Twisted Pseudodisk and Asymmetric Mass Accretion on the Circumstellar Disk

TL;DR

The study addresses how a misaligned magnetic field in a non-turbulent collapsing core shapes the long-term evolution of the pseudodisk and the pattern of mass accretion onto a circumstellar disk. Using three-dimensional resistive MHD simulations that run for about $10^5$ yr after protostar formation, the authors show that the magnetic field becomes strongly twisted around the disk, producing a deformed, non-axisymmetric pseudodisk and channelized, asymmetric accretion through multiple streams, with outflows carved along the flow. These intrinsic dynamics generate substantial temporal variability in the mass inflow to the disk and the accretion onto the protostar, with large-scale outflows roughly aligned with the initial field while inner flows follow the disk rotation axis. The results demonstrate that complex infalling envelope structures and channelized accretion can arise without turbulence or external asymmetries, having important implications for disk growth, protostellar variability, and interpretation of high-resolution observations.

Abstract

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.

Figures (8)

• Figure 1: (a)–(f): Density (color) distribution on the $x = 0$ (panels (a) and (d)), $y = 0$ (panels (b) and (e)), and $z = 0$ (panels (c) and (f)) planes. (g)–(i): Surface density (color) distributions on the $x = 0$ (panel (g)), $y = 0$ (panel (h)), and $z = 0$ (panel (i)) planes. The dotted curves in panels (a)–(c) indicate the rotationally supported disk. The elapsed time $t_{\rm ps}$ after protostar formation, the time $t$ since the onset of cloud collapse, and the protostellar mass are indicated in panel (a). The spatial scale in panels (a)–(c) differs from that in panels (d)–(f) and (g)–(i). An animated version of this figure is available. In the animation, the time sequence of the density and velocity distribution on the $x=0$, $y=0$, and $z=0$ plane as in Figure 1 from the beginning until the end of the simulation is shown. The duration of the animation is 25 s.
• Figure 2: Three-dimensional view of the iso-density surface at number density $n_{\rm surf}$, as indicated in each panel, at the same epoch as in Fig. \ref{['fig:1']}. The spatial scale, also noted in each panel, differs among the panels.
• Figure 3: Mass inflow rates $\tilde{\dot{M}}(\theta,\phi)$ onto spherical surfaces at radii of 600 au (panel (a)), 3,000 au (panel (b)), 6,000 au (panel (c)), and 12,000 au (panel (d)), at the same epoch as in Fig. \ref{['fig:1']}. The black regions correspond to outflowing gas with $v_r > 0$. The vertical and horizontal axes represent latitude ($-\pi/2 < \theta < \pi/2$) and longitude ($-\pi < \phi < \pi$), respectively, in radians. Note that the axes correspond to spherical angles, not to the disk plane. Since the disk orientation varies with radius as the inflow moves inward, a single well-defined disk midplane cannot be drawn in this projection.
• Figure 4: (a)–(c): Three-dimensional view of streamlines (blue lines) and high-density regions (brown), shown at different spatial scales, at the same epoch as in Fig. \ref{['fig:1']}. (d)–(f): Outflowing gas with $v_r > c_s$ (green surfaces) and magnetic field lines (red lines) are added to the structures shown in panels (a)–(c). The spatial scale is indicated in each panel. The Cartesian $x$-, $y$-, and $z$-axes are shown at the bottom right corner of each panel. The initial directions of the magnetic field $B_0$ and angular momentum $J_0$ are indicated in panel (a).
• Figure 5: (a)–(c): Time evolution of the mass inflow rate $\dot{M}_{\rm inf}$ (panel (a)), the mass outflow rate $\dot{M}_{\rm out}$ (panel (b)), and the ratio of outflow to inflow rates $\dot{M}_{\rm out}/\dot{M}_{\rm inf}$ (panel (c)) at different spatial scales, corresponding to grid levels $l = 8$–13, plotted against the elapsed time $t_{\rm ps}$ after protostar formation. (d): Time evolution of the mass accretion rate onto the protostar (sink) is shown on the left axis, while the protostellar mass and outflow mass are shown on the right axis, all as functions of $t_{\rm ps}$.