Computing the Parallax of the Pleiades from the [ITAL]Hipparcos[/ITAL] Intermediate Astrometry Data: An Alternative Approach

The inconsistency between the mean parallax of the Pleidaes open cluster from the Hipparcos catalog and that obtained from the stellar evolution theory and photometric measurements is probed by recomputing the Hipparcos data in a different way that reduces the propagation of the along-scan attitude errors. This is achieved by coupling observations of stars made nearly simultaneously in the two separate fields of view of the telescope. A direct calculation of astrometric quantities of 54 Pleiades members by the new method, based on the Intermediate Astrometry Data, provides a correction of -0.71 ± 0.14 mas to the weighted mean parallax of the cluster. The mean corrected parallax of the Pleiades is 7.75 ± 0.20 mas.