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

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