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Astron. Astrophys. 360, 391-398 (2000)

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1. Introduction

Accurate and reliable astrometric data are of crucial importance in many fields of Galactic astronomy. Any study which covers an area on the sky larger than a few tens of square degrees (the typical size of a photographic plate), or which needs absolute proper motions, might suffer from unknown systematic effects in data sets which are not absolute and coherent over the entire sky. These studies include, for example, investigations of the stellar kinematics and dynamics in the Solar neighbourhood (see, e.g., Dehnen & Binney 1998), the search for extended moving groups (see, e.g., Chereul et al. 1998and de Zeeuw et al. 1999), and the search for Galactic halo streamers (Helmi et al. 1999). For such studies the reliability of the conclusions depend sensitively on the quality of the astrometric data.

In this paper we discuss a comparison of the proper motions in four all-sky astrometric catalogues: the Hipparcos Catalogue (HIP) (ESA 1997), the Tycho Catalogue (TYC) (ESA 1997), the Tycho Reference Catalogue (TRC) (Kuzmin et al. 1999), and the Astrographic Catalogue plus Tycho Reference Catalogue (ACT) (Urban et al. 1998). This study was triggered by the discrepancies in proper motions between these astrometric catalogues found by Hoogerwerf (2000) in a study of astrometric membership determination of moving groups using the TRC and ACT catalogues.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000