The ages of the globular clusters M 71 and 47 Tuc from Strömgren uvby photometry ^*,**

Evidence for high ages

¹ Institute of Physics and Astronomy, Aarhus University, Ny Munkegade, 8000 Aarhus C, Denmark
² National Research Council, Herzberg Institute of Astrophysics, Dominion Astrophysical Observatory, 5071 W. Saanich Road, B.C., V9E 2E7, Canada
³ Division of Astronomy, University of Oulu, PO Box 3000, 90014 Oulun Yliopisto, Finland

Corresponding author: F. Grundahl, fgj@phys.au.dk

Received: 19 February 2002
Accepted: 21 May 2002

Abstract

New uvby CCD photometry for the fairly metal–rich globular clusters M 71 (NGC 6838) and 47 Tuc (NGC 104) is presented. We derive the cluster distances using a sample of field subdwarfs with metallicities determined from uvby photometry and accurate parallaxes from the Hipparcos mission. The biases associated with the main-sequence fitting technique are discussed and only that due to metallicity is found to be significant, corresponding to a -0.05 mag change in distance modulus. Our main results are that: 1) The distance moduli of 47 Tuc and M 71 are somewhat shorter than that derived by Reid ([CITE], AJ 115, 204). For M 71 and 47 Tuc we find (metallicity corrected) and , for adopted reddenings of and respectively (first errorbar denotes random errors and the second systematic errors). The main source of difference with Reid is the selection of subdwarfs with this study having more intrinsically faint field subdwarfs; 2) These values lead to ages of nearly 12 Gyr when using the isochrones of VandenBerg et al. ([CITE], ApJ, 532, 430); this estimate does not include the effects of He diffusion. 3) A differential comparison of the cluster colour-magnitude diagrams show that the age difference between the two is very small – less than one billion years. 4) The observed scatter in the c₁ index (due to star–to–star nitrogen variations) among main–sequence stars does not allow us to use the diagram for a distance-independent age determination.