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

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 12 C + 12 C fusion reactions that produce an alpha particle and neon-20 or a proton and sodium-23—that is, 12 C( 12 C, α) 20 Ne and 12 C( 12 C, p ) 23 Na—at temperatures greater than 0.4 × 10 9 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 × 10 9 kelvin relies on extrapolations that ignore the effects of possible low-lying resonances. Here we report the measurement of the 12 C( 12 C, α 0,1 ) 20 Ne and 12 C( 12 C, p 0,1 ) 23 Na reaction rates (where the subscripts 0 and 1 stand for the ground and first excited states of 20 Ne and 23 Na, respectively) at centre-of-mass energies from 2.7 to 0.8 megaelectronvolts using the Trojan Horse method 6 , 7 and the deuteron in 14 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 8 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 . The rate of carbon burning— 12 C + 12 C fusion—in stars is boosted by resonant behaviour at astrophysical energies.