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 ¹ (exceeding eight solar masses) and superbursts from accreting neutron stars^2,3. It proceeds through the ¹²C + ¹²C fusion reactions that produce an alpha particle and neon-20 or a proton and sodium-23-that is, ¹²C(¹²C, α)²⁰Ne and ¹²C(¹²C, p)²³Na-at temperatures greater than 0.4 × 10⁹ kelvin, corresponding to astrophysical energies exceeding a megaelectronvolt, at which such nuclear reactions are more likely to occur in stars. The cross-sections ⁴ 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 ⁴ below 2 megaelectronvolts because of exponential suppression arising from the Coulomb barrier. The reference rate ⁵ at temperatures below 1.2 × 10⁹ kelvin relies on extrapolations that ignore the effects of possible low-lying resonances. Here we report the measurement of the ¹²C(¹²C, α_0,1)²⁰Ne and ¹²C(¹²C, p_0,1)²³Na reaction rates (where the subscripts 0 and 1 stand for the ground and first excited states of ²⁰Ne and ²³Na, respectively) at centre-of-mass energies from 2.7 to 0.8 megaelectronvolts using the Trojan Horse method^6,7 and the deuteron in ¹⁴N. 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 × 10⁸ kelvin, the reaction rate is boosted to more than 25 times larger than the reference value ⁵ . This finding may have implications such as lowering the temperatures and densities ⁸ 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 ³ .