Major axis of the best-fitting Gaussian at the central velocity channel of the CO (2–1) emission as a function of a proxy of circumstellar density. Here, C-, S-, and M-type stars are red, green, and blue, respectively. The stars discussed in this paper are represented by triangles (ACA-only) and round (TP) symbols, while the star symbols show the data published in Ramstedt et al. (2020). Mira-type variables are marked with solid symbols and other variables are with open symbols. The dashed lines show the photodissociation diameter of C-type (red), S-type (green), and M-type (blue) stars from Saberi et al. (2019), while the thin lines are the fits to the results from Mamon et al. (1988). The solid lines show a spline-fit to the expected Gaussian FWHM of the CO(2–1) line determined from radiative transfer modelling of the models from Saberi et al. (2019).