Surface abundance of [C/N] for clump stars as a function of stellar ages (top panel) and stellar masses (bottom panel) for a synthetic thin disc computed with the BGM. Stars are divided in three metallicity-bins: −0.20 ≤ [Fe/H] ≤−0.10 (blue dots), −0.05 ≤ [Fe/H] ≤ +0.05 (green dots), and +0.15 ≤ [Fe/H] ≤ +0.25 (red dots). Relations between [C/N] and mass and age (Eqs. (3) and (4)) are also shown with the solid lines.