The Problem of Hipparcos Distances to Open Clusters. I. Constraints from Multicolor Main-Sequence Fitting*

摘要: Parallax data from the Hipparcos mission allow direct distance to open clusters be compared with inferred main-sequence (MS) fitting. There are surprising differences between two measurements, indicating either need for changes in cluster compositions or reddening, underlying problems technique of MS fitting, systematic errors parallaxes at 1 mas level. We examine different possibilities, focusing on fitting both metallicity-sensitive B-V and metallicity-insensitive V-I five well-studied systems (the Hyades, Pleiades, α Per, Praesepe, Coma Ber). The distances Hyades Per within σ MS-fitting V-I, while Ber Pleiades disagreement more than 3 measurements Praesepe; one is good agreement other disagrees 2 estimates colors conflict another but Pleiades. Changes relative metal abundances, age related effects, helium, reddening shown unlikely explain puzzling behavior present evidence spatially dependent level stars. implications this result discussed.