Line strengths of the CO J = 15−14 transition as a function of Ṁ_g/ ν_∞,g. The points with error bars give the measured CO line strengths, color coded according to the pulsational type or according to the values of the relevant quantity indicated in the legend. Each panel shows curves for different values of a given parameter in the model grid with similar color coding as the data points, if applicable: the CO abundance with respect to H₂ (n_CO/n_H2; upper left panel), gas terminal velocity (ν_∞,g; upper right panel), stellar effective temperature (T_⋆; lower left panel), and stellar luminosity (L_⋆; lower right panel). Adopted values for other parameters are listed in Table 4. The line strengths are scaled to a distance of 100 pc.