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Abundances and possible diffusion of elements in M67 stars

We present a spectroscopic study at high resolution, R~50,000, of 14 stars located on the main sequence, at the turn-off point and on the early subgiant branch in the cluster M67 in order to investigate its detailed chemical composition, for comparison with the Sun and solar twins in the solar neighbourhood, and to explore selective atomic diffusion of chemical elements as predicted by stellar-structure theory. We have obtained VLT/FLAMES-UVES spectra and analysed these strictly differentially in order to explore chemical-abundance similarities and differences between the M67 stars and the Sun, and among the M67 stars themselves. Individual abundances of 19 different chemical elements are obtained for the stars. They are found to agree very well with solar abundances, with abundance ratios closer to solar than those of most solar twins in the solar neighbourhood. An exception is Li which shows a considerable scatter among the cluster stars. There is a tendency for the cluster-star abundances to be depleted relative to the abundances in the field stars in correlation with the condensation temperature of the elements, a tendency earlier found also for the Sun. The results support the hypothesis that the gas of the proto-cluster was depleted by formation and cleansing of dust before the stars formed. They also add support to the proposal that the Sun was once formed in a dense stellar environment. Moreover, the observed minor reductions of heavy elements in the atmospheres of the dwarfs and turn-off point stars relative to our standard star M67-1194 and the subgiants seem to suggest that diffusion processes are at work in these stars, although the evidence is not compelling. Based on theoretical models the diffusion-corrected initial metallicity of M67 is estimated to be [Fe/H]=+0.06.