  Investigating the chemical homogeneity of stars born from the same molecular
cloud at virtually the same time is very important for our understanding of the
chemical enrichment of the interstellar medium and with it the chemical
evolution of the Galaxy. One major cause of inhomogeneities in the abundances
of open clusters is stellar evolution of the cluster members. In this work, we
investigate variations in the surface chemical composition of member stars of
the old open cluster M67 as a possible consequence of atomic diffusion effects
taking place during the main-sequence phase. The abundances used are obtained
from high-resolution UVES/FLAMES spectra within the framework of the Gaia-ESO
Survey. We find that the surface abundances of stars on the main sequence
decrease with increasing mass reaching a minimum at the turn-off. After
deepening of the convective envelope in sub-giant branch stars, the initial
surface abundances are restored. We found the measured abundances to be
consistent with the predictions of stellar evolutionary models for a cluster
with the age and metallicity of M67. Our findings indicate that atomic
diffusion poses a non-negligible constraint on the achievable precision of
chemical tagging methods.
