What Produces Dust Polarization in the HH 212 Protostellar Disk at 878 {\mu}m: Dust Self-Scattering or Dichroic Extinction?

TL;DR

This study presents high-resolution ALMA observations of dust polarization in the HH 212 protostellar disk, exploring whether dust self-scattering or dichroic extinction primarily causes the observed polarization patterns.

Contribution

It provides detailed polarization maps at 878 μm and discusses the potential roles of dust self-scattering and dichroic extinction in producing the polarization, indicating both mechanisms may be involved.

Findings

Polarization orientations are mainly parallel to the disk's minor axis.

Polarized intensity peaks near the central source and decreases outward.

Both dust self-scattering and dichroic extinction likely contribute to the observed polarization.

Abstract

We report new dust polarization results of a nearly edge-on disk in the HH 212 protostellar system, obtained with ALMA at ~ 0.035" (14 au) resolution in continuum at lambda ~ 878 um. Dust polarization is detected within ~ 44 au of the central source, where a rotationally supported disk has formed. The polarized emission forms V-shaped structures opening to the east and probably west arising from the disk surfaces and arm structures further away in the east and west that could be due to potential spiral arms excited in the outer disk. The polarization orientations are mainly parallel to the minor axis of the disk, with some in the western part tilting slightly away from the minor axis to form a concave shape with respect to the center. This tilt of polarization orientations is expected from dust self-scattering, e.g., by 50-75 um grains in a young disk. The polarized intensity and polarization degree both peak near the central source with a small dip at the central source and decrease towards the edges. These decreases of polarized intensity and polarization degree are expected from dichroic extinction by grains aligned by poloidal fields, but may also be consistent with dust self-scattering if the grain size decreases toward the edges. It is possible that both mechanisms are needed to produce the observed dust polarization, suggesting the presence of both grain growth and poloidal fields in the disk.