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

Ya-Wen Tang, Anne Dutrey, Patrick Koch, Stephane Guilloteau, Hsi-Wei Yen, Emmanuel di Folco, Eric Pantin, Takayuki Muto, Akimasa Kataoka, Robert Brauer

<div><br></div><div>The polarization of continuum emission from dust at mm wavelengths is a powerful way to sample the emitting medium properties.&nbsp;</div><div>We report ALMA observations of polarization from thermal dust emission at 3 and 0.9\,mm toward the GG Tau A system.</div><div>Significant polarization is detected both across the circumternary dust ring and toward the circumstellar disk of Aa.</div><div>In the ring, the polarization percentages and patterns differ between these two wavelengths.&nbsp;</div><div>The percentage is relatively homogeneous at 3 mm, 1.2%+-0.1%, while it exhibits a&nbsp;</div><div>clear radial variation at 0.9 mm, with a mean value of 0.6% at radius (r) &lt; 1.85" (280 au), but much larger of 2.8 % at r &gt; 1.85".</div><div>In addition, at large radii (r &gt;1.85") the pattern is nearly azimuthal at both wavelengths. Beyond 1.85",</div><div>the pattern remains azimuthal at 3 mm but becomes radial at 0.9 mm. &nbsp;</div><div><br></div><div><br></div><div>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.&nbsp;</div><div>This deviation agrees with the direction of the spiral pattern observed in the NIR,&nbsp;</div><div>but it is unclear how dust grains could be aligned along such spirals.</div><div>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.</div><div>Possible interpretations of the polarization are discussed, but there is no single mechanism which could explain the detected polarization simultaneously.</div>