Additional resonant contribution to the potential model for the 12C(alpha,gamma)16O reaction

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The additional resonant contribution to the potential model is examined in α+¹²C elastic scattering and the low-energy ¹²C(α,γ)¹⁶O reaction. The excitation function of elastic scattering below Ec.ₘ.= 5 MeV seems to be reproduced by the potential model satisfactorily, and it is not profoundly disturbed by the additional resonances. The weak coupling is good enough to describe the ¹⁶O structure in the vicinity of the α-particle threshold, especially below Ec.ₘ.= 8 MeV, corresponding to the excitation energy Eₓ ≈ 15 MeV. The additional resonances give the complement of the astrophysical S-factors from the simple potential model. The S-factor of ¹²C(α,γ)¹⁶O at Ec.ₘ.=300 keV is dominated by the E2 transition, which is enhanced by the subthreshold 2+₁ state at Eₓ= 6.92 MeV. The contribution from the subthreshold 1-₁ state at Eₓ= 7.12 MeV is predicted to be small. The additional resonances do not give the large contribution to the thermonuclear reaction rates of ¹²C(α,γ)¹⁶O at helium burning temperatures.