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

Abstract

Carbon burning powers pivotal scenarios that influence the fate of stars, such as the late evolutionary stages of massive stars (exceeding eight solar masses), superbursts from accreting neutron stars and progenitors of Type Ia supernovae. 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, \( \alpha \) )²⁰Ne and ¹²C(¹²C, \( p \))²³Na— at temperatures greater than \( 0.4 \cdot 10^9 \) K, corresponding to astrophysical energies exceeding a megaelectronvolt (MeV), 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 never been measured below 2 MeV because of exponential suppression arising from the Coulomb barrier (the Coulomb barrier is around 6 MeV). The reference rate at temperatures below \( 1.2\cdot 10^9 \) K relies on extrapolations that ignore the effects of possible low-lying resonances. In Tumino et al. (2018), we report the measurement of the ¹²C(¹²C, \( \alpha_{0,1} \)) 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 MeV using the Trojan Horse method and the deuteron in ¹⁴N. This is an indirect technique aiming at measuring low-energy nuclear reactions unhindered by the Coulomb barrier and free of electron screening. The deduced cross-sections exhibit several resonances that are responsible for a very large increase of the reaction rate at the relevant temperatures. In particular, around \( 5\cdot 10^8 \) K, the reaction rate is more than 25 times larger than the reference value. This finding may have significant 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 in the direction to reconcile observations with theoretical models.