The Transition of Polarized Dust Thermal Emission from the Protostellar Envelope to the Disk Scale

TL;DR

This study investigates the transition of polarized dust emission from protostellar envelopes to disks, revealing that magnetic alignment alone cannot explain the observed polarization decrease and orientation changes, suggesting scattering dominates disk polarization.

Contribution

It combines ALMA observations with non-ideal MHD simulations to analyze polarization trends, highlighting the limitations of magnetic alignment in explaining disk-scale polarization.

Findings

Magnetic field alignment weakens from envelope to disk scale due to density increase.

Magnetic susceptibility enhancement is required for grain alignment at disk scales.

Scattering likely dominates polarization at the disk scale, not magnetic alignment.

Abstract

Polarized dust continuum emission has been observed with ALMA in an increasing number of deeply embedded protostellar systems. It generally shows a sharp transition going from the protostellar envelope to the disk scale, with the polarization fraction typically dropping from ${\sim} 5\%$ to ${\sim} 1\%$ and the inferred magnetic field orientations becoming more aligned with the major axis of the system. We quantitatively investigate these observational trends using a sample of protostars in the Perseus molecular cloud and compare these features with a non-ideal MHD disk formation simulation. We find that the gas density increases faster than the magnetic field strength in the transition from the envelope to the disk scale, which makes it more difficult to magnetically align the grains on the disk scale. Specifically, to produce the observed ${\sim} 1\%$ polarization at ${\sim} 100\,\mathrm{au}$ scale via grains aligned with the B-field, even relatively small grains of $1\,\mathrm{\mu m}$ in size need to have their magnetic susceptibilities significantly enhanced (by a factor of ${\sim} 20$) over the standard value, potentially through superparamagnetic inclusions. This requirement is more stringent for larger grains, with the enhancement factor increasing linearly with the grain size, reaching ${\sim} 2\times 10^4$ for millimeter-sized grains. Even if the required enhancement can be achieved, the resulting inferred magnetic field orientation in the simulation does not show a preference for the major axis, which is inconsistent with the observed pattern. We thus conclude that the observed trends are best described by the model where the polarization on the envelope scale is dominated by magnetically aligned grains and that on the disk scale by scattering.