The Distances to Open Clusters as Derived from Main‐Sequence Fitting. II. Construction of Empirically Calibrated Isochrones

In the first of a new series of papers on open cluster distances, we useupdated stellar evolution models to construct an isochrone appropriate for theHyades, and compare it with the Hyades eclipsing binary system vB 22. We findthat the absolute and relative luminosities of the two stars are in goodagreement with the model, but the radii do not match the values inferred fromeclipse data. We present evidence that there is a consistency problem with theflux ratios and the inferred radii, and discuss possible theoretical effectsthat could be responsible for the mismatch in the radii. We derive a heliumabundance for the Hyades of Y = 0.271 \pm 0.006, which is equal within theerrors to the Sun's initial helium abundance even though the Hyades isconsiderably more metal-rich.Comment: Accepted for publication in Astrophysical Journa