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Astron. Astrophys. 360, 399-410 (2000)

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1. Introduction

Polaris ([FORMULA] Ursae Minoris, HR 424, HD 8890, ADS 1477, FK 907, HIP 11767) is a very interesting and important object, both from the astrophysical point of view and from the astrometric one. For astrophysics, the most remarkable feature of the multiple stellar system Polaris is the fact that its main component, namely [FORMULA] UMi A, is a Cepheid variable with a very unusual behaviour. In astrometry, Polaris is one of the most frequently and accurately observed objects, mainly because it is located so close to the North celestial pole and can be used for calibration purposes.

Up to now, the binary nature of Polaris was essentially neglected in ground-based fundamental astrometry, e.g in the FK5 (Fricke et al. 1988). This was justified by the limited accuracy reached by the meridian-circle observations. Now, the high-precision astrometric measurements carried out with the HIPPARCOS satellite (ESA 1997) require strongly to take into account the binary nature of Polaris in order to obtain an adequate astrometric description of [FORMULA] UMi. Similar procedures are required for many other binaries among the fundamental stars in order to be included properly into the Sixth Catalogue of Fundamental Stars (FK6; Part I: Wielen et al. 1999c; see also Wielen et al. 1998).

The main purpose of the present paper is to obtain a reliable astrometric orbit for Polaris, and to use this astrometric orbit for obtaining high-precision values for the position and proper motion of Polaris. This is done by combining the known spectroscopic orbit of Polaris with ground-based astrometric data given in the FK5 and with the HIPPARCOS results. Before doing so in the Sects. 3 and 4, we present in Sect. 2 an overview of the Polaris system.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000