Left: schematic plot of the bow-shock evolution of the DSO along the orbit. The mass-loss rate was taken to be ṁ_w = 10^-8M_⊙ yr^-1 and the terminal wind velocity is v_w = 100 km s^-1. The ambient density was colour-coded according the distribution expressed by Eq. (13). Right: total linear polarization degree in K_s band for different inclinations (0–180 degrees) of the DSO composite model. According to the key, the solid lines represent different epochs, 2008–2012, with a gradually increasing bow-shock density (see also Fig. 8). The three orange lines associated with the largest bow-shock density represent the set-ups for three different position angles of the bipolar outflow, δ = 0°, 45°, and 90°. The shaded rectangular region represents different angles of the bow-shock axis with respect to the line of sight for the period of 10 yr before the pericentre passage of the DSO, assuming a negligible motion of the ambient medium in comparison with the orbital velocity of the DSO. The horizontal dashed line marks the polarization degree value of p_L = 30%, which is approximately the observationally inferred degree for the DSO (Shahzamanian et al. 2016).