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.