A high relative precision colour-magnitude diagram of M67

Abstract

We have calibrated and combined an extensive set of BVI observations of M67 to produce a colour-magnitude diagram of stars measured with high relative precision. We have selected stars that are most likely to be single-star members of the cluster using proper motion, radial velocity, and variability information from the literature, and an examination of the most probable colour-magnitude diagram locations of unresolved stellar blends. We have made detailed comparisons of our photometry of the selected stars with theoretical models, and discuss the most notable discrepancies. Observations of M67 turn-off stars are a severe test of algorithms attempting to describe convective cores in the limit of small extent, and we find strong evidence of a 'hook' just fainter than the turn-off gap. The stars in M67 support assertions that the degree of convective core overshooting decreases to zero for stars with masses in the range 1.0 < (M/M ○. ) ≤ 1.5, but that the degree of overshoot is smaller than currently used in published isochrones. We also verify that all current theoretical models for the lower main sequence (with the exception of Baraffe et al. 1998) are too blue for M V ? 6, even when the sequences are shifted to match M67 near the M V of the Sun, probably due to a combination of problems with colour-T eff transformations and realistic surface boundary conditions for models. Finally, we identify a subset of cluster members with unusual photometry (candidate red giant binaries, blue straggler stars, and triple systems) deserving of further study.