Relative difference between the squares of observed ¹²CO and ¹³CO rotation curves predicted by the thermally stratified model (blue line), the isothermal model (orange line), and the data (red dots). Except for AS 209, where this quantity is negative in the inner part, it is clearly visible that data are well reproduced by the stratified model. Indeed, the difference of speed between the two curves cannot be explained just in terms of different height.