WIYN Open Cluster Study — XVI. Optical/infrared photometry and comparisons with theoretical isochrones

We present combined optical/near-infrared photometry (BVIK) for six open clusters - M35, M37, NGC 1817, NGC 2477, NGC 2420 and M67. The open clusters span an age range from 150 Myr to 4 Gyr and have metal abundances from[Fe/H] = -0.27 to +0.09 dex. We have utilized these data to test the robustness of theoretical main sequences constructed by several groups as denoted by the following designations - Padova, Baraffe, Y 2 , Geneva and Siess. The comparisons of the models with the observations have been performed in the [M V , (B - V) 0 ], [M V , (V - I) 0 ] and [M V , (V - K) 0 ] colour-magnitude diagrams as well as the distance-independent [(V - K) 0 , (B - V) 0 ] and [(V - K) 0 , (V - I) 0 ] two-colour diagrams. We conclude that none of the theoretical models reproduces the observational data in a consistent manner over the magnitude and colour range of the unevolved main sequence. In particular, there are significant zero-point and shape differences between the models and the observations. We speculate that the crux of the problem lies in the precise mismatch between theoretical and observational colour-temperature relations. These results underscore the importance of pursuing the study of stellar structure and stellar modelling with even greater intensity.