On the use of the Hipparcos intermediate astrometric data

With the publication of the Hipparcos catalogue, astrometric data of unprecedented quality and quantity has become available, providing parallaxes, positions and proper motions free from systematic errors down to a level of at least 0.1 mas and 0.1 mas y-1. The Hipparcos catalogue, however, contains not only these "ready-to-use” positions, parallaxes and proper motions, but also intermediate astrometric data or abscissa residuals, which are the data from which the astrometric solutions were obtained. These data allow alternative solutions to be made for the astrometric parameters, for example, through the use of additional information. When combining data from stars in a small area on the sky, it becomes possible to account for correlations that exist between the abscissa residuals for stars measured on the same great circle. This is relevant for stars in open clusters and the Magellanic Clouds, where such correlations will be very frequent. The intermediate data also provide the possibility to add external constraints to an astrometric solution, such as an approximate but small parallax value, one that would have been too small to measure with Hipparcos. In that case the parallax can be fixed at the estimated small value, giving a better constrained solution for the proper motion. Similarly, when for a group of stars the absolute magnitudes are linked through a period-luminosity relation or by being all closely the same, as for RR Lyrae stars, such a condition can be superimposed on the parallax solution for all stars in this group, providing a distance scale calibration well beyond the range of direct parallax measurements. An example of how to use the data for solar system objects, which are provided only in the form of intermediate astrometric data, is shown