The 12 C + 12 C fusion reaction at astrophysical energies

The fusion of two carbon nuclei is a key reaction during the late evolution of massive stars, in explosive nucleosynthesis in supernovae and superbursts in x-ray binary systems. The precise experimental determination of the ultra low carbon fusion cross sections is extremely challenging so that the quantification of this critical reaction still lacks the necessary accuracy to constrain astrophysical models. The STELlar LAboratory (STELLA) experiment has been developed to increase the accuracy of direct carbon fusion reaction measurements, as compared to conventional experiments, by using the coincident detection of the evaporation residues characteristic gamma rays and the emitted charged light particles, which drastically suppresses the backgrounds. STELLA furthermore combines nano seconds timing with this approach for unambigious exit channel identification with timing gates of tens of nanoseconds. 12 C+ 12 C measurements have been performed at the Andromede facility in Orsay, France using the STELLA setup, right in the astrophysics region of interest of 25 solar masses stars. These data complement an earlier experiment at the lowest-energy direct measurement carried out so far, where partly only limits could be established, and largely improves the understanding of the fusion excitation function. The results are discussed in terms of molecular resonances in the 24 Mg compound nucleus as well as hindrance of the fusion process at the lowest energies. The impact of the STELLA results on the chemical structure and evolution scenarios of massive stars are discussed, based on hydrodynamics calculations.