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Astron. Astrophys. 335, L65-L68 (1998)

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1. Introduction

The Tycho Reference Catalogue (TRC) is presented, containing high-quality positions and proper motions for [FORMULA] stars of the Tycho Catalogue. It is suited as a source of astrometric reference stars and of proper motions for galactic kinematic studies. The catalogue is made available from the CDS.

The proper motions were derived from Tycho positions and Astrographic Catalogue (AC) positions reduced to the Hipparcos system. This undertaking was envisaged when the Tycho project was proposed (Hog et al. 1982) and was elaborated later on (R"oser & Hog 1993). The resulting TRC Catalogue contains between 4 and 117 stars per square degree, depending on galactic latitude.

A new reduction of all Tycho observations is progressing which will result in a Tycho2 Catalogue (Hog 1997) with about 2.5 million stars, including proper motions derived from the AC positions. In parallel, a second version of TRC, the TRC2, is in preparation for which a more accurate (block-adjustment) reduction of the AC will be applied (Kuimov 1997; Kuzmin et al. 1997b).

The progress in terms of size and quality of astrometric reference catalogues during the recent years is illustrated in Table 1. The catalogues are listed in their sequence of publication, including those expected in the near future.


Table 1. Astrometric reference catalogues. Date of publication, number of stars, and median precision at the Hipparcos epoch of 1991.25 are given. Systematic errors are about 150 mas and 2.5 mas/yr for the PPM and ACRS, but less than 1 mas and 1 mas/yr for the catalogues based on Hipparcos.

The SAO, ACRS and PPM catalogues are based on meridian and photographic observations. The PPM (Röser & Bastian 1991, Bastian et al. 1993) included, for the first time, the century-old Astrographic Catalogue observations. This catalogue actually comprises about two dozen catalogues of positions at mean epoch B1905.0, containing 8.6 million photographic observations of about 4.5 million stars brighter than [FORMULA]. The observations were only available in printed volumes until the Sternberg Astronomical Institute (Nesterov et al. 1990, Kuzmin et al. 1997a) and, in parallel, the US Naval Observatory in 1987-1994 produced their machine readable versions. Independent reductions of the data sets onto the ICRS/Hipparcos system were recently performed (Kuzmin et al. 1998, Urban et al. 1998a).

The Astrographic Catalog Reference Stars, ACRS (Corbin & Urban 1991) was prepared especially for the planned re-reduction of the AC and therefore did not include that data set, as PPM did. The ACRS was also exploited in the construction of the TRC (see Sect. 2.1), because HIP does not provide a sufficient density of stars.

The Hipparcos and Tycho Catalogues, HIP and TYC, (ESA 1997) provide a much more accurate coordinate system of positions and proper motions. They were immediately exploited to improve the positional coordinate system of the AC and in combination with the Tycho positions to produce the proper motions contained in the `Astrographic Catalog and Tycho' (ACT) catalogue (Urban et al. 1998b) and TRC.

The comparison in Sect. 3 of the proper motions in TRC with those in ACT and with the proper motions in the Hipparcos and Tycho Catalogues assesses its quality as far as presently possible. A truly external comparison is not possible, but the comparison with the TYC proper motions is of particular interest because these did not contribute to the construction of TRC. The comparison with ACT illustrates, only too well, the occasional ambiguities in identifying TYC stars in the AC, which in spite of all efforts have lead to errors in either ACT or TRC.

Positions published in TRC are given for the epoch J2000.0. They were derived by applying proper motion for the short time interval between J2000.0 and the respective weighted mean epoch. The mean epochs are typically about 1987, quite close to the Tycho epoch, J1991.25, because the Tycho position was assigned a much higher weight in the adjustment than the AC positions. TRC positions calculated for the TYC epoch will not quite coincide with the TYC positions. The difference is generally below 2 mas, but larger differences, supposedly improvements of TYC, of about [FORMULA], may occur when a star has two or more widely separated AC epochs. The median external standard error of positions in TRC at J1991.25 is the same as for TYC, i.e., about 40 mas.

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998