An increase in the 12C + 12C fusion rate from resonances at astrophysical energies

Abstract

Carbon burning powers scenarios that influence the fate of stars, such as the late evolutionary stages of massive stars (exceeding eight solar masses) and superbursts from accreting neutron stars. It proceeds through the C +C fusion reactions that produce an alpha particle and neon-20 or a proton and sodium-23-that is, C(C, α)Ne and C(C, p)Na-at temperatures greater than 0.4 × 10 kelvin, corresponding to astrophysical energies exceeding a megaelectronvolt, at which such nuclear reactions are more likely to occur in stars. The cross-sections for those carbon fusion reactions (probabilities that are required to calculate the rate of the reactions) have hitherto not been measured at the Gamow peaks below 2 megaelectronvolts because of exponential suppression arising from the Coulomb barrier. The reference rate at temperatures below 1.2 × 10 kelvin relies on extrapolations that ignore the effects of possible low-lying resonances. Here we report the measurement of the C(C, α)Ne and C(C, p)Na reaction rates (where the subscripts 0 and 1 stand for the ground and first excited states of Ne andNa, respectively) at centre-of-mass energies from 2.7 to 0.8 megaelectronvolts using the Trojan Horse method and the deuteron in N. The cross-sections deduced exhibit several resonances that are responsible for very large increases of the reaction rate at relevant temperatures. In particular, around 5 × 10 kelvin, the reaction rate is boosted to more than 25 times larger than the reference value . This finding may have implications such as lowering the temperatures and densities required for the ignition of carbon burning in massive stars and decreasing the superburst ignition depth in accreting neutron stars to reconcile observations with theoretical models .

Status on $$^{\mathbf {12}}\mathbf {C}+{}^{\mathbf {12}}{\mathbf {C}}$$ fusion at deep subbarrier energies: impact of resonances on astrophysical $$S^{*}$$ factors

“…But a more serious problem with the THM data is that the mechanisms contributing to the THM data at E > 2. 16 MeV are not identified in [8]. From Figs.…”

“…Moreover, the trend of the THM data at higher energies E > 2.1 MeV causes the main concern because such a trend cannot be explained by the THM mechanism and can be a result of the contribution of the non-THM mechanisms (see [20] and discussion in Section III). As we know, in the THM experiment [8] only the energy dependence of the S * factor was measured and its absolute value was determined by the normalization of the THM data to the direct ones in the energy interval 2.50 − 2.63 MeV. Then, based on the data shown in Fig.…”

“…Then, based on the data shown in Fig. 2 we calculated the χ 2 test of the indirect THM [8] and direct data from [5,7] taking into account 19 points in the energy interval between E = 2.16 and 2.7 MeV. The uncertainty of the THM data is ±21% (see caption to Fig.…”

“…The criticism of the THM in this Letter is not aimed to taint the whole method which demonstrated its power in more than hundred publications. We critically review only the analysis of the data in [8].…”

“…Recently the Trojan Horse Method (THM) has been applied to determine the 12 C+ 12 C fusion S * -factor at energies below E = 2.7 MeV [8]. The THM S * -factor shows a profound rise in both α− and p− channels as energy E decreases, which is much faster than any data and models presented in Fig.…”