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Abstract:

Using 54 well defined members of the Pleiades cluster, a common proper motion and mean parallax of these stars is determined. The solution made use of the Hipparcos intermediate astrometric data in order to eliminate correlations that occur between astrometric parameter determinations for stars in a small area on the sky. This method also made it possible to incorporate ground-based relative proper motions with 10 times higher precision than the Hipparcos proper motions as additional constraints. The parallax result for the cluster is in considerable disagreement with previous (mainly photometric) determinations, putting the cluster some 15 per cent nearer to the Sun than was expected. At this moment there are no spectral or photometric indications that explain this difference between the photometric and the astrometric distance modulus for the cluster. This then creates a considerable uncertainty in other photometric distance calibration, which may contain similar discrepancies at the level of 0.1 to 0.4 mag. Key words: space astrometry; Pleiades; luminosity calibrations. 1.

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