Oral Presentation

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

Author(s): Ya-Wen Tang

Presenter: Ya-Wen Tang (ASIAA)

The polarization of continuum emission from dust at mm wavelengths is a powerful way to sample the emitting medium properties.
We report ALMA observations of polarization from thermal dust emission at 3 and 0.9 mm toward the GG Tau A system.
Significant polarization is detected both across the circumternary dust ring and toward the circumstellar disk of Aa.
In the ring, the polarization percentages and patterns differ between these two wavelengths.
The percentage is relatively homogeneous at 3 mm, 1.2+-0.1%, while it exhibits a
clear radial variation at 0.9 mm, with a mean value of 0.6% at radius (r) \< 1.85" (280 au), but much larger of 2.8% at r \> 1.85".
In addition, at large radii (r \>1.85") the pattern is nearly azimuthal at both wavelengths. Beyond 1.85",
the pattern remains azimuthal at 3 mm but becomes radial at 0.9 mm.


A closer inspection of the nearly azimuthal pattern reveal that polarization orientations are systematically tilted by -8 degree from the tangent of the orbit ellipses.
This deviation agrees with the direction of the spiral pattern observed in the NIR,
but it is unclear how dust grains could be aligned along such spirals.
For the scenario where the -8 degree deviation measures the radial component of the drift motion, the expected inward drifting velocity would be ~12% of the Keplerian speed, a factor of 2.5 larger than the theoretical predictions.
Possible interpretations of the polarization are discussed, but there is no single mechanism which could explain the detected polarization simultaneously.