Full-width half-maximum (FWHM) major axis of the best-fit Gaussian at the center velocity channel of the CO(2−1) emission (marked by triangles and in astronomical units by multiplying by the distance from Table 1) as a function of circumstellar density (as measured by Ṁ/v_∞ from Table 1). M-type stars are blue. C-type stars are red. Mira-type variables are marked with solid symbols and other variables appear with open symbols. The dashed lines show the photodissociation diameter for M-type (blue) and C-type (red) stars from the model grid by Saberi et al. (2019) (calculated with v_∞ = 7.5 km s⁻¹). The dotted lines are the parametrized fits to the results from Mamon et al. (1988). The solid lines show a spline-fit to the expected Gaussian FWHM determined from radiative transfer modeling of the models from Saberi et al. (2019). See text for a further explanation.