Formation of Large Circumstellar Discs in Multi-scale, ideal-MHD Simulations of Magnetically Critical Pre-stellar Cores

Abstract: The formation of circumstellar discs is a critical step in the formation of stars and planets. Magnetic fields can strongly affect the evolution of angular momentum during prestellar core collapse, potentially leading to the failure of protostellar disc formation. This phenomenon, known as the magnetic braking catastrophe, has been observed in ideal-MHD simulations. In this work, we present results from ideal-MHD simulations of circumstellar disc formation from realistic initial conditions of strongly magnetised, massive cores with masses between $30 ~{\rm M}\odot$ and $300 ~{\rm M}\odot$ resolved by zooming into Giant Molecular Clouds with masses $\sim 10^4 \ {\rm M}_\odot$ and initial mass-to-flux ratios $0.6 \le \mu_0 \le 3$. Due to the large turbulence caused by the non-axisymmetric gravitational collapse of the gas, the dominant vertical support of discs is turbulent motion, while magnetic and turbulent pressures contribute equally in the outer toroid. The magnetic field topology is extremely turbulent and incoherent, reducing the effect of magnetic braking by roughly one order of magnitude and leading to the formation of large Keplerian discs even in magnetically critical or near-critical cores. Only cores in GMCs with $\mu_0 < 1$ fail to form discs. Instead, they collapse into a sheet-like structure and produce numerous low-mass stars. We also discuss a universal $B-\rho$ relation valid over a large range of scales from the GMC to massive cores, irrespective of the GMC magnetisation. This study differs from the vast literature on this topic which typically focus on smaller mass discs with idealised initial and boundary conditions, therefore providing insights into the initial conditions of massive prestellar core collapse and disc formation.