The emerging brightness temperatures (T_bΔΩ) as function of the linear polarization fraction (P_l). The blue and red circles indicate the unsaturated and saturated masers, respectively, detected in NGC 7538 (Surcis et al. 2011a), W51, W48, IRAS 18556+0138, and W3(OH). The red arrows indicate that the T_bΔΩ values obtained from the radiative transfer method code are lower limits. The red full line is the limit of emerging brightness temperature above which the CH₃OH masers are considered saturated (T_bΔΩ > 2.6 × 10⁹ K sr; Surcis et al. 2011a), and the dotted line gives the lower limit to the linear polarization fraction for saturated masers (P_l ≈ 4.5%).