Polarisation fractions for different grain alignment models. The long horizontal lines show the observed average values of p (left y-axis) for the Planck (p_P, blue line) and the POL-2 (p_S, red line) observations. The black long horizontal line stands for the observed ⟨p_P ⟩∕⟨p_S⟩ ratio (right y-axis). The three curves with markers show the corresponding quantities for models that assume a loss of grain alignment above the density threshold n₀. The results for models with constant grain alignment are indicated with short horizontal lines near n = 10³ cm⁻³. The RAT models are shown as short horizontal lines near n = 10⁴ cm⁻³, for the default model (solid lines), for the case with larger grain sizes (dashed line), and for an alternative model with a smaller τ (250 μm)∕τ (J) ratio (dotted line).