A Precision test of Hipparcos systematics towards the Hyades

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  • 25 August 1998
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)> = 0.49 +- 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. We also find that the parallaxes determined using the Hipparcos proper motions and the common cluster velocity are consistent with the Hipparcos parallaxes themselves, thus confirming that the Hipparcos astrometric results towards the Hyades cluster are self consistent. Along the way, we determine the velocity of the Hyades cluster in equatorial coordinates to be (Vx,Vy,Vz) = (-5.70 +- 0.19, 45.67 +- 0.11, 5.61 +- 0.08)km/s and its velocity dispersion to be (330 +- 30) m/s.

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