Non-Resonant Reaction Rates of 13C(α,n)16O and 22Ne(α,n)25Mg reactions in AGB Stars

Abstract

Both 13C 16O and 22Ne 25Mg reactions perform a cosmic role in the production of neutrons in AGB stars, which significantly contributes to the nucleosynthesis via the s-process. The astrophysical S-factor for both reactions has been calculated in this research, utilizing EMPIRE code and depending on two parameter sets for the optical potential: McFadden and Satchler (MFS), and Avrigeanu and Hodgson (AH) for the non-resonant region of the spectrum, and over a temperature range . The extrapolated S-factor at zero energy is derived to be and for 13C 16O, and for 22Ne 25Mg from using MFS and AH parameter sets respectively, and they show a reasonable agreement with the most recommended value. The differences in the S-factor, S(E), values obtained from these two adopted parameters set, were attributed to the variations of the real potential term's diffuseness parameter that affecting the reaction cross section, hence S-factor, and specifically at low energy region. Moreover, the present results imply an influential enhancement of the rates by the electron shielding effect at the low-temperature region in which 13C 16O reaction activated, and especially on 22Ne 25Mg reaction. In addition, for both adopted reactions and overall selected temperature range, the reaction rates using values based on MFS showed acceptable results with other previous compilations and reference libraries. While that obtained from AH exceed all other compilations even though the resonance contributions are currently unconsidered.

Impact of Neutron Sources of Stellar Reactions on Fluorine Abundances

“…In Figure (3) the present results of reaction rates are shown, compared with those of NACRE II [21] experimental library as well as theoretical calculations of Ali and Selman [7]. Again, there have been no modifications used in the calculations of the reaction rates; therefore, it can be seen that the present results differ from that of the literature in the range 1 ≤ 𝑇 9 ≤ 8.…”

“…2, pp: 984-993 in the light mass number region. Only 7 Li isotope in this family is thought to be produced during the Big Bang [1]. However, among other processes, 19 F can be generated in massive stars during the branch of the slow neutron capture, or the s-process.…”

“…The low abundance of 19 F isotope is thought to be due to the limited sites where it is produced in the universe. Being the rarest amongst light elements in the universe, 19 F is possibly generated from three different mechanisms, namely [2]: (i)Type II Supernovae through the reaction Ne(𝜈, 𝜈 ′ 𝑝) F 19 20 (neutrino spallation), (ii) in Asymptotic Giant Branch (AGB) stars inside the He burning shell from the reaction N(𝛼, 𝛾) F(𝛽 + 𝑣) O(𝑝, 𝛼) N(𝛼, 𝛾) F [6,7]. Neutron sources, in return, are sensitive to many parameters, one of which is the optical model potential parameter.…”

“…To eliminate the strong dependence of 𝜎 on the entrance energy E (or temperature T), it is usual [7] to use the Astrophysical S-Factor using the transformation 𝐸𝜎(𝐸) = 𝑆(𝐸) exp[−2𝜋𝜂 (𝐸)] where [10],…”

“…In the range E=0.6 to 1.4 MeV the results were in a good agreement with experimental data taking into account that no modifications have been applied to the program's input, i.e., all the input of EMIRE was the default except energy, mass, and atomic numbers. However, few possible modifications could be made [7]. Ali [20] made a few modifications to better improve these results.…”