7 Conclusions

  The Hipparcos Transit Data, published as part of the Hipparcos and Tycho Catalogues (ESA 1997), provide data from an intermediate step in the data reduction process of the NDAC data analysis consortium. Transit Data are given for all known or suspected double or multiple star systems in the Hipparcos Catalogue, or about a third of the objects in the catalogue. There were several reasons to include these data in the published catalogue. A main reason was the realization that every stellar system could not be examined for every possible type of solution within the time available for completing the catalogue. Ideally, such examination should also take into account ground-based observations. The Transit Data allow the user to re-examine such solutions, should the need arise.

Basically the Transit Data contain the Fourier coefficients which describe the modulation of the detector signal caused by the object's motion across the modulating grid. As such they retain all the photometric and astrometric information on the object gathered during its transit across the grid. The Transit Data have been carefully calibrated and referred to the Hipparcos photometric and astrometric systems, so that any data derived from them should be directly comparable with other results in the published catalogue.

By giving this review of how the Transit Data were recorded, what they physically represent and examples of their practical uses, we hope to encourage readers to utilize the data in their own exploitations of the Hipparcos results. To aid this process, we have made programs available which provide interfaces with publicly available software packages, in particular Difmap and GaussFit. Many other applications could be thought of - the combination of Transit Data with ground-based speckle observations of double stars to improve orbits, parallaxes and mass ratios by Söderhjelm (1999) is an example. What has been covered and demonstrated here might just be a stepping stone to new and creative exploitations of the Transit Data.

Acknowledgements

Part of this work was supported by the Kungl. Fysiografiska Sällskapet i Lund and the Swedish National Space Board. We would also like to thank John Conway for his expertise and help in adapting the Transit Data for aperture synthesis imaging and to Martin Shepherd for his help with Difmap.