tokens: left. Fig. 2 Schematic overview of the features in the outer circumbinary ring detected in our SPHERE PI image. The image is centered on the location of GG Tau Aa and was smoothed for illustration purposes. Any unresolved circumstellar material may create a halo of polarization signal around the star. Because this unresolved polarized signal can affect the analysis of the immediate circumstellar environments, we subtracted these polarized signals (i.e., the total intensity halo multiplied by the degree of polarization) individually for Aa and Ab. While subtracting the polarization signal of Aa slightly increases the contrast of the fine structures in the immediate stellar environment, subtracting the polarization signal of Ab instead blurs these structures. This can be explained by the fact that the measured polarization degree of Ab is somewhat higher than that of Aa. Therefore, subtracting the polarization signal of Abadds an artificial polarization halo around Aa, which weakens the fine structures in its environment. Subtracting the (less strongly) polarized signal of Aa, however, does not noticeably affect the environment of Ab. Figure 3 shows the resulting image after subtraction of the polarization signal of Aa with annotations of the detected features. The immediate stellar environments that are affected by the diffraction pattern are masked out. In all the images, the inner region appears highly structured, as highlighted in Fig. 3 by the dotted lines. Most prominently, the “north arc”, an extended structure to the northeast of Ab observed in previous scattered light images (e.g., Krist et al. 2002 , 2005 ; Itoh et al. 2014 ; Yang et al. 2017 ), is clearly detected and appears in our SPHERE image to be composed of a double-arc structure at projected separations of ~ 0.38′′ and ~0.48′′. This double-arc structure may extend along the entire eastern side to the south, interrupted by a dark lane extending from Ab toward the east (see the dotted gray lines in Fig. 3 ). This dark lane seems to be connected to the shadow observed in the outer disk at a similar position angle (see Sect. 4.4 ). We furthermore detect two additional filament structures northwest and southwest of Ab. It is unclear, however, whether they are related to the double-arc system on the eastern side. Finally, another filament is detected immediately southeast of Aa, pointing toward the south. Fig. 3 Zoom on the inner region after subtraction of the stellar polarization on Aa. The dotted lines highlight the detected features: the “north arc”, revealing a double-arc structure (green), several filaments (light blue) and a possible shadow lane (gray dashed). See Sect. 4.1 for details. The immediate stellar environments (120 mas) are masked out. North is up and east is to the left. Figure 4 (left panel) shows the angles of linear polarization overplotted on the inner disk region. The polarization angles θ were calculated according to θ = 0.5 ×arctan( U ∕ Q ), within bins of 3
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