The first 3D magnetohydrodynamic core-collapse progenitors. I. General properties, convection, and nuclear burning

The most energetic core-collapse supernovae are thought to arise from rapidly rotating, magnetised progenitors, yet the 3D structure of their pre-collapse interior remains poorly constrained, and realistic distributions of magnetic fields, angular momentum, and convective asphericities are still lacking. We constructed physically consistent 3D pre-supernova progenitors, from the stellar core to the helium shell, including rotation and magnetic fields. In this first paper, we focus on the behaviour of turbulence and nuclear burning in the shells surrounding the stellar core. We assess their deviations from 1D stellar-evolution models. The magnetorotational properties of the progenitors are discussed in the second paper of this series. We used to perform 3D magnetohydrodynamic (MHD) simulations of two compact Wolf--Rayet progenitors obtained from the stellar evolution codes and The models were mapped onto the multidimensional domain several minutes before collapse and evolved until the onset of core collapse. We find that in extended oxygen-burning shells, turbulent velocities exceed the standard mixing-length-theory (MLT) predictions by approximately a factor of 2. In contrast, a thin silicon-burning shell is poorly described by MLT: mixing is reduced near both shell boundaries, and the inferred effective diffusion profile departs significantly from the standard 1D prescription. These differences directly affect the spatial extent and efficiency of nuclear burning. In one of our models, a late shell merger present in the 1D stellar-evolution calculation does not occur in the 3D MHD evolution, indicating that such events may be sensitive to the specific modelling framework adopted. We present the first 3D MHD pre-supernova progenitors of this kind, suitable for subsequent collapse and explosion calculations. We show that multidimensional effects can significantly modify turbulent mixing and shell burning during the final stages of massive-star evolution. We propose prescriptions to account for these effects in the advanced phases of stellar evolution.