WIYN Open Cluster Study. XIX. Main-Sequence-Fitting Distances to Open Clusters Using V−K Color-Magnitude Diagrams

We have combined existing optical magnitudes for stars in seven open clusters and 54 field stars with the corresponding JHKs photometry from the Two Micron All Sky Survey (2MASS). Combining optical with near-IR photometry broadens the color baseline, minimizing the influence of photometric errors and allowing better discrimination between cluster stars and contaminating foreground and background populations. The open clusters in this study include NGC 2516, M35, M34, NGC 3532, M37, M67, and NGC 188. The field stars we are using possess high-quality Hipparcos parallaxes and well-determined metal abundances, allowing us to empirically determine the dependence of V-K color on metal abundance in the range -0.45 ≤ [Fe/H] ≤ +0.35. Using this relation along with the parallaxes of the field stars, we are able to construct an unevolved main sequence in the [MV, (V-K)0] diagram for a specific abundance. These diagrams are then used to fit to the cluster main sequences in the (V, V-K) color-magnitude diagram in order to estimate a distance for each open cluster. We find that the resultant distances are within the range of distances found in the literature via the main-sequence–fitting technique. It is hoped that this will spur an expansion of the current (limited) database of star clusters with high-quality V-K photometry down to the unevolved main sequence.