Re-evaluation of the $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg and $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction rates

TL;DR

This study re-evaluates key neutron-producing reactions in stars using recent experimental data, leading to revised reaction rates that improve agreement between stellar models and isotopic measurements in presolar grains.

Contribution

It provides updated reaction rates for $^{22}$Ne reactions based on new experimental data, refining stellar nucleosynthesis models.

Findings

Revised $^{22}$Ne($ alpha, gamma$)$^{26}$Mg reaction rate similar to previous evaluations.

New $^{22}$Ne($ alpha,n$)$^{25}$Mg rate is lower at relevant temperatures.

Updated rates lead to decreased weak s-process production and better match isotopic ratios in presolar grains.

Abstract

The competing $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg and $^{22}$Ne($\alpha,n$)$^{25}$Mg reactions control the production of neutrons for the weak $s$-process in massive and AGB stars. In both systems, the ratio between the corresponding reaction rates strongly impacts the total neutron budget and strongly influences the final nucleosynthesis. The $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg and $^{22}$Ne($\alpha,n$)$^{25}$Mg reaction rates was re-evaluated by using newly available information on $^{26}$Mg given by various recent experimental studies. Evaluations of The evaluated $^{22}$Ne($\alpha,\gamma$)$^{26}$Mg reaction rate remains substantially similar to that of Longland {\it et al.} but, including recent results from Texas A\&M, the $^{22}$Ne($\alpha,n$)$^{25}$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 $^{22}$Ne as a neutron source. Using the new reaction rates in the MESA model results in $^{96}$Zr/$^{94}$Zr and $^{135}$Ba/$^{136}$Ba ratios in much better agreement with the measured ratios from presolar SiC grains.