Residuals of K_s-band polarization fraction maps in Fig. 6 (smoothed by Savitzky-Golay filter in Sect. C.5) subtracted by the models in Fig. D.1, with dimensions of 2″ × 2″ with identical color bars in linear scale. Combining the standard deviation of each residual map of ≈0.05 statistically, with an uncertainty of ≈0.03 systematically from the three MWC 758 observations in Fig. 7, we assign a total uncertainty of 0.06 for the maximum polarization fraction values.