Do magnetized winds or turbulence drive accretion in protoplanetary disks?
In plain words
Gas in a disk can only fall onto the star if it loses its spin, either by passing it outward through turbulent stirring or by carrying it away in a magnetized wind (gas flung off the disk surface along magnetic field lines). Measurements suggest the disks are too calm for turbulence to do the job, but this is not settled.
Precise statement
Classical T Tauri disks accrete at $\mathrm{mdot} \sim 10^{-9}-10^{-7}\ M_{\mathrm{sun}}/\mathrm{yr}$; a turbulent-viscosity model needs $\alpha \sim 10^{-3}-10^{-2}$, while non-ideal MHD (low ionization, Ohmic, ambipolar and Hall effects) suppresses the magnetorotational instability over much of $1-30\ \mathrm{AU}$. Determine the fraction of the angular-momentum flux carried by magnetized disk winds versus radial turbulent transport as a function of radius, using nonthermal molecular line widths, dust vertical settling, and measured wind mass-loss rates. The answer is the radial profile of the wind-to-turbulent torque ratio, or a demonstration that one channel dominates at $1-100\ \mathrm{AU}$ in typical disks.
What would settle it
ALMA turbulence and settling measurements combined with spatially resolved wind tracers (JWST molecular hydrogen and forbidden-line imaging) for the same disks, compared with the measured stellar accretion rates.
Status in the literature
ALMA line-width and dust-settling studies of 2015-2020 find $\alpha$ below about $10^{-3}$ in most disks examined, which favors wind-driven accretion, but turbulence with $\alpha$ of order 0.1 was reported in DM Tau (Flaherty et al. 2020).