A Precision Test ofHipparcosSystematics toward the Hyades

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Abstract
We propose and apply a test that can detect any systematic errors in the Hipparcos parallaxes towards the Hyades cluster at the level of 0.3 mas. We show that the statistical parallax method subsumes the classical moving cluster methods and provides more accurate and robust estimates of the distance and the first two moments of the velocity distribution of the Hyades cluster namely, its bulk space velocity and the velocity dispersion tensor. We predict the parallaxes of Hyades cluster members using the common cluster space velocity derived from the statistical parallax method and their individual Hipparcos proper motions. We show that the parallaxes determined in this manner (pi_pm) are consistent at the 1-sigma level with the parallaxes (pi_orb) of three Hyades spectroscopic binary systems with orbital solutions. We find that <(pi_pm - pi_orb)> = 0.52 +- 0.47 mas, where the error is dominated by the errors in the orbital parallaxes. A reduction in this error would allow a test of the systematic errors in the Hipparcos parallaxes at the 0.3 mas level. If the Hyades distance scale is fixed by Hipparcos parallaxes, then its bulk velocity in equatorial coordinates is (Vx,Vy,Vz) = (-5.70 +- 0.20, 45.62 +- 0.11, 5.65 +- 0.08)km/s, its velocity dispersion is 320 +- 39 m/s, and the distance modulus to the centroid of our sample of 43 cluster members is 3.34 +- 0.02 mag.Comment: 23 pages including 2 Figures and 2 Tables, revised version to appear in ApJ, Apr 10, 1999, Vol. 515#1, includes a new section listing the "Assumptions" and a more detailed description of cluster membershi