Disc formation in magnetized dense cores with turbulence and ambipolar diffusion

Discs are essential to the formation of both stars and planets, but how they form in magnetized molecular cloud cores remains debated. This work focuses on how the disc formation is affected by turbulence and ambipolar diffusion (AD), both separately and in combination, with an emphasis on the protostellar mass accretion phase of star formation. We find that a relatively strong, sonic turbulence on the core scale strongly warps but does not completely disrupt the well-known magnetically induced flattened pseudo-disc that dominates the inner protostellar accretion flow in the laminar case, in agreement with previous work. The turbulence enables the formation of a relatively large disc at early times with or without AD, but such a disc remains strongly magnetized and does not persist to the end of our simulation unless a relatively strong AD is also present. The AD-enabled discs in laminar simulations tend to fragment gravitationally. The disc fragmentation is suppressed by initial turbulence. The AD facilitates the disc formation and survival by reducing the field strength in the circumstellar region through magnetic flux redistribution and by making the field lines there less pinched azimuthally, especially at late times. We conclude that turbulence and AD complement each other in promoting disc formation. The discs formed in our simulations inherit a rather strong magnetic field from its parental core, with a typical plasma-β of order a few tens or smaller, which is 2-3 orders of magnitude lower than the values commonly adopted in magnetohydrodynamic simulations of protoplanetary discs. To resolve this potential tension, longer term simulations of disc formation and evolution with increasingly more realistic physics are needed.