2025MNRAS.543.3321M

We present simulations of the supernova-driven turbulent interstellar medium (ISM) in a simulation domain of volume $(256\, {\rm pc})^3$ within which we resolve the formation of protostellar accretion discs and their stellar cores to spatial scales of $\sim 10^{-4}$ au, using the moving-mesh code AREPO. We perform simulations with no magnetic fields, ideal magnetohydrodynamics (MHD) and ambipolar diffusion, and compare the resulting first Larson cores and their associated structures, including the accretion discs, their location within the larger-scale structure and the streamers connecting these. We find that discs of sizes $10{\small --}100\, {\rm au}$ form early in the simulations without magnetic fields, while there are no discs larger than 10 au with ideal MHD. Ambipolar diffusion causes large discs to form in a subset of cases (two out of six cores), and generally reduces the strength of outflows, which are seen to play a central role. When they are able to carry away significant angular momentum, they prevent the formation of a rotationally supported disc. Magnetic fields strengths grow from $0.1 {\small --} 1$ mG in the protostellar core to more than 10 G in the first Larson core in all simulations with ideal MHD. The rotationally supported discs which form can have rotation speeds $> 1$ km s$^{-1}$ even out to further than 100 au from the centre, become gravitationally unstable and form complex spiral substructures with Toomre $Q < 1$. We conclude that the impact of magnetic fields and non-ideal MHD on the formation of protostellar discs is substantial in realistic formation scenarios from the turbulent ISM.