We report on the progress of the Sixth Catalogue of Fundamental Stars (FK6) which combines the HIPPARCOS Catalogue with the ground-based data of the basic FK5 in the HIPPARCOS system. The FK6 will be the catalogue with the most accurate proper motions: Due to the combination of the long history of ground-based astrometry with the high accuracy of HIPPARCOS, the individual (random) accuracy of the FK6 proper motions is twice as good as that of the HIPPARCOS proper motions or the FK5 proper motions for truely single stars. However, many of the fundamental stars are binaries. A large fraction of these binaries are (individually) still not known to be double. Therefore the sample of `apparently single stars' is significantly affected by cosmic errors in the HIPPARCOS proper motions and positions. Using the concepts of statistical astrometry, we derive (in addition to the `single-star mode') also `long-term predictions' and `short-term predictions' for the positions of the FK6 stars as a function of time. On average, the predictions by the `long-term mode' of the FK6 should be better by a factor of about 4 than predictions based on the HIPPARCOS data for the sample of apparently single stars, which are affected by cosmic errors because of the contamination by undetected astrometric binaries.
The objects known to be binaries cause often severe problems in the process of combining the FK5 data with the HIPPARCOS data. From the 1535 basic fundamental stars, 97 stars have been treated as visual binaries (C solutions) by HIPPARCOS, 78 as astrometric binaries (O solutions), 10 as VIMs (variability-induced movers, V solutions), 95 as objects with acceleration terms (G solutions), and 22 as objects with stochastic (X) solutions. Hence in total, 20% of the FK5 stars are treated as binaries by HIPPARCOS. In addition, many other known double stars in the FK5 had no special handling by HIPPARCOS. We offer solutions for the two main problems: (1) to harmonize the FK5 and HIPPARCOS data with respect to a common reference point (component A, photo-center, center-of-mass), (2) to handle stars with motions known to be non-linear. Many of the double stars in the FK6 require a rather individual treatment.
Preprint (in Postscript format)
Roland Wielen, Heiner Schwan, Christian Dettbarn, Hartmut Jahreiß, Helmut Lenhardt: The Sixth Catalogue of Fundamental Stars (FK6) and the problem of double stars. In: Peter Brosche, Wolfgang R. Dick, Oliver Schwarz, Roland Wielen (Eds.): The Message of the Angles - Astrometry from 1798 to 1998. Proceedings of the International Spring Meeting of the Astronomische Gesellschaft, Gotha, May 11-15, 1998. (Acta Historica Astronomiae ; 3). Thun ; Frankfurt am Main : Deutsch, 1998, p. 123-132.
Html-Version: Wolfgang R. Dick. Created: 21 Jan 1999