Effect of a localised variation of the dust distribution on line emission. Top left: Azimuthal profiles of the ¹²CO J=2–1 integrated maps from two models with slightly different dust distributions between 27 au and 60 au. The blue dots are the observations. The shaded area indicates the region where the dust mass was artificially increased by 30% to boost heating by dust thermal accommodation. Bottom left: relative difference of modelled intensities with respect to the best representative model for the ¹²CO J = 2–1 case. Right: relative difference of integrated intensities for the ¹³CO J = 2–1 and C¹⁸O J = 2–1 cases.