Twisted Pseudodisk and Asymmetric Mass Accretion on the Circumstellar Disk

We model gas inflow patterns onto circumstellar disks and the evolution of the pseudodisk using three-dimensional resistive MHD simulations. Starting from a prestellar core without turbulence and with a misalignment between the initial magnetic field and rotation axis, the simulations are performed for $\sim10^5$ yr after protostar formation. After disk formation, the magnetic field around the disk becomes significantly distorted due to the disk rotational motion. Consequently, the structure of the pseudodisk also evolves into a complex morphology. As a result, both accretion onto the disk and outflow become asymmetric and anisotropic. Accretion to the disk occurs primarily through narrow-channel flows or streams. The time evolution of the infalling envelope leads to non-steady accretion onto the disk, which in turn causes variability in the mass accretion onto the central protostar. This study demonstrates that complex infalling envelope structures and channelized accretion flows onto the disk naturally arise even without assuming turbulence or external asymmetric inflows.

💡 Research Summary

This paper presents a comprehensive three‑dimensional resistive magnetohydrodynamic (MHD) study of gas inflow onto circumstellar disks and the evolution of the surrounding pseudodisk (often called a “pseudo‑disk”) in a collapsing molecular cloud core. The authors start from a non‑turbulent, magnetized Bonnor‑Ebert sphere (central density 10⁴ cm⁻³, temperature 10 K, mass 8.1 M☉, radius 4.8 × 10⁴ au) whose global magnetic field (B₀ = 5.5 µG) is inclined by 30° relative to the initial rigid rotation axis (Ω₀ = 2.5 × 10⁻¹⁴ s⁻¹). The mass‑to‑flux ratio is µ₀ = 3 (super‑critical), and the ratios of thermal, rotational, and magnetic energies to gravity are α₀ = 0.5, β₀ = 0.02, γ₀ = 0.1, respectively.

Using a nested‑grid code with 16 refinement levels (finest cell size 0.73 au), the collapse is followed for ~10⁵ yr after protostar formation. When the central density exceeds 10¹³ cm⁻³ a sink particle of radius 1 au is introduced, accreting 1 % of the gas above this threshold each timestep while preserving magnetic flux to maintain ∇·B = 0.

Key results:

1. Disk and Pseudodisk Morphology – By the end of the run the protostar has grown to 0.45 M☉. A rotationally supported circumstellar disk of ≈300 au radius forms at the centre. Surrounding it, a much larger flattened structure (the pseudodisk) extends to several thousand au. Because the magnetic field is twisted by the disk’s rotation, the pseudodisk’s normal vector is no longer aligned with the disk’s angular momentum vector; the pseudodisk becomes “twisted” and warped. High‑density spikes and filamentary channels appear within the pseudodisk, especially at radii ≳3 000 au.

2. Channelized Accretion – Gas does not accrete isotropically onto the disk. Instead, narrow, high‑density streams (the “channels”) funnel material from the pseudodisk onto the inner disk. These streams are visible as spiky density enhancements in the 3‑D iso‑density visualisations and dominate the mass flux onto the disk.

3. Asymmetric Inflow/Outflow Patterns – The authors compute mass flux on spherical shells of radii 600, 3 000, 6 000, and 12 000 au. At the largest scales (≥6 000 au) outflows are primarily bipolar along the original magnetic‑field direction (vertical in the simulation), reflecting the initial field geometry. At smaller radii, outflows re‑orient to align with the disk’s rotation axis, producing band‑like outflow structures near the equatorial plane. Inflow is highly non‑uniform, concentrated in the regions surrounding the outflow bands, and varies strongly with azimuth and latitude.

4. Temporal Variability – Because the pseudodisk is intrinsically warped and the accretion occurs through discrete channels, the mass accretion rate onto the protostar is unsteady, fluctuating between ~10⁻⁶ and 10⁻⁴ M☉ yr⁻¹ over the simulated 10⁵ yr. This variability arises without invoking external turbulence, clump infall, or asymmetric boundary conditions; it is a natural consequence of the magnetic‑rotation misalignment.

5. Implications for Observations – Recent high‑resolution ALMA observations have reported asymmetric gas streams, warped outer disk structures, and episodic accretion in young protostars. The present simulations demonstrate that such phenomena can be reproduced in a purely magnetically regulated collapse with an initial misalignment, offering an alternative to explanations that require turbulent driving or external perturbations.

Overall, the study advances our understanding of how magnetic field geometry influences the long‑term evolution of pseudodisks, the morphology of circumstellar disks, and the nature of mass delivery onto nascent stars. It highlights that even in the absence of turbulence, misaligned magnetic fields can generate twisted, non‑axisymmetric envelope structures and channelized accretion flows, thereby naturally producing the asymmetric and time‑variable accretion signatures now being uncovered by modern interferometric facilities.