Accretion across scales: Streamers, surface-layer transport, and rapid replenishment in young protoplanetary discs

Protoplanetary discs evolve around newly formed stars through an interplay of infall from surrounding turbulent cloud material, accretion towards the young star, and outflow-driven mass loss. Determining whether discs are fed predominantly through infall along the disc midplane or along the poles is a key challenge, along with the question of whether accretion occurs in a steady or burst-like fashion. Here, we present a suite of 3D ideal magnetohydrodynamic (MHD) simulations of protoplanetary disc formation and evolution in a dynamic, large-scale molecular cloud environment using the adaptive mesh refinement framework . We focus on nine stellar systems, with their discs resolved down to a scale of 0.8,au. Across the sample, stellar accretion proceeds at rates of ∼10-5,M_⊙$,yr^-1 over 10^5 yr, with significant variability. Discs grow to 100,au scales and remain gravitationally stable in time, with disc-to-star mass ratios below 10,%. Transient high-density streamers, with 10,kyr infall times, have the capacity to drive anisotropic mass delivery at rates comparable to the background accretion flow. Their interaction with discs typically results in a temporary reduction of the disc size by half and disc mass by 40,%. During later quiescent disc evolution stages ($t≳50,kyr), accretion predominantly occurs through the midplane and disc surface layers. This is associated with the development of a toroidal magnetic field morphology, which includes field reversals across both disc surfaces. In this way, the full disc mass reservoir is replenished on timescales of 10,kyr. These findings support the notion that when the outer parts of very young discs are well-ionised and close to the ideal MHD regime, they are not yet conducive to planet formation, due to high replenishment rates, strong turbulence, and disruptive streamer infall events. dispatch