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 (RIS) fitting. There are surprising differences between the two distance measure ments, indicating either the need for changes in the cluster compositions o r reddening, underlying problems with the technique of MS fitting, or syste matic errors in the Hipparcos parallaxes at the 1 mas level. We examine the different possibilities, focusing on MS fitting in both metallicity-sensit ive B-V and metallicity-insensitive V-I for five well-studied systems (the Hyades, Pleiades, alpha Per, Praesepe, and Coma Ber). The Hipparcos distanc es to the Hyades and alpha Per are within 1 sigma of the MS-fitting distanc e in B-V and V-I, while the Hipparcos distances to Coma Ber and the Pleiade s are in disagreement with the MS-fitting distance at more than the 3 sigma level. There are two Hipparcos measurements of the distance to Praesepe; o ne is in good agreement with the MS-fitting distance and the other disagree s at the 2 sigma level. The distance estimates from the different colors ar e in conflict with one another for Coma but in agreement for the Pleiades. Changes in the relative cluster metal abundances, age related effects, heli um, and reddening are shown to be unlikely to explain the puzzling behavior of the Pleiades. We present evidence for spatially dependent systematic er rors at the 1 mas level in the parallaxes of Pleiades stars. The implicatio ns of this result are discussed.