A Precision Test of Hipparcos Systematics toward the Hyades

We propose a test that can in principle detect any systematic errors in the Hipparcos parallaxes toward the Hyades cluster at the level of 0.3 mas. We show that the statistical parallax algorithm subsumes the classical moving cluster methods and provides more precise 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. To test the Hipparcos parallaxes, we first rescale the bulk velocity determined from statistical parallax to force agreement with the distance scale determined from Hipparcos parallaxes. We then predict the parallaxes of Hyades cluster members using this common cluster space velocity and their Hipparcos proper motions. We show that the parallaxes determined in this manner (π_pm) are consistent at the 1 σ level with the parallaxes (π_orb) of three Hyades spectroscopic binary systems with orbital solutions. We find that ⟨π_pm-π_orb⟩=0.52±0.47 mas, where the error is dominated by the errors in the orbital parallaxes. A reduction in these errors 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 (V_x,V_y,V_z)=(-5.70±0.20, 45.62±0.11, 5.65±0.08) kms⁻¹, its velocity dispersion is 320±39 m s⁻¹, and the distance modulus to the centroid of our sample of 43 cluster members is 3.34±0.02 mag.