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Astron. Astrophys. 330, 515-520 (1998)

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1. Introduction

RR Lyraes are one of the primary distance indicators. Within the Galaxy they are used to determine the distances to the Globular Clusters and hence their ages by matching their Colour-Magnitude diagrams with theoretical isochrones (eg Chaboyer et al. 1996). Since the Globular Clusters are amongst the oldest objects in the Galaxy this sets a lower limit to the age of the Universe. RR Lyraes are also used to determine distances to other Local Group galaxies where they serve as a check on brighter distance indicators, eg Cepheids. They are thus an important step on the path to determining the Hubble Constant (eg Freedman et al. 1997). In combination with a Cosmological Model, the Hubble Constant sets an upper limit to the age of the Universe and a comparison of this age with that of the Globular Clusters provides an important check on several key areas of astronomy and astrophysics (the distance scale, cosmology, stellar structure and evolution, etc.).

For these reasons there is a considerable body of work in the literature on the subject of RR Lyrae absolute magnitudes (see Feast 1997 for a review of this). In the present paper we use HIPPARCOS trigonometric parallaxes to estimate [FORMULA] directly (Sect. 3) and HIPPARCOS proper motions to estimate it using the method of Statistical Parallax (Sect. 4). Finally the implications of these results for firstly, the distance to the LMC and secondly, the ages of the Globular Clusters are discussed (Sect. 5) and summarised (Sect. 6). We begin by presenting the basic data (Sect. 2).

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998