2. Determining cluster parallaxes and proper motions from Hipparcos data
The combined abscissae method used to determine the parallax and proper motion of an open cluster from the Hipparcos data has been presented by van Leeuwen (1997) and van Leeuwen and Evans, 1998 (VLE98). It combines the intermediate astrometric data (abscissa residuals, ESA, 1997) for single cluster members, and accounts for correlations between measurements obtained on the same great circle. The degrees of freedom are reduced with respect to the sum of the individual star solutions, which makes the astrometric-parameter estimates more robust. Correlations between position and parallax determinations (pointed out by Pinsonneault as a possible explanation for the larger than expected parallax for the Pleiades) are much smaller in the combined solution, which also provides, through its variance (column 10 in Table 1), a parameter that indicates whether the modelling assumptions (a shared proper motion and parallax) were justified.
Table 1. Hipparcos astrometric paramaters for open clusters
Similar methods have been developed by F. Arenou and N. Robichon in Paris, and earlier results were in some cases discrepant. This was in particular the case for Praesepe, where VLE98 presented a parallax 5.04 mas, while Merm97 gave 5.65 mas (the value now agreed is ). In a joint effort with N. Robichon, discrepancies between the two implementations were all sorted out and explained as due to minor implementation mistakes. The results presented in the present paper have been derived with the corrected implementation, and are essentially the same as would be obtained from the same selection of stars by the implementation of Arenou and Robichon.
© European Southern Observatory (ESO) 1999
Online publication: December 16, 1998