The Pleiades question, the definition of the zero-age main sequence, and implications

Abstract

Abstract. Determinations and measurements of the parallax of the Pleiades, obtained with ground-based studies and with Hipparcos data, are reviewed. A number of uncertainties in both sets of data are found. Although a further correction for abscissa correlations brings the Hipparcos determination closer to the ground-based value, the difference still seems too large. The new Hipparcos parallax determinations seem to reinforce a possible age related effect. A new reduction of the Hipparcos data is in progress. It reduces significantly the contribution of the attitude noise, providing higher accuracies and lower correlation levels for the brighter stars.