Equivalent widths for six M67 stars

The sub-giants in our sample show negligible abundance variations (<=0.02dex), which implies that M67 was born chemically homogeneous. We note there is a significant abundance difference (~0.1-0.2dex) between sub-giants and turn-off stars, which can be interpreted as the signature of atomic diffusion. Qualitatively stellar models with diffusion agree with the observed abundance results.

Some turn-off stars do not follow the general pattern, which suggests that in some cases diffusion can be inhibited. Our results thus pose a challenge for chemical tagging using the turn-off stars since they are likely not chemically homogeneous. Meanwhile, our results suggest that the effect of atomic diffusion could be as large as 0.1-0.2dex, and must be taken into account when using the current surface abundances of stars for chemical tagging.