Correlated Errors inHipparcosParallaxes toward the Pleiades and the Hyades

We show that the errors in the Hipparcos parallaxes towards the Pleiades andthe Hyades open clusters are spatially correlated over angular scales of 2 to 3deg, with an amplitude of up to 2 mas. This correlation is stronger thanexpected based on the analysis of the Hipparcos catalog. We predict theparallaxes of individual cluster members, pi_pm, from their Hipparcos propermotions, assuming that all cluster members have the same space velocity. Wecompare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there aresignificant spatial correlations in pi_Hip. We derive a distance modulus to thePleiades of 5.58 +- 0.18 mag using the radial-velocity gradient method. Thisvalue, agrees very well with the distance modulus of 5.60 +- 0.04 magdetermined using the main-sequence fitting technique, compared with the valueof 5.33 +- 0.06 inferred from the average of the Hipparcos parallaxes of thePleiades members. We show that the difference between the main-sequence fittingdistance and the Hipparcos parallax distance can arise from spatiallycorrelated errors in the Hipparcos parallaxes of individual Pleiades members.Although the Hipparcos parallax errors towards the Hyades are spatiallycorrelated in a manner similar to those of the Pleiades, the center of theHyades is located on a node of this spatial structure. Therefore, the parallaxerrors cancel out when the average distance is estimated, leading to a meanHyades distance modulus that agrees with the pre-Hipparcos value. We speculatethat these spatial correlations are also responsible for the discrepantdistances that are inferred using the mean Hipparcos parallaxes to some openclusters. Finally, we note that our conclusions are based on a purely geometricmethod and do not rely on any models of stellar isochrones.Comment: 33 pages including 10 Figures, revised version accepted for publication in Ap