Correlated errors in Hipparcos parallaxes towards the Pleiades and the Hyades

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  • 21 October 1998
Abstract
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 deg, 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, pi_pm, from their Hipparcos proper motions, assuming that all cluster members have the same space velocity. We compare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there are significant spatial correlations in pi_Hip. We derive a distance modulus to the Pleiades of 5.58 +-0.18 mag using the radial-velocity gradient method. This value agrees very well with the distance modulus of 5.60 +- 0.04 mag using the main-sequence fitting technique, but it is in conflict with the value of 5.33 +- 0.06 mag inferred from the mean Hipparcos parallaxes of the Pleiades members. The Hipparcos parallaxes of all the Pleiades members that are within a 3 deg x 3 deg region of the cluster center are all systematically larger than pi_pm, thereby leading to a shorter mean distance to the Pleiades. Although the Hipparcos parallax errors towards the Hyades are also spatially correlated, the center of the Hyades is on a node of this spatial structure. Therefore, the parallax errors cancel out in an average distance estimate, leading to a mean Hyades distance that agrees with the pre-Hipparcos value. We speculate that the spatial correlations are also responsible for the discrepant distances that are inferred using the mean Hipparcos parallaxes to other open clusters, although an agreement with the pre-Hipparcos distances does not necessarily preclude spatially correlated Hipparcos parallax errors.

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