Left panel: sample stars from M 67 in the T_eff–log  g plane. Middle panel: [Na/Fe] ratio of each star in M 67 as a function of its surface gravity. Right panel: [Al/Fe] ratio of each star in M 67 as a function of its surface gravity. Stars are color-coded according to the surface gravity: red for giants with log  g ≤ 3.5, blue for turn-off and subgiant stars with 3.5 < log  g ≤ 4.0, and black for main-sequence stars with log  g> 4.0. A typical error bar (±0.14 dex for [Elem./Fe] and ±0.05 dex for log  g) is shown in the upper left part of the middle and right panels.