Polarization fraction as a function of the “mean" optical depth for different values of Cn_H2/A for the multilevel and two-level cases (red and black points, respectively). With triangles, stars and squares we show the polarization fraction for Cn_H2/A = 10, 1 and 0.1, respectively. With blue hexagons we show a multilevel case with Cn_H2/A = 1 where we only consider the radiative processes between levels J = 1 → 0 (collisional processes are considered between all sublevels). A schematic representation of the physical model used for these radiative-transfer simulations is shown in panel (a) of Fig. 7.