Polarization in the GG Tau Ring—Confronting Dust Self-scattering, Dust Mechanical and Magnetic Alignment, Spirals, and Dust Grain Drift

We report Atacama Large Millimeter/submillimeter Array (ALMA) polarization observations at 3 and 0.9 mm toward the GG Tau A system. In the ring, the percentage is relatively homogeneous at 3 mm, being 1.2%, while it exhibits a clear radial variation at 0.9 mm with a mean increasing from 0.6% to 2.8% toward larger radius (r). The polarization orientation at r > 1farcs85 appears nearly azimuthal at both wavelengths. At r < 1farcs85, the pattern remains azimuthal at 3 mm but becomes radial at 0.9 mm. The dust self-scattering model with ${a}_{\max }$ of 1 mm could reproduce the observed polarization orientation and percentage at 0.9 mm, but the expected polarization percentage at 3 mm would be 0.2%, much smaller than the detected 1.2%. Dust alignment with poloidal magnetic field could qualitatively reproduce the flip in polarization at r < 1farcs85 and also the detected polarization percentage. A closer inspection of the nearly azimuthal pattern reveals that polarization orientations are systematically deviating by −9fdg0 ± 1fdg2 from the tangent of the orbit ellipses. This deviation agrees with the direction of the spiral pattern observed in the near-infrared, but it is unclear how dust grains could be aligned along such spirals. For the scenario where the −9° deviation (−7fdg3 after considering the inclination effect) measures the radial component of the dust drift motion, the expected inward drifting velocity would be ∼12.8% of the Keplerian speed, a factor of 2.8 larger than the theoretical predictions. Possible additional interpretations of the polarization are discussed, but there is no single mechanism that could explain the detected polarization simultaneously.