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The mass at which a transition is made between stars that have radiative orconvective cores throughout the core H-burning phase is a fairly sensitivefunction of Z (particularly the CNO abundances). As a consequence, the ~4 Gyr,open cluster M67 provides a constraint on Z_\odot (and the solar heavy-elementmixture) because (i) high-resolution spectroscopy indicates that this systemhas virtually the same metal abundances as the Sun, and (ii) its turnoff starshave masses just above the lower limit for sustained core convection on themain sequence. In this study, evolutionary tracks and isochrones using thelatest MARCS model atmospheres as boundary conditions have been computed for0.6-1.4 solar masses on the assumption of a metals mix (implying Z_\odot =0.0125) based on the solar abundances derived by M. Asplund and collaboratorsusing 3-D model atmospheres. These calculations do not predict a turnoff gapwhere one is observed in M67. No such difficulty is found if the analysis usesisochrones for Z_\odot = 0.0165, assuming the Grevesse & Sauval (1998) mix ofheavy elements. Our findings, like the inferences from helioseismology,indicate a problem with the Asplund et al. abundances. However, it is possiblethat low-Z models with diffusive processes taken into account will be lessproblematic.

A Constraint on Z_\odot from Fits of Isochrones to the Color-Magnitude Diagram of M67