Solar twins in M 67

Context. The discovery of true solar analogs is fundamental to a better understanding of the Sun and of the solar system. Despite a number of efforts, this search has brought only limited results for field stars. The open cluster M 67 offers a unique opportunity to search for solar analogs, because its chemical composition and age are very similar to those of the Sun. Aims. We analyse FLAMES spectra of a large number of M 67 main sequence stars to identify solar analogs in this cluster. Methods. We first determined cluster members that are not likely binaries, by combining proper motions and radial velocity measurements. We concentrate our analysis on determining stellar effective temperature, using analyses of line-depth ratios and H alpha wing and making a direct comparison to the solar spectrum obtained with the same instrument. We also computed the lithium abundance for all the stars. Results. Ten stars have the temperature derived both by line-depth ratios and by the Ha wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening E(B - V) = 0.041, the solar color (B - V)(circle dot) = 0.649 +/- 0.016 and a cluster distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun and candidates to be true solar twins. These stars also have a low Li content, comparable to the photospheric abundance of the Sun, likely indicating a similar mixing evolution. Conclusions. We find several candidates for the best solar analogs ever. These stars are amenable to further spectroscopic investigations and planet searches. The solar colors are determined with fairly high accuracy with an independent method, as is the cluster distance modulus.