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

TL;DR

This paper uses the color-magnitude diagram of M67 to constrain the solar metallicity, finding that models with higher Z_f are more consistent with observations, challenging recent low-Z abundance estimates.

Contribution

It provides a new constraint on the solar metallicity by comparing isochrone fits to M67 with different solar abundance models.

Findings

Models with Z_f=0.0165 fit the M67 data better.

Low-Z models based on Asplund et al. abundances face difficulties.

Helioseismology and isochrone fitting both suggest higher solar metallicity.

Abstract

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 ~4 Gyr, open cluster M67 provides a constraint on Z_\odot (and the solar heavy-element mixture) because (i) high-resolution spectroscopy indicates that this system has virtually the same metal abundances as the Sun, and (ii) its turnoff 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 solar masses on the assumption of a metals mix (implying Z_\odot = 0.0125) based on the solar abundances derived by M. Asplund and collaborators using 3-D model atmospheres. These calculations do not predict a turnoff gap where one is observed in M67. No such difficulty is found if the analysis uses isochrones for Z_\odot = 0.0165, assuming the Grevesse & Sauval (1998) mix of heavy elements. Our findings, like the inferences from helioseismology, indicate a problem with the Asplund et al. abundances. However, it is possible that low-Z models with diffusive processes taken into account will be less problematic.