617 research outputs found
The combination of ground-based astrometric compilation catalogues with the HIPPARCOS Catalogue. II. Long-term predictions and short-term predictions
The combination of ground-based astrometric compilation catalogues, such as
the FK5 or the GC, with the results of the ESA Astrometric Satellite HIPPARCOS
produces for many thousands of stars proper motions which are significantly
more accurate than the proper motions derived from the HIPPARCOS observations
alone. In Paper I (Wielen et al. 1999, A&A 347, 1046) we have presented a
method of combination for single stars (SI mode). The present Paper II derives
a combination method which is appropriate for an ensemble of 'apparently
single-stars' which contains undetected astrometric binaries. In this case the
quasi-instantaneously measured HIPPARCOS proper motions and positions are
affected by 'cosmic errors', caused by the orbital motions of the photo-centers
of the undetected binaries with respect to their center-of-mass. In contrast,
the ground-based data are 'mean values' obtained from a long period of
observation. We derive a linear 'long-term prediction' (LTP mode) for epochs
far from the HIPPARCOS epoch T_H ~ 1991.25, and a linear 'short-term
prediction' (STP mode) for epochs close to T_H. The most accurate prediction
for a position at an arbitrary epoch is provided by a smooth, non-linear
transition from the STP solution to the LTP solution. We present an example for
the application of our method, and we discuss the error budget of our method
for the FK6 (a combination of the FK5 with HIPPARCOS) and for the combination
catalog GC+HIP. For the basic fundamental stars, the accuracy of the FK6 proper
motions in the LTP mode is better than that of the HIPPARCOS proper motions
(taking here the cosmic errors into account) by a factor of more than 4.Comment: Slightly revised version. Accepted for publication in Astronomy and
Astrophysic
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