Chemical (in)homogeneity and atomic diffusion in the open cluster M67

Context. The benchmark open cluster M67 is known to have solar metallicity and similar age as the Sun. It thus provides us a great opportunity to study the properties of solar twins, as well as the evolution of Sun-like stars. Aims. Previous spectroscopic studies claimed to detect possible subtle changes in stellar surface abundances throughout the stellar evolutionary phase, namely the effect of atomic diffusion, in M67. In this study we attempt to confirm and quantify the effect of atomic diffusion, as well as to explore the level of chemical (in)homogeneity in M67. Methods. We presented a strictly line-by-line differential chemical abundance analysis of two groups of stars in M67: three turn-off stars and three sub-giants. Stellar atmospheric parameters and elemental abundances were obtained with very high precision using the Keck/HIRES spectra. Results. The sub-giants in our sample show negligible abundance variations ($\le$ 0.02 dex), which implies that M67 was born chemically homogeneous. We note there is a significant abundance difference ($\sim$ 0.1 - 0.2 dex) 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. Conclusions. 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 - 0.2 dex, and must be taken into account when using the current surface abundances of stars for chemical tagging.