Polarization Substructure in the Spiral-Dominated HH 111 Disk: Evidence for Grain Growth

TL;DR

This study uses high-resolution ALMA observations to analyze dust polarization in the HH 111 protostellar disk, revealing evidence of grain growth and dust self-scattering as the polarization mechanism, especially within spiral arms.

Contribution

First detection of polarization substructure in the HH 111 disk, linking polarization patterns to dust self-scattering and grain growth to ~150 microns.

Findings

Polarization is aligned with the minor axis in the envelope, indicating magnetic alignment.

Polarization degree is lower in spiral arms and higher in interarm regions.

Modeling supports dust self-scattering by grains ~150 microns as the polarization mechanism.

Abstract

The HH 111 protostellar disk has recently been found to host a pair of spiral arms. Here we report the dust polarization results in the disk as well as the inner envelope around it, obtained with the Atacama Large Millimeter/submillimeter Array in continuum at lambda ~ 870 micron and ~ 0. 05" resolution. In the inner envelope, polarization is detected with a polarization degree of ~ 6% and an orientation almost everywhere parallel to the minor axis of the disk, and thus likely to be due to the dust grains magnetically aligned mainly by toroidal fields. In the disk, the polarization orientation is roughly azimuthal on the far side and becomes parallel to the minor axis on the near side, with a polarization gap in between on the far side near the central protostar. The disk polarization degree is ~ 2%. The polarized intensity is higher on the near side than the far side, showing a near-far side asymmetry. More importantly, the polarized intensity and thus polarization degree are lower in the spiral arms, but higher in between the arms, showing an anticorrelation of the polarized intensity with the spiral arms. Our modeling results indicate that this anticorrelation is useful for constraining the polarization mechanism and is consistent with the dust self-scattering by the grains that have grown to a size of ~ 150 micron. The interarms are sandwiched and illuminated by two brighter spiral arms and thus have higher polarized intensity. Our dust self-scattering model can also reproduce the observed polarization orientation parallel to the minor axis on the near side and the observed azimuthal polarization orientation at the two disk edges in the major axis.