Constraint on Z⊙ from fits of isochrones to the color-magnitude diagram of M67

The mass at which a transition is made between stars that have radiative or convective cores throughout the core H burning phase is a fairly sensitive function of Z ( particularly, the CNO abundances). As a consequence, the similar to 4 Gyr, open cluster M67 provides a constraint on Z(circle dot) (and the solar heavy-element mixture) because (1) high-resolution spectroscopy indicates that this system has virtually the same metal abundances as the Sun, and (2) its turn off stars have masses just above the lower limit for sustained core convection on the main sequence. In this study, evolutionary tracks and isochrones using the latest MARCS model atmospheres as boundary conditions have been computed for 0.6-1.4 M-circle dot, on the assumption of a metals mix (implying) Z(circle dot) approximate to 0.0125, based on the solar abundances derived by M. Asplund and collaborators using 3D model atmospheres. These calculations do not predict a turn off gap where one is observed in M67. No such difficulty is found if the analysis uses isochrones for, assuming the Grevesse and Sauval mix of heavy elements. Our findings, like Z(circle dot) = 0.0165, the inferences from helioseismology, indicate a problem with the abundances of Asplund and collaborators. However, it is possible that low-Z models with diffusive processes taken into account will be less problematic.