The Problem ofHipparcosDistances to Open Clusters. I. Constraints from Multicolor Main‐Sequence Fitting

Parallax data from the Hipparcos mission allow the direct distance to openclusters to be compared with the distance inferred from main sequence (MS)fitting. There are surprising differences between the two distancemeasurements, which could lead to consequences of significant astrophysicalimportance. We examine the different possibilities, focusing on MS fitting inboth metallicity-sensitive B-V and metallicity-insensitive V-I for fivewell-studied systems (the Hyades, Pleiades, Alpha Per, Praesepe, and Coma Ber). The Hipparcos distances to the Hyades and Alpha Per agree with the MS fittingdistances in both colors; there is a possible conflict for Praesepe and ComaBer. The Hipparcos distance to the Pleiades disagrees with the MS fittingdistance in both colors at more than the three sigma level. Changes in thecluster metal abundance, helium abundance, reddening and age-related effectsare all shown to be unlikely to explain the puzzling behavior for the Pleiades.We present evidence for spatially dependent systematic errors at the 1 maslevel in the parallaxes of Pleiades stars. The implications of this result arediscussed.Comment: 52 pages, 24 figures. To appear in ApJ. Figures also available by ftp at ftp://ftp.astronomy.ohio-state.edu/pub/pinsono/hipparco