Context. Nuclear reactions drive stellar evolution and contribute to stellar and galactic chemical abundances. New determinations of the nuclear reaction rates in key fusion reactions of stellar evolution are now available, paving the way for improved stellar model predictions. Aims. We explore the impact of new 12 C + 12 C reaction rates in massive star evolution, structure, and nucleosynthesis at carbon ignition and during the core carbon-burning phase. We analyse the consequences for stars of different masses including rotation-induced mixing. Methods. We computed a grid of massive stars from 8 to 30 M ⊙ at solar metallicity using the stellar evolution code GENEC, and including the new reaction rates. We explored the results using three different references for the rates, with or without rotation. We studied the effect in terms of evolution, structure, and the critical mass limit between intermediate and massive stars. We explored the consequences for heavy-element nucleosynthesis during the core carbon-burning phase by means of a one-zone nucleosynthesis code. Results. We confirm the significant impact of using the recent nuclear reaction rates following the fusion suppression hypothesis at deep sub-barrier energies (hindrance hypothesis) as well as the mass-dependent effect of a resonance at 2.14 MeV with dominant feeding of the α exit channel of 12 C + 12 C fusion reaction. This impacts the characteristics of the core of stars from the C-ignition and during the entire core C-burning phase (temperature and density, lifetime, size, convective or radiative core). The change in nuclear reaction rates modifies the central nucleosynthesis of the stars during the core-carbon burning phase, resulting in an underproduction of s -process elements, especially when including the rotation-induced mixing that amplifies the effects. Conclusions. The correct and accurate determination of the nuclear reaction rates, especially with the existence and location of resonances, impacts stellar evolution in many respects, affecting models’ predictions. The choice of the nuclear reaction rates reference for the 12 C + 12 C fusion reaction significantly changes the behaviour of the core during the carbon-burning phase, and consequently drives changes in the nucleosynthesis and end-of-life of stars. This choice needs, then, to be made carefully in order to interpret stellar evolution from the super asymptotic giant branch phase and its massive white dwarf remnants to the core-collapse supernovae of massive stars.

The measurement of C-12+C-12 at astrophysical energies is mandatory to well understand stellar evolution. First, fusion hindrance has been observed in most medium-heavy fusion systems, but the effect on light-medium systems is still unclear. Second, the presence of resonances in the C-12+C-12 fusion reaction, that can be a strong indication of molecular states in the Mg-24, can also have an impact on our understanding of stellar evolution of massive stars. The precise measurements of these reaction cross sections at deep sub-barrier energies are highly challenging, as the cross section is at a sub-nanobarn level with a dominating background. To overcome this challenge, the STELLA (STELla LAboratory) experiment combined with the UK-FATIMA (FAst TIMing Array) allows for a coincident measurement of gammas and charged particles from the fusion reaction to efficiently suppress the background and achieve precise measurements at deep sub -barriers energies in the astrophysical region of interest. The first experimental campaign in 2016/2017 revealed hints of hindrance and a potential resonance at low energies. This contribution will discuss data from the 2019 experimental campaign and give details about the experimental setup.

Fusion reactions with light nuclei play an essential role in understanding the energy production, the nucleosynthesis of chemical elements and the evolution of massive stars. The measurement of key fusion reactions at stellar energies is thus of interest, but highly challenging since the associated cross sections are extremely small, of the sub-nanobarn range. Among these reactions, the fusion of carbon nuclei, which drives the stellar carbon burning phase, is deeply connected with essential microscopic features such as the impact of symmetries, the access to quantum states, emerging of resonances or Pauli repulsion effects. These may manifest themselves in exceptional behaviour of the S-factor of this particular system and the precision of extrapolations to deep sub-barrier energies is limited. The present contribution discusses recent experimental results of the nuclear astrophysics community on the measurement of the carbon + carbon fusion reaction down to the astrophysics region. The interplay between nuclear structure, nucleosynthesis and stellar evolution is addressed.

Among the reactions driving stellar evolution during carbon burning, 12C + 12C fusion provides the key ingredients. This system reveals many resonances, but also regions with suppressed fusion cross-sections. The reaction was recently measured by the STELLA collaboration utilizing the gammaparticle coincidence technique for precise cross-section measurements reaching down to the Gamow window of massive stars. From the experimental data, reaction rates were determined by approximating a hindrance parametrization and by adding on top a resonance at the lowest measured energy. The impact of these reaction rates on the evolution of massive stars was explored with models of 12 and 25 M⊙ using the stellar evolution code GENEC, and a detailed study of the resulting nucleosynthesis with a 1454 elements network was performed. The sensitivity of the STELLA experimental cross-sections on the temperature range for C-burning for the stellar models studied were presented. The final abundances and their impacts on stellar evolution were discussed.

We present 12C+12C direct fusion measurements with STELLA UKFATIMA, that reach into the region of astrophysics interest relevant to massive stars (M⊙ ≈ 25) using self-supporting thin rotating carbon foils [1]. We demonstrate that detecting gammas and light charged particles in coincidence with nanosecond timing is key for effective background reduction achieving reliable measurements in the sub-nanobarn range. We give details about core developments of the detection apparatus as well as the coincidence-analysis procedure of low count statistics. The present data largely follows the phenomenological hindrance interpolation and shows indication for resonant behaviour at the lowest energy explored.