr}$ and $^{135}\mathrm{Ba}/^{136}\mathrm{Ba}$ ratios in much better agreement with the measured ratios from presolar SiC grains.Conclusion: The $^{22}\mathrm{Ne}+\phantom{\rule{0.16em}{0ex}}\ensuremath{\alpha}$ reaction rates $^{22}\mathrm{Ne}(\ensuremath{\alpha},\ensuremath{\gamma})^{26}\mathrm{Mg}$ and $^{22}\mathrm{Ne}(\ensuremath{\alpha}.n)^{25}\mathrm{Mg}$ have been recalculated based on more recent nuclear data. The $^{22}\mathrm{Ne}(\ensuremath{\alpha},\ensuremath{\gamma})^{26}\mathrm{Mg}$ reaction rate remains substantially unchanged since the previous evaluation but the $^{22}\mathrm{Ne}(\ensuremath{\alpha}.n)^{25}\mathrm{Mg}$ reaction rate is substantially decreased due to updated nuclear data. This results in significant changes to the nucleosynthesis in the weak branch of the $s$ process.