The astrophysical S factor of 12C(α,γ)16O at stellar energy

Stellar models show an exceptional sensitivity on the cross section of 12C(α,γ)16O and a precision of about 10% is required to provide adequate constraints on stellar evolution. A measurement at the astrophysical energy, E0≈300 keV, is unfeasible due to the extremely low cross section. Furthermore, the extrapolation of existing high-energy data is complicated by a complex reaction scheme. Besides direct measurements of 12C(α,γ)16O, the β-delayed α-decay of 16N and 12C + 4He elastic scattering provide additional information for the relevant 16O levels. In this Letter we present a new R-matrix analysis, where systematic uncertainties of the included data sets, in particular the absolute normalization, were treated in the fitting procedure. The data were selected according to rigorous criteria in order to reduce uncontrolled systematic effects and, finally, a Monte Carlo approach was used to evaluate the uncertainty at astrophysical energy. The resulting S factor, S(300)=161±19stat−2sys+8 keVb, is, for the first time, close to the required precision.