Microscopic Estimation of Nuclear Reaction Rate in Astrophysical Phenomena*

Nuclear reactions are the energy source for various astrophysical phenomena: 12C+12C and 12C+16O fusion reactions play a significant role in explosive astrophysical phenomena such as X-ray bursts and superbursts, and type Ia supernovae, where nuclear reaction rates determine recurrence times and the abundance of the produced elements.

The astrophysical is low energy for nuclear physics, and the contribution of resonances to nuclear reactions is significant. Direct measurement of low-energy resonances by accelerator experiments is challenging because of the tiny cross section due to the thick Coulomb barrier. Therefore, experimentally determined reaction rates at low temperatures are uncertain and need to be estimated by theory. However, the accuracy of theoretical evaluations is also still insufficient. In particular, heavy-ion fusion reactions are multinucleon rearrangement reactions, and it is necessary to clarify the properties of the resonance state, treating the effects of entrance and exit channel coupling. However, in the current situation where general coupling potentials have not been developed, it is not possible to treat the coupling phenomenologically.

We use a microscopic model, antisymmetrized molecular dynamics, to evaluate the nuclear reaction rate. Using the microscopic model, we can, in principle, treat the coupling of any channels from the nucleon-nucleon interactions. By treating the couplings of the entrance and exit channels and deformed structures using the generator coordinate method, the wave functions of the resonances are calculated. Then, their contribution to the fusion reaction is evaluated from the properties of the resonances.

In 24Mg, the resonances are obtained near the threshold of 12C+12C. They are essentially 12C+12C molecular resonance states, and the fragments that appear due to the coupling with the exit channels, a+20Ne and p+23Na channels, are distributed close to the threshold. They increase the 12C+12C fusion reaction rate in the typical temperature of X-ray superbursts. In 28Si, 12C+16O resonances are obtained close to the threshold through coupling with 12C+16O, a+24Mg, and other channels.