Reevaluation of the Ne-23(alpha, gamma)Mg-26 and Ne-22(alpha, n)Mg-25 reaction

Abstract

Background: The competing 22Ne(a,?)26Mg and 22Ne(a,n)25Mg 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 22Ne(a,?)26Mg and 22Ne(a,n)25Mg 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 22Ne(a,?)26Mg and 22Ne(a,n)25Mg 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 22Ne(a,?)26Mg reaction rate remains substantially similar to that of Longland et al. but, including recent results from Texas A&M, the 22Ne(a,n)25Mg 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 22Ne as a neutron source. Using the new reaction rates in the mesa model results in 96Zr/94Zr and 135Ba/136Ba ratios in much better agreement with the measured ratios from presolar SiC grains.

Conclusion: The 22Ne+a reaction rates 22Ne(a,?)26Mg and 22Ne(a.n)25Mg have been recalculated based on more recent nuclear data. The 22Ne(a,?)26Mg reaction rate remains substantially unchanged since the previous evaluation but the 22Ne(a.n)25Mg 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.