The Hipparcos Pleiades parallax error is also a proper motion error

TL;DR

This paper reassesses the Hipparcos Pleiades parallax error using Gaia data, confirming it is also a proper motion error, and discusses its implications for current astrometry.

Contribution

It demonstrates that the Hipparcos Pleiades parallax error is primarily a proper motion error, validated by Gaia data, and proposes a correction method based on vector spherical harmonics.

Findings

Gaia-Hipparcos differences are consistent with previous estimates.

Proper motion error in Hipparcos is about (+0.39, -0.74) mas/yr.

Implications for small-scale errors in current astrometry are discussed.

Abstract

The mean parallax of the Pleiades open cluster from the Hipparcos catalog is larger than the true value by approximately 1 mas. The origin of this error, as well as a possible algorithm of correcting it, was proposed by Makarov (2002). The problem is reassessed using the more accurate Gaia data with a focus on the predicted correction to the Pleiades proper motions. The accurately determined differences Gaia - Hipparcos for 52 common stars are close to these estimates within the formal uncertainties for all three parameters, which strongly suggests that the proposed interpretation was correct. With adjustments for the systematic vector field fitted with 126 vector spherical harmonics to degree 7, these differences amount to $(+0.39,\,-0.74)$ mas yr$^{-1}$. The implications of small-scale proper motion and position errors in Hipparcos for present day astrometry are briefly discussed.