We neglected perspective effects and assumed that all cluster stars have similar proper motions and parallaxes. This is justified by the large distances and resulting small angular diameters of the clusters studied. Due to the large distances the internal motions of the cluster stars can also be neglected.
Stars included in the Double and Multiple Systems Annex of the Hipparcos Catalogue, i.e. with C, G, V, O or X entries in field H59, were not used to derive the mean astrometric parameters, since the astrometric solution derived by Hipparcos is affected by the binary nature of these stars, as described in Robichon et al. (1999).
The mean astrometric parameters were kindly calculated for us by Floor van Leeuwen. For clusters containing more than one star, he used the method described in van Leeuwen & Evans (1998) to calculate the mean astrometric parameters. This method takes into account the correlations that exist between the abscissae residuals of Hipparcos stars measured on the same reference great circles. These correlations increase with decreasing angular separation and are important in our case due to the small angular diameters of the clusters studied. No such correlations exist for clusters containing only one star, so we took the solution for these clusters directly from the solution given for the single member in the Hipparcos Catalogue.
Alternatively, we also calculated the astrometric parameters without taking small-scale correlations into account. This was done to estimate the influence of these correlations. For that purpose, the cluster solutions were obtained from the member stars by taking the mean of their Hipparcos solutions.
Copyright The European Southern Observatory (ESO)