Left: comparison of the polarization degree of the DSO (black dots) with those of GC S-stars located close to the DSO position (S7, S57, S19, S20, S40, S23, and S63; shown with X, asterisk, plus sign, diamond, triangle, violet point, and square, respectively). Right: comparison of the polarization angle of the DSO (black dots) with those of the S-stars similar to the left panel. In both panels, some of the considered stars are not isolated in some years in which it is difficult to calculate their polarization parameters, therefore we did not show them as data points. The regions between two dashed red lines and dotted lines present the 1 and 3σ confidence intervals of the K_s-band polarization degree and angle distributions of the stars reported in Buchholz et al. (2013), respectively. For the significance and uncertainties of the measured points see Sects. 3.3 and 3.4.