Element abundances of unevolved stars in the open cluster M 67

Context. The star-to-star scatter in lithium abundances observed among otherwise similar stars in the solar-age open cluster M 67 is one of the most puzzling results in the context of the so called lithium problem. Among other explanations, the hypothesis has been proposed that the dispersion in Li is due to star-to-star differences in Fe or other element abundances which are predicted to affect Li depletion. Aims. The primary goal of this study is the determination of the metallicity ([Fe/H]), alpha- and Fe-peak abundances in a sample of Li-poor and Li-rich stars belonging to M 67, in order to test this hypothesis. By comparison with previous studies, the present investigation also allows us to check for intrinsic differences in the abundances of evolved and unevolved cluster stars and to draw more secure conclusions on the abundance pattern of this cluster. Methods. We have carried out an analysis of high resolution UVES/VLT spectra of eight unevolved and two slightly evolved cluster members using MOOG and measured equivalent widths. For all the stars we have determined [Fe/H] and element abundances for O, Na, Mg, Al, Si, Ca, Ti, Cr and Ni. Results. We find an average metallicity [Fe/H] = 0.03 +/- 0.01, in very good agreement with previous determinations. All the [X/Fe] abundance ratios are very close to solar. The star-to-star scatter in [Fe/H] and [X/Fe] ratios for all elements, including oxygen, is lower than 0.05 dex, implying that the large dispersion in lithium among cluster stars is not due to differences in these element abundances. We also find that, when using a homogeneous scale, the abundance pattern of unevolved stars in our sample is very similar to that of evolved stars, suggesting that, at least in this cluster, RGB and clump stars have not undergone any chemical processing. Finally, our results show that M 67 has a chemical composition that is representative of the solar neighborhood.