Correlated Errors in Hipparcos Parallaxes towards the Pleiades and the Hyades

We show that the errors in the Hipparcos parallaxes towards the Pleiades and the Hyades open clusters are spatially correlated over angular scales of 2 to 3 degrees, with an amplitude of up to 2 mas. We suggest that this correlation on small angular scales is a generic feature of the Hipparcos parallaxes all over the sky. We predict the parallaxes of individual cluster members, πpm, from their Hipparcos proper motions, assuming that all the cluster members move with the same space velocity. We compare these parallaxes with their Hipparcos parallaxes, πHip, and find that there are significant spatial correlations in the latter quantity. We derive a distance modulus to the Pleiades of 5.58± 0.18 mag from the gradient in the radial velocities of the Pleiades members in the direction parallel to the proper motion of the cluster. This value, derived using a geometric method, agrees very well with the distance modulus of 5.60± 0.04 mag determined using the main-sequence fitting technique, but it is in conflict with the value of 5.33± 0.06 mag inferred from the average of the Hipparcos parallaxes of the Pleiades members. The Hipparcos parallaxes of all the Pleiades members that are within a 3◦×3◦ region of the cluster center are all systematically larger than πpm, thereby leading to a shorter mean distance to the Pleiades. Although the Hipparcos parallax errors towards the Hyades are spatially correlated in a manner similar to those of the Pleiades, the center of the Hyades is located on a node of this spatial structure. Therefore, the parallax errors cancel out when the average distance is estimated, leading to a mean Hyades distance modulus that agrees with the pre-Hipparcos value. We speculate that these spatial correlations are also responsible for the discrepant distances that are inferred using the mean Hipparcos parallaxes to some open clusters, although an agreement between the mean Hipparcos parallax distance and the main-sequence fitting distance to other clusters does not necessarily preclude spatially correlated Hipparcos parallax errors. Subject headings: Astrometry: Parallaxes, Methods: Analytical, Statistical, Galaxy: Open clusters and Associations: Individual (Hyades, Pleiades) Alfred P. Sloan Foundation Fellow