Reevaluation of the Ne22(α,γ)Mg26 and

Abstract

Background: The competing 22 Ne(α, γ) 26 Mg and 22 Ne(α, 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.Purpose: The 22 Ne(α, γ) 26 Mg and 22 Ne(α, n) 25 Mg reaction rates must be re-evaluated by using newly available information on 26 Mg given by various recent experimental studies. 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. Methods:The new nuclear data are evaluated and, where possible, correspondence between states observed in different experiments are made. 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 Ne(α, γ) 26 Mg reaction rate remains substantially similar to that of Longland et al. but, including recent results from Texas A&M, the 22 Ne(α, 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. Conclusion:The 22 Ne+α reaction rates 22 Ne(α, γ) 26 Mg and 22 Ne(α.n) 25 Mg have been recalculated based on more recent nuclear data. The 22 Ne(α, γ) 26 Mg reaction rate remains substantially unchanged since the previous evaluation but the 22 Ne(α.n) 25 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.

Isoscalar monopole and dipole transitions in Mg24 , Mg26 , and SiAdsley1, Nesterenko2, Kimura3 et al. 2021Phys. Rev. CSelf Cite9281View full textAdd to dashboardCite

“…unless stated otherwise b From Ref [56]. c See Ref [61]. for a discussion of the energy of this level.d Present experiment B.…”

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

“…A formalism to mimic in 1D the mixing induced by IGWs was developed and tested by [25], which was used by [26] to compute a grid of stellar evolution and fullnucleosynthesis models of low-mass AGB stars, with around solar metallicity and different initial masses. Furthermore, in this case, the majority of isotopic ratios from SiC presolar grains was well reproduced, except for 84 Sr/ 86 Sr and 137 Ba/ 138 Ba (the comparison with the measured 96 Zr/ 94 Zr was eventually much improved by adopting the new 22 Ne(α,n) 25 Mg reaction rate presented in [27], based on the recent measurement of [28]).…”

“…The nuclear reaction rates adopted are the same as in [26]. Exceptions relevant for this work are the 22 Ne(α,n) 25 Mg and the 22 Ne(α,γ) 26 Mg reaction rates, for which we use [27].…”

“…We also included the M ini = 3 M , Z = 0.001 model from RI18 in Table 2 as a reference. Notice that the full nucleosynthesis of the RI18 model has been recomputed, adopting the newest 22 Ne(α,n) 25 Mg and 22 Ne(α,γ) 26 Mg nuclear reaction rates from [27]. The grid of models presented explores the impact of different combinations of mixing parameters.…”

Slow White Dwarf Mergers as a New Galactic Source of Trans-Iron Elements

“…The CO WD model 0.927-from-6.0-z2m2.mod, included in the MESA data folder, was cooled down for 10 5 yr before being used as a starting model for our calculations. The chemical composition in mass fraction of the accreted material, added at a constant accretion rate Ṁ=2.4 10 −6 M ⊙ yr −1 , consisted of 0.48809 of 12 C, 0.48809 of 16 O, 0.002 of 20 Ne, 0.021 of 22 Ne, and the remaining 0.00082 in iron-group isotopes between 50 Cr and 64 Ni (with X( 56 Fe)=0.0007). The nuclear reaction network used in our simulations is co-burn-extras.net, which mainly includes the isotopes needed for carbon burning up to 28 Si coupled by more than 50 reactions.…”